Photochemistry of the Atmospheric Aqueous Phase

Abstract

The coupled gas- and aqueous-phase photochemistry of a stratiform cloud in a remote region of the marine atmosphere is investigated with a time dependent box model. Both scavenging of ambient acidic aerosols and gases as well as aqueous-phase chemical reactions within droplets are found to be important sources of acidity to cloud water and can lead to pH levels in cloudwater in the remote marine atmosphere well below 5.6. The major sources of acidity via aqueous-phase chemical reactions are the generation of sulfuric acid from dissolved SO2 and the generation of formic acid from dissolved formaldehyde. In both cases aqueous-phase free radicals can play a significant role either by directly oxidizing dissolved SO2 and HCHO or indirectly by producing the aqueous-phase oxidant H2O2. While the transfer of HO2 and OH from the gas to the aqueous phase during the daylight hours can represent a significant source of free radicals to cloudwater, calculations indicate that another, potentially important source of aqueous-phase free radicals may arise from coronal discharges from droplets in electrified clouds. The rate of SO2 conversion to sulfuric acid is sensitive to a variety of parameters including the accommodation or sticking coefficient for SO2, H2O2, O2, HO2, and OH, the droplet growth rate and liquid water content, and the ambient levels of SO2, HNO3, and other acidic or basic gases. Because the SO2-to-sulfuric acid conversion rate is inhibited by low pH, the possibility exists that the pH of precipitation in polluted regions will respond non-linearly to reduced S emissions.