A field study of the oxidation of SO2 in a cap cloud at Great Dun Fell

Abstract

AbstractA series of field experiments has been performed in natural cloud on Great Dun Fell to investigate the multiphase oxidation of sulphur dioxide by hydrogen peroxide and ozone. This has been done by observing the decay of hydrogen peroxide and the generation of sulphate ions in the cloud water. It is demonstrated that the apparent field reaction rate between SO2 and hydrogen peroxide is considerably faster than would be expected from recent laboratory studies. It was found that hydrogen peroxide was generally the most important oxidant for sulphate in the cloud. Significant quantities of this oxidant can be entrained from the free troposphere into the cloud and this may be rate‐determining for the production of sulphate over longer timescales. When the pH of the cloud water was maintained above about 4.8 (probably due to gas phase ammonia) ozone was found to be the major oxidant. It is also clear that these processes will also be important in free clouds remote from the ground with oxidation by hydrogen peroxide being dominant away from ground‐based ammonia sources.