A field study of the oxidation of SO₂ in cloud

Abstract

This paper describes the execution and analysis of one of a series of field experiments performed at Great Dun Fell in the northern Pennines of England designed to study the oxidation of SO 2 in cloud. The experiments are designed to isolate the effect on cloud chemistry by artificially releasing SO 2 gas into the air entering cloud base. Observations made in cloud on April 30, 1986, clearly and unequivocally demonstrated the production of sulphate ions in cloudwater. A simultaneous decrease in H 2 O 2 concentration demonstrated that about 40% of this arises from rapid reaction between H 2 O 2 and SO 2 . The results of a comprehensive model of the cloud physical and chemical development at the site show good agreement with the observations, and indicate that the remaining fraction of sulphate produced resulted from the reaction of O 3 with SO 2 in cloudwater, promoted by the relatively high pH found in the cloud. Extrapolation of the results to other input gaseous concentrations by means of the model demonstrates the importance of gaseous ammonia in controlling cloudwater pH and hence the oxidation of SO 2 by O 3 , in that it is found that over a wide range of SO 2 concentrations, the total sulphate produced by the time air reaches a given point in the cloud is virtually independent of the initial SO 2 concentration, and roughly proportional to the ambient ammonia concentration.