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Role of Water in the Formation, Transformation and Fate of
Secondary Organic Aerosol
by
Jenny Pui Shan Wong
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Graduate Department of Chemistry
University of Toronto
© Copyright by Jenny Pui Shan Wong (2015)
ii
Role of Water in the Formation, Transformation and Fate of
Secondary Organic Aerosol
Jenny Pui Shan Wong
Doctor of Philosophy
Department of Chemistry
University of Toronto
2015
Abstract
Particle-phase water is the most abundant atmospheric aerosol constituent, yet the importance of
water for the formation, transformation and fate of secondary organic aerosol (SOA) remains not
fully characterized. In order to address this knowledge gap, this thesis explores the role of water
in SOA formation, chemical aging and fate as cloud condensation nuclei.
The formation of SOA from the photooxidation of isoprene in the presence of various sulfate
seed particles was investigated using a flow tube reactor. Under constant environmental
conditions, particle-phase water was found to have the largest effect on the amount of SOA
formed where this additional organic material was highly oxidized, likely arising from enhanced
uptake of organic acids due to their high water solubility. The amount of high molecular weight
compounds increases with acidity, suggesting the role of acidity in governing organic
composition.
The relative humidity (RH) dependence of SOA aging by photolysis was examined using
particles containing water-soluble α-pinene SOA material in an environmental chamber at three
RH conditions (5, 45 and 85 %). Photolysis led to substantial mass loss where the rate of mass
loss increased with increasing RH, suggesting that moisture-induced changes in SOA phase have
implications to particle reactivity.
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Aging of ambient SOA sampled at Whistler, British Columbia found that aging by both gas and
aqueous-phase OH increased the degree of oxygenation and CCN activity of the organic
material, confirming the hypothesis that there is a simple relationship between the hygroscopicity
of organic aerosol and its oxygen-to-carbon ratio.
Addition of various types of organic material onto sulfate particles resulted in the suppression of
water uptake during droplet growth. Experiments using sulfate particles with different acidity
suggest that high molecular weight compounds, formed via acid-catalyzed condensed phase
reactions, are the species affecting water uptake kinetics.
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Acknowledgements
First and foremost, this thesis would not be possible without the support and guidance from my
supervisor, Jon Abbatt. A short acknowledgement simply does not reflect the magnitude of
appreciation I have for the role Jon has played in my development as a scientist throughout the
years. Being believed in – it made a huge difference for me. Jon, thank you for everything.
Many thanks to my committee members, Jennifer Murphy and Jamie Donaldson, for their
mentorship and encouragements. I've had the opportunity to work as a RAP student at
Environment Canada under the supervision of John Liggio - thank you for making the
opportunity possible and for all your advice. I am lucky to have had the opportunity to work in
Jamie's and Rebecca Jockusch's research groups as an undergraduate. Thank you for fostering
my interest in research and leading me to believe that my ideas are worth pursuing.
I am so grateful to have shared this journey with past and present members of the Abbatt group.
There is great camaraderie among the members of the group and I value all the interactions I
have with each and every one of you, both in and outside the lab. Special thanks to the
phenomenal post-docs I have had the pleasure to work with: Jay Slowik, Shouming Zhou and
Alex Lee. From working with all of you, not only have I learned a lot about the science itself, but
how to be great scientists as well.
The environmental chemistry bunch has been an amazing - the people really makes this a very
special place.
Friends and family - thank you. Joshua, you have been a pillar of support. Your kindness and
patience are immeasurable. Lastly, I would like to thank my siblings and mother - for your
unwavering strength, courage and unity, despite all odds. I am so immensely proud and honoured
to be your sister and daughter.
v
Table of Contents
ACKNOWLEDGMENTS IV
TABLE OF CONTENTS V
LIST OF TABLES VIII
LIST OF FIGURES IX
PREFACE XIV
1 INTRODUCTION 1
1.1 Motivation 2
1.2 Formation and Aging of SOA 2
1.2.1 SOA Formation 3
1.2.2 SOA Aging Processes 5
1.3 Secondary Organic Aerosol Particles as Cloud Condensation Nuclei 10
1.3.1 CCN Activation 11
1.3.2 Droplet Growth Kinetics 14
1.4 Experimental Methods for SOA Studies 15
1.4.1 Approaches for SOA Formation and Aging 15
1.4.2 Chemical Analysis of SOA by Mass Spectrometry 17
1.4.3 Approaches for CCN Measurements 18
1.5 Summary of Research Objectives 19
1.6 References 20
2 IMPACTS OF SULFATE SEED ACIDITY AND WATER CONTENT ON
ISOPRENE SECONDARY ORGANIC AEROSOL FORMATION 30
2.1 Abstract 31
2.2 Introduction 31
2.3 Experimental Section 32
2.3.1 Seed Particle Generation and Phase Characterization 33
2.3.2 SOA Formation 34
2.3.3 Particle Measurements 35
2.4 Results and Discussion 37
2.4.1 Ammonium Sulfate Particles 37
2.4.2 Acidic Sulfate Particles 40
2.5 Atmospheric Implications 42
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2.6 Supporting Material 43
2.7 References 43
3 CHANGES IN SECONDARY ORGANIC AEROSOL COMPOSITION AND MASS
DUE TO PHOTOLYSIS: RELATIVE HUMIDITY DEPENDENCE 48
3.1 Abstract 49
3.2 Introduction 49
3.3 Experimental Section 51
3.3.1 SOA Generation and Collection 51
3.3.2 UV/Vis Absorption Spectroscopy 52
3.3.3 Photolysis of SOA 52
3.3.4 Particle Measurements 55
3.4 Results and Discussion 56
3.4.1 Absorption Cross Section of SOA 56
3.4.2 Photolytic Degradation of SOA 57
3.4.3 Kinetics of Photolysis 63
3.5 Conclusions and Atmospheric Implications 65
3.6 Supporting Material 67
3.6.1 Absorption Cross Section Calculations 67
3.7 References 67
4 OXIDATION OF AMBIENT BIOGENIC SECONDARY ORGANIC AEROSOL BY
HYDROXYL RADICALS: EFFECTS ON CLOUD CONDENSATION NUCLEI
ACTIVITY 71
4.1 Abstract 72
4.2 Introduction 72
4.3 Experiment 73
4.3.1 Gas-phase OH Oxidation 74
4.3.2 Aqueous-phase OH Oxidation 75
4.3.3 Particle Composition and CCN Measurements 76
4.4 Results and Discussion 77
4.5 Summary 81
4.6 Supporting Material 81
4.7 References 84
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5 SUPPRESSION IN DROPLET GROWTH KINETICS BY THE ADDITION OF
ORGANICS TO SULFATE PARTICLES 87
5.1 Abstract 88
5.2 Introduction 88
5.3 Experiment 90
5.4 Results and Discussion 94
5.4.1 α-Pinene SOA 94
5.4.2 Carbonyls 97
5.4.3 Engine Exhaust 99
5.5 Interpreting the Degree of Droplet Growth Suppression 100
5.6 Conclusions and Atmospheric Implications 101
5.7 Supporting Material 103
5.8 References 103
6 CONCLUSIONS AND FUTURE RESEARCH 109
6.1 Summary and Future Direction of SOA Formation Studies 109
6.2 Summary and Future Direction of SOA Aging Studies 109
6.3 Summary and Future Direction of SOA-CCN Studies 111
6.4 References 112
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List of Tables
Table 3.1 Total Organic Mass Loss Rate Constants for Different RH
Conditions
64
Table 4.1 Control experiments: particle properties of ambient organic aerosol.
Absolute mass (µg m-3
) of the various components is noted in the
brackets.
81
Table 4.2 Particle properties, and CCN activity of ambient organic aerosol.
Absolute mass (µg m-3
) of the various components is noted in the
brackets for the gas-phase OH oxidation experiments. For aqueous
OH oxidation experiments, 24-hr particle filter sample was obtained
from 08:00 to 08:00 of the following day.
83
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List of Figures
Figure 1.1 General schematic of the SOA formation pathway (adapted from
Seinfeld & Pankow6).
3
Figure 1.2 Reaction scheme for gas-phase OH + isoprene21–23
and products
formed in the particle-phase following reactive uptake.16,17,19
4
Figure 1.3 Schematic diagram of the processes involved in the reactive uptake
of a gas molecule onto a particle (adapted from Abbatt et al.31
).
6
Figure 1.4 Non-oxidative condensed phase reactions that increases the
molecular weight of organic compounds in SOA (adapted from
Hallquist et al.3).
7
Figure 1.5 The evolution of aerosol mass (left axis) and O:C ratio (right axis)
with OH exposure (data from Kroll et al.34
).
9
Figure 1.6 An illustrated representation of the formation, transformation and
fate of SOA in the atmosphere. Research questions addressed in
specific chapters are shown.
20
Figure 2.1 a) Schematic of the experimental setup. For each corresponding
region of the setup, the physical state of the particles is illustrated in
b) for sulfate seed particles, effloresced AS is represented by the red
hexagon and deliquesced AS is represented as the small red circles.
Abbreviations are described in the main text.
33
Figure 2.2 Mass spectra (normalized by total sulfate mass) for the organics on
wet AS (blue) and dry AS (red) seed particles. The molecular
formulae of the dominant organic fragment for significant ions are
identified in the main text.
37
Figure 2.3 Mass spectral difference (Wet AS – Dry AS) of the aerosol organics
normalized by total organic mass. Positive values (blue) indicate the
mass fragments that are enhanced for organics on wet AS seed, and
the negative values (red) indicate enhancement for dry AS seed. The
molecular formulae of the dominant organic fragments for
significant ions are labeled.
38
Figure 2.4 Mass spectral difference of the organics (normalized by total
organic mass) for a) wet AS and b) dry AS to illustrate the
reversibility of uptake due to water evaporation. Positive values
(blue) indicate the mass fragments that are enhanced for organics on
dried sulfate-SOA particles, and the negative values (red) indicate
enhancement for not-dried sulfate-SOA particles. The molecular
formulae of the dominant organic fragment for significant ions are
40
x
identified in the main text.
Figure 2.5 Mass spectral difference (wet ABS – wet AS) of the organics
(normalized by total organic mass). Positive values (blue) indicate
the mass fragments that are enhanced for organics on wet ABS seed,
and the negative values (red) indicate enhancement for wet AS seed.
The insert is a zoomed-in view of the mass spectrum at higher m/z.
41
Figure 2.6 Mass spectral difference of the organics (normalized by total organic
mass) for wet ABS to illustrate the reversibility of uptake due to
water evaporation. Positive values (blue) indicate the mass
fragments that are enhanced for organics on dried sulfate-SOA
particles, and the negative values (red) indicate enhancement for
not-dried sulfate-SOA particles. The molecular formulae of the
dominant organic fragment for significant ions are identified in the
main text.
42
Figure 2.7 Mass spectra (normalized by total organics) of the organics on wet
ABS (blue) and wet AS (red) seed particles. The insert illustrates
the zoomed-in view at higher m/z. The molecular formulae of the
dominant organic fragment for significant ions are identified in the
main text.
43
Figure 3.1 Typical volume distribution of SOA formed during filter collection,
measured by SMPS.
52
Figure 3.2 Sized-resolved mass distribution of organics (green) and sulfate
(red) measured by the AMS in PTOF mode for mixed SOA-AS
particles before UV light exposure
53
Figure 3.3 Measured photon flux in the chamber from UVB and UVA lamps
compared to the actinic flux for a clear-sky summer day. The actinic
flux was obtained from “Quick TUV Calculator”, available at
http://cprm.acd.ucar.edu/Models/TUV/Interactive_TUV/ using the
following parameters: SZA = 0, June 30, 2000, 300 Dobson
overhead ozone, surface albedo of 0.1, and 0 km altitude.
54
Figure 3.4 Typical time trace of org:sulfate ratio of the mixed particles due to
evaporation in the dark (control) and exposure to UV irradiation
under high RH conditions.
56
Figure 3.5 Absorption cross section of bulk SOA solution (this work) and
aqueous cis-pinonic acid.20
57
Figure 3.6 Time series profiles of the org:sulfate ratios measured by the AMS
for the photolysis of SOA by UVB radiation under high RH (solid
line), mid RH (dashed line), and low RH (dotted line) conditions.
The shaded areas for each RH condition represent the variability
(±1σ) between multiple experiments.
58
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Figure 3.7 Time series profiles of the sulfate normalized (a) m/z 29, (b) m/z 43,
and (c) m/z 44 for the photolysis of mixed SOA–sulfate particles by
UVB radiation at high RH (solid line), mid RH (dashed line), and
low RH (dotted line) conditions. The shaded areas for each RH
condition represent the variability (±1σ) between multiple
experiments.
59
Figure 3.8 Time series profiles of the sulfate normalized (a) total organic mass
(org:sulfate ratio, black), (b) m/z 29, (c) m/z 43, and (d) m/z 44 for
the photolysis of mixed SOA–sulfate particles by UVB (solid lines)
and UVA (dashed lines) radiation under high RH conditions. The
shaded areas for each light source represent the variability (±1σ)
between multiple experiments.
60
Figure 3.9 Aerosol composition described by the fraction of organic mass at
m/z 43 and 44 (f43 vs f44) before (open circles), during (closed
circles), and after (open circles) UV light exposure for (a) high RH,
(b) mid RH, (c) and low RH conditions. The inset shows the
zoomed-out view of the f43 vs f44 space (with dimensions of Figure
3.9a–c illustrated by the red box), where ambient SOA is distributed
within the triangular area.
62
Figure 3.10 Kinetic plots for the loss of total organic mass due to photolysis of
SOA at high RH (solid line), mid RH (dashed line), and low RH
(dotted line). The shaded region for each RH condition represents
the variability (±1σ) between experiments. The mass loss rate
constant was obtained from the linear best fit of the first 10 minutes
of photolysis.
63
Figure 4.1 Toronto Photo-Oxidation Tube (TPOT) setup for the study of the
gas-phase OH oxidation of ambient organic aerosol and their CCN
activity.
74
Figure 4.2 Activated particle fraction (CCN/CN) plotted as a function of the
water supersaturation (SS) for unoxidized and oxidized ambient
particles by gas-phase OH oxidation (Dm = 75 nm; July 22, 2010,
00:02–02:50) (red circle) and aqueous-phase OH oxidation (Dm =
100 nm; July 8, 2010) (blue square).
78
Figure 4.3 Relationship between O:C and κorg,CCN from the current study and
from several laboratory studies of model POA and biogenic
SOA,14,22,31,32
and a field CCN closure study21
for comparison. The
Lambe et al.32
solid line represents a fit for SOA from a number of
precursors. For each pair of points, the samples of the lowest and
highest O:C values are shown from each study. Dashed lines are
included only to indicate the relationship between these two points.
The following were the OH exposures used for the laboratory
studies of: BES (3.0 × 1012
molecules cm−3
s),31
O3-α-pinene SOA
80
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(1.3 × 1012
molecules cm−3
s),22
OH-α-pinene SOA (1.2 × 1012
molecules cm−3
s)14
and OH-α-pinene SOA (1.7 × 1012
molecules
cm−3
s).32
The shaded region illustrates the uncertainty of the
κorg,CCN and O:C relationships derived from the CCN closure study21
and laboratory generated organic aerosol.32
The uncertainties in the
calculated values of O:C and κorg,CCN are mainly influenced by the
systematic errors associated with each measurement, as discussed in
the main text. Also, daily averages are shown for the gas-phase OH
oxidation experiments, and the average of 4 one-day aqueous-phase
OH oxidation experiments (results from individual experiments
included in the supporting material).
Figure 4.4 AMS mass spectrum of organic and inorganic component of
particles subjected to “hv light” condition.
82
Figure 4.5 AMS mass spectrum of organic and inorganic component of
particles subjected to “OH Oxidation” condition.
82
Figure 5.1 Geometric mean diameter of droplets plotted as a function of seed
particle number concentration (number cm-3) for α-pinene SOA
condensed onto (a) sulfuric acid and (b) ammonium sulfate particle.
Each data point and the corresponding error bars for seed particle
number concentration and droplet diameters reflect the 5 min
averages of the measurements and ±1σ of measurement variability.
The results for controls (bare inorganic particles) are indicated by
the dashed lines (grey area, ±1σ).
94
Figure 5.2 Fraction of the geometric mean diameter of droplets with (Dd) and
without (Dd, control) α-pinene SOA on sulfuric acid (red circle) and
ammonium sulfate (blue square) seed particles, plotted as a function
of the org:sulfate mass ratio. The dashed line indicates no change in
droplet diameter, drawn to guide the eye.
95
Figure 5.3 Time series of various particle components measured by the AMS
for α-pinene SOA condensed onto (left) sulfuric acid and (right)
ammonium sulfate seed particles. The decrease in the mass loadings
for the various components is due to particle loss to the chamber
walls.
96
Figure 5.4 Fraction of the geometric mean diameter of droplets with (Dd) and
without (Dd, control) 2-pentanone and 1-octanal on sulfuric acid at RH
11 % (red circle) and RH 40 % (open circle), as well as ammonium
sulfate (blue square) seed particles, plotted as a function of the
org:sulfate ratio.
98
Figure 5.5 Time series of various particle components measured by the AMS
for carbonyls condensed onto sulfuric acid seed particles at (left)
RH 11 % and (right) 40 %.
99
xiii
Figure 5.6 Fraction of the geometric mean diameter of droplets with (Dd) and
without (Dd, control) engine exhaust on acidic sulfate with NH4+:SO4
2−
ratio of 1:4 (red circle), and NH4+:SO4
2− ratio of 2:5 (green circle),
plotted as a function of the org:sulfate ratio.
100
Figure 5.7 Mass spectra of the organic component measured by the AMS for
engine exhaust condensed on acidic sulfate particles with
NH4+:SO4
2- ratio of 1:4 (red) and 2:5 (green).
100
Figure 5.8 Mass spectra of the organic component measured by the AMS for
carbonyls condensed onto sulfuric acid seed particles at RH 11 %
(red) and 40 % (pink).
103
xiv
Preface This thesis was arranged in a series of chapters that are based upon several manuscripts in
preparation for submission to (Chapter 2) or have been published in peer-reviewed scientific
journals (Chapters 3 - 5). The following outlines the contributions made to each chapter.
Chapter 1: Introduction
Contributions: The chapter was written by Jenny P. S. Wong with critical comments from
Jonathan P. D. Abbatt.
Chapter 2: Impacts of Sulfate Seed Acidity and Water Content on Isoprene Secondary
Organic Aerosol Formation
Contributions: All experiments were designed and performed by Jenny P. S. Wong. Data
analysis was performed by Jenny P. S. Wong with data interpretation assistance from Alex K. Y.
Lee. The manuscript was written by Jenny P. S. Wong with critical comments from Alex K. Y.
Lee and Jonathan P. D. Abbatt.
Chapter 3: Changes in Secondary Organic Aerosol Composition and Mass due to
Photolysis: Relative Humidity Dependence
Contributions: All experiments were designed and performed by Jenny P. S. Wong and
Shouming Zhou. Data analysis and interpretation were by performed by Jenny P. S. Wong. The
manuscript was written by Jenny P. S. Wong with critical comments from Shouming Zhou and
Jonathan P. D. Abbatt.
Chapter 4: Oxidation of Ambient Biogenic Secondary Organic Aerosol by Hydroxyl
Radicals: Effects on Cloud Condensation Nuclei Activity
Contributions: All gas-phase OH oxidation experiments were designed, performed and analyzed
by Jenny P. S. Wong and Jay G. Slowik. All aqueous-phase OH oxidation experiments were
designed, performed and analyzed by Alex K. Y. Lee. Experiments were conducted at Raven’s
Nest site at Whistler, British Columbia with the assistance of W. Richard Leaitch and Ann-Marie
Macdonald from Environment Canada. The CCN instrument was provided by Droplet
xv
Measurement Technologies with the support from Dan J. Cziczo. The manuscript was written by
Jenny P. S. Wong with critical comments from Alex K. Y. Lee, Jay G. Slowik, Dan J. Cziczo,
W. Richard Leaitch, Ann-Marie Macdonald and Jonathan P. D. Abbatt.
Chapter 5: Suppression in Droplet Growth Kinetics by the Addition of Organics to Sulfate
Particles
Contributions: All experiments were designed, performed and analyzed by Jenny P. S. Wong
except for experiments conducted at Environment Canada’s engine lab, where substantial
assistance was provided by John Liggio and Shao-Meng Li. Modelling of experimental results
was performed by Athanasios Nenes. The manuscript was written by Jenny P. S. Wong with
critical comments from John Liggio, Shao-Meng Li, Athanasios Nenes and Jonathan P. D.
Abbatt.
Chapter 6: Conclusions and Future Research
Contributions: The chapter was written by Jenny P. S. Wong with substantial comments from
Jonathan P. D. Abbatt.
1
Chapter 1
1 Introduction
Introduction
2
1.1 Motivation
An aerosol is defined as a suspension of solid or liquid in a gas. The sizes and chemical
compositions of atmospheric aerosols are highly variable, both temporally and spatially. Particle
size ranges from 10-9
m to 10-5
m and can be comprised of inorganic (e.g., sulfate, nitrates),
carbonaceous matter (organic matter and elemental carbon), and water.1 Not only are organic
components ubiquitous in the atmosphere where they can constitute > 90 % of particle by mass,
they consist of a highly complex and variable mixture, most of which remains uncharacterized.2,3
The term organic aerosol refers to the organic fraction of the total aerosol mass. Furthermore,
most of the components in organic aerosol are secondary in nature (i.e., formed in the
atmosphere) and are referred to as secondary organic aerosol (SOA).
Atmospheric aerosols play important roles in climate and air quality. On one hand, particles
partially counteract the warming induced by greenhouse gases directly by interaction with solar
radiation or indirectly by acting as cloud nuclei. In fact, these climatic effects represent one of
the largest uncertainties in the assessments of global radiative forcing.4 On the other hand,
particles contribute to air pollution, with detrimental effects on human health.5 Ultimately, the
influence aerosols have on these effects is controlled by their physical and chemical properties. It
is thus critical to understand the formation, transformation, and fate of SOA in the atmosphere.
