Contribution of scavenged sulfur dioxide to the sulfate content of rain water

The rate of formation of sulfate ion in raindrop size distilled water drops was measured for partial pressures of SO2 ranging from 10−6 to 4 × 10−3 atm. A model was developed based on the results and on the assumption that the rate of formation of sulfate is determined by the rate of the first dissociation of sulfurous acid and by the rate of oxidation of sulfite to sulfate. The results of the model agree with published data for the rate of solution of SO2 at low partial pressures. The model was applied to rainout and washout of SO2 by cloud and raindrops. The contribution of the sulfate formed by gaseous absorption to the sulfate content of rain water was calculated. This contribution is important in polluted areas, although it is not predominant. It is also shown that fog droplets, after absorbing SO2 in relatively polluted areas, produce particles of cloud nucleus size by evaporation.