The effects of microphysical parameterization on model predictions of sulfate production in clouds

Model predictions of sulfate production by an explicit cloud chemistry parameterization are compared with corresponding predictions by a bulk chemistry model. Under conditions of high SO₂ and H₂O₂, the various model predictions are in reasonable agreement. For conditions of low H₂O₂, the explicit microphysical model predicts sulfate production as much as 30 times higher than the bulk model, though more commonly the difference is of the order of a factor of 3. The differences arise because of the size-dependent distribution of pH in the droplets, which is in turn a consequence of the size-dependent distributions of sulfate and ammonium ions. Experimental evidence for differing size-dependent distributions of ammonium and sulfate ions is reviewed and compared with model predictions. The results of the sulfate production comparison suggest that bulk cloud water pH is not always a reliable indicator of aqueous reaction conditions in clouds. Related to this, the ozone oxidation of aqueous S(IV) appears to be of more significance than would be suggested by pH measurements of bulk cloud water.