Aerosols

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Aerosols
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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.
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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)

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Aerosol Effects

• Climate
• Weather
• Visibility
• Health Effects

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Saharan Dust affects the West African Monsoon
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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 1500
Sea Salt 1000-10000 1300
Botanical Debris 26-80 50
Volcanoes 4-10000 30
Forest Fires 3-150 20
Gas conversion 100-260 180
Photochem 40-200 60
Total 2200-24000 3100

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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
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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.

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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

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Mount Pinatubo, 1991
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Urban Aerosol

• Dominated by anthropogenic sources
• Three Modes
• Nuclei Aitken
• Accumulation Large
• Coarse Giant

What is meant by the size of an aerosol? What
does a size distribution mean?
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ORIGIN OF THE ATMOSPHERIC AEROSOL
AerosolSize 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
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AEROSOL NUCLEATION
1 2 3
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molecules
DG
Surface tension effect
cluster size
Critical cluster size
Thermo driving force
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Atmospheric Aerosols
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Question?

• Considering the Urban Aerosol, where are most of
  the particles? Where is the most mass?
• How many 0.01 ?m particles are necessary to have
  the same mass as one 1?m particles?

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Urban Aerosol Size Distribution
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Nuclei Mode (lt0.1?m)

• 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
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Accumulation Mode (0.1 µm lt particle size lt 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 coarse-particle
  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
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Coarse-particle mode (particle size gt 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
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Dynamic Processes of Atmospheric Aerosol

• Formation
• Gas to particle conversion
• Photochemical processes
• Growth
• Coagulation, condensation, evaporation
• Removal
• Settling
• Deposition
• Rainout, washout

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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