1.2 Formation and Aging of SOA
SOA is formed in the atmosphere following the oxidation of volatile organic compounds (VOCs)
when oxidation products of sufficiently low vapour pressure partition to the condensed phase. As
the air mass gets transported away from the source region, following its formation, SOA is
subjected to further physical and chemical transformations (“aging”), such as evaporation and
heterogeneous oxidation. In fact, such aging processes can result in additional SOA formation or
loss. In this framework, the formation and aging processes of SOA are intrinsically coupled
processes. However, in the context of this thesis, formation refers to the processes that lead to the
initial generation of SOA while aging refers to processes that alter the properties of existing
SOA.
3
1.2.1 SOA Formation
Both biogenic (vegetation) and anthropogenic (human activities) sources of VOCs can form
SOA. However, biogenic VOCs are the dominant global source of SOA due to their larger
emissions.3 The formation of SOA is initiated from the oxidation of VOCs (see Figure 1.1),
where the vapour pressure of these oxidation products largely determines their potential to
partition to the condensed phase.
Figure 1.1. General schematic of the SOA formation pathway (adapted from Seinfeld &
Pankow6).
The gas-phase oxidation of SOA precursors by OH, O3, and NO3 favours the formation of
products that are less volatile as these reactions typically result in the addition of more
oxygenated/polar functional groups.7 Generally, multiple oxidation steps are required to form
products of sufficiently low volatility to condense onto existing particles. In pristine
environments, these low volatility products may undergo homogeneous nucleation to form new
particles.7,8
1.2.1.1 Emerging Research Questions
Using the “traditional” SOA formation framework shown in Figure 1.1, atmospheric models with
parameterizations based on gas and gas-particle reactions studied in the laboratory, cannot
predict ambient SOA mass loadings.9,10
This discrepancy may result from numerous reasons,
4
including: i) yields of SOA measured in the laboratory may under-represent those in the
atmosphere; ii) the existence of unidentified precursors; and iii) formation mechanisms from
known precursors not being well understood.
About a decade ago, it was hypothesized that water-soluble oxidation products, despite their high
volatility, can dissolve into particle-phase water where subsequent condensed phase reactions
can lead to the formation of SOA.11
In fact, the partitioning of glyoxal to particle-phase water
was estimated to contribute 10 – 30 % of SOA mass measured in Mexico City.10
The degree to
which water partitions to organic aerosol will be discussed later in Section 1.3. The following
discussion will focus on SOA formation from isoprene as the work in this thesis focuses on
isoprene SOA formation.
Figure 1.2. Reaction scheme for gas-phase OH + isoprene21–23
and products formed in the
particle-phase following reactive uptake.16,17,19
The oxidation of isoprene is estimated to be the largest source of SOA globally as it is the most
abundant non-methane hydrocarbon emitted to the atmosphere.12
In particular, isoprene
epoxydiols (IEPOX), which are second generation oxidation products of isoprene photooxidation
under low NOx conditions, have been identified as key contributors to SOA formation.13–15
As
illustrated in the upper reaction branch of Figure 1.2, the reactive uptake of IEPOX and
subsequent reactions in the condensed phase lead to the formation of low volatility products such
5
as alcohols, dimers, and organosulfates.16–18
Laboratory studies have shown that the reactive
uptake of IEPOX onto acidic seed particles is enhanced due to acid-catalyzed nucleophilic
addition to the epoxide ring.19,20
Contrary to laboratory evidence demonstrating the acid enhancement of the reactive uptake of
IEPOX, multiple ambient studies have observed weak correlations between particle acidity and
IEPOX-derived SOA,24–27
suggesting that outstanding factors are likely to modulate isoprene
SOA formation in the atmosphere. Given that acidic seed particles are hygroscopic, even at the
low relative humidity (RH) conditions used by the IEPOX studies mentioned previously, the
conclusions from these previous studies do not preclude the role of particle-phase water on the
reactive uptake of IEPOX.
In addition to IEPOX, the oxidation of isoprene generates other products, mostly carbonyl
compounds, as shown in the bottom branch of the reaction scheme in Figure 1.2. These products
can undergo hydration and form low volatility products via subsequent reactions.23
The relative
importance of particle acidity and water on these other oxidation products remains unknown and
will be explored in Chapter 2.
1.2.2 SOA Aging Processes
Away from its sources, SOA is continually subjected to chemical aging that transforms the
particles’ physico-chemical properties. Aging processes include heterogeneous oxidation, gas-
phase oxidation of semi-volatile components, and reactions in the condensed phase, such as
photolysis, oxidation, and oligomerization.28,29
Generally, aging processes increase the degree of
oxygenation of the SOA. The following sections include brief summaries of the various types of
SOA aging that can occur. For oxidation reactions, only aging processes by OH radicals will be
considered, as most SOA compounds react with OH much more quickly than they do with
ozone. While it is useful to understand these processes individually, these transformations are
dynamic. Therefore, a conceptual framework will also be discussed to understand the overall
implications of aging processes on the molecular properties and consequently the physico-
chemical properties of SOA.
6
1.2.2.1 Heterogeneous Oxidation
Oxidation of SOA can occur heterogeneously by gas-phase OH radicals. This process can be
kinetically parameterized by the reactive uptake coefficient (γ), which describes the probability
of a reaction following gas-surface collision. The reactive uptake of a molecule is governed by
several processes (illustrated in Figure 1.3): i) diffusion to and on particle surfaces; ii) reaction
on the surface; iii) diffusion and solubility of solvated radical species in the particle bulk; iv)
bulk phase reactions.30,31
The uptake coefficient is determined in the laboratory by monitoring
the loss of either gas-phase or particle-phase reactants. The uptake coefficient of gas-phase OH
on model organic aerosol/surfaces typically range from 0.1 to 1. These values of γ indicate that
reactive uptake of OH onto SOA is an efficient process.32,33
Heterogeneous oxidation is
important for the oxidation of very low volatility compounds that only exist in the particle-phase.
Laboratory studies of the heterogeneous oxidation of model organic compounds indicate that the
reaction can result in either an increase in oxygen content or the loss of carbon due to
fragmentation of the carbon-carbon backbone.29,34
This will be further explored in Section
1.2.2.4.
Figure 1.3. Schematic diagram of the processes involved in the reactive uptake of a gas
molecule onto a particle (adapted from Abbatt et al.31
).
1.2.2.2 Gas-phase Oxidation of Semi-Volatile Compounds
The semi-volatile nature of organic compounds in SOA means that they are present in significant
concentrations in both the gas and particle-phase, where the semi-volatile compounds can be
oxidized by OH in the gas-phase.35
For semi-volatile compounds, oxidation in the gas-phase will
7
be more rapid compared to diffusion-limited heterogeneous oxidation. Aging of laboratory SOA
has indicated that gas-phase oxidation results in a higher degree of oxygenation and the
generation of lower volatility products that results in additional SOA mass.36
1.2.2.3 Condensed Phase Reactions
Non-oxidative reactions in the condensed phase, such as condensation reactions, often increase
the molecular weight and thus decrease the volatility of organic compounds. For example,
formation of dimers from organic compounds with 6 – 8 carbon atoms can lead to a reduction in
vapour pressure by two orders of magnitude.7 Most of these condensed phase reactions are acid-
catalyzed and are summarized in Figure 1.4. Numerous laboratory and ambient measurements
have observed products of these condensation reactions, further demonstrating their
importance.37–39
Figure 1.4. Non-oxidative condensed phase reactions that increases the molecular weight of
organic compounds in SOA (adapted from Hallquist et al.3).
Oxidation in the condensed phase, particularly in the aqueous-phase has been an emerging area
of research in the last decade.40
Aqueous-phase oxidation of water-soluble compounds leads to
the formation of low volatility products that are highly oxidized. For example, Turpin and
coworkers have shown that aqueous oxidation of dicarbonyls leads to the formation of highly
8
oxidized compounds such as carboxylic acids and hydroxyhydroperoxides.41,42
Taking the lab to
the field, on-site aqueous OH oxidation of ambient aerosol filter extracts (i.e., water-soluble
components) resulted in the formation of highly oxidized products.43
Finally, direct photochemical aging in the presence of UV radiation (i.e., photolysis) has been a
less explored area of aging processes in the condensed phase. The importance of photolysis as a
process that can affect the reactivity of organic aerosol was suggested by early studies of SOA
yield, where exposure to direct UV radiation was observed to decrease α-pinene, d-limonene and
isoprene SOA yield by as much as 20 – 40 %.44–46
In particular, studies by Seinfeld and co-
workers observed that the fraction of peroxides in isoprene SOA decreased with UV irradiation
time, demonstrating that certain species in organic aerosol are photolabile and their
photodegradation can be a significant loss process.16,46
Studies by Nizkorodov and co-workers
also confirmed that the photolysis of carbonyls contributes significantly to the photodegradation
of d-limonene SOA.47,48
With the increasing awareness of the importance of cloud processing on SOA composition,
recent studies have investigated the photolysis of SOA solutions dissolved in water. Similar to
earlier studies conducted under dry conditions, the photolysis of aqueous SOA solutions leads to
significant photodegradation of carbonyl compounds and the formation of small molecules in the
gas-phase (i.e., CO, CH4, acetone).49,50
1.2.2.4 Conceptual Framework
It is important to note that the aging processes discussed previously do not occur in isolation, but
that they are dynamic and intrinsically coupled to each other. For example, heterogeneous
oxidation can lead to the formation of smaller, more volatile compounds, which following
partitioning to the gas-phase, can be further oxidized by gas-phase OH, reducing their volatility.
These coupled processes in the gas-phase, at the interface, and in the condensed phase are
referred to as multiphase processing and it has been shown that the evolution of SOA occurs via
a set of fundamental processes: functionalization, fragmentation, and oligomerization.8,34
Functionalization includes all processes that result in the addition of an oxygenated functional
group to the SOA whereas fragmentation results in the cleavage of carbon-carbon bonds. These
two steps have different implications for SOA physico-chemical properties, for example, both
processes will increase the oxygen-to-carbon atom ratio (O:C) but functionalization leads to a
9
reduction while fragmentation leads to an increase in average aerosol volatility.51
Oligomerization, will result in a decrease in O:C ratio, but as mentioned previously, will lead to
a reduction in volatility.52
Laboratory studies of the heterogeneous OH oxidation of squalane (C30H62) elucidated the
relative contributions of functionalization and fragmentation processes during different stages of
the reaction.34
The results of this study are shown in Figure 1.5 where both organic aerosol mass
and O:C ratio evolve with OH exposure. At low OH exposures, increases in both O:C and
aerosol mass suggest that the functionalization pathway dominates. However, at higher OH
exposure (~ 1.5 × 1012
molec cm-3
s), a decrease in aerosol mass and less net change in O:C were
observed, suggesting that fragmentation-induced volatilization was the dominant process. These
results also suggest that for less oxidized organic compounds, functionalization is the dominant
pathway whereas fragmentation is the dominant pathway for more oxidized compounds (i.e.,
more aged SOA is more susceptible to volatilization).
Figure 1.5. The evolution of aerosol mass (left axis) and O:C ratio (right axis) with OH exposure
(data from Kroll et al.34
).
1.2.2.5 Emerging Research Questions
As noted previously, research into the importance of photolysis as an aging process for SOA is
still in its infancy. The photolysis studies discussed previously have demonstrated that photolysis
can lead to the fragmentation of organics under both dry/low relative humidity (RH) and
saturated conditions. Recent work in the Abbatt group illustrated that the rate of heterogeneous
10
oxidation of benzo[a]pyrene embedded within an SOA matrix is suppressed, where the extent of
this suppression was shown to depend on RH.53
These results suggest that RH affects particle
reactivity as the viscosity of the SOA material has been shown to decrease with increasing water
content.54,55
In this context, diffusion of reactants is more rapid in low viscosity material,
resulting in the observed higher reactivity. Recent work by Lignell et al., have also shown that
matrix effects, such as changes in viscosity, can have implications to the photolysis of an
embedded substrate.56
In fact, there is growing experimental evidence that the viscosity of SOA
material varies with environmental conditions such as RH.54
While the study by Zhou et al. has
shown that the reactivity of a substrate buried in SOA matrix is dependent on RH, it remains
unclear whether the reactivity of the SOA itself is dependent on the RH conditions. This question
will be addressed in Chapter 3.
From the discussion in the previous sections, it can be seen that aging processes generally lead to
more functionalized and/or smaller compounds that are more water-soluble (i.e., more
hygroscopic). The hygroscopicity of organics will be discussed in detail in Section 1.3. The
degree to which aging processes increase the hygroscopicity of SOA remains unclear. This
question will be addressed in Chapter 4.
1.3 Secondary Organic Aerosol Particles as Cloud Condensation
Nuclei
The average atmospheric lifetime of SOA is approximately one week, where the particles can be
removed from the atmosphere either via dry deposition onto Earth’s surfaces or wet deposition
that is initiated both below and in clouds.57
SOA can initiate cloud formation by acting as cloud
condensation nuclei (CCN) and can eventually be lost via precipitation. SOA below the cloud
can also be scavenged by the precipitation. The following sections include an overview of the
theory that describes the ability of organics to act as CCN at supersaturated water vapour
conditions (i.e., RH < 100 %) and the kinetics of droplet growth. It is important to note that
water uptake on particles can also occur under subsaturated conditions (i.e., RH < 100 %). While
the hygroscopic response of pure inorganic particles has been well studied, recent studies have
demonstrated that the water content of organic-containing particles also increases with increasing
RH conditions.58,59
11
1.3.1 CCN Activation
At supersaturated water vapour (S) conditions (i.e., RH > 100 %), aerosol particles can activate
as CCN and grow into cloud droplets. This process occurs via the condensation of water vapour
and is described as an equilibrium process using the Kohler equation:60
Equation 1.1
where s is the ratio of the partial pressure of water above the droplet to the equilibrium vapour
pressure of water, S is the supersaturation, aw is the water activity of the solution, σ is the surface
tension of the droplet, Mw is the molecular weight of water, ρw is the density of water, R is the
universal gas constant, T is temperature, and D is the diameter of the droplet. Typical
supersaturations in clouds in the atmosphere are 0.2 – 2.0 %.61
The water activity can also be
expressed using Raoult’s Law:
Equation 1.2
where nsolutes is the moles of soluble species in the initial particle, and D is the diameter of the
droplet. Combining equation 1.1 and 1.2, it can be seen that two competing factors affect the
condensation of water, which occurs when ambient water vapour pressure exceeds the
equilibrium water vapour pressure over the aerosol particle: the Raoult effect (lowering of
vapour pressure of water by a solute) and the Kelvin effect (increased vapour pressure over a
curved surface). A particle requires a minimum or critical supersaturation (Sc) in order to convert
into a CCN. When exposed to the Sc, the particle must grow to a critical diameter (Dpc) to
overcome the Kelvin effect, following activation as a CCN, the droplet will reach a mode of
spontaneous growth (up to approximately 10 µm). As such, the Sc and Dpc are controlled by the
particle size and its chemical affinity for water (commonly referred to as “hygroscopicity”).
To better understand the CCN activity of organic aerosol, the earliest CCN-studies examined the
hygroscopicity of model organic compounds.62–64
These studies have demonstrated that highly
water-soluble compounds do indeed act as good CCN by increasing the concentration of solutes
(e.g., Raoult’s effect) and that the CCN activity of these compounds can be predicted using the
Kohler equation if their molecular properties are known. However, given that ambient organic
12
aerosols contains a myriad of compounds, most of which are still unidentified and unquantified,
their CCN activity cannot be predicted based on first principles. As a result, a modified Kohler
theory was developed by Petters and Kreidenweis65
where water activity is represented as:
Equation 1.3
where D is the droplet wet diameter, Di is the initial particle diameter, and κ is the hygroscopicity
parameter of the particle. This extension of the Kohler theory uses one parameter (κ) to describe
average molecular properties (e.g., molecular weight and solubility) that control the particle’s
water uptake characteristics. Using single component organic aerosols composed of model
compounds, κ values were empirically determined by numerous laboratory studies. For example,
κ of a hydrophobic compound, bis(2-ethylhexyl) sebacate, is 0,66
and for a hydrophilic
compound, malonic acid, the κ value is 0.23.64
In comparison, κ values of typical inorganic
particles in the atmosphere are 0.61 for ammonium sulfate and 0.90 for sulfuric acid.67
Given that ambient particles are typically composed of both inorganic and organic components,
the overall κ of the particle (κtotal) can be represented as a volume-fraction weighted average κ of
individual components:65
Equation 1.4
where εinorg, εorg are the volume fractions of the inorganic and organic components, and κinorg, κorg
are the average κ values of the individual components for inorganic and organic compounds.
This approach is useful to determine the average κorg of ambient organic aerosols since the
organic component contains many compounds that have yet to be completely speciated.
Additionally, from equation 1.4, it can be seem that the hygroscopicity of organics can contribute
significantly to the total aerosol hygroscopicity if they constitute a large fraction of the total
particle material and/or if the organics are highly hygroscopic (i.e., the product of the organic
terms is comparable to the product of the inorganic terms).
While the above discussion considers the contribution of organics to particle hygroscopicity
through solubility, organic compounds can also affect CCN activity by altering surface tension,
and thus the water activity. Surface active organics, which have both hydrophilic and
13
hydrophobic moieties, will partition to the gas-particle interface, resulting in a reduction in the
aerosol surface tension (i.e., reduction in the Kelvin term) and a more CCN-active particle.
Laboratory studies show that bulk solutions of organics do indeed have lower surface tension
compared to water.68–70
These findings have also been corroborated by laboratory studies of
organic aerosol composed of pure slightly soluble compounds such as large organic acids.71
Reactions of organics in the condensed phase have been shown to enhance the CCN activity of
inorganic particles.72
However, the same organic acids that were shown to increase CCN activity,
when mixed with hygroscopic inorganic particles, actually decreased the CCN activity.73,74
It is
thought that mixtures of surface active organics and inorganic compounds result in a decrease in
CCN activity due to a decrease in soluble ions when the surfactant partitions to the surface. At
this point, uncertainties remain for the overall effect surface active organics have on CCN
activity.
1.3.1.1 Emerging Research Questions
Although it is a simplification, unless one assumes that one single κ value is accurate for all
aerosol organic matter, the use of κ-Kohler theory still requires the quantification and some
chemical speciation of organic aerosol, which despite recent analytical advances, remains
challenging. As discussed earlier, organic aerosol undergoes chemical transformations during its
atmospheric lifetime, which can affect its CCN activity, thereby further complicating the
prediction of the CCN activity of ambient particles.8 Motivated by the need to simplify the
experimental approach to studying organic hygroscopicity, laboratory and ambient studies have
proposed that organic aerosol hygroscopicity can be characterized based on the organic
component’s degree of oxygenation, as quantified by the average O:C ratio.8,75,76
In this context,
oxidation of organic aerosols in the atmosphere, which typically gives rise to more
functionalized/smaller solute molecules (as discussed in Section 1.2.2), is likely to render the
aerosol more hygroscopic. This approach not only simplifies predictions of CCN activity of
ambient particles, but it also accounts for chemical reactivity.
Previous work in the Abbatt group has postulated that there is a simple linear relationship
between the hygroscopicity of organic aerosol and its O:C ratio.75
However, the extent to which
aging processes actually result in an increase in the O:C ratio and the hygroscopicity of ambient
organic aerosol remain unclear. The goal of Chapter 4 is to test this hypothesis.
14
1.3.2 Droplet Growth Kinetics
While the thermodynamic properties of organic aerosol and their relationship to droplet
activation are becoming well understood, much less is known regarding how organics affect the
rate of water vapor transport during CCN activation (i.e., kinetic effects). Changes to the droplet
growth rates will change the amount of water in the droplet, thus affecting cloud droplet size
distributions. In fact, the assumption that the activation of aerosols to CCN can be modelled
purely as an equilibrium process has been shown to be invalid in some conditions.77–79
While
highly uncertain, it appears that organics can result in kinetic limitations due to slow solute
dissolution associated with highly viscous organic particles54,80,81
or that organics can form a film
at the surface, affecting the mass-transfer rate of water vapor.82,83
1.3.2.1 Emerging Research Questions
Slow droplet growth rates have been observed in the laboratory and ambient measurements, yet
the extent to which growth rate suppression occurs in the atmosphere remains unclear as only a
limited number of studies have investigated this process, with contrasting results. In this context,
suppression in droplet growth kinetics is assumed to be responsible if the addition of an organic
compound to an inorganic particle gives rise to a smaller droplet size compared to that of the
bare inorganic particles, which have known rapid growth kinetics. Laboratory studies have
demonstrated that the following organic particles exhibited slow droplet growth kinetics:
carboxylic acid particles,84
highly viscous sucrose/sodium chloride particles,85
organic films
consisting of hydrocarbons with carbon chain length larger than 12,86
and secondary organic
aerosol from β-caryophylene ozonolysis.80
In addition, ambient observations of kinetic
limitations have been observed from various sources, including bovine emissions,87
and
continental particles containing anthropogenic organic components.88
However, using
computational fluid dynamics models of the conditions inside a CCN counter, Raatikainen et al.
analyzed 10 ambient data sets and represented growth kinetics via an effective water uptake mass
accommodation coefficient (α), a parameter describing the probability that a water molecule
striking the particle surface will be taken up by the growing droplet. This study constrained α
between the values of 0.1 and 1, indicating that rapid growth kinetics are globally prevalent.89
These contrasting results from laboratory and field data pose a challenge to determining whether
the kinetic limitation of droplet growth is an important process, and to what degree it actually
occurs in the atmosphere. While previous work has demonstrated that certain compounds and
15
particle sources can give rise to suppression in growth kinetics, one of the goals of Chapter 5 is
to investigate the droplet growth kinetics of various organic compounds with different
functionalities.
1.4 Experimental Methods for SOA Studies
In order to investigate SOA in the laboratory, a suite of experimental techniques is required.
Generally, a reactor is required to generate or age the SOA and subsequent measurement
techniques are required to investigate the physico-chemical properties of the SOA. To investigate
ambient SOA, other technical requirements must be met, generally those regarding the
portability of instrumentation and good time resolution. While there are many existing
approaches and techniques, the following sections focus the discussion on the techniques and
instrumentation used for this thesis.
