Factors Governing the pH, Availability of H+, and Oxidation Capacity of Rain
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, H2SO4, HNO3, NH3, and CaCO3 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 CO2 background, with some influence by SO2(aq), smaller concentrations of HNO2 and weak organic acids and minor bases, e.g., Fe2O3, yielding acid aquo metal ions. Total global emissions to the atmosphere of H2SO4 precursors outweigh those of HNO3 by a factor of 2–3 on an equivalent basis. In specific settings HNO3 may be comparable to H2SO4 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 HNO3 and H2SO4 that feedback among S and N species may be important. Heterogeneous oxidations of SO2 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 SO2 and NOx fluxes account for a pH lowering of ∼ 0.5 to 1.5, depending upon source location and transport-conversion rates.
KeywordsOxidation Capacity Liquid Water Content Acidity Constant Peroxynitric Acid Photochemical Mechanism
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