Field evidence for the oxidation of SO₂ by H₂O₂ in cap clouds

Abstract

The reaction of SO₂, dissolved in cloud droplets, with atmospheric oxidants such as H₂O₂ and ozone is thought to be an important pathway for the production of atmospheric sulphate, and may be significant in determining the pattern of acidic deposition over a region. Direct observation of in-cloud oxidation of SO₂ by these species, however, has proved extremely difficult. This is especially true for H₂O₂ because the reaction is so rapid. We have used a new experimental design to investigate the dominant oxidation pathways of SO₂ in the conditions that occur in natural clouds. This involved modifying SO₂ levels in capping cloud on a peak in the northern Pennine hills of England by releasing SO₂ locally upwind and then measuring changes in cloud and air composition. Here we present initial results from an experiment performed on 12 December 1985. Under the conditions encountered it was found that sulphate production by both ozone and H₂O₂ was negligible. The destruction of H₂O₂ by reaction with SO₂, however, was clearly demonstrated. By comparing H₂O₂ in the presence and absence of released SO₂, the rate of destruction was estimated and found to be comparable with that expected from laboratory measurements of the reaction rate constant, and our knowledge of the cloud physics.