Aus der englischen Wikipedia:
There are a large number of aqueous reactions that oxidize sulfur from S(IV) to S(VI), leading to the formation of sulfuric acid. The most important oxidation reactions are with ozone, hydrogen peroxide and oxygen (reactions with oxygen are catalyzed by iron and manganese in the cloud droplets).
In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction :
SO2 + OH· → HOSO2·
which is followed by:
HOSO2· + O2 → HO2· + SO3
Scheint also tatsächlich zu entstehen.