Availability of H2O2 for oxidation of SO2 in clouds in the Northeastern United States

A technique using inert tracers was used to determine SO4 formed from in situ oxidation of SO2 by H2O2 in 20 summer clouds at Whiteface Mountain, New York. The amount of in situ SO4 formed, along with H2O2 present in cloud water, was used to determine the total available H2O2. In 11 of the clouds, for which SO2 concentration was < 1 ppb, total available H2O2 could oxidize about 0.5 ppb SO2. In the other nine clouds, in which SO2 varied from 1 to 13 ppb with a weighted mean of 2.9 ppb, total available H2O2 could oxidize only 0.9 ppb SO2 or 31 % of the SO2 present. These results unequivocally show a deficiency of H2O2 in the atmosphere in the Northeastern U.S. that would result in a non-linear response between reductions in SO2 emissions and downwind acid deposition, for scenarios where the aqueous-phase oxidation by H2O2 is the dominant mechanism for forming sulfuric acid in this region.