The characterization of atmospheric particulate matter
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The Characterization of Atmospheric Particulate Matter. Richard F. Niedziela DePaul University 16 May 00. The atmosphere. Have you thought about your atmosphere today? Physical dimensions m atm  5.2  10 18 kg  10 -6 m earth h atm  100 km V atm  1.0  10 11 km 3  10 -1 V earth

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The Characterization of Atmospheric Particulate Matter

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The characterization of atmospheric particulate matter

The Characterization ofAtmospheric Particulate Matter

Richard F. Niedziela

DePaul University

16 May 00


The atmosphere

The atmosphere

Have you thought about your atmosphere today?

  • Physical dimensions

    • matm 5.2  1018 kg  10-6mearth

    • hatm  100 km

    • Vatm  1.0  1011 km3  10-1Vearth

  • Thermal profile

    • Several different thermal gradients


The atmosphere1

The atmosphere


The atmosphere2

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


Aerosols1

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

Aerosol characteristics

An aerosol is characterized by

  • Composition

  • Size

  • Phase

  • Shape


Aerosol composition

Aerosol composition

  • Organic materials

    • Long-chained hydrocarbons

    • Large carboxylic acids

  • Inorganic materials

    • Mineral acids

    • Metals

  • Organic/inorganic mixtures


Aerosol size

Aerosol size

Particle diameters range from submicron to tens of microns

Atmospheric background aerosols

Average atmospheric aerosols

Smallest detectable particles

Atoms, small molecules

Very fine aerosols

Cloud droplets

Raindrops

Drizzle

Hail

10-4

10-3

.01

.1

1

10

100

103

104

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


Aerosol phase

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

Aerosol shape

  • Liquids: spherical droplets

  • Solids: crystals and complex structures

  • Shape can impact physical, chemical, and optical properties of aerosols


Some actual aerosols

Some actual aerosols

Sulfate particle

Aluminum particle

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


Aerosol sources

Aerosol sources

  • Natural sources

    • Vegetation

    • Oceans

    • Volcanoes

  • Anthropogenic sources

    • Vehicle and industrial emissions

    • Agricultural practices


Aerosol production

Aerosol production

  • Mechanical action

    • Abrasion of plant leaves

    • Sea spray

    • Wind

  • Nucleation and condensation

    • Cloud formation


Aerosols and the environment

Aerosols and the Environment


Aerosols and the environment1

Aerosols and the Environment

  • Ozone depletion

  • Global climate change


The atmosphere3

The atmosphere

thermosphere

upper atmosphere

mesopause

80

mesosphere

60

altitude (km)

middle atmosphere

stratopause

40

stratosphere

20

tropopause

troposphere

lower atmosphere


Ozone

Ozone

  • 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

O

O

O

O3


Ozone1

Ozone

  • Chapman mechanism

    • Proposed in 1930

    • Qualitative prediction of atmospheric ozone profile

O2 + h

O + O

O + O2 + M

O3 + M

O3 +h

O2 + O

O3 + O

O2 + O2


Ozone depletion

Ozone depletion

There has been a recent overall

decrease in the stratospheric ozone

concentration.

