Kinetic studies of raindrop chemistry: 1. Inorganic and organic processes

Abstract

The chemistry of raindrops, thought not long ago to be adequately represented by a few simple ions and their equilibrium constants, is now recognized to include the complex reactions of ions, radicals, and molecules of both acidic and basic moieties. To describe this system, we have formulated a model which includes both inorganic and organic processes (but not metal catalysis) totaling 94 reactions in 53 species. Three sets of computations were performed for raindrops of different diameters. In the first two, the initial chemistry of the raindrops was set by the cloud chemistry measurements of APEX (Atmospheric Precipitation Experiment) in the northeastern United States. In the simulations the drops were released into tropospheric air characteristic of either the suburban or urban northeast. In the third set of computations, designed to simulate urban centers on the U.S. west coast, raindrops with composition typical of those over the oceans were dropped in urban tropospheric air. Among the results are the following: (1) Photochemical processes, including the photolysis of H2O2 to produce OH˙ and the photolysis of NO3− to produce ozone, are significant in raindrops chemistry. (2) Organic chemical reactions of interest and importance occur in raindrops, particularly those of small size (and thus long lifetime), with the oxidation of aldehydes to organic acids being perhaps the most important. (3) Significant differences in some species concentrations occur at different altitudes during the raindrop's fall and for drops of different diameters. These differences are related to the scavenging efficiencies and lifetimes of the drops. (4) Given constant initial concentrations of trace species in raindrops, the calculations show that ion concentrations in rainwater increase as the rain rate decreases and that in a steady rainfall the concentrations of scavenged ions decrease as the concentrations of the gaseous precursors decrease. Neither effect is large, however. (5) The model results are consistent with experimental data, suggesting that below‐cloud processes do not significantly affect the concentration of strong inorganic acids in rain at ground level unless acid concentrations are low in droplets at cloud base.