Observations of H2O2 and S(IV) in Air, Cloudwater and Precipitation and Their Implications for the Reactive Scavenging of SO2

Daum, P.H., 1990. Observations of H2O2 and S(IV) in air, cloudwater and precipitation and their implications for the reactive scavenging of SO2. Atmos. Res., 25: 89-102. In-cloud oxidation of atmospheric SO2 by H2O2 to produce H2SO4 has been demonstrated by several recent field studies. This reaction appears rapid and quantitative with the amount of sulfate determined by the concentration of either SO2 or H2O2 whichever reagent is present in lesser amount. We report here measurements of H2O2 and S(IV) in air, precipitating clouds and in surface precipitation made in the several field studies conducted as part of the DOE PRECP program, and during a field study in southern Ontario in the winter of 1984. Clear air concentrations measured in the midwest during June 1987 were found to be quite variable from day to day, and even within days when different air masses were sampled. The SO2/H2O2 vertical profiles show that while SO2 is largely confined to the boundary layer, H2O2 is present over a much wider range of altitudes. The H2O2 in the boundary layer was seldom present at concentrations sufficient to completely oxidize the available SO2. This implies that mixing processes associated with clouds may be an important factor in controlling the extent to which the reaction of these two species occurs. The mutual exclusiveness of H2O2 and SO2 previously found in non-precipitating clouds was not observed in precipitating clouds; substantial quantities of these two species frequently coexisted. This is attributed to material dissolved in falling hydrometeors which has not had time to react with SO2. Analysis of the seasonal dependency of cloudwater and precipitation H2O2 concentrations shows a strong seasonal trend with concentrations much higher in summer than winter, consistent with a photochemical source for H2O2.