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Sulphur dioxide

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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