Transcript Aerosols and the Environment

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
– 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
10-4
10-3
.01
.1
1
10
100
micron = 1 mm = 10-4 cm = 10-6 m
103
104
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
Sulfate particle
Aluminum particle
T. Reichhardt, Environ. Sci. Tech., 29(8), 360A, (1995).
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
Aerosols and the Environment
Ozone depletion
 Global climate change

The atmosphere
thermosphere
upper atmosphere
mesopause
altitude (km)
80
mesosphere
60
stratopause
middle atmosphere
40
stratosphere
20
tropopause
troposphere
lower atmosphere
Ozone

O
O

Pungent gas (named after the
Greek word ozein, “to smell”)
“Good” vs. “Bad”

O
– 90% of all ozone
– 10 ppmv peak concentration
– UV screening

O3
Stratosphere
Troposphere
– 10 ppbv peak concentration
– Disinfectant
– Respiratory stress
Ozone
O2 + h
O + O2 + M
O3 + h
O3 + O

O+O
O3 + M
O2 + 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.
CF2Cl2 + h
Cl + O3
ClO + O
O3 + O
Ozone measured over Payerne, Switzerland
CF2Cl + Cl
ClO + O2
Cl + O2
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 tropospheric
trace gases in the stratosphere
Polar ozone depletion theories

Reactive nitrogen species chemically
destroy ozone
Discounted due to low concentrations of
nitrogen species during depletion events
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
2ClO + M
Cl2O2 + h
ClOO + M
2Cl + 2O3
2O3 + h
Cl2O2 + M
ClOO + Cl
Cl + O2 + M
2ClO + 2O2
3O2
These reactions are thought to be responsible
for 70% of the observed ozone depletion
Homogeneous reactions
CFCs
h
ClONO2
ClO
h
NO2
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)
PSCs
Cl2(g) + HNO3(s)
ClONO2(s) + H2O(s)
PSCs
HOCl(g) + HNO3(s)
Chlorine is released into the gas phase
Nitrogen is chemically removed
Nitrogen is physically removed
Heterogeneous reactions
h
HCl
HNO3
CFCs
ClONO2
Polar Stratospheric Clouds
PSCs
H2O
Cl2
h
HOCl
h
Cl
Sedimentation
Cl
Polar stratospheric chemistry
h
HCl
CFCs
ClONO2
HNO3
h
ClONO2
h
NO2
H2O
PSCs
HOCl
ClO
Cl2
h
ClO + ClO
h
Cl2O2
h
ClO
Cl
Sedimentation
O3
O2
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
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



k is the imaginary component of the refractive index



determines how fast light moves through material
n=c/v
determines how light is absorbed by material
k = al / 4p
Optical constants
PSC spectra
Ice
NAD
NAT
O.B.Toon and M.A. Tolbert, Nature, 375, 218, (1995).
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H2O
 Nitric acid trihydrate (NAT): HNO33H2O
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

Retrieve optical constants from infrared
spectra of model PSC aerosols
 Frequency
 Temperature