1.4.1 Approaches for SOA Formation and Aging
Multiple approaches have been used to study the formation and aging of SOA in the laboratory.
Historically, environmental chambers were used to simulate atmospheric conditions and these
seminal experiments have led to much of our current understanding of SOA formation. The use
of flow tube reactors has emerged more recently. Note that other techniques have also been
employed to study SOA formation and aging, for example, reactions on SOA collected on filters
or dissolved in bulk aqueous solutions.48
The following sections provide a brief overview of
environmental chamber and flow tube techniques as they were used for the work in this thesis.
1.4.1.1 Environmental Chambers
Environmental chambers are generally bags made out of Telfon film with large volumes (0.01 –
250 m3) in which the reactions take place.
90 The large volume of these chambers result in a large
volume-to-surface area ratio to minimize wall effects, which is important for studying SOA
formation as it minimizes the interaction of condensable organic vapours with walls. However,
recent work by Zhang et al. demonstrated that SOA formation can still be suppressed due to the
loss of SOA-forming vapours onto the walls.91
To investigate photochemical reactions, broad-
band artificial light sources are placed outside the chambers. While these light sources are used
to simulate actinic radiation, the relative intensities and wavelengths may not be comparable.
There are also outdoor environmental chambers that utilize actual solar radiation as their light
16
source.92,93
Environmental chambers are typically operated in batch or continuous modes, which
can theoretically provide aerosol residence times from hours to days. However, aerosol residence
times are generally limited by the wall loss of particles, resulting in stimulations of ambient
reaction times up to a few (2 - 3) days at most. Environmental chambers were used in the work
described in Chapters 3 and 5.
1.4.1.2 Flow Tubes
The use of flow tube reactors to investigate aerosol chemistry emerged in the 1980’s as the
importance of heterogeneous reactions on the surfaces of polar stratospheric clouds was
recognized.94
The use of flow tubes to study the chemistry of organic aerosol only began in the
last decade. The volumes of these reactors (generally < 0.001 – 0.02 m3) are much smaller
compared to environmental chambers, which result in short residence times (minutes) and
potentially significant wall effects. Indeed, an evaluation of two flow tube reactors with different
volume-to-surface area ratios demonstrated that differences in wall-interactions resulted in
different measured SOA yields.94
However, compared to environmental chambers, the total wall
surface areas are much smaller, and therefore it is possible to pacify the walls through
conditioning. Sheath flows have also been used in order to minimize wall interactions.95
Given
the small volumes, high concentrations of gas and particle species are typically used to drive up
reaction rates, resulting in simulation of atmospheric processing times of up to two weeks. While
this property renders flow tubes useful for investigating SOA aging processes that occur
following its formation, the chemical reactions occur on an accelerated time scale compared to
the rate at which they occur in the atmosphere. Flow tube reactors have been constructed out of
Pyrex and stainless steel, with the former subject to electrostatic wall loss of charged particles.
Similar to environmental chambers, photochemical reactions have been investigated using lamps
placed inside or outside the flow tubes, however, the light sources are not necessarily comparable
to actinic radiation (e.g., UV light from mercury lamps). From a practical standpoint, flow tubes
enable rapid switching between experimental conditions (e.g., seed particle type, RH, light
source) compared to environmental chambers due to differences in volumes. This is particularly
advantageous for the study described in Chapter 2. Also, being compact in size compared to
environmental chambers, flow tubes can be transported and used in the field to investigate aging
processes on ambient aerosols. These properties of flow tube reactors enabled the study
described in Chapter 4.
17
1.4.2 Chemical Analysis of SOA by Mass Spectrometry
The analytical techniques used for the chemical characterization of SOA span a wide range, from
optical spectroscopy to mass spectrometry. Nevertheless, complete characterization of SOA
chemical composition is still impossible.3 This section will focus on mass spectrometry
techniques used to investigate SOA processes in the Abbatt group. Note that other mass
spectrometry ionization techniques have been used to characterize SOA, such as electrospray
ionization (ESI), tunable vacuum ultraviolet (VUV) ionization, etc.96
The commercially developed Aerosol Mass Spectrometer (AMS) by Aerodyne, Inc. has become
central to many SOA studies over the past decade.97
The AMS generates gas-phase species by
thermal desorption and ionization by electron impact ionization. This quantitative technique
provides size-resolved mass spectra of inorganic and organic compounds with fast time
resolution (~ one minute). The instrument consists of three main regions: i) particle inlet and
sizing, ii) vaporization and ionization, and iii) time-of-flight mass spectrometer. After passing
through the inlet, the particle beam is focused by a series of aerodynamic lenses with 100 %
transmission for particles with diameters between 70 – 500 nm. The resulting focused particle
beam passes through a chopper which sends bursts of particles through the sizing chamber, from
which the particle flight time can be used to determine the particle aerodynamic vacuum
diameter. The non-refractory components of the particles (e.g., sulfate, nitrate, organics) are
vapourized upon impaction on a heated surface (~600°C) and are ionized by 70 eV electrons.
While earlier versions of the AMS used a quadrupole mass spectrometer, new versions employ
time-of-flight (ToF) mass spectrometers, of which a compact version (c-ToF) as well as a high
resolution version (HR-ToF) are available. While quantitative, the use of EI results in extensive
fragmentation of molecular compounds, where the identity of the parent compound is lost.
However, classes of compounds (e.g., hydrocarbons vs. oxidized carbons) can be identified from
the mass spectra, and coupled with factor analysis, the mass spectra can be deconvoluted into
components with different chemical or source properties. The use of the AMS is central to this
thesis.
More recently, the use of chemical ionization mass spectrometry (CIMS) to investigate SOA has
been increasing. The chemical information provided by the CIMS is generally complementary to
that provided by the AMS.98
Similar to the AMS, aerosol-CIMS employs thermal desorption to
18
volatilize the organic constituents of the particle, but only temperatures up to around 200°C have
been used (i.e., inorganics will not be vapourized).99
Along with the relatively low temperatures
used for vapourization, the soft ionization technique used by the CIMS means the analyte ions
will not fragment significantly. Various reagent ions have been used (i.e., (H2O)nH+, I
-,
CH3COO-) and they provide selective detection of various classes of organic compounds.
100
Recently, an inlet has been developed (FIGAERO, Filter Inlet for Gas and Aerosols) that enables
semi-continuous detection of both particle-phase and gas-phase compounds by the CIMS.101
Here, particles are collected on a particle filter while gas-phase measurements are conducted.
When particle-collection is completed, the filter is heated gradually as the organics volatilize as a
function of their vapour pressure, allowing for particle molecular composition measurements.
While not employed for any of the studies in this thesis, this upcoming technique is a powerful
tool for future SOA studies.
1.4.3 Approaches for CCN Measurements
In order to investigate the ability of particles to act as CCN, all CCN counters (CCNc) must be
able to generate a known supersaturation of water vapour. While the approach to generating a
supersaturation is not identical for all CCNc, because the CCN work in this thesis is based on the
CCNc commercially developed by Droplet Measurement Technology, the following will focus
on the operational principles of that instrument.102
Particles, which are continuously sampled by
the CCNc, enter a CCN column, a region where supersaturation is generated and the particles
activate into droplets. In the column, which is coated by liquid water, a positive temperature
gradient is generated in the streamwise direction and a supersaturation results since the rate of
mass transfer is greater than heat transfer (i.e., because water molecules diffuse faster than do
nitrogen and oxygen molecules). A range of supersaturations can be achieved (0.07 – 2.0 %) by
changing the temperature gradient down the column. The supersaturated water vapour condenses
onto particles and the newly formed droplets. The droplets are counted and sized using an optical
particle counter, which can measure droplets from 0.75 – 10 µm.
The following includes a brief overview of the two different approaches commonly used to
investigate the CCN activity of organic aerosol. The main difference between the two approaches
is that one uses polydisperse aerosols (hence measuring the average hygroscopicity of the aerosol
population) while the other measures the hygroscopicity of monodisperse (size-selected)
19
particles. For the first approach, polydisperse particles are sampled by a CCNc at a fixed
supersaturation. Paired with particle size distribution measurements, the minimum particle
diameter that corresponds to the fixed supersaturation of the CCNc is considered to be the
critical diameter (Dpc). This approach, called CCN-closure, compares the measured CCN
concentration to that predicted using Kohler theory. Closure is achieved if the measurements
agree with predictions. However, the presence of multiply charged particles has been
demonstrated by Petters et al. to skew the results from this approach.103
Additionally, another
challenge of this type of CCN study is that the particle mixing state can play an important role in
activation. For example, the composition of the particles with the critical diameter could be
different from those measured or assumed.104
For the second approach, size-selected particles are
sampled both by the CCNc operated at a range of supersaturations, and by a particle counter.
Here, the supersaturation at which 50 % of the counted particles activate as CCN is considered
the critical supersaturation (Sc). Ambient studies have shown that monodisperse CCN
measurements can be used to discern the mixing state of hygroscopic aerosols.105
For the CCN
work in this thesis, the second approach was used.
1.5 Summary of Research Objectives
Particle-phase water is an abundant atmospheric constituent that is estimated to exceed the total
dry particle mass by 2 – 3 times globally.106
Despite its abundance, the importance of water on
the formation, aging and fate of SOA remains poorly characterized and the goal of this thesis is
to address the gaps in knowledge shown in Figure 1.6.
20
Figure 1.6. An illustrated representation of the formation, transformation and fate of SOA in the
atmosphere. Research questions addressed in specific chapters are shown.
In Chapter 2, the first question is examined by quantifying the amount of isoprene SOA formed
on different sulfate seed particles with different acidity and water content. The importance of
photolysis as an aging process of SOA and the dependence of the kinetics of this process on
relative humidity are examined in Chapter 3. The modification of CCN activity of ambient SOA
by OH oxidative aging was observed and a simple relationship of organic CCN activity and
degree of oxygenation was demonstrated in Chapter 4. The last question is addressed in Chapter
5, where the droplet growth kinetics of neutral and acidic sulfate particles with three types of
organics (SOA, carbonyls, and diesel engine exhaust) were determined. The results obtained in
these studies provide insight into the role of water on the formation, transformation and fate of
SOA in the atmosphere. A summary of the main results along with a discussion of future
research directions conclude this thesis in Chapter 6.
1.6 References
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Climate Change; John Wiley & Sons, Inc.: New York, 1998.
21
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(8) Jimenez, J. L.; Canagaratna, M. R.; Donahue, N. M.; Prevot, A. S. H.; Zhang, Q.; Kroll, J.
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Chapter 2
2 Impacts of Sulfate Seed Acidity and Water Content on
Isoprene Secondary Organic Aerosol Formation
Impacts of Sulfate Seed Acidity and Water Content on
Isoprene Secondary Organic Aerosol Formation
J. P. S. Wong, A. K. Y. Lee, J. P. D. Abbatt
In preparation for submission to Environmental Science and Technology
31
2.1 Abstract
The effects of particle-phase water and acidity of pre-existing sulfate seed particles on the
formation of isoprene secondary organic aerosol (SOA) was investigated. SOA was generated
from the photooxidation of isoprene in a room temperature flow tube reactor at 70 % relative
humidity (RH) in the presence of 3 different sulfate seeds (effloresced and deliquesced
ammonium sulfate, ammonium bisulfate). Particle-phase water had the largest effect on the
amount of SOA formed, with 60 % more SOA formation occurring with deliquesced ammonium
sulfate seeds as compared to effloresced ones. The additional organic material was highly
oxidized. While the amount of SOA formed did not exhibit a dependence on the range of seed
particle acidity examined, the amount of high molecular weight material increases with acidity,
indicating the role of acidity in governing organic composition. While the uptake of organics was
partially reversible upon drying, the results nevertheless indicate that the presence of particle-
phase water will enhance the amount of organic aerosol material formed in the atmosphere and
that the RH cycling of sulfate particles mediates the extent of isoprene SOA formation.
2.2 Introduction
Isoprene is the most abundant non-methane hydrocarbon emitted to the atmosphere, and its
oxidation is estimated to be the largest source of secondary organic aerosol (SOA).1,2
Despite the
importance of SOA on climate and human health, there is a lack of understanding of its
atmospheric formation and evolution pathways.3 Traditionally, SOA formation is thought to
occur via the partitioning of semi-volatile organics from gas-phase photochemistry. Current
atmospheric models, based on this volatility-driven partitioning theory, underestimate observed
SOA mass, suggesting unidentified formation mechanisms.4
Previous studies have shown that water-soluble isoprene photooxidation products, even if highly
volatile, can readily partition to particle-phase water where subsequent condensed phase
reactions can lead to the formation of SOA.5 In particular, the formation of epoxydiols (IEPOX)
from isoprene photooxidation under low NOx conditions and their reactive uptake to the particle-
phase have been proposed as key processes for isoprene SOA formation.6,7
Studies have shown
that the reactive uptake of IEPOX is enhanced under acidic seed particle conditions, leading to
substantial SOA formation via acid-catalyzed nucleophilic addition to the epoxide ring.8–10
However, ambient studies have observed weak correlations between particle acidity and IEPOX-
32
derived SOA,11–14
suggesting that outstanding factors are likely to modulate isoprene SOA
formation. Given that acidic seed particles are hygroscopic, even at the low RH conditions used
by most isoprene SOA studies, the relative importance of seed particle acidity and particle-phase
water on the formation of isoprene SOA remains uncertain.
Recent laboratory studies have focused on elucidating the effects of particle acidity and particle-
phase water on the reactive uptake of IEPOX.15,16
Collectively, these studies have shown that
particle acidity is the dominant factor affecting the reactive uptake rate of IEPOX for particles
with pH values < 3 while for less acidic particles (pH > 3), the availability of efficient
nucleophiles (e.g., NH3) is key for the reactive uptake process to occur. Recently, Liu et al.,
quantified that the uptake of IEPOX onto acidic sulfate seed particles under low RH conditions
accounts for 50 % of the SOA mass formed.17
Although the reactive uptake process of IEPOX
and its contribution to the SOA burden are becoming better understood, it is important to note
that other isoprene oxidation products are water-soluble (e.g., carbonyls), and that the effects of
acidity and particle-phase water on such oxidation products remain unknown.
In this chapter, we present laboratory measurements of SOA formation from the photooxidation
of isoprene under high RH (70 %) conditions onto various seed particles in a flow tube reactor:
effloresced and deliquesced ammonium sulfate, as well as acidified sulfate particles. In
particular, by operating at constant relative humidity, experimental issues related to varying gas-
phase chemistry and/or wall loss rates are minimized. And by taking advantage of the hysteresis
in the phase of ammonium sulfate aerosol particles as a function of increasing and decreasing
relative humidity, we can operate with seeds that have the same chemical composition but
different phases (i.e., solid vs. aqueous). Overall, this study aims to i) isolate the roles of
particle-phase water and acidity on the nature of isoprene SOA formation and ii) investigate the
reversibility of the uptake processes leading to SOA formation that occurs upon changes in
relative humidity.
2.3 Experimental Section
A schematic of the experimental setup used to investigate seed particle effects on isoprene SOA
formation is shown in Figure 2.1. Key components of the setup are described in detail below. We
emphasize that the overall experimental requirements to achieve our science goals are i) to have
33
control over the phase of the ammonium sulfate seeds, i.e., whether the particles are deliquesced
or effloresced, and ii) for the oxidation chemistry that forms the SOA to be conducted at the
same relative humidity independent of seed phase or composition.
Figure 2.1. a) Schematic of the experimental setup. For each corresponding region of the setup,
the physical state of the particles is illustrated in b) for sulfate seed particles, effloresced AS is
represented by the red hexagon and deliquesced AS is represented as the small red circles.
Abbreviations are described in the main text.
2.3.1 Seed Particle Generation and Phase Characterization
Sulfate seed particles were generated by atomizing aqueous sulfate solutions with a constant
output atomizer (TSI 3076) with N2 as carrier gas. For ammonium sulfate seed particles, a dilute
solution (0.12 mM) of ammonium sulfate (AS, Sigma-Aldrich) was used. For acidified sulfate
seed particles, the above AS atomizer solution was acidified using sulfuric acid (Fisher
Scientific) resulting in a 1:1 ammonium-to-sulfate molar ratio. After atomization, 300 sccm of
the atomizer output was dried using a silica gel diffusion dryer, which reduced the RH to < 30 %,
where the AS effloresced. The polydisperse particles were subsequently charge neutralized and
size selected at a mobility diameter of 100 nm (for AS) or 120 nm (for acidified AS) by a
differential mobility analyzer (DMA, TSI-3081). 120 nm was chosen for the size of the acidified
AS particles to match the size of the deliquesced AS particles (see below).
34
Following size-selection, the monodisperse particle flow, controlled by a 3-way valve, was
alternatively mixed with a humidified N2/O2 flow (1550 sccm) in the conditioning tube (RH 82
%) or in a bypass tube (RH < 30 %). AS particles mixed with the humidified flow at this point
deliquesce in the conditioning tube (residence time approximately 1 min), otherwise they remain
effloresced in the bypass tube.
The resulting deliquesced AS particle flow was combined with a dry flow containing ozone (150
sccm, generated by passing O2 through a mercury lamp) in a mixing volume. For the effloresced
AS particle flow, the same dry ozone flow was first mixed with the 1550 sccm N2/O2 humidified
flow (see above) which was then added to the particle flow in the mixing volume. It is important
to note that i) the RH of the latter particle flow never exceeds 80 % and so the AS particles
remain fully effloresced, and that ii) within the mixing flow, the relative humidities are the same
for both configurations, but the phase of the AS particles is not.
Control experiments were conducted to ensure that the AS particles deliquesced in the
conditioning tube. The particle size distributions were measured from the outflow of the mixing
volume using a scanning mobility particle sizer (SMPS, TSI-3075) whose sheath flow was
conditioned to the same RH conditions as the mixing volume for several hours. For effloresced
AS particles size-selected at a mobility diameter of 100 nm, exposure to RH 82 % in the
conditioning tube resulted in an increase of particle diameter to 120 nm, suggesting the AS
particles have deliquesced. For particles that flowed through the bypass tube, no change in
particle mean diameter was observed, indicating the particles remain effloresced. The observed
increase in particle diameter due to deliquescence is consistent with that expected from the
hygroscopic behaviour of AS.18
For the remainder of this paper, deliquesced AS will be referred
to as “wet AS”, effloresced AS as “dry AS,” and acidic sulfate as “wet ABS” for simplicity.
These sulfate seed particles were chosen as they are ubiquitous in the atmosphere and are
commonly used seed particles in previous isoprene SOA/IEPOX studies.
2.3.2 SOA Formation
SOA was formed from the oxidation of isoprene with OH radicals in the Toronto Photo-
Oxidation Tube (TPOT), which was previously described in detail elsewhere.19,20
Briefly, TPOT
is a flow tube reactor that produces high OH exposures via the photolysis of ozone (λ = 254 nm)
35
in the presence of water vapor. Since the use of this UV lamp increased the temperature inside
the reactor by 2 – 3 degrees above room temperature, the temperature inside the TPOT was
continuously measured using a thermocouple. The RH and temperature of the reactor’s outflow
(at room temperature) were continuously measured as well (Vaisala). These measurements were
used to calculate the dew point, and subsequently, the RH inside the TPOT. All the reported RH
values in this paper correspond to the corrected RH inside the TPOT reactor. Specifically, the
TPOT was operated using 8.5 × 1012
molecules cm-3
(350 ppb) of ozone, at RH 70 %, and for a
residence time of 100 seconds. These operating conditions resulted in an OH exposure of 3.4 ×
1011
molecules cm-3
s, which is equivalent to 2 - 3 days of OH exposure with ambient
concentrations of 1.5 × 106 molecule cm
-3. The OH concentration was determined by monitoring
the decay of SO2 (2.4 × 1012
molecules cm-3
) by the reaction of OH using a SO2 monitor (Thermo
40c), as described previously.19,20
The TPOT was cleaned before and after each experiment by
exposure to OH for several hours, until the SMPS and AMS measurements (described below)
reach background levels.
To initiate SOA formation, a small flow (20 sccm) of an isoprene mixture (17.5 ppm in nitrogen,
Scott Specialty Gas) was added to the particle flow in the mixing volume upstream of the TPOT,
resulting in a final isoprene concentration of 4.3 × 1012
molecules cm-3
(175 ppb). The system
was conditioned for 30 minutes, until stable SMPS and AMS measurements were observed.
From the SMPS measurements, no homogeneous nucleation (i.e., formation of small particles 10
nm and larger) was observed following the reaction of isoprene and OH. Control experiments
were conducted in which isoprene was oxidized solely by ozone in the TPOT and no organic
aerosol material formation was observed, indicating that the OH oxidation of isoprene generated
the SOA during typical experiments.
2.3.3 Particle Measurements
Following organic condensation onto seed particles, the resulting SOA-sulfate particles were
alternatively dried by passing through a silica gel diffusion dryer or not dried by passing through
a bypass tube. These conditions were investigated for two reasons. First, since it is hypothesized
that the presence of particle-phase water would result in the uptake of soluble organic gases,
water removal by the dryer explores the reversibility of the uptake of soluble organic gases
leading to SOA formation. Second, these conditions allowed for the characterization of the types
36
of organic materials that are associated with the particle-phase water. We note that all sulfate
particles (wet and dry AS, as well as wet ABS) experienced a 10 % particle loss in the diffusion
dryer and that this effect has been accounted for in the results presented below.
After passing through the dryer or dryer bypass, the particles were sampled online by a time-of-
flight aerosol mass spectrometer (ToF-AMS, Aerodyne, Inc.) for particle mass and composition
measurements. Most experiments were conducted with a lower mass resolution AMS (cToF-
AMS) whereas a high mass resolution AMS (HR-ToF-AMS), which can resolve different ions at
the same nominal m/z, was used on occasion to confirm the identity of the dominant organic ion
fragments. The operating and analysis procedure of the AMS has been reviewed previously in
detail.21
Briefly, the AMS provides size-resolved mass and chemical composition of the non-
refractory components of particles, including organics, sulfate, nitrate, and ammonium. The
ionization efficiency of the AMS (IENO3 and RIENH4) was calibrated routinely using size-selected
ammonium nitrate particles. Signals from major gases (N2, O2, H2O, Ar, and CO2) were
accounted for by obtaining multiple filter measurements (i.e., zero particle signal) throughout
each experiment.
