AerosolsAerosols

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

Aerosols: General Comments

Any solid, liquid (or mixture) in the atmosphere

SourcesNaturalAnthropogenic (urban, construction,

agriculture)Primary (introduced directly into the

atmosphere)Secondary (formed in the attmosphere)

Any solid, liquid (or mixture) in the atmosphere

SourcesNaturalAnthropogenic (urban, construction,

agriculture)Primary (introduced directly into the

atmosphere)Secondary (formed in the attmosphere)

Aerosol EffectsAerosol Effects

Climate Weather Visibility Health Effects

Climate Weather Visibility Health Effects

Clouds?

Saharan Dust affects the West African Monsoon

Natural Sources and Estimates of Global Emissions of Atmospheric

Aerosols

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

Anthropogenic Sources of AerosolsAnthropogenic 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: 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.

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

Natural Background Aerosol

StratosphericMajor volcanic activity injects sulfur

dioxide (SO2) into the stratosphere

Gas to particle conversion, SO2 into sulfuric acid (H2SO4)

TroposphericVegetation, deserts and oceanPrimarily in the lowest few km

StratosphericMajor volcanic activity injects sulfur

dioxide (SO2) into the stratosphere

Gas to particle conversion, SO2 into sulfuric acid (H2SO4)

TroposphericVegetation, deserts and oceanPrimarily in the lowest few km

Mount Pinatubo, 1991Mount Pinatubo, 1991

Urban AerosolUrban Aerosol

Dominated by anthropogenic sources

Three ModesNuclei AitkenAccumulation LargeCoarse Giant

Dominated by anthropogenic sources

Three ModesNuclei AitkenAccumulation LargeCoarse Giant

What is meant by the size of an aerosol? What does a size distribution mean?

ORIGIN OF THE ATMOSPHERIC AEROSOL

ORIGIN OF THE ATMOSPHERIC AEROSOL

Soil dustSea salt

Aerosol:Size range: 0.001 m (molecular cluster) to 100 m (small raindrop)

Environmental importance: health (respiration), visibility, radiative balance,cloud formation, heterogeneous reactions, delivery of nutrients…

AEROSOL NUCLEATIONAEROSOL NUCLEATION

# molecules 1 2 3 4

G

cluster sizeCritical cluster size

Surface tension effect

Thermo driving force

Atmospheric AerosolsAtmospheric AerosolsAtmospheric AerosolsAtmospheric Aerosols

Question?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 1m particles?

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 1m particles?

Urban Aerosol Size Distribution

Urban Aerosol Size Distribution

Nuclei Mode (<0.1m)Nuclei Mode (<0.1m)

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

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)

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 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.

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

Coarse-particle mode (particle size > 2.5 μm)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.

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

Dynamic Processes of Atmospheric Aerosol

Formation Gas to particle conversion Photochemical processes

Growth Coagulation, condensation, evaporation

Removal Settling Deposition Rainout, washout

Formation Gas to particle conversion Photochemical processes

Growth Coagulation, condensation, evaporation

Removal Settling Deposition Rainout, washout

Global Effects of AerosolsGlobal Effects of Aerosols Global Cooling

Direct effectIndirect effect

Ozone depletionPolar stratospheric clouds

(PSC)Surfaces of PSC act to

catalyze Cl compounds to atomic Cl

Global CoolingDirect effectIndirect effect

Ozone depletionPolar stratospheric clouds

(PSC)Surfaces of PSC act to

catalyze Cl compounds to atomic Cl