Availability of H 2O 2 for oxidation of SO 2 in clouds in the Northeastern United States

Abstract

A technique using inert tracers was used to determine SO 4 formed from in situ oxidation of SO 2 by H 2O 2 in 20 summer clouds at Whiteface Mountain, New York. The amount of in situ SO 4 formed, along with H 2O 2 present in cloud water, was used to determine the total available H 2O 2. In 11 of the clouds, for which SO 2 concentration was < 1 ppb, total available H 2O 2 could oxidize about 0.5 ppb SO 2. In the other nine clouds, in which SO 2 varied from 1 to 13 ppb with a weighted mean of 2.9 ppb, total available H 2O 2 could oxidize only 0.9 ppb SO 2 or 31 % of the SO 2 present. These results unequivocally show a deficiency of H 2O 2 in the atmosphere in the Northeastern U.S. that would result in a non-linear response between reductions in SO 2 emissions and downwind acid deposition, for scenarios where the aqueous-phase oxidation by H 2O 2 is the dominant mechanism for forming sulfuric acid in this region.