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

Abstract

Two case studies are presented which investigate the aqueous phase oxidation of sulfur dioxide (SO 2 ) 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 (H 2 O 2 ) concentrations in the free troposphere were much greater than near the ground and that entrainment of air containing H 2 O 2 , into the cap cloud could provide a significant supply of oxidant. These findings suggest that the entrainment of H 2 O 2 into cloud systems may be important to the oxidation of SO 2 , 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 H 2 O 2 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, (SO 4 2− ), production was observed when significant concentrations of SO 2 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 SO 2 oxidation by H 2 O 2 and O 3 and an enhanced H 2 O 2 ‐SO 2 reaction rate.