The Characterization of Atmospheric Particulate Matter Richard F. Niedziela DePaul University 16 May...

The Characterization ofAtmospheric Particulate Matter

Richard F. Niedziela

DePaul University

16 May 00

The atmosphere

Have you thought about your atmosphere today?

Physical dimensions– matm 5.2 1018 kg 10-6 mearth

– hatm 100 km

– Vatm 1.0 1011 km3 10-1 Vearth

Thermal profile– Several different thermal gradients

The atmosphere

The atmosphere

The atmosphere is made out of...– 78% N2 (3.9 1018 kg)

– 21% O2 (1.2 1018 kg)

– 1% trace gases and suspended matter, or aerosols (0.1 1018 kg)

Aerosols

Aerosols

Aerosols are small particles of condensed matter that are found throughout the environment, from the surface of the Earth to the upper reaches of the atmosphere.

Brilliant red sunsets Blue hazes in forests Fog

Aerosol characteristics

An aerosol is characterized by Composition Size Phase Shape

Aerosol composition

Organic materials Long-chained hydrocarbons Large carboxylic acids

Inorganic materials Mineral acids Metals

Organic/inorganic mixtures

Aerosol size

Particle diameters range from submicron to tens of microns

micron = 1 m = 10-4 cm = 10-6 m

10-4 10-3 .01 .1 1 10 100 103 104

Atom

s, sm

all m

olecu

les

Small

est d

etec

table

par

ticles

Very f

ine a

eros

ols

Atmos

pher

ic ba

ckgr

ound

aer

osols

Avera

ge a

tmos

pher

ic ae

roso

ls

Cloud

drop

lets

Drizzle

Raindr

ops

Hail

Aerosol phase

Liquids Oil droplets from vegetation Sulfuric acid aerosols

Solids Suspended crust material Water ice particles in cirrus clouds

Liquid/solid mixtures

Aerosol shape

Liquids: spherical droplets Solids: crystals and complex structures Shape can impact physical, chemical,

and optical properties of aerosols

Some actual aerosols

T. Reichhardt, Environ. Sci. Tech., 29(8), 360A, (1995).

Sulfate particle

Aluminum particle

Aerosol sources

Natural sources Vegetation Oceans Volcanoes

Anthropogenic sources Vehicle and industrial emissions Agricultural practices

Aerosol production

Mechanical action Abrasion of plant leaves Sea spray Wind

Nucleation and condensation Cloud formation

Aerosols and the Environment


Ozone depletion Global climate change

The atmosphereal

titud

e (k

m)

20

40

60

80

tropopause

stratopause

mesopause

stratosphere

mesosphere

thermosphere upper atmosphere

middle atmosphere

lower atmospheretroposphere

Ozone

O

O O Pungent gas (named after

the Greek word ozein, “to smell”)

“Good” vs. “Bad” Stratosphere

– 90% of all ozone– 10 ppmv peak concentration– UV screening

Troposphere– 10 ppbv peak concentration– Disinfectant– Respiratory stress

O3

Ozone

O2 + h O + O

O + O2 + M O3 + M

O3 + h O2 + O

O3 + O O2 + O2

Chapman mechanism Proposed in 1930 Qualitative prediction of atmospheric ozone profile

Ozone depletion

There has been a recent overall decrease in the stratospheric ozone concentration.

Ozone measured over Payerne, Switzerland

CF2Cl2 + h CF2Cl + Cl

Cl + O3 ClO + O2

ClO + O Cl + O2

O3 + O 2 O2

Polar ozone depletion

The loss of ozone over the South Pole is more dramatic

Polar ozone depletion theories

Atmospheric motions Stratospheric air replaced with

tropospheric air

Discounted due to lack of tropospherictrace gases in the stratosphere


Reactive nitrogen species chemically destroy ozone

Discounted due to low concentrations of nitrogen species during depletion events


Chlorine compounds are responsible for the ozone depletion Produced from CFCs Persist for up to 100 years

Polar ozone depletion cycle

2ClO + M Cl2O2 + MCl2O2 + h ClOO + ClClOO + M Cl + O2 + M2Cl + 2O3 2ClO + 2O2

2O3 + h 3O2

These reactions are thought to be responsible for 70% of the observed ozone depletion

Homogeneous reactions

CFCs

ClONO2

h

ClONO2

h

Polar stratospheric chemistry

Homogenous chemistry cannot provide all of the ClO needed to deplete ozone

Ozone depletion occurs in the presence of polar stratospheric clouds or PSCs

Polar stratospheric clouds

Type I Formed near 195 K Composed of nitric acid and water Exist in different phases

– Type Ia: Solid nitric acid particles– Type Ib: Supercooled liquid droplets (sulfuric acid,

nitric acid, water)

Type II Formed near 185 K Water ice particles

Heterogeneous reactions

ClONO2(s) + HCl(s) Cl2(g) + HNO3(s)

