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 SO 4 − radical with a number of species commonly found in cloud water. Much lower radical concentrations were employed than in all previous direct studies of SO 4 − reaction kinetics. In the zero ionic strength limit, rate coefficients for SO 4 − reactions with the anions HSO 3 − , Cl − , NO 2 − , HCOO − , and CH 3 COO − are found to be 7.5 × 10 8 , 2.6 × 10 8 , 9.8 × 10 8 , 1.1 × 10 8 , and 3.7 × 10 6 M −1 s −1 , respectively. Rate coefficients for SO 4 − reactions with the neutral species H 2 O 2 , CH 3 OH, HCOOH, and CH 3 COOH are found to be 1.2 × 10 7 , 8.8 × 10 6 , 4.6 × 10 5 , and 1.4 × 10 4 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 SO 4 − + HSO 3 − reaction proceeds more slowly than previously believed, while the SO 4 − + 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.