Aerosols
Atmospheric Aerosols
Bibliography
Seinfeld & Pandis, Atmospheric Chemistry and Physics, Chapt. 7-13
Finlayson-Pitts & Pitts, Chemistry of the Upper and Lower Atmosphere,
Chapt. 9.
Classic papers:
Prospero et al. Rev. Geophys. Space Phys., 1607, 1983; Charlson et al. Nature
1987; Charlson et al., Science, 1992.
Recent Papers:
Ramanathan et al., Science, 2001; Andreae and Crutzen, Science, 1997;
Dickerson et al., Science 1997; Jickells et al., Global Iron Connections Between
Desert Dust, Ocean Biogeochemistry and Climate, Science, 308 67-71, 2005.
Aerosols: General
Comments
 Any solid, liquid (or mixture) in the
atmosphere
 Sources
 Natural
 Anthropogenic (urban, construction,
agriculture)
 Primary (introduced directly into the
atmosphere)
 Secondary (formed in the attmosphere)
Aerosol Effects
 Climate
 Weather
 Visibility
 Health Effects
Clouds?
Saharan Dust
affects the West
African Monsoon
Natural Sources and Estimates of Global
Emissions of Atmospheric Aerosols
Source
Amount-range (Tg yr-1)
Amount -best
estimate (Tg yr-1)
Soil Dust
1000-3000
Sea Salt
1000-10000
Botanical Debris 26-80
1500
1300
50
Volcanoes
4-10000
30
Forest Fires
Gas conversion
Photochem
Total
3-150
100-260
40-200
2200-24000
20
180
60
3100
Anthropogenic Sources of Aerosols
Source
Amount Range
(Tg yr-1)
Best Estimate
Direct Emission
50-160
120
Gas to particle
260-460
330
Photochemistry
5-25
10
Total
320-640
460
Reference: W.C. Hinds, Aerosol Technology, 2nd
Edition, Wiley Interscience
Gas-to-particle conversion:
 Certain gas phase reactions result in
formation of low-vapor-pressure
reaction products.
 Because of their low vapor pressure,
they exist at high supersaturations
and can form particles.
Natural Background
Aerosol
 Stratospheric
 Major volcanic activity injects sulfur
dioxide (SO2) into the stratosphere
 Gas to particle conversion, SO2 into
sulfuric acid (H2SO4)
 Tropospheric
 Vegetation, deserts and ocean
 Primarily in the lowest few km
Mount Pinatubo, 1991
Urban Aerosol
 Dominated by anthropogenic
sources
 Three Modes
 Nuclei
 Accumulation
 Coarse
Aitken
Large
Giant
What is meant by the size of an aerosol?
What does a size distribution mean?
ORIGIN OF THE ATMOSPHERIC AEROSOL
Aerosol:Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop)
Soil dust
Sea salt
Environmental importance: health (respiration), visibility, radiative balance,
cloud formation, heterogeneous reactions, delivery of nutrients…
AEROSOL NUCLEATION
# molecules 1
2
3
4
DG
Surface
tension
effect
Thermo
driving
force
Critical
cluster size
cluster size
Atmospheric Aerosols
Question?
 Considering the Urban Aerosol,
where are most of the particles?
Where is the most mass?
 How many 0.01 mm particles are
necessary to have the same mass as
one 1mm particles?
Urban Aerosol Size Distribution
Nuclei Mode (<0.1mm)
 Consist of:
 Direct combustion particles emitted
 Particles formed by gas-to-particle conversion
 Usually found near sources of combustion
(e.g. highways!)
 Due to their high number concentration:
 Coagulate rapidly.
 End up in accumulation mode
 Relatively short lifetime
Aitken Particles
Accumulation Mode (0.1 μm <
particle size < 2.5 μm)
 Includes combustion particles, smog particles,
and coagulated nuclei-mode particles.
(Smog particles are formed in the atmosphere by
photochemical reactions)
 Particles in this mode are small but they
coagulate too slowly to reach the coarseparticle mode.
 they have a relatively long lifetime in the
atmosphere
 they account for most of the visibility effects of
atmospheric aerosols.
 The nuclei and accumulation modes together
constitute “fine” particles.
Large Particles
Coarse-particle mode
(particle size > 2.5 μm)
 Consist of
 Windblown dust, large salt particles from sea
spray,
 Mechanically generated anthropogenic
particles such as those from agriculture and
surface mining.
 Due to their large size
 Readily settle out or impact on surface,
 Lifetime in the atmosphere is only a few hours.
Giant Particles
Dynamic Processes of
Atmospheric Aerosol
 Formation
 Gas to particle conversion
 Photochemical processes
 Growth
 Coagulation, condensation, evaporation
 Removal
 Settling
 Deposition
 Rainout, washout
Global Effects of Aerosols
 Global Cooling
 Direct effect
 Indirect effect
 Ozone depletion
 Polar stratospheric clouds
(PSC)
 Surfaces of PSC act to
catalyze Cl compounds to
atomic Cl