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)
Natural Sources and Estimates of Global Emissions of Atmospheric Aerosols SourceAmount-range (Tg yr -1 ) Amount -best estimate (Tg yr -1 ) Soil Dust1000-30001500 Sea Salt1000-100001300 Botanical Debris26-8050 Volcanoes4-1000030 Forest Fires3-15020 Gas conversion100-260180 Photochem40-20060 Total2200-240003100
Anthropogenic Sources of Aerosols SourceAmount Range (Tg yr -1 ) Best Estimate Direct Emission50-160120 Gas to particle260-460330 Photochemistry5-2510 Total320-640460 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
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: 0.001 m (molecular cluster) to 100 m (small raindrop) Environmental importance: health (respiration), visibility, radiative balance, cloud formation, heterogeneous reactions, delivery of nutrients…
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