A field study of the cloud chemistry and cloud microphysics at Great Dun Fell

Abstract

A comprehensive series of experiments designed to investigate the oxidation of SO 2 to sulphate by H 2O 2 and O 3 are being performed in the cap cloud at Great Dun Fell. In this paper results of the first set of experiments are presented. These took place during November 1985. The aim of these experiments was chiefly to monitor the H 2O 2 oxidation process by measuring its depletion with time within the cloud in the presence of SO 2. Increases in sulphate content of the cloud water were not observed during this experiment because H 2O 2 levels were too low and oxidation by O 3 was inhibited by the low cloud water pH. The concentrations of aqueous phase H 2O 2 measured were typically 100 nmol and O 3 gas phase concentrations 20 ppbv. It was found that, in the presence of SO 2, the concentration of H 2O 2 declined much more rapidly with height above cloud base than predicted by simple dilution by liquid water. Assuming this to indicate reaction with SO 2, a comparison was made with the predictions of the model of Hill, Choularton and Penkett (1986, Atmospheric Environment 20, 1763–1771). It was found that the rate of reaction was consistent with a value for the second order rate constant κ H 2 O 2 of 2 ± 1 × 10 5 s −1 where κ H 2 O 2 is defined by: d[Svl] dt = H kH 2O 2[H 2O 2][SO 2·H 2O] 0.1+[H + (Martin and Damschen, 1981, Atmospheric Environment 15, 1615–1621). This was determined with a cloud temperature of 8.5°C and a pH of 4.8. This is about three times larger than the laboratory determined rate constant found in Martin and Damschen (1981) when corrected to the same temperature and pH. On some occasions microphysical measurements in the cap cloud indicated that tropospheric air from above the cloud top was being entrained into the cloud. Increases in H 2O 2 concentrations with altitude within the cap cloud on these occasions showed that extra H 2O 2 was being simultaneously introduced to the cloud system. It is suggested that this entrainment process may play a very important role in SO 2 oxidation in clouds when the reaction is oxidant limited.