Sulphur dioxide (a primary pollutant) is oxidised in the atmosphere to sulphur trioxide (a secondary pollutant) which then reacts with water to form sulphuric acid.
The oxidation of sulphur dioxide can occur in various ways
1. Reaction in water droplets
SO2(g) + H2O(l) --> HSO3-(aq) + H+(aq)
HSO3-(aq) + 1/2O2(aq) --> HSO4-(aq)
This process is slow but is catalysed by metal ions, such as manganese(II) found in atmospheric aerosols.
2. Photochemical reaction
SO2 + hf --> SO2* where * denotes an electronically excited species
SO2* + O2 --> SO3 + O
3. Gas phase reaction with ozone
SO2 + O3 --> SO3 + O2