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Optical Properties of Moderately Absorbing Organic and ...
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Optical Properties of Moderately Absorbing Organic and Mixed Organic/Inorganic
Aerosols at Relative Humidities up to 95%
Benjamin Brem (Measurements) Francisco Mena (Closure Modeling)
Tami Bond and Mark Rood (PIs)
DoE Grant No. DE-FG02-08ER64533
Department of Civil and Environmental Engineering University of Illinois at Urbana-Champaign
3rd ASR Science Team Meeting, Crystal City, VA
March 14, 2012
Relative Humidity Effects on Aerosol Optical Properties
• Aerosol water uptake and aerosol size = function of aerosol chemistry and relative humidity (RH)
• Water uptake alters light scattering and light absorption
• RH effects on scattering previously characterized (RH < 90%)
• Limited understanding of RH effects on absorption 2
Primary Light Absorbing Organic Carbon (LAOC) Aerosols
• Emission Sources of Primary OC: – 91% from open and residential biomass burning (Bond et al., 2004)
• Chemical Characteristics: – Complex mixture of polycyclic aromatic hydrocarbons (PAHs),
humic-like substances and biopolymers (Pöschl , 2003)
– Up to 70% water soluble by mass (Chen and Bond, 2010)
• Optical Properties: – Weakly absorbing at visible wavelengths
– Wavelength dependent absorption with strongest absorption towards blue\ ultraviolet
– Few in situ measurements at multiple wavelengths and variable RH
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LAOC on Filter
Research Motivations & Objectives
Instrumentation challenges limit light absorption measurement as a function of RH • Objective 1: Build and benchmark equipment to measure aerosol
absorption and scattering as a function of RH up to 95% in a laboratory setting (presented at previous ASR meetings)
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Research Motivations & Objectives
Instrumentation challenges limit light absorption measurement as a function of RH • Objective 1: Build and benchmark equipment to measure aerosol
absorption and scattering as a function of RH up to 95% in a laboratory setting (presented at previous ASR meetings)
Limited knowledge of LAOC absorption and its response to RH • Objective 2: Measure absorption and scattering of LAOC as a function of
RH and evaluate optical closure (main focus of this presentation)
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Research Motivations & Objectives
Instrumentation challenges limit light absorption measurement as a function of RH • Objective 1: Build and benchmark equipment to measure aerosol
absorption and scattering as a function of RH up to 95% in a laboratory setting (presented at previous ASR meetings)
Limited knowledge of LAOC absorption and its response to RH • Objective 2: Measure absorption and scattering of LAOC as a function of
RH and evaluate optical closure (main focus of this presentation)
Limited understanding of optical properties as a function of RH for internal LAOC/ inorganic solute mixtures • Objective 3: Perform closure study for LAOC mixed with inorganic solute
as a function of RH (in the works)
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Approach Overview
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LAOC generated by pyrolysis of wood @ 425°C and its mixtures with NaCl
Nephelometer Extinction Cell
H-TDMA SMPS Offline Analysis
Extinction Dry/Wet
Scattering Dry/Wet
Absorption Dry/Wet
Growth Factor
PSD Dry
Ionic Fraction
Approach Overview
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LAOC generated by pyrolysis of wood @ 425°C and its mixtures with NaCl
Nephelometer Extinction Cell
H-TDMA SMPS Offline Analysis
Extinction Dry/Wet
Scattering Dry/Wet
Absorption Dry/Wet
Growth Factor
PSD Dry
Ionic Fraction
Optical (Mie) Model
