Kinetics and Mechanisms of Chemical Reactions in Clouds and Fogs

Abstract

Clouds, fogs, and haze aerosols are subjected to the same chemical processes because of their physical similarities. For example, cloud- and fogwater droplets are found in the size range of 2 to 50 μm, while deliquescent haze aerosol will be in the range of 0.01 to 1 μm. On the other hand, raindrops are approximately 100 times larger than cloud and fog water droplets (0.1 to 3 mm). However, a more important determinant of aqueous-phase chemistry within the droplets is the liquid water content (LWC); values of LWC range from 0.1 to 1.0 g m^(-3) in clouds, from 0.01 to 0.5 g m^(-3) in fogs, and from 10 to 100 μg m^(-3) in haze aerosols. The presence of condensation nuclei, which are composed of both soluble and insoluble materials, is essential for the formation of atmospheric water droplets. Accretion or evaporation of water to or from the condensation nuclei or droplet is forced by the difference between the ambient and local humidities, and is affected by the droplet surface tension and the chemical potential of the solutes in the liquid phase. During droplet growth, the temperature within the droplet differs from the ambient temperature due to the release of latent heat, which in turn depends upon the instantaneous growth rate.