Field observations of SIV in cloud

A novel technique for measuring dissolved sulphur dioxide/bisulphite/sulphite (SIV) continuously in cloud water has been used in a study of the oxidation of SIV in a hill cap cloud. In March/April 1995 measurements were made at Great Dun Fell, Cumbria, of the concentrations of gases and particles entering and leaving a hill cap cloud, and of the ions dissolved in the cloud. An approximate sulphur balance was obtained, but the precision was not sufficient to demonstrate sulphate formation under the observed conditions of very low sulphur dioxide (SO2) concentrations (<1 ppb). However, measurements of SO2 and hydrogen peroxide (H2O2) upwind of the cloud, and of SIV and H2O2 in cloud water, showed that: (i) in conditions when H2O2 concentrations were small (<0.1 ppb in the gas phase below cloud), SIV concentrations in the cloud were consistent with Henryʹs Law equilibrium (cloud water pH was around 6, with ammonium (NH4+) concentrations over 100 μM), suggesting negligible loss of SIV between sampling and analysis; (ii) in the presence of larger concentrations of H2O2 the measured SIV concentration was much smaller than predicted from solution pH and measured SO2 concentrations, but consistent with losses between sampling and analysis caused by reaction with formaldehyde (HCHO) and metal-catalysed oxidation by O2. Use of the method in future should aim to minimise the time between sampling and analysis of SIV.