The importance of liquid water concentration in the atmospheric oxidation of SO2

The concentration of condensed water available for aqueous chemical reactions is viewed as a fundamental parameter of the heterogeneous conversion of gaseous SO₂ to particulate sulfate. New results from a series of dispersed-phase experiments in a cloud chamber, in which the magnitude of this parameter was allowed to vary widely, demonstrate that the heterogeneous SO₂ conversion rate in hazes is generally limited by the small concentration of condensed water. This limitation precludes the heterogeneous oxidation pathway from being important in the atmosphere during haze episodes except under extreme conditions of high humidities and aerosol loadings. In clouds, on the other hand, the liquid water concentrations are relatively large, permitting chemically related factors, such as pH-dependent equilibria and oxidant abundances, to limit the SO₂ conversion rate.