ClONO2(s) + H2O(s) HOCl(g) + HNO3(s)

Chlorine is released into the gas phase Nitrogen is chemically removed Nitrogen is physically removed

PSCs

PSCs

Heterogeneous reactions

CFCs

ClONO2

h

PSCs

HCl

HNO3

Sedimentation

Cl2

H2O

Polar Stratospheric Clouds

HOCl h

hCl

Cl


CFCs

ClONO2ClONO2

ClO

Cl2

NO2

PSCs

HCl

HNO3

Sedimentation

ClO + ClO

Cl2O2

Cl

ClO

O3 O2

h h

h

h

H2O

HOClhh


Heterogeneous reaction rates are dependent on PSC phase, composition, and size

Need to characterize PSCs to fully investigate depletion process

PSC characterization

Collect infrared spectra of PSCs Mie scattering theory

Spherical particles Complex refractive indices for proposed

PSC components

Complex refractive indices

N n ik n is the real component of the refractive index

determines how fast light moves through material n = c / v

k is the imaginary component of the refractive index determines how light is absorbed by material k = / 4

Optical constants

PSC spectra

O.B.Toon and M.A. Tolbert, Nature, 375, 218, (1995).

IceNADNAT


Good fits were not obtained using known optical constants for Water ice Nitric acid monohydrate (NAM): HNO3H2O

Nitric acid dihydrate (NAD): HNO3H2O

Nitric acid trihydrate (NAT): HNO33H2O


PSCs are not pure water or nitric acid aerosols

Ternary mixtures with sulfuric acid Determine optical constants for ternary

mixtures

Retrieving optical constants

Retrieve optical constants from infrared spectra of model PSC aerosols Frequency Temperature

Optical constants for NAD

Aerosol flow cell II

F l o wIn je c t io n

P o r t

F T I RS p e c t ro m e te r

F l o wE x h a u s t

C o o l in gC o i ls

M C T

1 2 3

4

56


Collect many scatteringspectra representing

different particle sizes

Scattering spectra

700 1200 1700 2200 2700 3200 3700 4200 4700

0.0

0.2

0.4

0.6

0.8

1.0

Nitric Acid Dihydrate at 180 K

Exti

ncti

on

Wavenumber (cm-1)


Collect a non-scatteringspectrum to estimate k

k() = K()



Non-scattering spectrum

700 1200 1700 2200 2700 3200 3700 4200 4700

0.00

0.05

0.10

0.15

0.20


Exti

ncti

on

Wavenumber (cm-1)


Select a scatteringspectrum and guess

the particle size




k() = K()



the particle size

Use Kramers-Kronigrelationship tocalculate n()




k() = K()



the particle size


Use Mie scatteringtheory to calculate

scattering spectrum




k() = K()



the particle size



scattering spectrum

Compare calculated andexperimental spectra




k() = K()



the particle size



scattering spectrum

Compare calculated andexperimental spectraCorrect k() if necessary




k() = K()



the particle size



scattering spectrum


Vary k() scaling factor, K




k() = K()



the particle size



scattering spectrum


Vary k() scaling factor, K

Vary particle size




k() = K()

Final fit results

700 1200 1700 2200 2700 3200 3700 4200 4700

0.0

0.2

0.4

0.6

0.8

1.0

rmed

= 0.33 m


Exti

ncti

on

Wavenumber (cm-1)

Final optical constants

700 1200 1700 2200 2700 3200 3700 4200 4700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

k

n


Refr

acti

ve in

dex

Wavenumber (cm-1)

NAD optical constants

Overall good agreement with thin-film results

Some discrepancies do exist Comparison of several aerosol and thin-

film spectra suggest substrate interference

Aerosol vs. thin-film spectra

NAD thin-film spectra

NAD aerosol spectra

Wavenumber (cm-1)

Aerosol optical constants

Optical constants derived from aerosols arebetter suited for analyzing atmospheric particles

Aerosol composition

NAD aerosols have a fixed composition Composition of liquid sulfuric acid

aerosols can vary

Tunable diode laser

H eN e

T D L

T o V acuum

W indow to C ell

F ocusingO bjective

B eam splitterA lignm ent P inho le

O cular

S lit

B ypass O ptics

M onochrom ator V acuum Jacket

Tunable diode laser

Tunable diode laser

Diode laser beam samples the same aerosol stream as the FT-IR spectrometer

Determines water vapor pressure by applying Beer’s law to a single water absorption line

Tunable diode laser

1751.39 1751.40 1751.41 1751.42 1751.43 1751.44 1751.45

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Cell Pressure (Torr) 9.7 49.3 99.5 200.0 300.0T

ran

smis

sio

n (

I/Io)

Wavenumber (cm-1)


