Production of Sulfate in Rain and Raindrops in Polluted Atmospheres

A physico-chemical model for the accumulation of sulfur species in raindrops is developed in which account is taken of the mass transfer of SO1, 0,, NH, and CO1 into ideal raindrops containing the catalyst Fe(II1). The sulfur accumulation was calculated for the droplets as a function of fall distance. The model predicts the formation of sulfate due to the oxidation of dissolved SO2 by 0, and catalytic ions in the presence of NH, and C02. The initial pH of a drop was taken as 5.56. The final value depended on drop size, fall distance, and ambient concentrations, but in all cases was between 4.2 and 6.6. Sulfate values also depended on the same conditions and ranged from 2 to 2000 ymol 1-l for individual drops. For a precipitation rate of 10 mm h’ and a fall distance of 2000 m, the raindrop pH varied from 4.5 to 6.5 and the average sulfate concentration from 3 to 87 pmol I’ depending on ambient pollutant concentrations. These results conform to the experimentally measured values of the rain pH (3-9) and sulfate concentration (20-150 ymol I-‘), and corresponding air SO* concentration (l-10 ppb) as reported in the literature.