Droplet

Figure 17. Schematics of the interactions between iron and sulfur cycles. Solid arrows indicate conversions, while dotted arrows indicate influences.

clouds may induce similar effects by influencing NOk-related chemistry.

3.1. Microphysical and chemical modifications

Cloud water is an effective medium for chemical reactions. It can absorb trace gases, and dissociate them into ionic species, which may then react to form new species. Some of the aqueous phase reactions dominate the production or destruction of certain important chemical species in the atmosphere. Aqueous phase oxidation of sulfur and nitrogen species adds to the nonvolatile solute mass in cloud drops, such that larger and more efficient CCNs can be produced if they are not removed through rainout. Lelieveld (1993, 1994) estimated that up to 8090% of the atmospheric S02 is converted into sulfate within clouds, while the rest of the conversion proceeds mainly via reaction with OH radicals in the gas phase. Most of the condensed water in clouds does not precipitate out, and the remaining droplets unavoidably evaporate and return the remnants to the aerosol population. So, aqueous phase chemical production may increase the chemical mass load of aerosols. However, the increase in particle size also leads to a greater deposition velocity. So clouds are effective in shortening the lifetimes of atmospheric chemicals not only directly by in-cloud chemical conversion and wet deposition but also indirectly by enhancing dry deposition.

Cloud processes may modify the properties of mineral dust. Wurzler et al. (1997) measured aerosol composition in the Mediterranean and found that mineral dust particles often get coated with sulfate and other soluble substances. It has been suggested that the formation of the sulfate coating is related to the scavenging of aerosol particles by cloud droplets and the subsequent impaction scavenging of mineral dust particles. Certainly, some of the sulfate may also be produced within the cloud drops via the aqueous phase chemistry discussed above. Wurzler et al. (1997) also found that the processed dust particles may become effective giant CCNs which, upon entering subsequent clouds, enhance the development of precipitation through the collision-coalescence process. Yan et al. (2002), on the other hand, suggested that the sulfate-coated dust particles become effective CCNs after passing through a convective cloud, and these particles increase the concentration of the activated drops and widen the drop size distribution while entering the next cloud. The widening of drop size distribution also accelerates precipitation formation.

The dissolution of gaseous S02 into cloud water and its oxidation into sulfate are positively dependent on the pH, which means that the conversion of S02 into sulfate (a process that decreases the pH) is generally self-limiting. However, the ammonia or even the ammonium in aerosols may elevate the pH and boost the sulfur chemistry. During the condensation growth, the dilution of cloud drops causes a departure from Henry's equilibrium between the air and the droplet. So, ammonia in the air is immediately drawn into the cloud drops. As soon as the ammonia concentration in the air is reduced, the disequilibrium is relayed to the interstitial aerosols, which do not swell as much as cloud drops do. As a result, ammonia will be drawn from the interstitial aerosols into the air and, eventually, into the cloud drops. Such a redistribution of ammonia is in fact the main reason why in Fig. 13 the pH values of the interstitial aerosols decreased during the cloud formation shown. This ammonia input helps to fix more sulfate and other acidic materials in the cloud drops; if the acidic materials (such as sulfate) are nonvolatile, they also help to fix ammonia in the particle when cloud drops evaporate. In this way, cloud processes affect the chemical distribution among aerosols.

Aqueous phase oxidation in cloud water is not the only mechanism that increases the mass of nonvolatile chemicals in aerosols (when the cloud evaporates). The absorption of ammonia, an alkaline chemical as discussed above, also helps to absorb nitric acid in the air and fix it in the droplet. Figure 18 shows the partition of two acids in the gas and liquid phases. Sul-furic acid has very high solubility; thus, it mostly stays in haze particles under typical atmospheric liquid water contents (LWCs) and pH values [see Fig. 18(a)]. But nitric acid has relatively weak solubility in aerosols, particularly when the aerosol particle contains sulfuric acid, which makes the aerosol very acidic. From Fig. 18(b)

Figure 18. Partition of sulfuric acid and nitric acid between the gas phase and the liquid phase as a function of atmospheric liquid water content and droplet pH. The vertical axis is the fraction of these acids that exists in the gas phase.

one can see that under typical clear air LWCs of below 10~5 gkg-1 and aerosol pH of around 2 (see Fig. 11), about 90% of the nitric acid exists in the gas phase (point A). When clouds are formed, both the LWCs and the droplet pH increase significantly, and essentially all gas phase nitric acid will enter cloud drops (point B). As pointed out by Kulmala et al. (1993), Xue and Feingold (2004) and Romakkaniemi et al. (2004), the intake of nitric acid that reserved in the gas phase may enhance the activation of aerosols and the growth of cloud drops. Furthermore, the ammonia intake mechanism mentioned above can fix such nitric acid in the particle phase and alter the composition of aerosols.

Other alkaline substances that may enhance the mass production of aerosols through cloud processes include sea salt and mineral dust, as mentioned in Subsec. 2.3.2. These aerosols tend to induce stronger modification by cloud chemistry. Hegg et al. (1992) pointed out that alkaline sea salt particles significantly impact the sulfate production and modify the size distribution, thus enhancing the light-scattering efficiency of aerosols. Yuen et al. (1994) then suggested a different behavior from continental

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