Optical (Mie) Modeling Inputs
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• Dry size distribution & hygroscopic size growth factors humidified size distribution and aerosol water content
• Dry LAOC refractive index: – 1.57+0.017i (467nm), 1.57+0.01i (530nm), 1.57+0.002i (660nm)
n from literature, ki from spectroscopy from filter extracts
• Humidified refractive index mixing models:
– Linear Volume Average (LVA) : commonly used for OC, infers uniform mixture (completely soluble LAOC)
– Dynamic Effective Medium Approximation (DEMA) Infers non-uniform, composite media with randomly distributed insoluble inclusions of absorbing material (Chylek, 1984)
Increasing RH
Increasing RH
Measured LAOC Optical Properties
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40 50 60 70 80 90 1001000
1250
1500
1750
2000
40 50 60 70 80 90 1001000
1250
1500
1750
2000
40 50 60 70 80 90 100-50
0
50
100
150
200
250
40 50 60 70 80 90 100
0.90
0.95
1.00
1.05
Ex
tinct
ion
b ep(M
m-1)
RH (%)
Measured
467, 530, 660 λ (nm)
Scat
terin
g b sp
(Mm
-1)
RH (%)
Abso
rptio
n b ap
(Mm
-1)
RH (%)
Sing
le S
catte
ring
Albe
do ω
RH (%)
Error bars dotted for clarity Error bars dotted for clarity
FINDINGS: • Absorption increases by a factor of 2 at High RH (λ = 467 and 530 nm) • Absorption at λ = 660 nm is below detection limit of instrumentation
40 50 60 70 80 90 1001000
1250
1500
1750
2000
40 50 60 70 80 90 1001000
1250
1500
1750
2000
40 50 60 70 80 90 100
0
50
100
150
200
250
40 50 60 70 80 90 100
0.88
0.92
0.96
1.00
1.04
Ex
tinct
ion
b ep(M
m-1)
RH (%)
Measured
LVA Model
467, 530, 660 λ (nm)
Scat
terin
g b sp
(Mm
-1)
RH (%)
Abso
rptio
n b ap
(Mm
-1)
RH (%)
Sing
le S
catte
ring
Albe
do ω
RH (%)
Optical Closure Results (1): Linear Volume Average (LVA) RI Mixing
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Error bars removed for clarity Error bars removed for clarity
FINDING: LVA predicts extinction and scattering, but not absorption
Optical Closure Results (2) : Dynamic Effective Medium Approximation (DEMA) RI Mixing
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40 50 60 70 80 90 1001000
1250
1500
1750
2000
40 50 60 70 80 90 1001000
1250
1500
1750
2000
40 50 60 70 80 90 100-50
0
50
100
150
200
250
40 50 60 70 80 90 100
0.88
0.92
0.96
1.00
1.04
Ex
tinct
ion
b ep(M
m-1)
RH (%)
Measured
LVA Model
DEMA Model
467, 530, 660 λ (nm)
Scat
terin
g b sp
(Mm
-1)
RH (%)
Abso
rptio
n b ap
(Mm
-1)
RH (%)
Sing
le S
catte
ring
Albe
do ω
RH (%)
Error bars removed for clarity Error bars removed for clarity
FINDING: DEMA captures trends in absorption and SSA indicating that configuration of insoluble absorbing material might be responsible for absorption enhancement
30 40 50 60 70 80 90 100
1.0
1.1
1.2
1.3
1.4
1.5
30 40 50 60 70 80 90 100
1.0
1.1
1.2
1.3
1.4
1.5
30 40 50 60 70 80 90 100
1.0
1.5
2.0
2.5
30 40 50 60 70 80 90 100
0.88
0.92
0.96
Ext
inct
ion
Gro
wth
Fac
tor f
(RH
) bep
λ = 467 nm Experiment 1 Experiment 2 Experiment 3 Experiment 4 LVA DEMA
RH (%)
Sca
tterin
g G
row
th F
acto
r f(R
H) b
sp
RH (%)
Abs
orpt
ion
Gro
wth
Fac
tor f
(RH
) bap
RH (%)
Sing
le S
catte
ring
Albe
do ω
RH (%)
f RH bsp =bsp RH
bsp(RH<45%)
Normalized LAOC Optical Properties for Multiple Experiments (λ = 467nm)
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FINDINGS: • Low Variability in f(RH) ext. and scat. particles have same hygroscopicity • Same trend in absorption growth and SSA reduction, represented best with DEMA
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Normalized LAOC Optical Properties for LAOC- NaCl Mixtures (λ = 467nm)
20 30 40 50 60 70 80 90 100
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
20 30 40 50 60 70 80 90 100
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
20 30 40 50 60 70 80 90 100
1.0
1.5
2.0
2.5
20 30 40 50 60 70 80 90 1000.88
0.90