F l o wIn je c t io n

P o r t

F T I RS p e c t ro m e te r

M C TD e te c to rs

F l o wE x h a u s t

1 2 3

C o o l in gC o i ls

T D L a n dO p t ic s B o x

4

56

Sulfuric acid optical constants

One optical constant study by Palmer and Williams in 1975

Bulk data for a few concentrations at room temperature

Widely used by atmospheric scientists Spectra change substantially at low

temperatures


800 1300 1800 2300 2800 3300 3800 4300

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

75 wt% Sulfuric Acid/Water

k

n

Refr

acti

ve In

dex

Wavenumber (cm-1)


800 1300 1800 2300 2800 3300 3800 4300

0.0

0.5

1.0

1.5

2.0

2.5

k

n

38 wt% Sulfuric Acid/Water

Refr

acti

ve in

dex

Wavenumber (cm-1)


30 40 50 60 70 80 90

200

220

240

260

280

300

Tem

per

atu

re (

K)

Weight % H2SO

4


The Palmer and Williams optical constants should not be used at low temperatures

Temperature and composition dependence indicate interesting ion equilibrium chemistry

Emphasize the need to perform similar studies on ternary systems


Ozone depletion Global climate change

The atmosphereal

titud

e (k

m)

20

40

60

80

tropopause

stratopause

mesopause

stratosphere

mesosphere

thermosphere upper atmosphere

middle atmosphere

lower atmospheretroposphere

Global climate change

Climate depends on the chemical composition of the atmosphere

Forecasting how the climate will change Will our current coastlines disappear? Will there be another ice age?

Over time, incoming solar energy is balanced by energy radiated from Earth

Energy balance

Climate Change 1994: Radiative Forcing of Climate Change and An Evaluation of the IS92Emission Scenarios (Cambridge University Press, Cambridge, 1995).

Eath

Sun

Earth

Energy imbalance

Anything which causes a change in the energy balance is known as a forcing

Climate responds to forcing by re-establishing energy balance

A forcing example

Doubling CO2 concentration Forcing of 4 Wm-2

Surface must warm up 1 Kto restore balance

Positive forcing warms the planet,while negative forcing cools the planet

Forcing sources

Solar output Surface characteristics of the Earth Greenhouse gases

H2O, CO2, O3, CH4, N2O, and halocarbons

Direct interaction with energy radiated from the Earth

Forcing sources

Aerosols “Direct” forcing

– Direct interaction with incoming or outgoing light

“Indirect” forcing– Affecting other components of the climate

Forcing contributions

S.E. Schwartz and M.O. Andreae, Science, 272, 1121, (1996).

Aerosol forcing uncertainties

Interaction with light is largely unknown Lack of optical constant information

Hygroscopic properties are unknown Important gauge of indirect effects

Complex spatial and temporal distributions throughout the atmosphere

Aerosol forcing effects

Aerosol forcing could offset greenhouse forcing

Cooling of 2 - 3 K due to “background aerosols”

Mt. Pinatubo eruption Peak forcing of -4.5 Wm-2

A temporary, calculated and observed cooling of 0.5 K

Tropospheric aerosols

Materials: soil dust, sulfates, sea salt, soot, and organics

Only sulfates have been “characterized” Soot and organic aerosols are perhaps

the most important

Present laboratory work

Apply optical constant retrieval method to organic aerosols

Study hygroscopic properties of organic aerosols

Characterize multi-component organic aerosols

Organic aerosols

Primary organic aerosols (POAs) Emitted from source as an aerosol

Secondary organic aerosols (SOAs) Condensation of gas-phase species on pre-

existing particles Composed of terpenes, PAHs, alkanes,

and carboxylic acids

Organic aerosols - terpenes

Organic aerosols - terpenes

Natural sources are nearly ten times greater than anthropogenic sources

C=C bonds are susceptible to attack by O3, NO3, and OH

Model organic aerosols

Determine optical constants for single-component organic aerosols

Start with easily obtained materials that closely represent actual organic aerosols

Model organic aerosols

Carvone1000 2000 3000 4000 5000

0.0

0.5

1.0

1.5

2.0

Abs

orba

nce

Wavenumber (cm-1)

o

Aerosol flow cell III

First spectra

1000 2000 3000 4000 5000

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Abs

orba

nce

Wavenumber (cm-1)

Humidity dependence

Add water vapor along with organic aerosols

Optical constants as a function of relative humidity

Hygroscopic vs. hygrophilic Evaluate the indirect effect of organic

aerosols

Multi-component aerosols

Prepare known mixed organic and mixed organic/inorganic aerosols

Use single-component optical constants to determine refractive index mixing rules

Test rules on unknown aerosols Apply rules to real tropospheric aerosols

Acknowledgments

PSCs (UNC - Chapel Hill) R.E. Miller, D.R. Worsnop, and M.L. Norman NASA Upper Atmosphere Research Program

Organic aerosol studies (DePaul University) Elena Lucchetta LA&S Summer Research Program (1999)

The Characterization of Atmospheric Particulate Matter Richard F. Niedziela DePaul University 16 May...

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