Aerosols Trude Storelvmo Aerosols 1 / 21
Aerosols: Definition Definition of an aerosol: disperse system with air as carrier gas and a solid or liquid or a mixture of both as disperse phases. Aerosol particles (AP) have diameters in the range from 10 3 µm to several hundred µm. They are larger than atmospheric small ions: diameter (µm) mass (g) concentr. (cm 3 ) N 2 0.00038 4.6 10 23 10 19 AP 0.01-10 10 18-10 9 < 10 8 nucleation (Aitken) mode: 10 3 µm - 10 1 µm in diameter accumulation mode: 10 1 µm - 1 µm in diameter coarse mode AP: > 1µm in diameter Trude Storelvmo Aerosols 2 / 21
Principle aerosol processes Atmospheric aerosol size distributions are usually presented as a sum of n log-normal distributions because D p typically varies over several orders of magnitude Trude Storelvmo Aerosols 3 / 21
Aerosol processes in the different modes For AP > 1 µm gravitational settling is important as a removal mechanism (1 µm AP 0.003 cm/s ; 10 µm AP 0.3 cm/s) Anthropogenic AP are mainly in submicron range; natural aerosols are mainly in supermicron range Nucleation mode AP originate primarily from combustion processes, of which the most important sources are probably related to human activities (but forest fires and volcanoes also contribute) Coagulation of nucleation mode AP is a major source for accumulation mode AP, but does not remove AP from the atmosphere. The coarse mode peak is attributable to mechanical processes, such as wind erosion which produces dust. The mobility of AP decreases rapidly as their size increases confining coagulation due to Brownian motion essentially to AP < 0.2 µm Trude Storelvmo Aerosols 4 / 21
Aerosol size distributions (Kaufman et al., Nature, 2002) Trude Storelvmo Aerosols 5 / 21
Different aerosol types (Source: IGAC, 2003) Trude Storelvmo Aerosols 6 / 21
Formation of aerosols Gas-to-particle conversion: nucleation of aerosols from supersaturated gases Bulk-to-particle conversion: wind blown dust (arid regions), emissions of pollens and spores by plants Liquid-to-particle conversion: Sea salt aerosols originate from drops ejected into the air when air bubbles in breaking waves burst at the ocean surface. Trude Storelvmo Aerosols 7 / 21
Formation of aerosols in Atlanta Trude Storelv Aerosols 8 / 21
Global Source Strength, Lifetime and Burden Aerosol Type Flux Lifetime Burden (Tg/yr) (d) (mg/m 2 ) Natu- Pri- Dust (desert) 900-1500 4 19-33 ral mary Sea salt 2300 1 3 Biological debris 50 4 1 Sec. Sulfates from biogenic gases 70 5 2 Sulfates from volcanic SO 2 20 10 1 Sulfates from Pinatubo (40) (400) (80) Biogenic organics 20 5 0.6 Total 3400-4000 27-41 Anth- Pri. Dust (soil+desert+industry) 40-640 4 1-14 ropo- Black carbon 14 7 0.6 genic Organic carbon in smoke 54 6 1.8 Sec. Sulfates from SO 2 140 5 3.8 Organics from hydrocarbons 20 7 0.8 Total 270-870 8-21 Sum 3670-4870 35-62 (Source: Ramanathan et al., Science, 2001) Trude Storelvmo Aerosols 9 / 21
Global aerosol sources [Stier et al., ACP, 2005] Trude Storelvmo Aerosols 10 / 21
Removal of aerosols 80%-90% of the aerosol mass is removed from the atmosphere by precipitation particles (wet scavenging) Prior to that aerosols serve as cloud condensation nuclei (CCN). As these particles grow, aerosols tend to be forced onto their surface by diffusion fields associated with the flux of water vapor to the growing cloud droplet (diffusiophoretic force) Precipitation particles collect aerosols by direct impaction, the better the larger the aerosols (best for aerosols > 2 µm). Aerosols are also removed by gravitational settling and subsequent impaction onto obstacles on Earth s surface (dry deposition) which accounts for 10-20% of the aerosols mass removed from the atmosphere. Trude Storelvmo Aerosols 11 / 21
Wet deposition (Fig 20.10 Seinfeld&Pandis, 1998) Trude Storelvmo (Yale G &G) Aerosols Oct 15, 2015 12 / 21
Vertically integrated aerosol burden [Stier, ACP, 2005] Trude Storelvmo Aerosols 13 / 21
Aerosols AP sizes AP types Sources Sinks Amount and lifetime Observed total aerosol optical depth Trude Storelvmo Aerosols Aerosol radiative effects [IPCC, Ch.2, 2007] 14 / 21
Importance of aerosol particles (AP) Essential for cloud formation: Aerosol particles act as centers for cloud droplets and ice particles Effects on pollution: Photochemical smog (ozone) Degradation of visibility Winter smog (solid aerosols provide surface upon which trace gases can be absorbed and then react, e.g. London smog) Effects on radiative transfer and climate: Direct effect: Aerosols scatter and absorb radiation Indirect cloud albedo effect: More aerosols lead to more and smaller cloud droplets per given liquid water path more reflection of solar radiation (Twomey, 1959) Indirect cloud lifetime effect: The more and smaller cloud droplets do not collide efficiently less drizzle longer cloud lifetime more reflection of solar radiation (Albrecht, 1989) Trude Storelvmo Aerosols 15 / 21
London smog Trude Storelvmo Aerosols 16 / 21
Aerosol radiative effects Trude Storelvmo Aerosols 17 / 21
Shiptracks off the coast of Washington Trude Storelvmo Aerosols 18 / 21
Evidence of the indirect cloud albedo effect Figure: Durkee et al., JAS, 2000 Trude Storelvmo Aerosols 19 / 21
Evidence of the indirect cloud lifetime effect Figure: Peng et al., JGR, 2002 Trude Storelvmo (Yale G &G) Aerosols Oct 15, 2015 20 / 21
Figure: Direct (upper panel) and indirect cloud albedo effect (lower panel) (W m 2 ) [Boucher and Pham, GRL 2002] Trude Storelvmo Aerosols 21 / 21