0.92
0.94
0.96
Ext
inct
ion
Gro
wth
Fac
tor f
(RH
)bep
λ = 467nm 17% vol. NaCl 23% vol. NaCl 27% vol. NaCl
RH (%)
Sca
tterin
g G
row
th F
acto
r f(R
H)b
sp
RH (%)
Abso
rptio
n G
row
th F
acto
r f(R
H)b
ap
RH (%)
Sing
le S
catte
ring
Albe
do ω
RH (%)
FINDINGS • Observable deliquescence ~ 76% RH • Absorption increase is in the same range as for pure LAOC
Summary & Outlook • LAOC absorption↑ and SSA↓ with RH↑ at the 467 and 530
nm wavelengths. (660 nm below detection limit)
• Change in LAOC absorption and SSA is currently not implemented in models
• Widely used LVA RI mixing rule is unable to catch trends in absorption and SSA with RH
• DEMA captures trends in absorption and SSA indicating that configuration of insoluble absorbing material might be responsible for absorption enhancement
• Addition of NaCl affects scattering but no specific trend (in comparison to pure LAOC) was observed for absorption
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QUESTIONS? CONTACT INFORMATION: [email protected] WWW.HIWATER.ORG HTTP://AQES.CEE.ILLINOIS.EDU/
Thank you for your attention
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Acknowledgements
• Bond / Rood Research Groups • DoE-ASP Grant No. DE-FG02-08ER64533 • DoE-ASR Grant No. DE-SC0006689
(current ASR support)
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Appendix: Normalized LAOC Optical Properties for Multiple Experiments (λ = 530nm)
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30 40 50 60 70 80 90 100
1.0
1.1
1.2
1.3
1.4
1.5
30 40 50 60 70 80 90 100
1.0
1.1
1.2
1.3
1.4
1.5
30 40 50 60 70 80 90 100
1.0
1.5
2.0
2.5
30 40 50 60 70 80 90 1000.92
0.94
0.96
0.98
1.00
f(R
H)b
ep
a) f(RH)bep 530nm 11/22/2010 12/03/2010 12/04/2010 11/11/2011 LVA DEMA
RH (%)
f(R
H)b
sp
b) f(RH)bsp 530nm
RH (%)
f(R
H)b
ap
c) f(RH)bap 530nm
RH (%)
ω
d) ω 530nm
RH (%)
Appendix: Normalized LAOC Optical Properties for Multiple Experiments (λ = 660nm)
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30 40 50 60 70 80 90 100
1.0
1.1
1.2
1.3
1.4
1.5
30 40 50 60 70 80 90 100
1.0
1.1
1.2
1.3
1.4
1.5
30 40 50 60 70 80 90 100
-6-4-202468
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30 40 50 60 70 80 90 100
0.96
0.98
1.00
1.02
f(R
H)b
ep
a) f(RH)bep 660nm 11/22/2010 12/03/2010 12/04/2010 11/11/2011 LVA DEMA
RH (%)
f(R
H)b
sp
b) f(RH)bsp 660nm
RH (%)
f(R
H)b
ap
c) f(RH)bap 660nm
RH (%)
ω
d) ω 660nm
RH (%)
Appendix: Methods: Aerosol Generation and Treatment
• LAOC aerosol was generated by pyrolyzing wood in an electrically-heated combustor at 425 ± 10 ºC
• Flaming (formation of BC) was avoided by using nitrogen sheath flow
• Aerosol was sampled at a 10:1 dilution into a 220 L storage chamber
• After pyrolysis event (~4-6 min), chamber was disconnected from sampling system and connected to optical instrumentation
• For mixing experiments inorganic solute was atomize into diluent
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Appendix: Instrumentation • Optical Properties:
– bep Short path extinction cell (SPEC) λ= 467, 530 and 660 nm – bsp λ and temperature modified nephelometer – bap Difference between bep and bsp
• Physical Properties: – Dp SMPS (TSI) – Gf(Dp) H-TDMA
• Environmental Variables – RH / dew point temperature 2 RH (Vaisala), 2 Dew point (GE) – Temperature 4 Thermocouples
• Offline Filter Measurements (Chemical Properties) – OC/EC, Ions, OC functional groups 1H and 13C NMR
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Appendix: Instrumentation
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Humidifier
Cooler
Dew Point Meters
Extinction Cell
Nephelometer
RH Control Box
Impactor
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Appendix: Benchmarking Ammonium Sulfate System Performance under purely Light Scattering Conditions
