Kinetics of aqueous phase reactions of the SO4 - radical with potential importance in cloud chemistry

A laser flash photolysis/long path absorption technique has been employed to study at 298 K the kinetics of aqueous phase reactions of the SO₄^- radical with a number of species commonly found in cloud water. Much lower radical concentrations were employed than in all previous direct studies of SO₄^- reaction kinetics. In the zero ionic strength limit, rate coefficients for SO₄^- reactions with the anions HSO₃^-, Cl^-, NO₂^-, HCOO^-, and CH₃COO^- are found to be 7.5 × 10⁸, 2.6 × 10⁸, 9.8 × 10⁸, 1.1 × 10⁸, and 3.7 × 10⁶ M^-1 s^-1, respectively. Rate coefficients for SO₄^- reactions with the neutral species H₂O₂, CH₃OH, HCOOH, and CH₃COOH are found to be 1.2 × 10⁷, 8.8 × 10⁶, 4.6 × 10⁵, and 1.4 × 10⁴ M^-1 s^-1, respectively. For many of the reactions studied, agreement with previously reported rate data is poor. Of particular importance for cloud chemistry is the fact that the SO₄^- + HSO₃^- reaction proceeds more slowly than previously believed, while the SO₄^- + Cl^- reaction is somewhat faster than previously thought. Incorporation of these results into cloud chemistry models should reduce the calculated efficiency of free radical chain reactions as an oxidation mechanism for S IV.