The photochemistry of a remote marine stratiform cloud

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 p H 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 SO 2 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 SO 2 and HCHO or indirectly by producing the aqueous‐phase oxidant H 2 O 2 . The rate of SO 2 conversion to sulfuric acid is sensitive to a variety of parameters including the accommodation or sticking coefficient for SO 2 , H 2 O 2 , HO 2 , and OH, the liquid water content, and the ambient levels of SO 2 , HNO 3 , and other acidic or basic gases. Because high levels of SO 2 tend to deplete cloud water of H 2 O 2 , the possibility exists that the p H of precipitation in polluted regions will respond nonlinearly to reduced SO 2 emissions.