Aerosols

Atmospheric Aerosols Bibliography Seinfeld & Pandis, Atmospheric Chemistry and Physics, Chapt 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, 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, , 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 SourceAmount-range (Tg yr -1 ) Amount -best estimate (Tg yr -1 ) Soil Dust Sea Salt Botanical Debris Volcanoes Forest Fires Gas conversion Photochem Total

Anthropogenic Sources of Aerosols SourceAmount Range (Tg yr -1 ) Best Estimate Direct Emission Gas to particle Photochemistry Total 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 (SO 2 ) into the stratosphere  Gas to particle conversion, SO 2 into sulfuric acid (H 2 SO 4 )  Tropospheric  Vegetation, deserts and ocean  Primarily in the lowest few km

Mount Pinatubo, 1991

Urban Aerosol  Dominated by anthropogenic sources  Three Modes  NucleiAitken  AccumulationLarge  CoarseGiant What is meant by the size of an aerosol? What does a size distribution mean?

ORIGIN OF THE ATMOSPHERIC AEROSOL Soil dust Sea salt Aerosol:Size range:  m (molecular cluster) to 100  m (small raindrop) Environmental importance: health (respiration), visibility, radiative balance, cloud formation, heterogeneous reactions, delivery of nutrients…

AEROSOL NUCLEATION # molecules GG cluster sizeCritical cluster size Surface tension effect Thermo driving force

Atmospheric Aerosols

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?

Urban Aerosol Size Distribution

Nuclei Mode (<0.1  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

Accumulation Mode (0.1 μ m < particle size < 2.5 μ m)  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 μ m)  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