To quantify the amount of SOA material condensed onto the sulfate seed particles, the organic
mass loading measurements from the AMS were used. For all experiments, the contribution from
background organics have been subtracted from those arising from SOA formation. Given that
the collection efficiency (CE, related to the particle bounce characteristics on the AMS
vaporizer) of wet AS is higher than that of dry AS,22
the sulfate mass was used to normalize the
organic signal for different aerosol types under the assumption of no sulfate reactivity. However,
for the comparison of SOA formation onto wet AS and wet ABS seed particles, given that they
have similar CE in the AMS, SOA formation will be reported as an absolute yield (using CE of
1.0), in order to facilitate comparison to previous isoprene SOA formation studies.5
In additional to AMS measurements, particle size distributions and total surface area (Sa)
concentrations were measured by a SMPS (TSI-3075). Efforts were made to conduct the
experiments under the same operating conditions as much as possible, and so typical Sa ranged
only from 1.0 – 1.4 × 109 nm
2 cm
-3, with the small variation arising from experimental variability
in particle generation efficiency and variations in Sa due to the presence of water for wet seed
37
particles. One goal for working at constant aerosol surface areas is that recent work has shown
that SOA formation is sensitive to seed particle Sa as the condensation of oxidation products is a
competitive process of all available surface area (i.e., seed particles, reactor’s walls).23
To
evaluate the effect of Sa on SOA formation in the TPOT, control experiments with varying Sa
were conducted, and the measured isoprene SOA yields were compared. Similar SOA yields
(within the experimental uncertainties) were observed when Sa was increased by 200 %,
indicating that the range of Sa used in this study does not affect SOA formation in the TPOT.
2.4 Results and Discussion
2.4.1 Ammonium Sulfate Particles
The formation of SOA onto dry and wet AS was compared in order to gain insight into the role
of particle-phase water. A major result is that a greater amount of isoprene SOA was observed
for wet AS compared to dry AS by a factor of 1.6 ± 0.2. This value is the average of 4
measurements and the uncertainty reflects one standard deviation in those results. We note that
the SOA mass yield for dry AS [(5.3 ± 3.6) × 10-3
] is consistent with the SOA yield observe by
Kroll et al. from isoprene photooxidation under NOx free conditions (0.001 – 0.004).24
Analysis
of the corresponding AMS organic mass spectra (normalized by sulfate to reflect absolute
signals) indicates that carbonyl compounds contribute significantly to the SOA formed, given the
high intensity of m/z 29 (Figure 2.2). High resolution fitting of the mass spectra confirm the peak
at m/z 29 as arising from the CHO+ fragment, which has previously been used as a tracer for
organic compounds with carbonyl functional groups.25
Figure 2.2. Mass spectra (normalized by total sulfate mass) for the organics on wet AS (blue)
38
and dry AS (red) seed particles. The molecular formulae of the dominant organic fragment for
significant ions are identified in the main text.
In addition to the absolute changes in SOA mass, to better understand the impact of particle-
phase water on SOA composition, the spectral differences between the organic-normalized
spectra of SOA on dry and wet AS are shown on Figure 2.3. Note that this spectrum represents
the fractional differences in organic composition. The organics associated with particle-phase
water (i.e., wet AS) were more oxidized, where the strong positive differences in m/z 28 (CO+)
and m/z 44 (CO2+) indicate that organic acids are likely associated with the presence of particle-
phase water.26
While the specific organic acids cannot be identified using an AMS, we speculate
that the uptake of isoprene oxidation products such as methylglyceric acid are likely to be
enhanced due to the presence of particle-phase water, due to their high water solubility.5 For dry
AS, the strong negative difference for m/z 29 (CHO+) indicates enhancement in carbonyl
compounds.
Figure 2.3. Mass spectral difference (Wet AS – Dry AS) of the aerosol organics normalized by
total organic mass. Positive values (blue) indicate the mass fragments that are enhanced for
organics on wet AS seed, and the negative values (red) indicate enhancement for dry AS seed.
The molecular formulae of the dominant organic fragments for significant ions are labeled.
It is important to note that the enhancement in organic acids due to particle-phase water could
also have arisen from aqueous photochemical reactions of carbonyls subsequent to uptake.27,28
For this study, the use of the UV lamp in the TPOT may have initiated these photochemical
reactions. We note that the organic fragment m/z 82 (C5H6O+) has been suggested as a tracer for
IEPOX-derived organics if it contributed to more than 4 ‰ of the total organic mass (f82) as the
contributions from other types of SOA are normally less than that amount.29
For the current
39
study, the f82 for SOA material on both dry and wet AS were less than 4‰, indicating that
IEPOX did not contribute to the SOA material in the current study. Additionally, given that the
lifetime of gas-phase IEPOX with respect to OH oxidation (kOH ~ 1 × 10-11
cm3 molec
-1 s
-1)
in the
TPOT is estimated to be less than a second,30,31
its reactive uptake is unlikely to be the dominant
fate in the TPOT.
The reversibility of the uptake processes leading to additional SOA ultimately determines their
importance for SOA formation in the atmosphere. This was determined by passing the SOA-
sulfate particles through a dryer, qualitatively simulating the evaporation of water in the
atmosphere due to RH cycling. For wet AS, the drying process led to a loss of 20 ± 15 % of SOA
organics, whereas a loss of 35 ± 15 % was observed for dry AS, indicating that the uptake of
SOA materials is partially reversible for both seed particles. The composition of the organics that
was removed can be observed from the spectral differences in the organic spectra (Figures 2.4a-
b). The drying process removed similar types of organics for both dry and wet AS, mainly
comprised of organic acids. It is important to note that the drying process can also accelerate the
rate of condensed phase reactions due to increasing concentrations of solutes during water
evaporation.32,33
Given that a net loss of organic mass was observed upon drying, we believe the
loss of organic acids was largely due to the reversibility of their uptake.
Considering the experiments described above, we conclude that the uptake of SOA material was
enhanced due to the presence of particle-phase water, and that this enhanced uptake of organic
acids is somewhat reversible. Accounting for the partial reversibility of SOA uptake on both wet
and dry AS, the net increase in the amount of SOA that can be taken up by wet AS was 1.7 times
the dry AS value.
40
Figure 2.4. Mass spectral difference of the organics (normalized by total organic mass) for a)
wet AS and b) dry AS to illustrate the reversibility of uptake due to water evaporation. Positive
values (blue) indicate the mass fragments that are enhanced for organics on dried sulfate-SOA
particles, and the negative values (red) indicate enhancement for not-dried sulfate-SOA particles.
The molecular formulae of the dominant organic fragment for significant ions are identified in
the main text.
2.4.2 Acidic Sulfate Particles
While the previous set of experiments demonstrates that particle-phase water affects the
formation of isoprene SOA, the role of acidity also needs to be investigated in order to
understand the relative importance of the two factors on aerosol yield. At 70 % RH, wet ABS
has similar particle water content compared to wet AS, while being more acidic.34
The pH of the
particles used in the current study was estimated using AIM-II from AMS measured ammonium
and sulfate mass, where the pH of wet ABS is estimated to be -0.31 (ammonium-to-sulfate molar
ratio: 0.94) and 0.59 for wet AS (ammonium-to-sulfate molar ratio: 1.72). Note that the
ammonium-to-sulfate molar ratio for AS deviates from the expected value of 2, indicating loss of
ammonium via the partitioning of ammonia to the gas-phase, likely due to flow dilution.
b)
a)
41
The observed isoprene SOA yield for wet AS is (9.3 ± 6.3) × 10-3
and (4.3 ± 0.3) × 10-3
for wet
ABS. The values are the average of 4 measurements for AS and 3 measurements for ABS - the
uncertainties reflect one standard deviation in those results. Thus, similar isoprene SOA yields
were observed for both sulfate seeds, indicating changes in particle acidity did not affect SOA
formation within experimental uncertainty. We note that the IEPOX tracer at m/z 82 was not
observed for wet ABS (i.e., < 4 ‰ of the total organic mass), further suggesting that the uptake
of IEPOX did not contribute to the SOA formed in the current study. The spectral differences of
the SOA on the different seed particles shown in Figure 2.5 indicate that carbonyls contribute a
higher fraction of the total organics in wet AS while a higher fraction of organic mass at m/z >
100 was observed for wet ABS (Figure 2.5 insert). Given the hard ionization technique used in
the AMS, organic fragments > m/z 100 arise from the fragmentation of even larger molecular
weight compounds. We speculate that the high molecular weight species were formed via acid-
catalyzed condensed phase reactions. These observations are consistent with previous studies
where high molecular weight species (i.e., esters and organosulfates) have been previously
identified in isoprene SOA on acidic seed, where their contributions increase with acidity.8,35
Figure 2.5. Mass spectral difference (wet ABS– wet AS) of the organics (normalized by total
organic mass). Positive values (blue) indicate the mass fragments that are enhanced for organics
on wet ABS seed, and the negative values (red) indicate enhancement for wet AS seed. The
insert is a zoomed-in view of the mass spectrum at higher m/z.
The reversibility of the uptake process leading to isoprene SOA formation on wet ABS was also
investigated, where the drying process resulted in a loss of 30 ± 10 % organics. Thus, the drying
process resulted in comparable mass loss to that of wet and dry AS, along with similar changes
42
in chemical composition (see Figure 2.6). These observations suggest that the differences in
particle-phase water and acidity do not affect the reversibility uptake behavior of isoprene SOA.
Figure 2.6. Mass spectral difference of the organics (normalized by total organic mass) for wet
ABS to illustrate the reversibility of uptake due to water evaporation. Positive values (blue)
indicate the mass fragments that are enhanced for organics on dried sulfate-SOA particles, and
the negative values (red) indicate enhancement for not-dried sulfate-SOA particles. The
molecular formulae of the dominant organic fragment for significant ions are identified in the
main text.
2.5 Atmospheric Implications
In this work, the importance of particle-phase water and acidity on isoprene SOA formation is
demonstrated and quantified for the first time at high relative humidity. Particle-phase water was
the dominant factor controlling the amount of SOA formed as it gave rise to 60 % more mass.
The resulting SOA material is more enriched in organic acids, likely due to their enhanced
uptake due to presence of particle-phase water. The uptake of these organic acids was also
observed to be partially reversible upon the removal of particle-phase water. Particle acidity
mainly contributed to differences in chemical composition of the resulting SOA likely due to
acid catalyzed condensed phase reactions where high molecular weight products were formed.
Considering all experiments in sum, the results indicate that the RH cycling of sulfate seed
particles in the atmosphere affects the SOA burden, and further illustrates how anthropogenic
components (i.e., sulfate) mediate the formation of SOA from biogenic precursors.36,37
The results of this study are largely consistent with ambient measurements where weak
correlations of isoprene-SOA and particle acidity have been observed.11–14
We note that IEPOX
did not contribute to the SOA formed in this study due to its short lifetime in the reactor flow
43
tube. Because of this, the distribution of gas-phase isoprene oxidation products probably
resembles that further away from isoprene sources, where IEPOX has been oxidized.
Also, we note that the presence of pre-existing organics on sulfate particles may affect the role of
particle-phase water and acidity on isoprene SOA formation. Gaston et al. observed that the
uptake of IEPOX on mixed sulfate/polyethylene glycol particles was suppressed even for acidic
sulfate seeds at high RH conditions.16
Given the complexity of seed particle effects on just one
SOA precursor, further investigations using a wide range of seed particle composition, SOA
precursors, and conditions (i.e., RH, effects of UV light) are warranted in order to understand
how seed particles mediate SOA formation in the atmosphere.
Acknowledgements
The authors thank Natural Sciences and Engineering Research Council for funding.
2.6 Supporting Material
Figure 2.7. Mass spectra (normalized by total organics) of the organics on wet ABS (blue) and
wet AS (red) seed particles. The insert illustrates the zoomed-in view at higher m/z. The
molecular formulae of the dominant organic fragment for significant ions are identified in the
main text.
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Inorganic Seeds. Atmos. Chem. Phys. 2014, 14, 3497–3510.
(16) Gaston, C. J.; Riedel, T. P.; Zhang, Z.; Gold, A.; Surratt, J. D.; Thornton, J. A. Reactive
Uptake of an Isoprene-Derived Epoxydiol to Submicron Aerosol Particles. Environ. Sci.
Technol. 2014, 48, 11178–11186.
(17) Liu, Y.; Kuwata, M.; Strick, B. F.; Thomson, R. J.; Geiger, F. M.; McKinney, K.; Martin,
S. T. Uptake of Epoxydiol Isomers Accounts for Half of the Particle-phase Material
Produced from Isoprene Photooxidation via the HO2 Pathway. Environ. Sci. Technol.
2014, 49, 250-258.
(18) Tang, I. N.; Fung, K. H.; Imre, D. G.; Munkelwitz, H. R. Phase Transformation and
Metastability of Hygroscopic Microparticles. Aerosol Sci. Technol. 1995, 23, 443–453.
(19) Wong, J. P. S.; Lee, A. K. Y.; Slowik, J. G.; Cziczo, D. J.; Leaitch, W. R.; Macdonald, A.;
Abbatt, J. P. D. Oxidation of Ambient Biogenic Secondary Organic Aerosol by Hydroxyl
Radicals: Effects on Cloud Condensation Nuclei Activity. Geophys. Res. Lett. 2011, 38,
L22805.
(20) Slowik, J. G.; Wong, J. P. S.; Abbatt, J. P. D. Real-Time, Controlled OH-Initiated
Oxidation of Biogenic Secondary Organic Aerosol. Atmos. Chem. Phys. 2012, 12, 9775–
9790.
(21) Canagaratna, M. R.; Jayne, J. T.; Jimenez, J. L.; Allan, J. D.; Alfarra, M. R.; Zhang, Q.;
Onasch, T. B.; Drewnick, F.; Coe, H.; Middlebrook, A.; et al. Chemical and
Microphysical Characterization of Ambient Aerosols with the Aerodyne Aerosol Mass
Spectrometer. Mass Spectrom. Rev. 2007, 26, 185–222.
(22) Docherty, K. S.; Jaoui, M.; Corse, E.; Jimenez, J. L.; Offenberg, J. H.; Lewandowski, M.;
Kleindienst, T. E. Collection Efficiency of the Aerosol Mass Spectrometer for Chamber-
Generated Secondary Organic Aerosols. Aerosol Sci. Technol. 2013, 47, 294–309.
(23) Zhang, X.; Cappa, C. D.; Jathar, S. H.; McVay, R. C.; Ensberg, J. J.; Kleeman, M. J.;
Seinfeld, J. H. Influence of Vapor Wall Loss in Laboratory Chambers on Yields of
Secondary Organic Aerosol. Proc. Natl. Acad. Sci. 2014, 111, 5802–5807.
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(24) Kroll, J. H.; Ng, N. L.; Murphy, S. M.; Flagan, R. C.; Seinfeld, J. H. Secondary Organic
Aerosol Formation from Isoprene Photooxidation. Environ. Sci. Technol. 2006, 40, 1869–
1877.
(25) Lee, A. K. Y.; Hayden, K. L.; Herckes, P.; Leaitch, W. R.; Liggio, J.; Macdonald, A. M.;
Abbatt, J. P. D. Characterization of Aerosol and Cloud Water at a Mountain Site during
WACS 2010: Secondary Organic Aerosol Formation through Oxidative Cloud Processing.
Atmos. Chem. Phys. 2012, 12, 7103–7116.
(26) Ng, N. L.; Canagaratna, M. R.; Zhang, Q.; Jimenez, J. L.; Tian, J.; Ulbrich, I. M.; Kroll, J.
H.; Docherty, K. S.; Chhabra, P. S.; Bahreini, R.; et al. Organic Aerosol Components
Observed in Northern Hemispheric Datasets from Aerosol Mass Spectrometry. Atmos.
Chem. Phys. 2010, 10, 4625–4641.
(27) Lim, Y. B.; Tan, Y.; Perri, M. J.; Seitzinger, S. P.; Turpin, B. J. Aqueous Chemistry and
Its Role in Secondary Organic Aerosol (SOA) Formation. Atmos. Chem. Phys. 2010, 10,
10521–10539.
(28) Zhao, R.; Lee, A. K. Y.; Abbatt, J. P. D. Investigation of Aqueous-phase Photooxidation
of Glyoxal and Methylglyoxal by Aerosol Chemical Ionization Mass Spectrometry:
Observation of Hydroxyhydroperoxide Formation. J. Phys. Chem. A 2012, 116, 6253–
6263.
(29) Allan, J. D.; Morgan, W. T.; Darbyshire, E.; Flynn, M. J.; Williams, P. I.; Oram, D. E.;
Artaxo, P.; Brito, J.; Lee, J. D.; Coe, H. Airborne Observations of IEPOX-Derived
Isoprene SOA in the Amazon during SAMBBA. Atmos. Chem. Phys. 2014, 14, 11393–
11407.
(30) Jacobs, M. I.; Darer, A. I.; Elrod, M. J. Rate Constants and Products of the OH Reaction
with Isoprene-Derived Epoxides. Environ. Sci. Technol. 2013, 47, 12868–12876.
(31) Bates, K. H.; Crounse, J. D.; St. Clair, J. M.; Bennett, N. B.; Nguyen, T. B.; Seinfeld, J.
H.; Stoltz, B. M.; Wennberg, P. O. Gas-phase Production and Loss of Isoprene
Epoxydiols. J. Phys. Chem. A 2014, 118, 1237–1246.
(32) Ervens, B.; Turpin, B. J.; Weber, R. J. Secondary Organic Aerosol Formation in Cloud
Droplets and Aqueous Particles (aqSOA): A Review of Laboratory, Field and Model
Studies. Atmos. Chem. Phys. 2011, 11, 11069–11102.
(33) Lee, A. K. Y.; Zhao, R.; Li, R.; Liggio, J.; Li, S.-M.; Abbatt, J. P. D. Formation of Light
Absorbing Organo-Nitrogen Species from Evaporation of Droplets Containing Glyoxal
and Ammonium Sulfate. Environ. Sci. Technol. 2013, 47, 12819–12826.
(34) Schlenker, J. C.; Martin, S. T. Crystallization Pathways of Sulfate-Nitrate-Ammonium
Aerosol Particles. J. Phys. Chem. A 2005, 109, 9980–9985.
(35) Surratt, J. D.; Lewandowski, M.; Offenberg, J. H.; Jaoui, M.; Kleindienst, T. E.; Edney, E.
O.; Seinfeld, J. H. Effect of Acidity on Secondary Organic Aerosol Formation from
Isoprene. Environ. Sci. Technol. 2007, 41, 5363–5369.
47
(36) Carlton, A. G.; Turpin, B. J. Particle Partitioning Potential of Organic Compounds Is
Highest in the Eastern US and Driven by Anthropogenic Water. Atmos. Chem. Phys. 2013,
13, 10203–10214.
(37) Pye, H. O. T.; Pinder, R. W.; Piletic, I. R.; Xie, Y.; Capps, S. L.; Lin, Y.-H.; Surratt, J. D.;
Zhang, Z.; Gold, A.; Luecken, D. J.; et al. Epoxide Pathways Improve Model Predictions
of Isoprene Markers and Reveal Key Role of Acidity in Aerosol Formation. Environ. Sci.
Technol. 2013, 47, 11056–11064.
48
Chapter 3
3 Changes in Secondary Organic Aerosol Composition and
Mass due to Photolysis: Relative Humidity Dependence
Changes in Secondary Organic Aerosol Composition
and Mass due to Photolysis: Relative Humidity
Dependence
J. P. S. Wong, S. Zhou, J. P. D. Abbatt, The Journal of Physical Chemistry A, Article ASAP,
doi:10.1021/jp506898c
Reprinted (adapted) with permission from The Journal of Physical Chemistry A
Copyright 2014 American Chemical Society
49
3.1 Abstract
This study is focused on the relative humidity (RH) dependence of water-soluble secondary
organic aerosol (SOA) aging by photolysis. Particles containing α-pinene SOA and ammonium
sulfate, generated by atomization, were exposed to UV radiation in an environmental chamber at
three RH conditions (5, 45 and 85 %), and changes in chemical composition and mass were
monitored using an aerosol mass spectrometer (AMS). Under all RH conditions, photolysis leads
to substantial loss of SOA mass, where the rate of mass loss decreased with decreasing RH. For
all RH conditions, the less oxidized components of SOA (e.g., carbonyls) exhibited the fastest
photodegradation rates, which resulted in a more oxidized SOA after photolytic aging. The
photolytic reactivity of SOA material exhibited a dependence on RH likely due to moisture-
induced changes in SOA morphology or phase. The results suggest that the atmospheric lifetime
of SOA with respect to photolysis is dependent on its RH cycle, and that photolysis may be an
important sink for some SOA components occurring on an initial time scale of a few hours under
ambient conditions.
3.2 Introduction
Organic compounds are ubiquitous in ambient aerosol, yet their formation and transformation
mechanisms in the atmosphere remain not fully characterized.1 These organic compounds are
largely secondary in nature (i.e., secondary organic aerosol, SOA), produced from the
condensation of oxidation products of volatile organic compounds. Away from sources, SOA is
continually subjected to further chemical processing, or aging, that transforms the particulate
physico-chemical properties, which are important for climate, air quality, and human health.
Current atmospheric models, with parametrizations based largely on gas- and gas-particle
reactions studied in the laboratory, have difficulty predicting the mass and the degree of
oxidation of ambient SOA, suggesting unidentified formation and aging mechanisms.2,3
Aging processes include heterogeneous oxidation, gas-phase oxidation of semi-volatile
components, and reactions in the condensed phase, such as photolysis, polymerization, and
aqueous oxidation.3 Historically, the aging of SOA material has been investigated using SOA
particles in environmental chambers or flow tube reactors, or particles collected on filters.
