Chemical Reactions in Cloud and Fog Waters

The notion that chemical reactions in the aqueous phase may be important to atmospheric chemistry dates back at least 30 yr, when Junge and Ryan (1958) called attention to the great potential of cloud water for the oxidation of dissolved S02 by heavy-metal catalysis. At that time the process appeared to be the only viable oxidation mechanism for atmospheric S02. Later, when the concept of OH radical reactions gained ground, the gas-phase oxidation of S02 by OH was recognized to be equally important. The recent revival of interest in aqueous phase reactions is connected with efforts to achieve a better understanding of the origins of rainwater acidity. An oxidation of N02 to nitric acid also takes place in cloud water. Contrary to previous ideas, however, this process was recently shown to have little influence on atmospheric reactions of N02. 

As discussed in Section 8.4.1, the dissolution of S02 in water yields S02aq, HSOJ, and S03, in addition to undissociated H2S03, which is indistinguishable from S02aq. The sum of these species is designated S(IV). The oxidation of S(IV) occurs by ozone, H202, and by oxygen in the presence of a catalyst. Rate expressions and coefficients are summarized in Table 8-8, which is discussed below. 

The recognition that ozone is an important oxidant in clouds is due to Penkett (1972). The reaction with S(IV) may be written 

Ozone reacts with each of the S(IV) species, albeit with different velocities. The resulting rate expression is 

The dependence on proton concentration shown in Table 8-8 is obtained by virtue of the equilibria 3 and 4 of Table 8-3. The reaction rate is greatest at high pH because the most rapid reaction occurs between ozone and 

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