CF2Cl2 + h

CF2Cl + Cl

Cl + O3

ClO + O2

ClO + O

Cl + O2

O3 + O

2 O2

Ozone measured over Payerne, Switzerland


Polar ozone depletion

Polar ozone depletion

The loss of ozone over the South Pole is more dramatic


Polar ozone depletion theories

Polar ozone depletion theories

  • Atmospheric motions

  • Stratospheric air replaced with tropospheric air

Discounted due to lack of tropospheric

trace gases in the stratosphere


Polar ozone depletion theories1

Polar ozone depletion theories

  • Reactive nitrogen species chemically destroy ozone

Discounted due to low concentrations of

nitrogen species during depletion events


Polar ozone depletion theories2

Polar ozone depletion theories

  • Chlorine compounds are responsible for the ozone depletion

    • Produced from CFCs

    • Persist for up to 100 years


Polar ozone depletion cycle

Polar ozone depletion cycle

2ClO + M

Cl2O2 + M

Cl2O2 + hn

ClOO + Cl

Cl + O2 + M

ClOO + M

2Cl + 2O3

2ClO + 2O2

2O3 + hn

3O2

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


Homogeneous reactions

Homogeneous reactions

hn

CFCs

ClONO2

hn

NO2

ClO


Polar stratospheric chemistry

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

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

Heterogeneous reactions

  • Chlorine is released into the gas phase

  • Nitrogen is chemically removed

  • Nitrogen is physically removed

ClONO2(s) + HCl(s)

Cl2(g) + HNO3(s)

PSCs

ClONO2(s) + H2O(s)

HOCl(g) + HNO3(s)

PSCs


Heterogeneous reactions1

Heterogeneous reactions

hn

HCl

CFCs

HNO3

ClONO2

Polar Stratospheric Clouds

PSCs

H2O

Cl2

hn

HOCl

hn

Cl

Sedimentation

Cl


Polar stratospheric chemistry1

Polar stratospheric chemistry

hn

hn

CFCs

HCl

ClONO2

ClONO2

HNO3

hn

NO2

H2O

PSCs

ClO

Cl2

HOCl

ClO + ClO

hn

Cl2O2

hn

ClO

hn

Cl

Sedimentation

O3

O2


Polar stratospheric chemistry2

Polar stratospheric chemistry

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

  • Need to characterize PSCs to fully investigate depletion process


Psc characterization

PSC characterization

  • Collect infrared spectra of PSCs

  • Mie scattering theory

    • Spherical particles

    • Complex refractive indices for proposed PSC components


Complex refractive indices

Complex refractive indices

  • 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 = al / 4p

  • Optical constants


Psc spectra

PSC spectra

Ice

NAD

NAT

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


Polar stratospheric clouds1

Polar stratospheric clouds

  • Good fits were not obtained using known optical constants for

    • Water ice

    • Nitric acid monohydrate (NAM): HNO3·H2O

    • Nitric acid dihydrate (NAD): HNO3·2H2O

    • Nitric acid trihydrate (NAT): HNO3·3H2O


Polar stratospheric clouds2

Polar stratospheric clouds

  • PSCs are not pure water or nitric acid aerosols

  • Ternary mixtures with sulfuric acid

  • Determine optical constants for ternary mixtures


Retrieving optical constants

Retrieving optical constants

  • Retrieve optical constants from infrared spectra of model PSC aerosols

    • Frequency

    • Temperature

  • Optical constants for NAD


Aerosol flow cell ii

Aerosol flow cell II


Aerosol flow cell ii1

Aerosol flow cell II


Aerosol flow cell ii2

Aerosol flow cell II


Retrieving optical constants1

Retrieving optical constants

Collect many scattering

spectra representing

different particle sizes


Scattering spectra

Scattering spectra


Retrieving optical constants2

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

k(n) = Ka(n)


Non scattering spectrum

Non-scattering spectrum


Retrieving optical constants3

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

Select a scattering

spectrum and guess

the particle size

k(n) = Ka(n)


Retrieving optical constants4

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

Select a scattering

spectrum and guess

the particle size

k(n) = Ka(n)

Use Kramers-Kronig

relationship to

calculate n(n)


Retrieving optical constants5

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

Select a scattering

spectrum and guess

the particle size

k(n) = Ka(n)

Use Kramers-Kronig

relationship to

calculate n(n)

Use Mie scattering

theory to calculate

scattering spectrum


Retrieving optical constants6

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

Select a scattering

spectrum and guess

the particle size

k(n) = Ka(n)

Use Kramers-Kronig

relationship to

calculate n(n)

Use Mie scattering

theory to calculate

scattering spectrum

Compare calculated and

experimental spectra


Retrieving optical constants7

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

Select a scattering

spectrum and guess

the particle size

k(n) = Ka(n)