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• Instrumentation capable of measuring optical properties up to 95% RH
• Measured data lie within uncertainty of model
• Shift in bsp deliquescence due to nephelometer heating of 0.5°C
Tang (1994)
Appendix PSL Results
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Benchmarking with Nigrosin System Capable of Measuring Absorption as a Function of RH
• bap enhanced by a factor of 1.2 between 35 and 95% RH
• bap λ dependence similar to bulk measurements
• Closure modeling Poster Mena et al. Number: 2E8
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Appendix LAOC PSD
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Appendix LAOC Composition
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Table 1 Composition analysis of OC aerosol generated by the pyrolysis of oak wood at 425°C. Elemental carbon (EC) and the ions of Br-, PO4
3- and Mg2+ were also determined but the results were below the detection limit of the analysis OM* OM*/ OC Cl- SO4
2- NO3- NH4
+ Ca2+ K+ Na+
Mean 97.25 1.68 0.74 0.28 0.21 0.07 0.21 0.87 0.17 Std. 1.13 0.06 0.23 0.04 0.20 0.08 0.15 0.25 0.06 *Corrected by forcing closure with mass analyzed by gravimetric analysis (OM/ OC describes the correction factor).
LAOC HTDMA Results
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κ = 0.08 ± 0.015
Appendix: LAOC NMR Results
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0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
O=C-(R, H)
O=C-(OH/NH2)
C=C-(O,N)
C=C-(C,H)
O-C-O
C-O or C-N
C-C
High T water
High T methanol
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
O=C-(R, H)
O=C-(OH/NH2)
C=C-(O,N)
C=C-(C,H)
O-C-O
C-O or C-N
C-C
Low T water
Low T methanol
Percentage peak area
Percentage of total 13C NMR peak area under each spectral region for methanol and water extracts of OC generated at two 300 and 425C. Difference of contribution from two major functional groups is highlighted with green circles.
Appendix Global Relative Humidity (RH) Conditions
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Observation-derived average global surface RH for the years between January 1960 and January 2011. Image provided by NOAA-ESRL, Boulder, Colorado
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Appendix: Wood combustion process
Evans and Milne, 1986
Appendix Dry Data Processing
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Appendix: Humidification System Details & Function
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Dry Inflow with Aerosols
Humidified Outflow
HeatingTape Gore- Tex®
Membrane
RH Sensor
Controller
CirculatingWater
Appendix: RH Sensor Performance
V1, V2 = Capacitance based Vaisala Sensors DP1, DP2 = General Eastern Dew Point Meters (RH calculated with dry bulb temperatures )
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32
33
34
35
36a) Setpoint = 34%
RH [%
]
48
49
50
51
52b) Setpoint = 50%
RH [%
]
V1 V2 DP1
DP2
88
89
90
91
92c) Setpoint = 90%
RH [%
]
V1 V2 DP1
DP2
96
97
98
99d) Setpoint = 97.5%
RH [%
]
Appendix: Modified Nephelometer Performance
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• For all three wavelengths, the instruments differ less than 1.5%
Appendix: Extinction Cell and Nephelometer Sensitivity and Detection Limit
• Function of integration time • Best sensitivity is btw. 100
and 300s integration time • Corresponding absorption
detection limits: 57.3 Mm-1 (467 nm), 54.5 Mm-1 (530 nm) and 105 Mm-1 (660 nm) with a signal to noise ratio is assumed to be 3
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Appendix POA Sources
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Residential Biofuel
17% Residenti
al Coal 1%
Residential Other
0%
Transport: Non-road
1% Transport:
Road 4%
Industry 1%
Electricity Generatio
n 0%
Open vegetative
burning 74%
Other 2%
Total: 34 Tg/ yr (2000)