Typically, these studies have been conducted under dry or low RH conditions. On the other hand,
studies have also examined the aging of aqueous SOA solutions to investigate the role of cloud
50
processing (e.g., saturated RH conditions). Collectively, these studies have shown that aging can
affect the mass and the chemical composition of SOA, but the experimental conditions only
represent a narrow range of RH conditions found in the atmosphere. Only a limited number of
studies have investigated aging processes under a wide range of RH conditions where the
reactivity of model organic particles was observed to be dependent on RH, as aging by ozone4
and OH radical5 were enhanced under high RH conditions. For complex mixtures such as SOA,
recent work by Zhou et al. observed that the kinetics of the heterogeneous ozonolysis of
benzo[a]pyrene (BaP) within SOA material was suppressed under dry conditions, where the
kinetic behavior was between that expected of solids and liquids. Additionally, it was observed
that the reactivity of BaP was enhanced with increasing RH conditions. These results suggest
that RH affects particle reactivity, as water uptake by hygroscopic SOA under high RH condition
lowers the material’s viscosity, which allows for more rapid diffusion of reactants compared to
dry conditions where SOA is semisolid.6
Indeed, there is growing experimental evidence that SOA can be an amorphous solid with high
viscosity under dry/low RH conditions. With increasing RH, water acts as a plasticizer,
decreasing the viscosity of the organic material (i.e., liquid SOA).7–10
In addition to the study by
Zhou et al., other recent studies have shown that the phase/viscosity of SOA material can affect
gas-particle partitioning11
and aging by heterogeneous OH oxidation.12
Given that past
investigations have shown the importance of RH on the phase/viscosity of SOA, which has
implications on particle reactivity, it is necessary to characterize how variations in RH conditions
affect other aging processes. Of particular interest to the current study is the aging of SOA by
photolysis.
As with other SOA aging experiments, previous laboratory studies have focused on the
photolysis of dry SOA particles, or SOA dissolved in bulk solutions, observing a reduction in
SOA mass and significant photodissociation of compounds containing carbonyl and peroxide
functional groups.13–15
Although there is growing evidence that photolysis may play an important
role in the aging of SOA, the RH effects are still unknown. The objective of the current study is
to investigate the photolysis of particles containing water-soluble α-pinene SOA material and
ammonium sulfate under various RH conditions in an environmental chamber. The changes in
SOA mass and chemical composition were measured using an Aerodyne aerosol mass
spectrometer (AMS). The kinetics of SOA photolysis were examined in order to estimate the
51
atmospheric importance of direct photochemical aging. The use of mixed SOA–ammonium
sulfate particles was chosen for two reasons. In particular, the sulfate mass could be used to
normalize the organic signal under the assumption of no sulfate photochemical reactivity. Also,
the sulfate hygroscopic properties are well characterized ensuring that water was present in the
particles at high relative humidity and not present at the lowest relative humidity.
3.3 Experimental Section
3.3.1 SOA Generation and Collection
SOA was generated and collected separately from the photolysis experiments in order to remove
the effects of gas-phase oxidants and volatile α-pinene oxidation products. The ozonolysis of α-
pinene was used to generate SOA material in a 1 m3 Telfon (FEP) dark chamber, under dry
conditions (RH < 5 %) and in a continuous flow mode. The terpene was introduced into the
chamber via a 6 sccm flow from the headspace of a bubbler containing liquid α-pinene (Sigma-
Aldrich), chilled to −10 °C and mixed with a 9 slpm flow of purified air. Ozone was generated
by passing 0.5 slpm of purified air over a mercury lamp and introduced into the chamber to
initiate SOA formation. The final mixing ratios were 750 ppb of ozone, as monitored by an
ozone analyzer (Thermo Scientific) and 150 ppb of α-pinene, as measured by a proton-transfer-
reaction mass spectrometer (Ionicon). The residence time in the chamber during SOA generation
was 100 min. The resulting SOA particle size distribution was measured using a TSI Scanning
Mobility Particle Sizer (SMPS: DMA TSI-3080 and CPC, TSI-3225). A sample volume
distribution of the resulting SOA is shown in Figure 3.1. The SOA was collected on
polytetrafluoroethylene filters (47 mm, 2 um pore size, Pall Corporation) at 9.5 slpm for 48 h
after the O3 mixing ratio and SOA mass loading were stable. The amount of SOA collected on
filter (∼9 mg) was determined from the difference in mass measured before and after collection.
Immediately after collection, the water-soluble organic compounds in the SOA were extracted in
8.75 mL of purified water (18 mΩ) and stored in a freezer at −20 °C.
52
Figure 3.1. Typical volume distribution of SOA formed during filter collection, measured by
SMPS.
3.3.2 UV/Vis Absorption Spectroscopy
The UV/vis absorption properties of SOA were measured by an UV/vis dual beam spectrometer
(PerkinElmer). A bulk SOA solution was used, using a concentration identical to the solutions
used for generating particles for photolysis experiments (described below). Purified water in a
matching curvette (1 cm, quartz) was used as the reference. From the measured absorbance
(base-10), the effective absorption cross section (base-e) of SOA was determined (see the
Supporting Information for a detailed calculation). Since the UV lamps used in the current study
to initiate photolysis have a sharp emission cutoff at 284 nm, the absorbance below this
wavelength was not considered.
3.3.3 Photolysis of SOA
All photolysis experiments were conducted in the University of Toronto Mobile Concentration
and Aging (MOCA) chamber in batch mode. MOCA is a 1 m3 Teflon (FEP) bag mounted around
a cubic Teflon-coated frame, surrounded by 24 UV lamps (Sylannia) on four of its six sides,
cooled with multiple fans. The temperature and RH inside the chamber were continuously
monitored (Vaisala). Following each experiment, the chamber was flushed with clean air and
irradiated with UV light for a minimum of 12 h for cleaning.
Prior to each experiment, the chamber was conditioned to a specific RH by flowing purified air
through heated water bubblers into the chamber prior to particle introduction. To generate
particles, ammonium sulfate (AS, 0.04 w/v %) was added to the SOA water extract to a total
53
volume of 10 mL, resulting in a 3:2 inorganic-to-organic mass ratio. AS was chosen because its
hygroscopic response to variations in RH is well characterized.16
This bulk solution was
aerosolized into the chamber using a constant output atomizer (TSI 3076) for 20 min. From AMS
measurements (described below) obtained using particle time-of-flight (PTOF) mode, the
particles in the chamber had a typical mass mode of 230 nm prior to UV light exposure (size-
resolved mass distribution shown in Figure 3.2). Prior to photolysis, the typical particle mass
loading in the chamber was 30 μg m-3
. To study the photolysis of SOA material, the AS–SOA
particles were mixed in the dark chamber for 20 min before they were exposed to UV irradiation
for 33 min, following which the chamber was returned to dark conditions. Upon UV light
exposure, the temperature inside the chamber increased by 1° (above room temperature), which
resulted in a decrease of ∼5 % in RH. It is important to note that the effects of UV light exposure
can be due to volatilization (due to heating) and UV irradiation. Since this temperature increase
was identical for all photolysis experiments, the contributions of volatilization to changes in
SOA mass and composition are also identical. That being said, with the temperature increase so
small, we believe the changes are largely due to photolysis.
Figure 3.2. Sized-resolved mass distribution of organics (green) and sulfate (red) measured by
the AMS in PTOF mode for mixed SOA-AS particles before UV light exposure.
Two sets of experiments were conducted to characterize the photolysis of SOA material. For the
first set of experiments, the photolysis of SOA by UVB light centered at 310 nm at three
different RH conditions (85 % (“high RH”), 45 % (“mid RH”), and 5 % (“low RH”)) was
examined to elucidate the effects of RH. The polydisperse particles were fully deliquesced before
entering the chamber, and then equilibrated to each RH condition in the chamber. Given that
previous work has characterized the effects of RH on the particle-phase and viscosity of SOA.9,10
54
we speculate that the SOA material is likely to be liquid at high RH and semisolid at low RH
conditions, possibly having phase separated. At mid RH conditions, the particles were expected
to have intermediate liquid water content, and the resulting phase and viscosity of the SOA
material is likely to be between the values present under high and low RH conditions. For AS, it
is deliquesced at high RH and effloresced at low RH conditions.16
The second set of experiments was conducted to investigate the UV wavelength dependence of
SOA photolysis. Under the same high RH conditions, photolysis using UVA lamps centered at
360 nm was conducted, and compared to results obtained using UVB lamps centered at 310 nm.
The wavelength dependence of the photon flux inside the chamber was measured using a
spectroradiometer (StellaNet Inc.). The measured wavelength dependent photon fluxes from both
UVB and UVA lamps are shown in Figure 3.1, where the actinic flux at solar noon is provided
for comparison. It is important to note that the spectral flux of the UVB and UVA lamps is not
fully representative of that in the atmosphere. Compared to ambient conditions, the UVB lamps
are more intense at lower wavelengths while the UVA lamps are less intense for all wavelengths.
Figure 3.3 Measured photon flux in the chamber from UVB and UVA lamps compared to the
actinic flux for a clear-sky summer day. The actinic flux was obtained from “Quick TUV
Calculator”, available at http://cprm.acd.ucar.edu/Models/TUV/Interactive_TUV/ using the
following parameters: SZA = 0, June 30, 2000, 300 Dobson overhead ozone, surface albedo of
0.1, and 0 km altitude.
55
3.3.4 Particle Measurements
For particle mass and composition measurements, the particles were first dried using a silica gel
diffusion dryer and then sampled online by the time-of-flight aerosol mass spectrometer (ToF-
AMS, Aerodyne Research, Inc.). Experiments were conducted with the unit resolution AMS
(cToF-AMS) unless otherwise stated. The operating and analysis procedure of the AMS has been
previously reviewed in detail.17
Briefly, the AMS provides the size-resolved mass and chemical
composition of the non-refractory component of particles, including organics, sulfate, nitrate,
and ammonium. Since the resulting mass spectra of the sampled particles include signals from
major gases (N2, O2, H2O, Ar, and CO2), their contributions were accounted for by obtaining
multiple filter measurements (e.g., zero particle signal) throughout each experiment.
To understand the changes in organic particle mass due to photolysis, the loss of particle mass to
the chamber walls needs to be accounted for. Since the chamber experiments were conducted in
batch mode, where the particle mass concentration decreases over time due to deposition, the
individual organic fragments and total organic mass were normalized to the total sulfate mass
measured by the AMS in order to determine absolute changes due to photolysis, assuming sulfate
is nonreactive. Additionally, the mass loading of α-pinene SOA has been shown to affect its
chemical composition, as semi-volatile organic compounds will increasingly partition to the
particle at higher mass loadings.18,19
In the current study, where particle mass loading decreases
over time due to wall loss, we expect the evaporation of semi-volatile compounds over the
course of the experiment. Control experiments were conducted where particles were not exposed
to UV irradiation in the chamber, under each RH condition. These control experiments were
conducted at the same temperature as typical photolysis experiments. The observed changes to
the chemical composition of particles due to wall loss were accounted for in order to isolate the
effects due to UV irradiation (Figure 3.4).
56
Figure 3.4. Typical time trace of org:sulfate ratio of the mixed particles due to evaporation in the
dark (control) and exposure to UV irradiation under high RH conditions.
3.4 Results and Discussion
3.4.1 Absorption Cross Section of SOA
While SOA is a complex mixture of many different compounds, the absorption cross section of
the bulk SOA solution (Figure 3.5) provides information about the functional groups that absorb
light in the wavelength range emitted by the UVB and UVA lamps used in this study. The
observed main absorption region corresponds to characteristic absorptions of carbonyl and
peroxy functional groups, which are known products of SOA formed by α-pinene ozonolysis.
The absorption cross section of a known carbonyl product, cis-pinonic acid,20
is also shown in
Figure 3.5 for comparison. The absorption observed at higher wavelengths is likely due to the
presence of peroxide functional groups, as absorption cross sections of peroxides tend to exhibit
a broad tail into the actinic region.21
While it is ideal to measure the absorption cross section of
SOA material in particles, this was not possible with the instrumentation available. However, a
previous measurement of the mass absorption coefficient of SOA particles at RH 70 % observed
a similar spectral shape compared to the spectrum obtained in this study using bulk SOA
solution, suggesting the absorptive properties of SOA material in bulk solutions are similar to
those in particles under higher RH conditions.22
57
Figure 3.5 Absorption cross section of bulk SOA solution (this work) and aqueous cis-pinonic
acid.20
3.4.2 Photolytic Degradation of SOA
Absolute changes to the total organic mass, as indicated by the organic-to-sulfate (org:sulfate)
mass ratio, due to exposure to UVB light under different RH conditions are shown in Figure 3.6.
Org:sulfate ratios were normalized to pre-aging values to guide comparison of the changes for
different experimental conditions. Under all RH conditions, decreases in total organic mass were
observed due to UV light exposure, where the amount of SOA mass loss increases with RH.
Absorption of UV light by organic compounds can initiate photolysis, where the fragmentation
of their carbon skeleton results in the formation of smaller products (e.g., fewer carbons atoms)
that are more volatile.18,23
We speculate that these volatile photolysis products evaporate from
the particle, resulting in the observed loss of total organic mass. In fact, the formation of gas-
phase products, such as CO, CH4, acetone, and other small VOCs, has been observed following
the photodegradation of limonene–SOA on filters and in aqueous solutions.13,14
58
Figure 3.6. Time series profiles of the org:sulfate ratios measured by the AMS for the photolysis
of SOA by UVB radiation under high RH (solid line), mid RH (dashed line), and low RH (dotted
line) conditions. The shaded areas for each RH condition represent the variability (±1σ) between
multiple experiments.
In addition to the total organic mass, the absolute changes in specific organic fragments provide
additional insight into the photolysis of SOA. Due to the evaporation/ionization techniques used
in the AMS, most compounds are subject to extensive fragmentation; however, specific
fragments can be used as tracers for organic compounds with specific functional groups.17
The
CHO+ fragment detected at m/z 29 arises from the fragmentation of compounds containing
carbonyl functional groups,24
while two different fragments can contribute to the signal observed
at m/z 43, with the C2H3O+ fragment originating from functionalities with a single oxygen atom
(e.g., carbonyls, alcohols, and ethers) and the C3H7+ fragment originating from aliphatic
molecules.25
Control experiments using the high-resolution ToF-AMS, which can separate
different ions at the same nominal m/z, indicated that the C2H3O+ ion was the dominant signal at
m/z 43 for this study. The organic ion at m/z 44 (CO2+) arises from the fragmentation of highly
oxidized organic acids (e.g., carboxylic acids and esters)26
and peroxides.27
For all RH
conditions, upon UVB irradiation, loss of the oxidized components of SOA was observed, where
the decays of m/z 29 and m/z 43 (Figure 3.7a,b) were faster than that of m/z 44 (Figure 3.7c).
59
Figure 3.7. Time series profiles of the sulfate normalized (a) m/z 29, (b) m/z 43, and (c) m/z 44
for the photolysis of mixed SOA–sulfate particles by UVB radiation at high RH (solid line), mid
RH (dashed line), and low RH (dotted line) conditions. The shaded areas for each RH condition
represent the variability (±1σ) between multiple experiments.
Given that the absorption spectrum of SOA indicates the presence of carbonyl and peroxide
function groups, both of which absorb UVB radiation, it is not surprising that we observed
decays of m/z 29, 43, and 44, suggesting the photodegradation of these functional groups. To
60
gain further insight of the wavelength dependence of SOA photolysis, experiments under high
RH conditions were conducted using UVA lights. Compared to experiments using UVB
radiation at the same RH condition, SOA exposed to UVA radiation exhibited slower decays for
total organic mass (Figure 3.8a) and for all organic fragments considered (Figure 3.8b–d). In
particular, the loss of m/z 29 and 43 is greatly suppressed for UVA lights, suggesting that the
photolysis of less oxidized components (i.e., carbonyls) contributes significantly to the observed
photochemical aging with the UVB light. It is important to note that the slower decay of m/z 44
compared to m/z 29 and 43 can also arise from secondary reactions of photolysis which result in
the formation of highly oxidized organics. For example, photolysis of peroxides is known to
generate OH radicals, which can oxidize other SOA components, forming highly oxidized
components such as organic acids.28,29
Direct photolysis of aldehydes has also been observed to
generate organic acids as well.13,20
Given that both peroxides and carboxylic acids contribute to
the organic fragment at m/z 44, the combined result of photodissociation of peroxides and the
formation of carboxylic acids may have resulted in the slower decay of m/z 44.
Figure 3.8. Time series profiles of the sulfate normalized (a) total organic mass (org:sulfate
ratio, black), (b) m/z 29, (c) m/z 43, and (d) m/z 44 for the photolysis of mixed SOA–sulfate
particles by UVB (solid lines) and UVA (dashed lines) radiation under high RH conditions. The
shaded areas for each light source represent the variability (±1σ) between multiple experiments.
61
The relative changes in the chemical composition of SOA due to photolysis can be illustrated
using the framework developed by Ng et al. in Figure 3.9, where aging of SOA is described
using the fraction of organic mass at m/z 43 vs 44 (f43 and f44).26
Ambient SOA typically falls
within the triangle denoted by the dashed lines in Figure 3.9c (inset), with less oxidized SOA
mainly residing in the lower half of the triangle and highly oxidized SOA in the upper half. In
general, studies have shown that SOA aging processes, such as OH oxidation, result in
movement toward the upper left of the triangle space.30
The trends observed for this figure are
also consistent with those observed for Figure 3.7, with, for example, a stronger decrease for m/z
43 compared to m/z 44, with the magnitude of the change depending on RH conditions. This
suggests that enhanced loss of less oxidized particulate organics due to photolysis results in a
more oxidized SOA. Note that the composition of the SOA used in the current study resides in
the middle region of the triangle, suggesting that the organics are somewhat oxidized prior to
photolysis.
Considering the experiments discussed above, the loss of organic components and changes in
chemical composition due to SOA photolysis all show RH dependence, with enhanced loss
observed with increasing RH conditions. We speculate that the observed RH dependence for
SOA photolysis is likely due to the effects of viscosity on diffusion, as recent studies have shown
that SOA exhibits liquid-like behavior under high RH and is semisolid/solid under low RH.9,10
In
viscous semisolid/solid material, the slow diffusion of SOA photodissociation products can favor
their recombination, resulting in a reduction in the efficiency of photolysis. For example, the net
quantum yields for the photolysis of certain amines and iodine were previously observed to be
reduced in viscous media.31,32
Additionally, slow diffusion has been shown to kinetically hinder
the evaporation of SOA.11
Thus, the effects of RH on diffusion can lead to the results observed in
the current study, where, under lower RH conditions, slow diffusion of photolysis products
resulted in less mass loss compared to higher RH conditions.
62
Figure 3.9. Aerosol composition described by the fraction of organic mass at m/z 43 and 44 (f43
vs f44) before (open circles), during (closed circles), and after (open circles) UV light exposure
for (a) high RH, (b) mid RH, (c) and low RH conditions. The inset shows the zoomed-out view
of the f43 vs f44 space (with dimensions of Figure 3.9a–c illustrated by the red box), where
ambient SOA is distributed within the triangular area.
63
3.4.3 Kinetics of Photolysis
Figure 3.10. Kinetic plots for the loss of total organic mass due to photolysis of SOA at high RH
(solid line), mid RH (dashed line), and low RH (dotted line). The shaded region for each RH
condition represents the variability (±1σ) between experiments. The mass loss rate constant was
obtained from the linear best fit of the first 10 minutes of photolysis.
To further investigate the role of RH on the reactivity of SOA material on particles, the decay
rates of particulate SOA mass loss due to photolysis were determined. The kinetic plot (shown in
Figure 3.10) indicated that the reaction exhibited first order kinetics at early times, as expected
from direct photolysis. Linear fits to the first 10 min of data yield first-order mass loss rates
(kSOA) for all RH and light conditions, listed in Table 3.1. Assuming that the initial observed
mass loss is only due to the volatilization of small organic compounds from photolysis of SOA,
the observed mass loss rate constants were equated to the effective photolysis rate constant:
Equation 3.1
where σ(λ) is the absorption cross section of SOA, ϕ(λ) is the photolysis quantum yield, and F(λ)
is the photon flux. Using the measured absorption cross section of SOA and the measured photon
flux in the chamber, the quantum yield for the loss of organics due to SOA photolysis can be
estimated. Since the UV lamps used in the current study to initiate photolysis have a sharp
emission cutoff at 284 nm, the absorbance below this wavelength was not considered. We note
that this approach does not take into account the enhancement in photon flux that may occur
inside wet particles relative to values in air.33
64
Table 3.1. Total Organic Mass Loss Rate Constants for Different RH Conditionsa
kSOA (s–1
)
light source high RH mid RH low RH
UVB (1.67 ± 0.20) × 10–4
(1.64 ± 0.09) × 10–4
(7.91 ± 0.13) × 10–5
UVA (4.69 ± 0.14) × 10–5
aThe uncertainties in kSOA represent variability (±1σ) between multiple experiments.
From the observed mass loss rates under high RH conditions for both UVB and UVA lights, the
average effective photolysis quantum yield was determined to be 1.2 ± 0.2. This large quantum
yield suggests that either the primary photolytic process efficiently gives rise to volatile products
or secondary reactions (such as those driven by the photochemical formation of OH in the
particle) may have also contributed to the observed total mass loss rates. The photolysis quantum
yield for cis-pinonic acid, one of the identified compounds in α-pinene SOA, is 0.5 ± 0.3,20
i.e.,
also quite large and similar to our result. The factor of 2 difference could potentially be due to
the presence of other more photolabile compounds in SOA material.
At decreasing RH, the SOA becomes highly viscous and concentrated, where elevated
concentrations can accelerate condensed phase reactions (e.g., polymerization, reactions with
inorganics), thus potentially changing the absorptive properties of the material. Since the
absorption cross section of SOA measured from bulk solutions may not be identical compared to
particles under decreasing RH conditions, the current approach to determine the effective
quantum yield for mid and low RH conditions was not pursued. Nevertheless, the suppression in
mass loss rates with decreasing RH indicates that photolysis is dependent on RH conditions,
where the mass loss rate under low RH is a factor of 2 slower than that at high RH. It is
important to note that the observed mass loss rate at the low RH condition using UVB lights for
the current study (7.91 ± 0.13) × 10–5
s–1
] is similar to the photolytic mass loss rate constant of
dry α-pinene SOA–AS particles for UVA lights (6 × 10–5
s–1
), reported by Henry and Donahue.15
In comparison, for dry isoprene SOA, Kroll et al. also observed mass loss [(1 – 5) × 10–5
s–1
] due
to irradiation with UVA lights.34
However, this mass loss rate might have arisen due to reaction
with gas-phase OH radicals, due to the addition of H2O2 (both UV photons and OH radicals are
present during irradiation).
