Early studies of atmospheric chemistry emphasized trace gases and homogeneous gas-phase reactions. However, in the latter part of the twentieth century, increasing attention was given to atmospheric aerosols (see Chapter 6), chemical reactions on aerosol surfaces (see Chapter 10), and the role of clouds in atmospheric chemistry. In this chapter we consider how clouds remove particles and gases from the air, some of the chemical reactions that can occur within cloud droplets, and how these processes modify the chemical composition of cloud water and precipitation, as well as some other important properties of clouds. Finally, we will discuss the effects of cloud processing on modifying atmospheric aerosols.

Overview

We will organize our discussion of cloud and precipitation chemistry around the processes illustrated schematically in Figure 7.1.

Clouds form when air becomes slightly supersaturated with respect to liquid water (or in some cases with respect to ice). The most common means by which this is achieved in the atmosphere is through the ascent of air parcels, which results in the expansion and cooling of the air below its dew point. When the air becomes slightly supersaturated with respect to water (by a few tenths of 1%), water vapor begins to condense onto some of the articles in the air to form a cloud of small water droplets. This process, by which a portion of the atmospheric aerosol (called cloud condensation nuclei, or CCN) nucleate the formation of cloud droplets, and are thereby incorporated into cloud water as insoluble and soluble components, is referred to as nucleation scavenging.