S 10

10"6 10"5 10"4 10"3 10"2 10"1 1 10 Specific reaction volume (ml m"3)

FIGURE 8.12 Percentage of S02 converted to sulfate after a time interval At in small "haze" particles, fogs, and clouds as a function of the aqueous reaction volume; note that the time intervals for each one are different, reflecting how long they typically last in the atmosphere (adapted from Lamb et al., 1987).

In the high-pH (4-7) region, the reaction is first order in S(IV), but the dependence on the catalyst concentration is complex. Martin (1994) recommends the following rate expressions for pH around 4.0 and 5-6, respectively, where it is assumed that there is at least a trace of iron available for catalysis in the 5-6 pH range:

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