Oxidation of atmospheric sulphur dioxide can occur by homogeneous photochemically initiated gas-phase reactions as well as by heterogeneous reactions in cloud and fog droplets. Gas phase oxidation can result from reactions of excited SO
2
molecules formed by absorption of solar u.v. radiation by ground state SO
2
, from reactions of SO
2
with photochemically generated OH and RO
2
free radicals, and from its reaction with transient species produced in thermal ozone—alkene reactions. Evaluation of the available mechanistic and rate data reveals that, of these three processes, oxidation by free radicals, particularly OH, is likely to be the most important in the atmosphere. Oxidation rates of up to 4 % h
-1
are predicted for a hydrocarbon-NO
x
polluted atmosphere under western European summertime conditions. This can lead to the formation of elevated concentrations of sulphuric acid and sulphate aerosol in polluted air. In the natural background troposphere oxidation rates are much less,
ca
. 0.3 % h
-1
averaged over 24 h, but probably still significant as a source of atmospheric sulphates.