65
3.5 Conclusions and Atmospheric Implications
In this work, the RH dependence for the photolysis of a chemically complex mixture, water-
soluble SOA, was illustrated. Photolysis resulted in substantial organic mass loss, with the most
rapid loss observed under high RH conditions. In particular, the less oxidized components of
SOA (e.g., carbonyls) exhibited the fastest photodegradation, which resulted in a more oxidized
SOA particle after photolytic aging. We hypothesize that the dependence of photolysis on RH
conditions was due to moisture-induced viscosity changes of SOA material, affecting the
diffusion of reactants/products. In this regard, despite a change in several orders of magnitude in
the viscosity of the SOA material from high to low RH conditions, its photolysis was only slower
by a factor of 2. This suggests that, even at such high viscosity values, photolyzed SOA material
can nevertheless efficiently diffuse out of the particle and evaporate within the experimental time
scale of the current study.8,10
To assess the importance of photolysis as an aging mechanism for SOA in the atmosphere, the
atmospheric lifetime of SOA particles with respect to photolysis is estimated to be between that
observed with the UVB lamps and that for the UVA lamps. In particular, using the
experimentally determined absorption cross section and photolysis quantum yield, and calculated
actinic fluxes, the initial photolysis lifetime is ∼3.7 h at Solar Noon for high RH conditions.
Given that the average lifetime of particles in the atmosphere is approximately 1 week with
respect to deposition, this very rough calculation indicates that photolysis can be an important
sink of SOA material in the atmosphere due to volatilization.
Clearly, there are uncertainties in this estimate, and this initial lifetime should not be equated to
the overall lifetime for loss of SOA particles in the atmosphere. In particular, the contribution of
secondary photolysis to the effective quantum yield was not constrained. Therefore, the average
photolysis quantum yield determined in the current study is likely an upper limit to the primary
photolysis process, corresponding to a lower limit for this calculated lifetime. Nevertheless, the
effects of such secondary reactions of photolysis may be of importance to the fate of SOA. For
example, it is known that photolysis of organic compounds in cloudwater is an important source
of H2O2, which is an important oxidant.35
Also, we note that SOA reactivity may be dependent on its atmospheric age and the degree of
prior processing. In particular, the slopes of the mass loss decay curves (e.g., Figure 3.10)
66
decreased with time, indicating depletion of the most photochemically active species (i.e.,
carbonyls and peroxides) and formation of more stable compounds, such as carboxylic acids. On
the other hand, Henry and Donahue observed enhanced mass loss for the photolysis of SOA that
was aged via OH oxidation, as oxidative aging by OH may lead to the formation of more
carbonyl and peroxide compounds compared to photolytic aging.15
Additionally, brown carbon
formed via condensed phase reactions in aerosols has been observed to be photolabile.36
As such,
it is important to understand how the reactivity of SOA changes with atmospheric age.
Also, we point out that this work has implications to photooxidation SOA chamber and flow tube
studies, given that many of these reactors use UV lamps similar to those in this experiment.
Thus, the changes in SOA mass and chemical composition due to photolytic aging observed in
this study may also occur in other experiments.
Finally, the formation mechanism of the mixed SOA–AS is not necessarily the same as that in
the atmosphere, probably resembling the process by which particles are formed by cloud
processing more than that from condensation of terpene oxidation products on an ammonium
sulfate particle.
Overall, this is one of the first studies to quantify the changes in SOA that occur upon exposure
to ultraviolet light, and the first study to look at the effect of RH on this process. This RH
dependence needs to be further investigated with other aging processes, SOA types, and various
conditions (e.g., peroxides are more likely to be formed under low NOx conditions during SOA
formation), in order to understand the processing of such species under a wide range of
atmospherically relevant conditions.
Acknowledgements
The authors thank Natural Sciences and Engineering Research Council and Canada Foundation
for Innovation for funding.
67
3.6 Supporting Material
3.6.1 Absorption Cross Section Calculations
The effective absorption cross section ( base-e) of SOA was calculated from the measured
absorbance ( , base-10) using the following equation:
σ = ln (10) 1000
Equation 3.2
where is the cuvette path length in cm, is the concentration of the SOA solution in mol L-1
,
and is Avogrado’s number. In order to calculate the molar concentration of the SOA solution,
the density of the SOA solution was calculated from the mass of SOA material collected on the
filter dissolved in a known volume of purified water. The average molecular weight of this is
estimated to be 200 g mol-1
.14,37
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71
Chapter 4
4 Oxidation of ambient biogenic secondary organic aerosol
by hydroxyl radicals: Effects on cloud condensation nuclei
activity
Oxidation of ambient biogenic secondary organic
aerosol by hydroxyl radicals: Effects on cloud
condensation nuclei activity
J. P. S. Wong, A.K. Y. Lee, J.G. Slowik, D.J. Cziczo, W.R. Leaitch, A. MacDonald, J.P. D.
Abbatt, Geophysical Research Letters, 2011, 38, L22805, doi:10.1029/2011GL049351.
Reprinted (adapted) with permission from Geophysical Research Letters
Copyright 2014 American Geophysical Union
72
4.1 Abstract
Changes in the hygroscopicity of ambient biogenic secondary organic aerosols (SOA) due to
controlled OH oxidation were investigated at a remote forested site at Whistler Mountain, British
Columbia during July of 2010. Coupled photo-oxidation and cloud condensation nuclei (CCN)
experiments were conducted on: i) ambient particles exposed to high levels of gas-phase OH,
and ii) the water-soluble fraction of ambient particles oxidized by aqueous-phase OH. An
Aerodyne Aerosol Mass Spectrometer (AMS) monitored the changes in the chemical
composition and degree of oxidation (O:C ratio) of the organic component of ambient aerosol
due to OH oxidation. The CCN activity of size-selected particles was measured to determine the
hygroscopicity parameter (κorg,CCN) for particles of various degrees of oxygenation. In both cases,
the CCN activity of the oxidized material was higher than that of the ambient particles. In
general, κorg,CCN of the aerosol increases with its O:C ratio, in agreement with previous laboratory
measurements.
4.2 Introduction
One of the largest uncertainties in global radiative forcing assessment is the cooling effect of
atmospheric particulates via their ability to act as CCN, which is dependent on their size and
composition. Organic compounds constitute a large fraction of ambient aerosol, as demonstrated
at various locations in the Northern Hemisphere.1 Characterization of organic aerosol
hygroscopicity is thus crucial for reducing uncertainties in current climate models.
The earliest studies of organic aerosol CCN properties used model compounds, mainly chosen by
their solubility properties.2–5
These studies illustrated that highly water soluble compounds give
rise to good CCN – indicating that organic constituents could be of comparable importance to
cloud droplet formation as highly soluble, inorganic components. CCN closure studies were also
performed.6,7
Recent lab work has demonstrated that SOA, formed by photo-oxidation processes
from specific precursors, is quite hygroscopic.8–15
Given that a complete chemical characterization of ambient particles is not yet achievable, their
CCN properties cannot be calculated from first-principles. As a result, the particle hygroscopicity
is now typically described using the κ-Köhler method, where one parameter (κ) describes the
average properties (e.g., molecular weight and solubility) of all compounds present.16
This
73
eliminates the need for the specific chemical identities of the constituents found in ambient
aerosol and allows for a simple representation of CCN activation in global climate models.
In this context, a highly oxygenated organic particle is more likely to be a better CCN due to the
increased polarity and solubility of its constituents. Given that oxidative aging leads to more
functionalized solute molecules,17
it is reasonable to inquire whether such aging increases the
hygroscopicity of these particles. For laboratory data, a simple trend of increasing CCN activity
and sub-saturated hygroscopicity with degree of SOA oxidation (approximated by O:C atomic
ratios) has been reported.12,14,18,19
However, since the chemical composition of laboratory SOA is
different than that measured in the atmosphere,20
it is necessary to determine how oxidation
processes affect the CCN activation of ambient SOA. In particular, Chang et al.21
conducted a
CCN closure study using ambient aerosol measurements, where a direct relationship between the
organic aerosol's degree of oxygenation and its hygroscopicity parameter determined from CCN
activity (κorg,CCN) was proposed. An objective of the current study was to investigate the
postulate made by Chang et al.21
that there is a simple linear relationship between κorg,CCN of an
organic aerosol and its degree of oxidation.
Here we report the degree to which OH-initiated gas- and aqueous-phase oxidation affect the
CCN activity of ambient organic aerosol sampled in a biogenically-rich region. This marks the
first deployment of a portable flow tube apparatus that can generate controlled levels of gas-
phase OH, and it demonstrates the relationship between the CCN activity of ambient particles
and their O:C ratios due to OH oxidation. The study of ambient particles, in which their O:C
ratios were changed by OH exposure on-site, further supports the observations from earlier
laboratory experiments14,15,18,19,22
of increasing CCN activity with oxidation, and the relationship
of κorg,CCN vs. O:C postulated by Chang et al.21
and Lambe et al.15
4.3 Experiment
Experiments were conducted at the Raven's Nest site at Whistler, BC, Canada (1300 m ASL,
50°N 122°W) as part of Environment Canada's Whistler Aerosol and Cloud Study (WACS)
2010. The site is situated in a coniferous forest that produces high levels of biogenic organic
aerosol23
during warm summertime conditions, with minimal contributions from local pollution
and no evidence for biomass burning impact during the periods described below that had daytime
temperatures ranging from 20 to 27°C.
74
4.3.1 Gas-phase OH Oxidation
Gas-phase OH oxidation of ambient particles was conducted in the Toronto Photo-Oxidation
Tube (TPOT) (Figure 4.1), which is a modified version of a system15,24
that produces high OH
exposures (1.0 – 1.3 × 1012
molecules cm−3
s) via the photolysis (λ = 254 nm) of gas-phase O3
(1.3 – 2.7 × 1013
molecules cm−3
) in the presence of water vapour (relative humidity = 40 %) for
a residence time of 2.2 minutes in the oxidation region. Modifications from the technique
described in previous publications15,24
include the use of a wider diameter, stainless steel flow
reactor that was electrochemically coated with amorphous silicon material (SilcoTek Corp.) to
reduce gas-phase adsorption and the use of multiple point aerosol injection to accelerate the
mixing of particles with the gas flows. The sample (1100 sccm) was introduced through 3 m of
electrochemically coated 6 mm o.d. stainless steel tubing (no cyclone) that extended 0.5 m above
the roof of the sampling site. The UV lamp was placed in the centre of the flow tube, and was
surrounded by a quartz tube cooled with a flow of room air (∼20 lpm). The OH concentration
was determined by monitoring the decay of methyl-ethyl-ketone by reaction with OH using the
Ionicon PTR-MS. The OH exposures were equivalent to 8 – 10 days (24-h) of ambient OH
exposure of 1.5 × 106 molecules cm
−3.
Figure 4.1. Toronto Photo-Oxidation Tube (TPOT) setup for the study of the gas-phase OH
oxidation of ambient organic aerosol and their CCN activity.
75
During normal operation, ambient aerosol was sampled continuously. Aerosol is mixed with a
humidified flow (700 sccm). An automated switching valve alternately injects either an ozone-
containing flow (200 sccm) or a blank flow of O2 and N2 (200 sccm) with a period of 12 min.
These flows are allowed to mix and then pass into the reaction tube, where they are irradiated
with UV light. If the ambient aerosol was mixed with ozone, OH radicals are produced;
otherwise unreacted particles are measured. In addition to reaction with OH, ambient aerosol can
potentially react with ozone and/or be volatilized by heat and UV light produced from the lamp.
Ozone control experiments were conducted by comparing organic mass concentrations and mass
spectra with and without added ozone under dark conditions using a bypass tube. No reaction
with ozone was observed. Volatilization control experiments were conducted by comparing
ozone-free organic AMS mass spectra and concentrations in light (i.e., UV lamp on) vs. dark
conditions. With the lamp on, the temperature increased by approximately 4°C (25°C to 29°C).
In the presence of UV light, an 18 % decrease in total organic mass was observed, which can be
due to both increased temperature and photolysis of aerosol compounds leading to their
volatilization. More details on these control experiments are in the auxiliary material. Ambient
particles were subjected to OH and no OH oxidation alternatively for 12 minutes each, for the
total time period required to complete a CCN activation curve scan (described below). Three
time periods from each of two days of experiments are analyzed here.
4.3.2 Aqueous-phase OH Oxidation
A sampling inlet for particle filter samples was situated beside that for the gas-phase OH
oxidation studies. The particle samples passed through a cyclone with PM1 cut size (UGR,
Model 463) and were collected on 47 mm Teflon filters (2.0 μm pores) for 24 hour periods,
following which water extraction was conducted. The aqueous solutions were oxidized on-site
using our custom photoreactor,25
with a final H2O2 concentration of 70 mM (continuously
stirred) and then irradiated by a UV lamp (UVP, 254 nm) inserted into the solution. The
solutions were oxidized for 10 minutes, then nebulized (TSI 3076 atomizer) with compressed air
for the detection by particle instruments. Solutions without H2O2 were also nebulized under dark
conditions (i.e., UV lamp off). Four samples are considered for the current analysis.
76
4.3.3 Particle Composition and CCN Measurements
The non-refractory particle composition was measured by an Aerodyne Time-of-Flight aerosol
mass spectrometer (C-ToF-AMS). For the CCN measurements, particles were first dried using a
silica gel diffusion dryer and then size-selected at a mobility diameter of 75 nm (TPOT output)
or 100 nm (aqueous reactor output) by a TSI-3081 DMA before being measured by the CCNC.
The CCN active number fraction was determined from the ratio of the number of CCN-active
particles measured by one channel of a DMT-200 CCN counter to the total particle number
concentration, measured by a TSI-3010 Condensation Particle Counter (CPC).
To obtain a CCN activation curve, multiple supersaturations between 0.07 – 1.0 % were scanned
for a period of 24 minutes on each supersaturation for the gas-phase oxidation experiments, and
10 minutes for the aqueous-phase oxidation. CCN activation curves were constructed by
calculating the fraction of activated particles, determined from the CCN/CPC ratio at the various
supersaturation values. Using the dry mobility diameter (Di), and the critical supersaturation (Sc),
defined as the supersaturation in which 50 % of the size-selected particles act as CCN, the
kappa-parameter for the total particles was calculated as described by Petters and Kreidenweis,16
Equation 4.1
where D is the droplet wet diameter, Mw, ρw and σw are the molecular weight, density, and
surface tension of water.
The overall κ of the particles (κtotal,CCN) is calculated as the sum of the volume-fraction weighted
average κCCN's of the individual components. To arrive at the kappa-parameter for the organic
component of the particles, the following equation was used:16
Equation 4.2
where ɛorg is the volume fraction of the organic component determined from the C-ToF AMS,
assuming an initial organic density of 1200 kg m−3. Only samples with ɛorg ≥ 67 % (AMS mass
fraction, fOrg ≥ 0.63) are considered for the current analysis, so that the changes in κorg,CCN due to
oxidation are statistically significant and not dominated by the inorganic fraction. For example,
the estimated uncertainty in κorg,CCN is 25 % when ɛorg = 0.8, with smaller values for larger ɛorg
77
values. For the gas-phase oxidation experiments, the increase in the organic component density
due to oxidation was accounted for, using results obtained from George and Abbatt22
where they
determined changes in ρorg of laboratory α-pinene SOA as a function of OH exposure. This
amounts to a relative change in the κorg,CCN of only 0.5 % for the current analysis. For the
aqueous oxidation experiments, changes in organic density were not corrected for since the OH
exposures used were unknown. The aerosol inorganics measured by the AMS were considered in
the calculation as ammonium nitrate (κ = 0.72, density = 1730 kg m−3
kg) and ammonium sulfate
(κ = 0.59, density = 1770 kg m−3
).26–28
During the periods studied, ammonium sulfate accounted
for more than 80 % of the measured inorganic mass. At times the particle was not fully
neutralized but the sensitivity of calculated κorg,CCN to particle acidity was shown to be very low.
An assumption made in the analysis was that all particles were internally mixed. Molecular O:C
ratios were estimated from the fraction of the total organic signal measured at m/z 44 (f44), using
the empirical relationship formulated by Aiken et al.29
The AMS measurements were averaged
over the period of time required to complete the corresponding CCN activation scans.
4.4 Results and Discussion
Sample CCN activation curves from both gas- and aqueous-phase OH oxidation experiments are
shown in Figure 4.2. For all experiments, the critical superstaturation (Sc) decreased for samples
subjected to oxidation, compared to the control samples. The average κorg,CCN values for the
TPOT experiments were smaller than those for the aqueous-phase samples as there may be some
larger, less soluble organic species in the ambient particles compared to the water-soluble
component. The CCN activation curves for particles subjected to aqueous-phase OH oxidation
are steeper compared to those from gas-phase OH oxidation for two reasons. First, the aqueous
reactor is expected to produce more uniform OH exposures as the solution was constantly stirred.
Second, the extraction and dissolution process is likely to homogenize the samples.
78
Figure 4.2. Activated particle fraction (CCN/CN) plotted as a function of the water
supersaturation (SS) for unoxidized and oxidized ambient particles by gas-phase OH oxidation
(Dm = 75 nm; July 22, 2010, 00:02–02:50) (red circle) and aqueous-phase OH oxidation (Dm =
100 nm; July 8, 2010) (blue square).
For all the experiments considered, the O:C ratios increased after oxidation (see auxiliary
material for detailed results on all experiments, including sample aerosol mass spectra). The
mass fraction of m/z 44 (predominantly CO2+) to the total organic component (f44), an indicator
of highly oxidized species, increased while the mass fraction of m/z 43 (predominantly C2H3O+)
to the total organic component (f43), an indicator of less oxidized species, decreased slightly.30
For the gas-phase OH oxidation experiments, the changes in hygroscopicity can arise from
conversion of insoluble organic species to more soluble species, changes in the properties of
dissolved constituents, the loss of volatile components that are less hygroscopic, and the
condensation of oxidation products onto the aerosol. While it is possible there was some
condensation of oxidation products, an average decrease in the total organic mass of 10 % was
observed and so changes in hygroscopicity due to this process were likely to be minor. Note that
the small increase in the mass fraction of sulfate to total particle mass (fSO42−
) due to oxidation
are not large enough to explain the overall increase in κtotal,CCN observed.
Figure 4.3 shows the relationship between O:C to κorg,CCN. Literature values of other oxidation-
CCN measurements, mostly from lab studies, are included for comparison. We note that other
79
studies have examined this relationship through the hygroscopic growth factor (HGF 95
%).14,18,19
The κorg,CCN and O:C values from a laboratory oxidative study of O3-α-pinene SOA are
included; specifically, the “lamp” and “OH” flow tube conditions from Figure 9a of George and
Abbatt22
provided the best comparison of experimental conditions to the no OH and OH
oxidation conditions employed in this study. The κorg,CCN values from oxidation of bis-2-
ethylhexyl sebacate (BES) were also included as a model of primary organic aerosol,31
although
we recognize that this is a different starting material from the monoterpene. The corresponding
O:C ratios for BES were calculated from the measured f44 values, using the relationship in
Figure A1 from Lambe et al.15
Results from other lab CCN studies of α-pinene SOA are included
to further evaluate the generality of the relationship of κorg,CCN and O:C.14,32
Also included in the
plot are two relationships between κorg,CCN and O:C, i.e., from Chang et al.,21
which was derived
from CCN closure at a rural field site under the postulate of a direct linear relationship between
the two quantities (solid green line), and from Lambe et al.,15
which was derived from various
laboratory generated organic aerosol (solid blue line).
The uncertainties in the calculated values of O:C and κorg,CCN are mainly influenced by the
systematic errors associated with each measurement, including the aerosol's chemical
composition (AMS)33
, instrument supersaturation (CCNC), particle size selection (DMA), and
aerosol number (CPC). The uncertainties reported for the empirical relationship formulated by
Aiken et al.29
to calculate O:C ratios from f44 were also considered. For this study, the average
uncertainty was estimated to be 15 % for κorg,CCN and 10 % for O:C ratios. The results obtained
from the gas- and aqueous-phase OH oxidation of ambient organic aerosol follow a direct
positive relationship between O:C and κorg,CCN, as observed in previous lab studies. Also, the
CCN closure result from Chang et al.21
is consistent with the other measurements. Given the
range of potential organic aerosol precursors in the field measurements, it is not surprising that
there is some scatter in this plot that combines both lab and field data. Also, to match best with
the field data, we note that only the data from Massoli et al.14
and Lambe et al.32
that arise from
the α-pinene precursor are included (as single points). Nevertheless, correspondence between the
two quantities appears clear, extending down to low values of O:C. Thus, this study validates the
postulate made earlier by Chang et al.21
that there is a connection between the two quantities.
Indeed, the correspondence to the results from Whistler to those from Chang et al.21
may arise
because biogenic SOA from monoterpene precursors was present at the CCN closure field site.34
80
It is important to note, as mentioned by Jimenez et al.,18
the κorg,CCN and overall composition
from a variety of precursors may become increasingly similar with increasing O:C. Also, a
similar molecular weight for many of the SOA precursors in Figure 4.3 will tend to lead to
oxidation products of similar molecular weight, and thus similar hygroscopicities for the same
O:C values.
Figure 4.3. Relationship between O:C and κorg,CCN from the current study and from several
laboratory studies of model POA and biogenic SOA,14,22,31,32
and a field CCN closure study21
for
comparison. The Lambe et al.32
solid line represents a fit for SOA from a number of precursors.