Use Kramers-Kronig

relationship to

calculate n(n)

Use Mie scattering

theory to calculate

scattering spectrum

Compare calculated and

experimental spectra

Correct k(n) if necessary


Retrieving optical constants8

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

Select a scattering

spectrum and guess

the particle size

k(n) = Ka(n)

Use Kramers-Kronig

relationship to

calculate n(n)

Vary k(n) scaling factor, K

Use Mie scattering

theory to calculate

scattering spectrum

Compare calculated and

experimental spectra

Correct k(n) if necessary


Retrieving optical constants9

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

Select a scattering

spectrum and guess

the particle size

k(n) = Ka(n)

Vary particle size

Use Kramers-Kronig

relationship to

calculate n(n)

Vary k(n) scaling factor, K

Use Mie scattering

theory to calculate

scattering spectrum

Compare calculated and

experimental spectra

Correct k(n) if necessary


Retrieving optical constants10

Retrieving optical constants

Collect a non-scattering

spectrum to estimate k

Collect many scattering

spectra representing

different particle sizes

Select a scattering

spectrum and guess

the particle size

k(n) = Ka(n)

Vary particle size

Use Kramers-Kronig

relationship to

calculate n(n)

Vary k(n) scaling factor, K

Use Mie scattering

theory to calculate

scattering spectrum

Compare calculated and

experimental spectra

Correct k(n) if necessary


Final fit results

Final fit results


Final optical constants

Final optical constants


Nad optical constants

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

Aerosol vs. thin-film spectra

NAD thin-film spectra

NAD aerosol spectra

Wavenumber (cm-1)


Aerosol optical constants

Aerosol optical constants

Optical constants derived from aerosols are

better suited for analyzing atmospheric particles


Aerosol composition1

Aerosol composition

  • NAD aerosols have a fixed composition

  • Composition of liquid sulfuric acid aerosols can vary


Tunable diode laser

Tunable diode laser


Tunable diode laser1

Tunable diode laser


Tunable diode laser2

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 laser3

Tunable diode laser


Aerosol flow cell ii3

Aerosol flow cell II


Sulfuric acid optical constants

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


Sulfuric acid optical constants1

Sulfuric acid optical constants


Sulfuric acid optical constants2

Sulfuric acid optical constants


Sulfuric acid optical constants3

Sulfuric acid optical constants


Sulfuric acid optical constants4

Sulfuric acid optical constants

  • 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


Aerosols and the environment2

Aerosols and the Environment

  • Ozone depletion

  • Global climate change


The atmosphere4

The atmosphere

thermosphere

upper atmosphere

mesopause

80

mesosphere

60

altitude (km)

middle atmosphere

stratopause

40

stratosphere

20

tropopause

troposphere

lower atmosphere


Global climate change

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

Energy balance

Sun

Eath

Earth

Climate Change 1994: Radiative Forcing of Climate Change and An Evaluation of the IS92

Emission Scenarios (Cambridge University Press, Cambridge, 1995).


Energy imbalance

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

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

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 sources1

Forcing sources

  • Aerosols

    • “Direct” forcing

      • Direct interaction with incoming or outgoing light

    • “Indirect” forcing

      • Affecting other components of the climate


Forcing contributions

Forcing contributions

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


Aerosol forcing uncertainties

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 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

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

Present laboratory work

  • Apply optical constant retrieval method to organic aerosols

  • Study hygroscopic properties of organic aerosols

  • Characterize multi-component organic aerosols


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


Organic aerosols terpenes1

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

Model organic aerosols

  • Determine optical constants for single-component organic aerosols

  • Start with easily obtained materials that closely represent actual organic aerosols


Model organic aerosols1

Model organic aerosols

o

Carvone


Aerosol flow cell iii

Aerosol flow cell III


Aerosol flow cell iii1

Aerosol flow cell III


Aerosol flow cell iii2

Aerosol flow cell III


First spectra

First spectra


Humidity dependence

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

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

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)


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