Case studies of the oxidation of sulphur dioxide in a hill cap cloud using ground and aircraft based measurements

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Abstract
Two case studies are presented which investigate the aqueous phase oxidation of sulfur dioxide (SO2) within a cap cloud on the summit of Great Dun Fell (847 m above sea level) in England. Data gathered by aircraft showed that hydrogen peroxide (H2O2) concentrations in the free troposphere were much greater than near the ground and that entrainment of air containing H2O2, into the cap cloud could provide a significant supply of oxidant. These findings suggest that the entrainment of H2O2 into cloud systems may be important to the oxidation of SO2, particularly in the absence of large concentrations of ammonia gas, which may favor oxidation by ozone. It is also shown that the apparent reaction rate of H2O2 within the cloud is consistent with our previous estimates made and were substantially larger than would be predicted from laboratory studies in bulk solutions (Chandler et al., 1988b). Sulphate, (SO42−), production was observed when significant concentrations of SO2 were present. The measured concentrations of the principal species involved in one of the case studies were consistent with model predictions which take account of SO2 oxidation by H2O2 and O3 and an enhanced H2O2‐SO2 reaction rate.