Field observations of S-IV in cloud

A novel technique for measuring dissolved sulphur dioxide/bisulphite/sulphi te (S-IV) continuously in cloud water has been used in a study of the oxida tion of S-IV in a hill cap cloud. In March/April 1995 measurements were mad e 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 clo ud. 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, measur ements of SO2 and hydrogen peroxide (H2O2) upwind of the cloud, and of S-IV and H2O2 in cloud water, showed that: (i) in conditions when H2O2 concentr ations were small (< 0.1 ppb in the gas phase below cloud), S-IV concentrat ions in the cloud were consistent with Henry's Law equilibrium (cloud water pH was around 6, with ammonium (NH4+) concentrations over 100 mu M), sugge sting negligible loss of S-IV between sampling and analysis; (ii) in the pr esence of larger concentrations of H2O2 the measured S-IV concentration was much smaller than predicted from solution pH and measured SO, concentratio ns, but consistent with losses between sampling and analysis caused by reac tion with formaldehyde (HCHO) and metal-catalysed oxidation by O-2. Use of the method in future should aim to minimise the time between sampling and a nalysis of S-IV. (C) 1999 Elsevier Science B.V. All rights reserved.