The photochemistry of a remote marine stratiform cloud

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 cloud water 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 directly by oxidizing dissolved SO2 and HCHO or indirectly by producing the aqueous‐phase oxidant H2O2. The rate of SO2 conversion to sulfuric acid is sensitive to a variety of parameters including the accommodation or sticking coefficient for SO2, H2O2, HO2, and OH, the liquid water content, and the ambient levels of SO2, HNO3, and other acidic or basic gases. Because high levels of SO2 tend to deplete cloud water of H2O2, the possibility exists that the pH of precipitation in polluted regions will respond nonlinearly to reduced SO2 emissions.