Abstract
A comprehensive series of experiments designed to investigate the oxidation of SO2 to sulphate by H2O2 and O3 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 H2O2 oxidation process by measuring its depletion with time within the cloud in the presence of SO2. Increases in sulphate content of the cloud water were not observed during this experiment because H2O2 levels were too low and oxidation by O3 was inhibited by the low cloud water pH. The concentrations of aqueous phase H2O2 measured were typically l00nmol and O3 gas phase concentrations 20 ppbv.
It was found that, in the presence of SO2, the concentration of hydrogen peroxide declined much more rapidly with height above cloud base than predicted by simple dilution by liquid water. Assuming this to indicate reaction with SO2, a comparison was made with the predictions of the model of Hill, Choularton and Penkett (1986). It was found that the rate of reaction was consistent with a value for the second order rate constant \(\rm K_{H_{2}O_{2}}\) of 2±1 × 105 s−1 is \(\rm K_{H_{2}O_{2}}\) defined by:
This was determined with a cloud temperature of 8.5°C and a pH of 4,8. This is about 3x 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 H2O2 concentrations with altitude within the cap cloud on these occasions showed that extra hydrogen peroxide was being simultaneously introduced to the cloud system. It is suggested that this entrainment process may play a very important role in SO2 oxidation in clouds when the reaction is oxidant limited.
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© 1988 Kluwer Academic publishers
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Chandler, A.S. et al. (1988). Cloud Chemistry Research at Great Dun Fell. In: Unsworth, M.H., Fowler, D. (eds) Acid Deposition at High Elevation Sites. NATO ASI Series, vol 252. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-3079-7_10
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DOI: https://doi.org/10.1007/978-94-009-3079-7_10
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