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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

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 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)
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 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
retrieving optical constants1
Retrieving optical constants

Collect many scattering

spectra representing

different particle sizes

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)

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

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 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
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 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 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
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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