aThe process of aerosols turning into cloud drops was often termed "cloud nucleation." However, hygroscopic aerosols usually become wet (a process called "deliquescence"; see Subsec. 2.1.3 for details) long before reaching the cloud state. So, the formation of cloud drops from hygroscopic aerosols usually does not involve new-phase formation. Therefore, the proper term is "activation," instead of "nucleation."

Figure 2. Model-calculated number concentration of cloud drops activated from a CCN distribution of Nccn = C ASk, expressed as a function of the coefficients C and k. The updraft speed used for this calculation is 1 m s . (From Chen and Liu, 2004.)

such dependence (e.g. Hegg et al., 1991; Leaitch et al., 1992). In the previous section we quoted from Rosenfeld (2006) that sizes may be a more important factor than chemical composition for the activation capability of aerosols, but in terms of Twomey's indirect effects, the number concentration is certainly an even more critical attribute of aerosols.

More aerosols means more cloud drops, and more cloud drops tends to enhance the reflectance of clouds and therefore cool the atmosphere by reflecting more sunlight. A vivid illustration of such an effect is the co-called ship tracks, where the marine boundary layer clouds that are contaminated with particles of ship exhaust have brighter reflectivity (Coakley et al., 1987). Other examples are the reduced cloud particle size and even suppressed precipitation originating from major urban areas or industrial facilities (Rosenfield, 2000; Jirak and Cotton, 2006) and from downwind of biomass burning (Warner and Twomey, 1967; Rosenfeld, 1999). Twomey and others demonstrated theoretically that the cooling effect has a magnitude comparable, but opposite in sign, to that of greenhouse warming due to anthropogenic trace gases (Twomey et al., 1984; Charlson et al., 1992; Boucher and Lohmann, 1995). Using results from a mesoscale meteorological model and a regional air pollution model, Tsai (2001) diagnosed the indirect effect of aerosols at the Pacific Rim and found that anthropogenic aerosols significantly affect near-continent oceans that are located downwind of the major emission source under northeasterly monsoon winds. The cloud albedo averaged over the whole domain change (including areas without clouds) from 0.08 for natural conditions to 0.13 for polluted conditions. Apparently, anthropogenic aerosols of East Asia may significantly increase the CDNC, cloud optical thickness, and cloud albedo over the western Pacific Ocean and the South China Sea during the winter monsoon.

However, the relationship between the numbers of aerosols and cloud drops might not be linear or monotonic, but rather "jumpy," if a more complete microphysical cycle was considered. Baker and Charlson (1990) and

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