For each pair of points, the samples of the lowest and highest O:C values are shown from each
study. Dashed lines are included only to indicate the relationship between these two points. The
following were the OH exposures used for the laboratory studies of: BES (3.0 × 1012
molecules
cm−3
s),31
O3-α-pinene SOA (1.3 × 1012
molecules cm−3
s),22
OH-α-pinene SOA (1.2 × 1012
molecules cm−3
s)14
and OH-α-pinene SOA (1.7 × 1012
molecules cm−3
s).32
The shaded region
illustrates the uncertainty of the κorg,CCN and O:C relationships derived from the CCN closure
study21
and laboratory generated organic aerosol.32
The uncertainties in the calculated values of
O:C and κorg,CCN are mainly influenced by the systematic errors associated with each
measurement, as discussed in the main text. Also, daily averages are shown for the gas-phase OH
oxidation experiments, and the average of 4 one-day aqueous-phase OH oxidation experiments
(results from individual experiments included in the supporting material).
81
4.5 Summary
This study is the first direct measurement of the relationship between the degree of oxygenation
and the CCN hygroscopicity of the organic component of ambient biogenic aerosol (κorg,CCN)
subject to OH oxidation, using novel methods for exposure to both gas- and aqueous-phase OH.
When combined with lab measurements, the results support a general linear relationship between
the κorg,CCN of biogenic organic aerosols and their degree of oxygenation, as postulated by Chang
et al.21
and shown previously to prevail for hygroscopic growth factors18
. The results emphasize
that OH is an important aerosol oxidant. Additional field data with onsite OH processing should
provide an indication of whether OH oxidation is the main organic aerosol aging process that
drives the increases in hygroscopicity. While empirical in nature, this relationship provides a
simple method for relating organic aerosol hygroscopicity to its composition, one that could
potentially be included in atmospheric models. This trend needs to be further explored with
ambient aerosol arising from a variety of sources, especially in urban and biomass burning
regions, given that not all lab SOA precursors follow this relationship as closely.14,32
Acknowledgements
The authors would like to thank Environment Canada, NSERC and CFCAS-CAFC for funding.
4.6 Supporting Material
Table 4.1. Control experiments: particle properties of ambient organic aerosol. Absolute mass
(µg m-3
) of the various components is noted in the brackets. Date/Time
(GMT+8) Condition fOrg fSO4
2- fNO3
- fNH4
+ f43 f44 O:C
07/15/2010
14:21-15:00
Dark 0.75
(0.99)
0.20
(0.26)
0.03
(0.04)
0.02
(0.03) 0.1 0.1 0.47
hv 0.70
(0.78)
0.24
(0.27)
0.02
(0.02)
0.04
(0.04) 0.1 0.12 0.53
07/19/2010
09:05-09:38
Dark 0.72
(1.41)
0.20
(0.39)
0.04
(0.07)
0.05
(0.00) 0.1 0.1 0.46
Dark (O3) 0.71
(1.40)
0.20
(0.39)
0.04
(0.07)
0.05
(0.10) 0.1 0.1 0.47
82
Figure 4.4. AMS mass spectrum of organic and inorganic component of particles subjected to
“hv light” condition.
Figure 4.5. AMS mass spectrum of organic and inorganic component of particles subjected to
“OH Oxidation” condition.
83
Table 4.2. Particle properties, and CCN activity of ambient organic aerosol. Absolute mass (µg m-3
)
of the various components is noted in the brackets for the gas-phase OH oxidation experiments.
For aqueous OH oxidation experiments, 24-hr particle filter sample was obtained from 08:00 to
08:00 of the following day.
Date/Time
(GMT+8) Sample fOrg fSO4
2- fNO3
- fNH4
+ O:C
Di
(nm) Sc %
κtotal,
CCN
κorg,
CCN
Gas-phase OH Oxidation
07/21/2010
18:23-21:13
Unoxd. 0.83
(1.84)
0.12
(0.25)
0.02
(0.03)
0.04
(0.08) 0.52 75 0.425 0.18 0.12
Oxd. 0.79
(1.63)
0.15
(0.30)
0.01
(0.03)
0.05
(0.10) 0.69 75 0.369 0.24 0.17
07/21/2010
21:14-00:01
Unoxd. 0.81
(1.51)
0.13
(0.25)
0.02
(0.03)
0.04
(0.08) 0.53 75 0.421 0.18 0.11
Oxd. 0.78
(1.30)
0.16
(0.27)
0.01
(0.02)
0.05
(0.09) 0.7 75 0.374 0.23 0.15
07/22/2010
00:02-02:50
Unoxd. 0.78
(1.38)
0.15
(0.25)
0.03
(0.04)
0.04
(0.07) 0.52 75 0.397 0.21 0.12
Oxd. 0.74
(1.17)
0.19
(0.29)
0.02
(0.03)
0.05
(0.09) 0.72 75 0.334 0.29 0.21
07/25/2010
22:43-01:31
Unoxd. 0.87
(1.83)
0.08
(0.17)
0.02
(0.04)
0.02
(0.06) 0.48 75 0.43 0.17 0.13
Oxd. 0.84
(1.61)
0.10
(0.20)
0.02
(0.03)
0.02
(0.08) 0.64 75 0.397 0.21 0.16
07/25/2010
01:32-04:20
Unoxd. 0.88
(1.83)
0.08
(0.17)
0.02
(0.04)
0.02
(0.05) 0.48 75 0.43 0.17 0.13
Oxd. 0.85
(1.61)
0.10
(1.61)
0.02
(0.20)
0.04
(0.07) 0.65 75 0.397 0.22 0.16
07/26/2010
04:21-07:09
Unoxd. 0.88
(1.89)
0.08
(0.17)
0.02
(0.04)
0.02
(0.04) 0.49 75 0.416 0.19 0.14
Oxd. 0.85
(1.67)
0.10
(0.19)
0.02
(0.03)
0.02
(0.06) 0.66 75 0.383 0.22 0.17
Aqueous-phase OH Oxidation
07/06/2014 Unoxd. 0.87 0.05 0.06 0.02 0.52 100 0.24 0.25 0.2
Oxd. 0.81 0.07 0.07 0.05 0.98 100 0.33 0.34 0.29
07/08/2010 Unoxd. 0.9 0.01 0.03 0.06 0.47 100 0.15 0.16 0.13
Oxd. 0.89 0.02 0.03 0.06 0.97 100 0.24 0.25 0.23
07/10/2010 Unoxd. 0.88 0.03 0.07 0.03 0.41 100 0.24 0.23 0.18
Oxd. 0.69 0.06 0.16 0.08 0.86 100 0.2 0.36 0.26
07/19/2010 Unoxd. 0.73 0.18 0.04 0.06 0.49 100 0.23 0.27 0.17
Oxd. 0.63 0.23 0.05 0.09 0.84 100 0.2 0.34 0.22
84
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87
Chapter 5
5 Suppression in droplet growth kinetics by the addition of
organics to sulfate particles
Suppression in Droplet Growth Kinetics by the
Addition of Organics to Sulfate Particles
J. P. S. Wong, J. Liggio, S.-M. Li, A. Nenes, J.P. D. Abbatt, Journal of Geophysical Research
Atmospheres, 2014, 119, 12 222 – 12 232, doi:10.1002/2014JD021689.
Reprinted (adapted) with permission from Journal of Geophysical Research Atmospheres
Copyright 2014 American Geophysical Union
88
5.1 Abstract
Aerosol-cloud interactions are affected by the rate at which water vapor condenses onto particles
during cloud droplet growth. Changes in droplet growth rates can impact cloud droplet number
and size distribution. The current study investigated droplet growth kinetics of acidic and neutral
sulfate particles which contained various amounts and types of organic compounds, from model
compounds (carbonyls) to complex mixtures (α-pinene secondary organic aerosol and diesel
engine exhaust). In most cases, the formed droplet size distributions were shifted to smaller sizes
relative to control experiments (pure sulfate particles), due to suppression in droplet growth rates
in the cloud condensation nuclei counter. The shift to smaller droplets correlated with increasing
amounts of organic material, with the largest effect observed for acidic seed particles at low
relative humidity. For all organics incorporated onto acidic particles, formation of high
molecular weight compounds was observed, probably by acid-catalyzed Aldol condensation
reactions in the case of carbonyls. To test the reversibility of this process, carbonyl experiments
were conducted with acidic particles exposed to higher relative humidity. High molecular weight
compounds were not measured in this case and no shift in droplet sizes was observed, suggesting
that high molecular weight compounds are the species affecting the rate of water uptake. While
these results provide laboratory evidence that organic compounds can slow droplet growth rates,
the modeled mass accommodation coefficient of water on these particles (α > 0.1) indicates that
this effect is unlikely to significantly affect cloud properties, consistent with infrequent field
observations of slower droplet growth rates.
5.2 Introduction
Atmospheric particles affect the radiative budget of the Earth directly by absorbing and
scattering incoming radiation, or indirectly by influencing the microphysical properties,
abundance, and lifetimes of clouds. This indirect effect represents one of the largest uncertainties
in the assessment of global radiative forcing.1 The activation of aerosol particles as cloud
condensation nuclei (CCN) and their subsequent growth into cloud droplets by the condensation
of water vapor are key processes that determine the magnitude of the cloud indirect effect.2,3
The ability of particles to act as CCN depends on their size and composition.4 Given that organic
compounds constitute a large fraction of ambient particles, many laboratory studies have focused
on characterizing hygroscopic properties of model organic compounds. These studies have
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illustrated that organic constituents are quite hygroscopic and can be of importance to cloud
formation.5–8
Since the chemical composition of ambient organic particles is highly complex,
recent studies have shown that the influence of particle composition on CCN activity can be
represented by a single hygroscopicity parameter, κ.9 Additionally, studies have confirmed that
the hygroscopicity of organics (κorg) can contribute to the CCN activity of ambient particles.10–16
While the thermodynamic properties of organic-bearing particles are becoming well understood,
much less is known regarding how organic compounds affect the rate of water vapor transport
during droplet growth (i.e., kinetic effects). Changes to the growth rate will affect cloud droplet
size distribution, thus influencing the radiative properties as well as the lifetime of clouds.3,17,18
In fact, the assumption that the activation of particles to CCN can be modeled only as an
equilibrium process has been shown to be invalid in some conditions, leading to overestimates in
cloud radiative forcing.2,3
Being highly uncertain, such kinetic limitations may arise due to slow
solute dissolution associated with highly viscous organic particle-phase19–21
or the formation of
organic films at the droplet surface.18,22
To investigate whether kinetic limitations in droplet growth rate arise due to differences in
chemical composition, previous studies compared the droplet size from the particle being studied
to that grown from calibration inorganic particles (such as ammonium sulfate) with known rapid
growth kinetics.11,23,24
Studies have also coupled these measurements with a computational fluid
dynamics model, where the effects on the droplet size can be parameterized to an effective mass
accommodation coefficient (α), to which droplet growth is highly sensitive.25–27
The effective
mass accommodation coefficient accounts for all processes that determine the probability for a
water molecule striking the particle surface being taken up by the growing droplet, including
gas-phase diffusion, interfacial transport, and bulk phase diffusion. In this regard, smaller droplet
sizes arise due to the sample particles having lower values of α (i.e., slow droplet growth
kinetics) than the standard (e.g., ammonium sulfate, α > 0.2). On the other hand, if the droplet
size is indistinguishable from the standard, they are considered to have similar α values (i.e.,
rapid growth kinetics). It is generally assumed that only particles with values of α smaller than
0.1 will significantly impact cloud formation and shortwave cloud forcing.27,28
Slow droplet growth rates have been observed in both laboratory and ambient measurements, yet
it remains unclear the extent to which suppression in growth rates occur as only a limited number
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of studies have investigated this process. On the one hand, from the analysis of 10 ambient data
sets, Raatikainen et al.27
constrained the effective mass accommodation coefficient between the
values of 0.1 to 1, indicating that rapid growth kinetics is globally prevalent. On the other hand,
lab-generated carboxylic acid particles7 highly viscous sucrose/sodium chloride particles
29,
mineral dust particles30, and secondary organic aerosol from β-caryophylene ozonolysis
26
exhibited slow droplet growth kinetics. There are also ambient observations of kinetic limitations
where slow droplet growth rates were observed arising from different particles sources, such as
bovine emissions23
and continental particles containing anthropogenic organic components.24
These contrasting results, both from the laboratory and in the field, pose a challenge to determine
whether the kinetic limitation of droplet growth is an important process. While previous studies
have identified certain compounds and particle sources that can result in such kinetic limitations,
more work is required to further explore the droplet growth of a wider range of organic particles;
in particular, biogenic secondary organic aerosol (SOA) material, which is abundant in the
atmosphere.31
Additionally, given ambient observations that organic particles are chemically
reactive32
, it is necessary to investigate how the chemical evolution of organics affects their
droplet growth kinetics.
Given that the effects of many atmospherically relevant organic compounds on droplet growth
are currently unknown, the objective of the current study is to investigate the droplet growth
kinetics of particles composed of both inorganic hygroscopic components and various organic
compounds with different functionalities: α-pinene SOA, carbonyls, and diesel engine exhaust.
The size of droplets arising from organic-bearing sulfate particles was measured using a CCN
counter, from which droplet growth was inferred. The effects of chemical evolution of organic-
bearing particles on growth rates were also investigated. Finally, the mass accommodation
coefficient of water onto growing droplets was modeled to assess the potential atmospheric
impacts of delayed growth kinetics.
5.3 Experiment
To investigate whether the addition of organic compounds to particles affects their droplet
growth, size-selected sulfate seed particles were exposed to various organics in an environmental
chamber or flow tube. The chemical composition and droplet growth of these resulting particles
were monitored using online measurements. α-pinene, 2-pentanone, and 1-octanal (Sigma-
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Aldrich) were introduced into a 2 m3 dark Teflon chamber (batch mode) by slowly injecting a
small volume of the liquid organic into 10 L min−1
of zero air for 10 min (final concentrations:
34–78 ppb of α-pinene, 600 ppb of 2-pentanone, and 600 ppb of 1-octanal). Sulfuric acid (SA, 10
% w/v) and ammonium sulfate (AS, 0.1 % w/v) seed particles were generated using a TSI
atomizer (Model 3076), dried using a silica gel diffusion dryer, then size selected at a mobility
diameter of 200 nm by a TSI-3081 Differential Mobility Analyzer (DMA). Size-selected
particles were introduced into the chamber until sufficient number concentrations and mass
loading for particle measurements were achieved. SA and AS were chosen as they are ubiquitous
in the atmosphere and are commonly used seed particles in laboratory chamber studies.
To generate SOA, pulses of a 4 L min−1
ozone (2 ppm) flow were introduced into the chamber
for a time period of 30 s to 2 min. Multiple pulses of ozone were injected to generate increasing
amounts of SOA material (monitored by increasing organic-to-sulfate mass ratio by the Aerosol
Mass Spectrometer, described below) and to prevent homogeneous nucleation. Particle size
distribution measurements by a Scanning Mobility Particle Sizer (SMPS) (DMA, TSI-3080, and
Condensation Particle Counter (CPC) TSI-3076) during control experiments indicated no
homogeneous nucleation occurred upon the reaction of α-pinene and ozone. Unless specified, the
relative humidity (RH) for all experiments was under 12 % in the Telfon chamber (Vaisala
HMP60 probe), i.e., the SA particles were liquid and very acidic, whereas the AS particles were
solid and neutral. Multiple experiments were conducted to ensure reproducibility.
Similar experiments utilizing diesel engine exhaust were conducted at Environment Canada's
Environmental Technology Centre in Ottawa, ON, Canada, as part of the Diesel Engine Exhaust
Research Experiment (DEERE) 2012. Specific details of the engine conditions and flow tube
setup are discussed elsewhere.33
A Jetta Turbocharged Direct Injection engine equipped with a
diesel oxidation catalyst was operated under steady state driving conditions with the engine
exhaust directly vented into a constant volume sampling system, where it was mixed with
filtered laboratory air to maintain a constant volume flow. The diluted exhaust flow was then
filtered to remove particles prior to injection into an electropolished stainless steel flow tube with
an inner diameter of 0.2 m and 2.5 m in length. Five sampling ports were located along the length
of the flow tube at equal intervals. Additional details of the flow tube are discussed elsewhere.34
Acidic sulfate particles with 1:4 and 2:5 ammonium:sulfate molar ratio were generated (TSI
3081 atomizer) and size selected (Dm = 300 nm, TSI 3081 DMA) prior to injection into the flow
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tube. The total flow rate in the flow tube was 20 L min−1
, resulting in a total residence time of
10 min. For all the experiments considered for this study, the RH was below 20 % in the flow
tube, and samples were measured from the fifth sampling port, resulting in a residence time of
8 min.
For experiments conducted in both the Teflon chamber and the flow tube, the chemical
composition of the particles was measured online using the time-of-flight Aerosol Mass
Spectrometer (ToF-AMS, Aerodyne Research, Inc.), providing chemical composition of the non-
refractory component of particles.35,36
Both high-resolution (HR-ToF) and compact (C-ToF)
AMS instruments were used in this study; however, while the HR-ToF-AMS was operated by
alternating between V and W mode, only V mode data were analyzed from the high-resolution
version for comparable limit of detection to the C-ToF-AMS. The mode of operation of the HR-
AMS refers to the different ion paths in the ToF mass spectrometer, where V mode is more
sensitive, but offers lower mass resolution than W mode. Details of the AMS principles of
operation and typical data analysis procedures are reviewed in the literature.37
During these experiments, the CCN properties of particles were continuously measured by first
drying the particles using a silica gel diffusion dryer (residence time of 20 s), then sampling by a
Condensation Particle Counter (CPC, TSI-3075), and a Cloud Condensation Nuclei Counter
(CCNC, Droplet Measurement Technologies (DMT)-100). Particles were exposed to a maximum
water supersaturation of 0.3 % in the CCNC for the Teflon chamber experiments, and 1.0 % for
the DEERE experiments. For all experiments, 100 % of particles activated into droplets, as
measured by the ratio of the number of CCN (measured by the CCNC) to the total particle
number concentration (measured by the CPC).
To investigate droplet growth kinetics, the geometric mean diameters were calculated from the
measured water droplet size number distributions measured by the optical particle counter in the
CCNC. The calculated droplet sizes from inorganic particles with organics were compared to
those arising from bare inorganic particles (i.e., no organics). The addition of organic material
onto the sulfate seed particles increased the particle geometric mean diameter up to 10 %, as
measured by an SMPS (DMA, TSI-3080, and CPC, TSI-3076). This additional hygroscopic
material may affect droplet sizes by lowering the critical supersaturation of the particle due to
increased solute amount, whereby it activates into a droplet earlier in time, allowing for more
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growth in the CCNC, thus resulting in larger droplets. To evaluate the effects an increased
amount of hygroscopic material would have on droplet size, the droplet sizes of particles with
varying amounts of a highly hygroscopic material were compared. This was conducted by
comparing droplet sizes arising from pure sulfuric acid seed particles with different mobility
diameters (200 – 220 nm), at similar particle number concentrations (1100 – 1250 number cm-3
).
Similar particle concentrations were used to account for water vapor depletion in the CCNC, as
discussed below. No significant differences in the resultant droplet diameters were observed due
to increased amounts (up to 30 %) of sulfuric acid. Since sulfuric acid is more hygroscopic than
the organic compounds used in this study, we infer from these control experiments that the
addition of organic material onto sulfate seed particles does not affect droplet size due to changes
in particle hygroscopicity (i.e., thermodynamic processes). From these control experiments, we
know that if the addition of hygroscopic organic material resulted in smaller droplets, then a
suppression in droplet growth rates is implied.
We note that Lathem and Nenes38
observed that there is a small dependence of droplet size on
particle number concentration in the CCNC. When particle number concentration is large
enough, the rate of water uptake increases to a point where water vapor depletion in the CCNC
occurs, affecting the supersaturation and the size growing droplets can reach. Since chamber
experiments were done in batch mode, where particle concentration decreases over time due to
wall loss, this effect must be accounted for in order to study droplet growth kinetics. This
relationship was empirically measured for pure inorganic particles (e.g., dashed lines in Figure
5.1), and this is the reference state to which the sizes of droplets grown from particles with
organics are compared. This water vapor depletion effect is consistent with the observations by
Lathem and Nenes.38
Accounting for this effect, and that all particles were exposed to identical
supersaturations and residence times in the CCNC, differences in droplet sizes were inferred to
be due to differences in droplet growth kinetics. It is important to note that we cannot distinguish
whether differences in droplet size arose due to differences in overall or initial water uptake rate.
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Figure 5.1. Geometric mean diameter of droplets plotted as a function of seed particle number
concentration (number cm-3) for α-pinene SOA condensed onto (a) sulfuric acid and (b)
ammonium sulfate particle. Each data point and the corresponding error bars for seed particle
number concentration and droplet diameters reflect the 5 min averages of the measurements and
±1σ of measurement variability. The results for controls (bare inorganic particles) are indicated
by the dashed lines (grey area, ±1σ).
5.4 Results and Discussion
5.4.1 α-Pinene SOA
The comparison of the droplet sizes arising from bare inorganic particles (dashed line) with those
with α-pinene SOA material (colored square and circle points) is shown in Figure 5.1. From
these plots, at the same particle concentration, differences in droplet size between the bare
inorganic particle and those with organic material arise due to differences in droplet growth
kinetics. The color scale indicates the organic-to-sulfate (org:sulfate) mass ratio from AMS
measurements, as an indication of the amount of organics added to the sulfate seed particles. For
both SA and AS, the addition of SOA material (indicated by the increasing org:sulfate ratio)
resulted in smaller droplets than those of the bare inorganic particle, where this decrease in
droplet size is greater for SA than AS. The differences in droplet size arising from SA and AS
may have arisen due to differences in the amount of organics in the particles.
95
At the RH employed in these experiments (RH < 12 %), SA and AS have different liquid water
content, resulting in large differences in particle acidity as well as different physical states (i.e.,
liquid SA and effloresced AS). Previous studies have shown that particle acidity affects the
uptake of organics, where acidic particles enhanced the uptake of organics.39–41
The physical
state of particles can also affect the uptake of organic compounds, by changing their gas-particle
partitioning.42
These differences in particle properties can result in differences in the amount of
organics taken up by SA and AS. Note that for SA, the particles have an org:sulfate ratio of
approximately 0.3 prior to ozone addition, suggesting uptake of α-pinene, which was previously
observed by Liggio et al.,43
also resulted in smaller droplets in one case. For AS, slower droplet
growth was only observed following addition of ozone into the chamber, initiating the ozonolysis
of α-pinene to generate SOA material that condenses onto the particle.
