Factors Governing the pH, Availability of H^+, and Oxidation Capacity of Rain

Abstract

The acidity of rain is coupled to redox reactions in air and in atmospheric water. The pH, an intensive quantity, needs to be distinguished from the base neutralizing capacity. For acidic rain observed at most locations, H_2SO_4, HNO_3, NH_3, and CaCO_3 are dominant components. Their local availability or production rates govern net acidity. pH is thus almost entirely determined by these major "strong" components imposed on a CO_2 background, with some influence by SO_2(aq), smaller concentrations of HNO_2 and weak organic acids and minor bases, e.g., Fe_2O_3, yielding acid aquo metal ions. Total global emissions to tfie atmosphere of H_2SO_4 precursors outweigh those of HNO_3 by a factor of 2-3 on an equivalent basis. In specific settings HNO_3 may be comparable to H_2SO_4 in rain. Total atmospheric acidity appears to be a useful quantity for estimating potential acidity of rain at different locations. There are indications in photochemical models of HNO_3 and H_2SO_4 that feedback among S and N species may be important. Heterogeneous oxidations of SO_2 in cloud, fog, and rain play important roles in the acidification process. "Background" acidities of rain appear to be highly variable; pH values are expected to range from below 5 to above 6. Present-day SO_2 and NO_x fluxes account for a pH lowering of ~ 0.5 to 1.5, depending upon source location and transport-conversion rates.