In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction:
:SO + OH· → HOSO·
which is followed by:
:HOSO· + O → HO· + SO
In the presence of water, sulfur trioxide (SO) is converted rapidly to sulfuric acid:
:SO (g) + HO (l) → HSO (aq)
Nitrogen dioxide reacts with OH to form nitric acid:
:NO + OH· → HNO