Clouds present substantial concentrations of liquid-phase water, which can potentially serve as a medium for dissolution and reaction of atmospheric gases. The important precursors of acid deposition, S02, and nitrogen oxides NO and N02 are only sparingly soluble in clouds without further oxidation to sulfuric and nitric acids. In the case of S02, aqueous-phase reaction with hydrogen peroxide and to lesser extent ozone are identified as important processes leading to this oxidation, and methods have been described by which to evaluate the rates of these reactions. The limited solubility of the nitrogen oxides precludes significant aqueous-phase reaction of these species, but gas-phase reactions in clouds can be important especially at night.
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