The term cloud chemistry is considered here to comprise both cloud composition and reactions that take place in clouds. Clouds are a very special subset of the atmosphere because they present substantial amounts of condensed-phase water (liquid or solid) that can dissolve gases that would otherwise be present in the gas phase, and, as a consequence of condensed-phase reactions, permit reactions to occur that would not otherwise occur or would be much slower. In this sense clouds may be considered to serve as catalysts of atmospheric reactions.

The uptake and reaction of material in clouds, especially sulfur and nitrogen oxides and acids, has received particular attention in the context of gaining improved understanding of the processes responsible for acid deposition. Consequently, the examples developed here focus on these chemical systems. However, much of the resulting understanding of these phenomena is applicable more generally to other systems.

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