Exploring the chemical fate of the sulfate radical anion by reaction with sulfur dioxide in the gas phase

Abstract. The gas phase reaction between SO₄⁻(H₂O)_n and SO₂, n = 0–2, is investigated using ab initio calculations and kinetic modelling. Structures of reactants, transition states and products are reported. Our calculations predict that the SO₂SO₄⁻(H₂O)_n cluster ion, which is formed upon SO₂ and SO₄⁻(H₂O)_n collision, can isomerize to SO₃SO₃⁻(H₂O)_n. The overall reaction is SO₂ oxidation by the SO₄⁻(H₂O)_n anionic cluster. The results show that SO₄⁻(H₂O)_n is a good SO₂ oxidant, especially at low relative humidity, with a reaction rate constant up to 1.5 × 10⁻¹⁰ cm³ molecule⁻¹s⁻¹. At high relative humidity, instead, the re-evaporation of SO₂ from the SO₂SO4⁻(H₂O)_n cluster ion is favoured.