Optical constants for NAD
Aerosol flow cell II
Flow
Injection
Port
2
1
3
4
MCT
6
Flow
Exhaust
5
Cooling
Coils
FTIR
Spectrometer
Aerosol flow cell II
Aerosol flow cell II
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Scattering spectra
1.0
Nitric Acid Dihydrate at 180 K
0.8
Extinction
0.6
0.4
0.2
0.0
700
1200
1700
2200
2700
3200
-1
Wavenumber (cm )
3700
4200
4700
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Non-scattering spectrum
0.20
Extinction
0.15
Nitric Acid Dihydrate at 180 K
0.10
0.05
0.00
700
1200
1700
2200
2700
3200
-1
Wavenumber (cm )
3700
4200
4700
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Select a scattering
spectrum and guess
the particle size
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Select a scattering
spectrum and guess
the particle size
Use Kramers-Kronig
relationship to
calculate n()
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Select a scattering
spectrum and guess
the particle size
Use Kramers-Kronig
relationship to
calculate n()
Use Mie scattering
theory to calculate
scattering spectrum
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Select a scattering
spectrum and guess
the particle size
Use Kramers-Kronig
relationship to
calculate n()
Compare calculated and
experimental spectra
Use Mie scattering
theory to calculate
scattering spectrum
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Select a scattering
spectrum and guess
the particle size
Use Kramers-Kronig
relationship to
calculate n()
Correct k() if necessary
Compare calculated and
experimental spectra
Use Mie scattering
theory to calculate
scattering spectrum
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Select a scattering
spectrum and guess
the particle size
Use Kramers-Kronig
relationship to
calculate n()
Vary k() scaling factor, K
Correct k() if necessary
Compare calculated and
experimental spectra
Use Mie scattering
theory to calculate
scattering spectrum
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Select a scattering
spectrum and guess
the particle size
Vary particle size
Use Kramers-Kronig
relationship to
calculate n()
Vary k() scaling factor, K
Correct k() if necessary
Compare calculated and
experimental spectra
Use Mie scattering
theory to calculate
scattering spectrum
Retrieving optical constants
Collect many scattering
spectra representing
different particle sizes
Collect a non-scattering
spectrum to estimate k
k() = Ka()
Select a scattering
spectrum and guess
the particle size
Vary particle size
Use Kramers-Kronig
relationship to
calculate n()
Vary k() scaling factor, K
Correct k() if necessary
Compare calculated and
experimental spectra
Use Mie scattering
theory to calculate
scattering spectrum
Final fit results
1.0
Nitric Acid Dihydrate at 180 K
0.8
Extinction
0.6
0.4
rmed = 0.33 mm
0.2
0.0
700
1200
1700
2200
2700
3200
-1
Wavenumber (cm )
3700
4200
4700
Final optical constants
2.6
2.4
2.2
Nitric Acid Dihydrate at 180 K
2.0
Refractive index
1.8
1.6
n
1.4
1.2
1.0
0.8
0.6
0.4
0.2
k
0.0
700
1200
1700
2200
2700
3200
-1
Wavenumber (cm )
3700
4200
4700
NAD optical constants
Overall good agreement with thin-film
results
 Some discrepancies do exist
 Comparison of several aerosol and thinfilm 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 are
better suited for analyzing atmospheric particles
Aerosol composition
NAD aerosols have a fixed composition
 Composition of liquid sulfuric acid
aerosols can vary

Tunable diode laser
Window
to Cell
TDL
Focusing
Objective
Alignment
Pinhole
Beamsplitter
HeNe
Ocular
Bypass
Optics
Slit
Monochromator
To Vacuum
Vacuum 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
1.0
0.9
Transmission (I/Io)
0.8
0.7
0.6
0.5
0.4
Cell Pressure (Torr)
9.7
49.3
99.5
200.0
300.0
0.3
0.2
1751.39
1751.40
1751.41
1751.42
1751.43
-1
Wavenumber (cm )
1751.44
1751.45
Aerosol flow cell II
Flow
Injection
Port
2
1
3
4
6
5
MCT
Detectors
Flow
Exhaust
TDL and
Optics Box
Cooling
Coils
FTIR
Spectrometer
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 constants
2.6
2.4
2.2
75 wt% Sulfuric Acid/Water
2.0
Refractive Index
1.8
1.6
n
1.4
1.2
1.0
0.8
0.6
0.4
k
0.2
0.0
800
1300
1800
2300
2800
3300
-1
Wavenumber (cm )
3800
4300
Sulfuric acid optical constants
2.5
38 wt% Sulfuric Acid/Water
Refractive index
2.0
n
1.5
1.0
0.5
k
0.0
800
1300
1800
2300
2800
3300
-1
Wavenumber (cm )
3800
4300
Sulfuric acid optical constants
300
Temperature (K)
280
260
240
220
200
30
40
50
60
Weight % H2SO4
70
80
90
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 Environment
Ozone depletion
 Global climate change

The atmosphere
thermosphere
upper atmosphere
mesopause
altitude (km)
80
mesosphere
60
stratopause
middle atmosphere
40
stratosphere
20
tropopause
troposphere
lower atmosphere
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
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
Anything which causes a change in the
energy balance is known as a forcing
 Climate responds to forcing by reestablishing energy balance

A forcing example
Doubling CO2 concentration
 Forcing of 4 Wm-2
 Surface must warm up 1 K to 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

forcing of -4.5 Wm-2
 A temporary, calculated and observed
cooling of 0.5 K
 Peak
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 preexisting 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 singlecomponent organic aerosols
 Start with easily obtained materials that
closely represent actual organic
aerosols

Model organic aerosols
2.0
o
Absorbance
1.5
1.0
0.5
0.0
1000
2000
3000
4000
-1
Wavenumber (cm )
5000
Carvone
Aerosol flow cell III
Aerosol flow cell III
Aerosol flow cell III
First spectra
0.40
0.35
0.30
Absorbance
0.25
0.20
0.15
0.10
0.05
0.00
1000
2000
3000
4000
-1
Wavenumber (cm )
5000
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)