Figure 5.2 further illustrates the role of organics in affecting droplet growth for the condensation
of SOA material onto SA and AS. Here droplet growth suppression was calculated as the fraction
of the droplet geometric mean diameter with (Dd) and without organics present (Dd, control), at the
same particle number concentration. For both types of particles, increasing amounts of SOA
material result in a greater suppression in droplet growth kinetics, as indicated by the decreasing
droplet size with increasing org:sulfate ratio. Given the same amount of organic material (i.e.,
same org:sulfate ratio), the suppression in droplet growth is greater when SOA material
condensed onto SA, suggesting that this enhancement in droplet growth suppression was due to
differences in the nature of the organics in the particles.
Figure 5.2. Fraction of the geometric mean diameter of droplets with (Dd) and without (Dd, control)
α-pinene SOA on sulfuric acid (red circle) and ammonium sulfate (blue square) seed particles,
plotted as a function of the org:sulfate mass ratio. The dashed line indicates no change in droplet
diameter, drawn to guide the eye.
96
A different particle composition for SA seed particles is demonstrated by the aerosol mass
spectra of the organic component during SA and AS experiments. The mass loading at each m/z
is normalized to the total organic mass to account for particle wall loss in the Telfon chamber.
Over the course of the experiments, organic fragments at higher m/z increased for SA seeds. This
can also be observed in the time series of the various particle components measured by the AMS
(Figure 5.3), where the fraction of organic mass at high m/z (forg, high m/z) increased over the course
of the experiment only for SA. The decrease in the mass loading for the various components is
due to particle loss to the chamber walls. Given the harsh ionization and fragmentation
associated with the AMS, organic fragments at high m/z likely result from the fragmentation of
even higher molecular weight species. Here higher molecular weight products are defined as
organics that resulted in fragments at m/z higher than the molecular weight of pinic acid, a
common oxidation product of α-pinene ozonolysis.44
Figure 5.3. Time series of various particle components measured by the AMS for α-pinene SOA
condensed onto (left) sulfuric acid and (right) ammonium sulfate seed particles. The decrease in
the mass loadings for the various components is due to particle loss to the chamber walls.
The formation of high molecular weight products observed in the current study can occur in both
the gas-phase45
and the particle-phase. Given that the oxidant levels and the reaction time for α-
pinene ozonolysis are similar for SA and AS, we speculate that the formation of high molecular
weight compounds occurred due to differences in particle acidity. In fact, previous studies have
shown that in acidic particles, organic compounds can undergo various condensed phase
97
reactions, generating higher molecular weight compounds/oligomers.39–41,43,46,47
In particular,
organosulfate formation has been observed following the uptake of α-pinene SOA material onto
acidic particles40,41
, and these compounds are likely to be surface active.48
Surface-active species
can form an organic film, which can potentially inhibit the uptake of water vapor, resulting in
suppression in droplet growth as observed in the current study.18,49,50
The observed slower
growth kinetics may also have arisen from the SOA material being in a glassy, highly viscous
state, which has been reported for SOA particles originating from terpene oxidation at ambient
temperature and lower RH conditions comparable to those in the current study.51,52
The exact
mechanism that resulted in the observed slow droplet growth is difficult to assess at this time.
5.4.2 Carbonyls
To further investigate the effects of acid-catalyzed reactions in the particle-phase on droplet
growth kinetics, the growth of SA and AS seeds exposed to two organic compounds with ketone
(2-pentanone) and aldehyde (1-octanal) functionalities were examined. In the presence of acids,
ketones, and aldehydes undergo Aldol condensation reactions in bulk solutions, generating
higher molecular weight compounds.53
Such reactions forming oligomers and/or high molecular
weight species have been observed previously.39,54–56
As shown in Figure 5.4, under the same
relative humidity conditions, slower droplet growth only arises when the two carbonyl
compounds were exposed to SA. For AS, since negligible amounts of particulate organic
material were measured by the AMS, the time series and the organic mass spectrum are not
shown, and the org:sulfate ratio was taken to be zero. The differences in the amount of
particulate organic were due to the fact that the uptake of these volatile carbonyl compounds is
enhanced by acid-catalyzed reactions.55,57,58
These results suggest that the presence of organic
material results in the suppression of droplet growth rates.
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Figure 5.4. Fraction of the geometric mean diameter of droplets with (Dd) and without (Dd, control)
2-pentanone and 1-octanal on sulfuric acid at RH 11% (red circle) and RH 40 % (open circle), as
well as ammonium sulfate (blue square) seed particles, plotted as a function of the org:sulfate
ratio.
Since Aldol condensation reactions are reversible in the presence of water53
, the RH in the
chamber was increased to 40 % to demonstrate the effects of this reaction on droplet growth. At
higher RH, the liquid water content increases for SA particles,59
which will reverse the reaction,
decreasing the amount of high molecular weight products. As well, the acidity will be lower. As
demonstrated in Figure 5.4, for a given amount of organic material in the particles, the
suppression of droplet growth was diminished at higher RH, suggesting that the Aldol
condensation reaction forms compounds that contribute to the suppression in droplet growth
kinetics for carbonyls on SA. However, we note that it is possible that the role of water as a
plasticizer would have also given rise to the results observed at higher RH, if the particles were
less solid, leading to faster dissolution of organics compared to lower RH conditions. The AMS
time series shown in Figure 5.5 indicated that at similar org:sulfate ratios, the fraction of higher
molecular weight products is higher for SA under lower RH conditions, consistent with previous
observations.60,61
For carbonyls, higher molecular weight compounds are defined as organic
fragments having m/z higher than 1-octanal. These observations suggest that formation of higher
molecular weight compounds by acid-catalyzed reactions give rise to the suppression in droplet
growth kinetics and that the extent of suppression depends on RH.
99
Figure 5.5. Time series of various particle components measured by the AMS for carbonyls
condensed onto sulfuric acid seed particles at (left) RH 11 % and (right) 40 %.
5.4.3 Engine Exhaust
Previous ambient observations reported that particles containing organics of anthropogenic
origin exhibited suppressed droplet growth kinetics.24
The current study examined in a more
systematic manner the effects of organics on droplet growth from a complex source, i.e., vehicle
engine exhaust. In particular, the acidity of the sulfate particles was varied to further investigate
whether acid-catalyzed reactions of engine exhaust organics affect droplet growth. Following
exposure to diesel engine exhaust, suppression in droplet growth was observed for acidic sulfate
particles (Figure 5.6). Increased particle acidity did not give rise to larger suppression in droplet
growth. Comparison of the mass spectra (Figure 5.7) also suggests differences in the observed
organic components, where more organic mass at m/z > 150 was observed for sulfate particles of
lower acidity. The variability in the observations may have arisen due to variations in engine
operation conditions, which affects the composition of the exhaust62
. Nevertheless, particles with
a higher fraction of organics at high m/z (forg, high m/z) correlated with greater growth kinetics
suppression, where forg, high m/z for acidic sulfate (1:4 NH4+:SO4
2−) is 0.016 and 0.098 for acidic
sulfate (2:5 NH4+:SO4
2−). Here high molecular weight compounds are defined as giving rise to
organic fragments higher than m/z 150. These observations further suggest that higher molecular
weight organic compounds can lead to the suppression of droplet growth.
100
Figure 5.6. Fraction of the geometric mean diameter of droplets with (Dd) and without (Dd, control)
engine exhaust on acidic sulfate with NH4+:SO4
2− ratio of 1:4 (red circle), and NH4
+:SO4
2− ratio
of 2:5 (green circle), plotted as a function of the org:sulfate ratio.
Figure 5.7. Mass spectra of the organic component measured by the AMS for engine exhaust
condensed on acidic sulfate particles with NH4+:SO4
2- ratio of 1:4 (red) and 2:5 (green).
5.5 Interpreting the Degree of Droplet Growth Suppression
Using the coupled microphysical fluid dynamical model of the DMT CCNC63
, we modeled the
experimental results with the goal being to determine the effective mass accommodation
coefficient of water vapor onto the particles as they activate and grow into droplets. While the
data indicated clearly that smaller droplets were formed when organics were condensed onto the
sulfate cores, the degree of growth kinetics suppression will determine the overall atmospheric
impact of the results. SOA material condensed onto sulfuric acid seed particles was chosen for
modeling because the largest suppression in droplet growth kinetics was observed using this
system. The input to the model included the total pressure, temperatures, and flows of the
CCNC, as well as dry particle size distributions and assumed hygroscopicities. κ = 0.1 was used
for SOA material, although the mass accommodation coefficient was insensitive to the κ value,
consistent with the results from Raatikainen et al.27,63
Water depletion effects were fully
accounted for in the model. According to Raatikainen et al.,27,63
the value of the mass
101
accommodation coefficient, inferred from CCN size distributions, cannot be precisely
determined when shifts in droplet size are less than 0.3 µm. This was referred to as the
“unresolvable range” of mass accommodation coefficient by Raatikainen et al.63
For the system considered, the variations in droplet sizes were at most 0.3 µm. Given this, for the
system considered, the mass accommodation coefficient for water onto the growing droplets
ranges between the values of 0.1 and 1.0 – comparable to that of the standard reference particles
(ammonium sulfate). We note that the model predicts droplet size changes of 0.9 and 1.8 µm for
mass accommodation coefficients of 0.05 and 0.02, respectively. Changes of this magnitude are
clearly much larger than the measurement precision, which as shown in Figure 5.1, is very high.
As a result the data rule out mass accommodation coefficients this small.
A decrease in the mass accommodation coefficient from 1.0 to 0.1 has been modeled to result in
a relatively small (10 – 15 %) increase in the number of cloud droplets globally.27
This
observation is also consistent with laboratory measurements which show that the kinetics of
evaporation and condensation of water was negligibly influenced by the presence of an organic
film except for specific conditions (i.e., films of hydrocarbons with carbon chain length larger
than 12).50
5.6 Conclusions and Atmospheric Implications
Organic material, such as SOA, carbonyls, and diesel engine exhaust, when condensed onto
hygroscopic particles, resulted in the suppression of droplet growth kinetics, which was only
observed under low relative humidity conditions. We speculate that this was due to the formation
of higher molecular weight organic products in the particle-phase. While these experimental
observations show that the addition of organics to a highly hygroscopic core can slow the rate of
growth of particles into droplets, the modeled mass accommodation coefficient lower limit of 0.1
suggests that the observed suppression in droplet growth are not large enough that this process
will become rate limiting in the environment. In this manner, the results are fully consistent with
the analysis of field data by Raatikainen et al.27
which showed that kinetic effects larger than this
limit were infrequently observed in the environment. This conclusion is strengthened by the
observation that the strongest effects were only observed under the conditions of very high
particle acidity and low relative humidity, which only rarely prevail in the atmosphere.
102
Despite the fact that suppression in water uptake by the addition of organics to sulfate particles
appears to not be an important process governing cloud properties, the kinetic limitations of
transport driven by the presence of organic compounds observed in the current study may also
occur for the transport of other important atmospheric species. Such kinetic limitations of gas
uptake can influence the reactivity, and thus, properties of organic-containing particles. For
example, Liggio et al.34
observed that the uptake of ammonia onto acidic sulfate particles was
reduced in the presence of ambient organic gases, suggesting that ambient organic particles can
remain more acidic for longer than expected in the atmosphere. From flow tube experiments,
Shiraiwa et al.64
and Zhou et al.65,66
have shown that ozone uptake onto particles with organic
compounds can be kinetically hindered due to slow diffusion, which has implications for the
lifetime of organic compounds in the atmosphere. As such, it is important to understand whether
the uptake of other important atmospheric species, such as OH radicals, is affected by organic
compounds.
As well, we point out that this work has direct implications to smog chamber studies that study
SOA formation yields and aerosol processing given that many of these studies are performed
with aerosol seeds similar to those used in this experiment, i.e., either pure sulfuric acid or solid
ammonium sulfate, at low relative humidity. Thus, the high molecular weight organics species
and kinetics limitations in gas-phase uptake observed here may also occur in those experiments,
at least at early times.
Overall, the current study demonstrated that under specific conditions (e.g., low RH), organics
and their reactions in the particle-phase can affect the rate of water droplet growth but not to the
degree such that the process will be limiting to the growth of particles intro droplets formed in
atmospheric warm clouds. However, we note that this process was only studied for a limited
range of conditions (e.g., temperature, RH, and particle acidity) and that there may be other
atmospherically relevant conditions where droplet growth could be suppressed. Additionally,
likely arising via similar mechanisms, such kinetic limitations can be important to the gas-
particle transport for other atmospherically relevant species.
Acknowledgements
The authors thank Environment Canada, Natural Sciences and Engineering Research Council,
and Canada Foundation for Innovation for funding. We would also like to thank the Particles and
103
Related Emission Project (Project C12.007), a program administered by the Natural Resources
Canada for funding and logistical support from the staff of Emissions Measurement Research
Section of Environment Canada during DEERE2012 campaign.
5.7 Supporting Material
Figure 5.8. Mass spectra of the organic component measured by the AMS for carbonyls
condensed onto sulfuric acid seed particles at RH 11 % (red) and 40 % (pink).
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Chapter 6
6 Conclusions and Future Research
Conclusions and Future Research
6.1 Summary and Future Directions of SOA Formation Studies
As discussed in Section 1.2, the discrepancy in modeled and predicted SOA mass loadings may
result from numerous effects: i) SOA yields measured in the laboratory under-represent actual
yields in the atmosphere; ii) existence of unidentified precursors; and iii) unidentified formation
110
mechanisms from known precursors. One of the objectives of this thesis is to better understand
the impacts sulfate seed particle properties have on isoprene SOA formation. Using a flow tube
setup that has the capacity to control the phase of sulfate particles while maintaining all other
experimental conditions constant, we found that particle-phase water was the dominant factor
controlling the amount of SOA formed, giving rise to 60 % more mass. While we have identified
that organic acids likely contributed to the additional SOA mass formed, we were unable to
determine their molecular identity. It is critical to characterize the molecular identity of these
organic acids and their gas-phase yields from the photooxidation of isoprene in order to fully
assess their potential to contribute to ambient SOA mass. Additionally, the results from our
study suggest that sulfate particle acidity affects the chemical composition of the resulting SOA
due to acid-catalyzed condensed phase reactions. The products of these reactions, high molecular
weight compounds, have been previously shown to degrade due to aqueous OH oxidation.1
However, the kinetics of such degradation processes are uncertain, hence the atmospheric
lifetimes of these high molecular weight compounds remain unknown.
It is important to note that our results represent the SOA formation potential from the oxidation
products of isoprene under high OH exposures and low NOx conditions on sulfate seed particles
(i.e., characteristic of gas-phase oxidation products further away from isoprene sources, where
IEPOX has been degraded). The process by which IEPOX contributes to SOA mass remains
unclear. While recent studies have shown that particle acidity is the controlling factor for the
reactive uptake of IEPOX,2 its contribution to SOA mass was largely independent of acidity.
3
Whereas IEPOX is an important intermediate for isoprene formation under low NOx conditions,
it has been identified that methacryloyperoxynitrate (MPAN) is the key reactive intermediate for
isoprene SOA formation under high NOx conditions.4 The effects of sulfate seed particle
properties on the isoprene SOA formation under high NOx conditions remain unexplored.
While our results have indicated that particle acidity can be an important factor controlling SOA
chemical composition, the pH of ambient aerosols is, however, highly variable and poorly
characterized. This is largely due to the fact that one of the most common techniques used to
discern particle pH, ion balancing, has recently been demonstrated to not accurately estimate pH
levels under certain conditions.5–7
Accurate measurements of ambient particle acidity are
necessary in order to assess the extent to which particle acidity will actually control SOA
chemical composition in the atmosphere.
111
Finally, while our study focused on isoprene SOA, the effects of sulfate seed particle properties
on SOA formation from other precursors, both biogenic (e.g., α-pinene) and anthropogenic (e.g.,
toluene) remain unknown. In fact, recent work has shown that large multifunctional gas-phase
oxidation products of α-pinene may be sufficiently soluble such that they can partition
significantly to particle-phase water, contributing to SOA mass.8 Additionally, the oxidation of
other SOA precursors has previously been shown to generate organic acids,9 which as shown
from our study, can experience enhanced uptake due to the presence of particle-phase water. It is
important to note that while isoprene is the dominant biogenic SOA precursor in tropical regions,
monoterpenes (e.g., α-pinene) can contribute significantly to biogenic SOA in boreal regions.10
Also, emissions associated with fossil fuel combustion and biomass burning are likely to be the
important SOA precursors in countries such as China and India.11,12
Given the ubiquity of water as an atmospheric aerosol constituent, there is a need to investigate
SOA formation under high RH conditions such that the particle water content is representative of
that in the atmosphere.
6.2 Summary and Future Directions of SOA Aging Studies
One of the objectives of this thesis was to determine the importance of photolysis as an SOA
aging process in the atmosphere. In Chapter 3, we demonstrated that photolysis can be an
important sink of some SOA components, and that the atmospheric lifetime of SOA with respect
to photolysis is dependent on the RH cycles it experiences. The RH dependence of other aging
processes, such as heterogeneous oxidation have recently been explored. Chan et al., observed
that the aerosol phase, which is controlled by the liquid water content in the particle, affected the
kinetics of the heterogeneous OH oxidation of succinic acid particles.13
Similarly, the reactive
uptake coefficient of OH by levoglucosan particles was shown to increase by a factor of 4 with a
40 % increase in RH. Conversely, in the same study by Slade et al., a decrease in OH uptake by
methyl-nitrocatechol particles was observed with increasing RH, which can be attributed to the
accumulation of water on the surface of the particle due to low water solubility of methyl-
nitrocatechol.14
These contrasting results clearly demonstrate that more systematic studies on the
RH dependence of SOA aging processes are warranted.
From our study, we determined the average effective photolysis quantum yield for organic mass
loss to be 1.2 ± 0.2. The large quantum yield suggests that indirect photolysis likely played a role
112
but its contribution was not constrained in our work. The effects of such secondary reactions of
photolysis may be important to the fate of organic particles in the atmosphere. For example, the
photolysis of peroxides, which are known constituents of SOA,15,16
can be a potential
photochemical OH source. Additionally, recent work has shown that photosensitized reactions, a
secondary photolysis process, can be important to the SOA formation and processing.17
Further
mechanistic studies of SOA photolysis are necessary to better understand the importance of
photolysis as this area of photochemical aging remains relatively unexplored compared to other
aging processes.
It is important to note that most SOA photolysis studies were conducted using laboratory
generated SOA. Since it is well known that the chemical composition of laboratory generated
SOA is different than ambient SOA,9 it is important to photolyze ambient SOA as this is the
ultimate demonstration of the importance of photolysis as a SOA aging process in the
atmosphere.
6.3 Summary and Future Directions of SOA-CCN Studies
In Chapter 4, we have demonstrated that oxidative aging by both gas-phase and aqueous-phase
OH can increase the hygroscopicity of ambient SOA at Whistler, BC and that there is a general
linear relationship of organic hygroscopicity and the degree of oxygenation. Subsequent to our
work, Rickards et al., have compiled all relevant literature results, as illustrated in Figure 8 of
their paper, where they demonstrated that while there is indeed a general trend of increasing
organic hygroscopicity and its degree of oxidation, considerable scatter can be observed.18
This
suggests that the hygroscopic response of SOA cannot be completely captured by a single
particle bulk property. For example, different SOA precursors will different molecular weights,
where their oxidation will have different number of oxidation products (i.e., different number of
moles of solutes). Additionally, as discussed in Section 1.2.2.4, the evolution of SOA via
functionalization, fragmentation and oligomerization can result in different changes in O:C
ratios. Nevertheless, the predictive power of this linear relationship has yet to be tested using
ambient SOA (e.g., a CCN closure study using this linear relationship).
Many current studies of CCN activity of ambient aerosol, including our work described in
Chapter 4, were conducted at ground level. However, the ultimate test of our understanding of
CCN activation would be to conduct studies in situ (i.e., at cloud level) as the composition of
113
particles will be different aloft compared to ground level. Additionally, most CCN counters are
operated around room temperature (i.e., 297 K), while the temperatures at cloud level are much
lower. In this regard, the effects of temperature on CCN activation may not be well represented
in previous studies. For example, recent work has shown that the adsorption of gas-phase
surfactant onto the gas-particle interface can promote CCN activation.19
As partitioning to the
particle is favoured with decreasing temperature, this process can contribute significantly to
CCN activation at cloud level.
While the thermodynamics of the effect of organics on particle activation are generally well
understood, as discussed in Section 1.3, the impacts of organics on droplet growth kinetics
remain unclear. The contrasting results from both laboratory and ambient studies pose a
challenge to assess the importance of droplet growth kinetics in the atmosphere. One of the goals
of this thesis is to determine the impacts of organics on droplet growth kinetics by comparing
growth kinetics of water on sulfate particles containing simple mixtures of model organic
compounds (carbonyls) and complex organic mixtures (α-pinene SOA and diesel engine
exhaust). In Chapter 5, we demonstrated that in most cases, smaller droplets arise when particles
contained organics, where increasing amounts of particulate organics resulted in greater
suppression in growth kinetics. However, the modeled results indicate that the observed
suppression in droplet growth is not large enough such that this process will be rate limiting in
the atmosphere. Nevertheless, the process was only studied for a limited range of conditions
(e.g., temperature, RH, and types of organics) and there are still likely to be other conditions
such that kinetic limitations can prevail.20
Given that only a limited number of studies have
investigated the effects of organics on droplet growth kinetics, additional laboratory studies
investigating the effects of droplet growth by other types of organics and ambient measurements
are warranted. In particular, more ambient observations of such kinetic limitations are important
as they will enable an assessment of the extent to which slow droplet growth actually occurs in
the atmosphere.
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