Parameterization of Sulfate Removal by Precipitation

Abstract

The concentration of sulfate in precipitation is predicted to be directly proportional to the sulfate concentration of the air ingested into the cloud and inversely proportional to cloud water concentration. In addition, for light to moderate precipitation rates, the removal of airborne sulfate by precipitation is forecast to be strongly dependent upon the mechanism of precipitation formation. If the precipitation is in the form of snow or originated as snow in the upper portions of the clouds, then the surface sulfate concentration of precipitation is proportional to R^−0.3, where R is the precipitation rate. Should the precipitation form without the benefit of an initial ice growth stage, then the sulfate concentrations in precipitation water can increase by a factor of 2 to 10 or more over the ice-dependent predictions. Limited surface observations give support to these predictions. The analyses leading to the prediction of washout ratios are also used to forecast the mean oxidation rate of SO₂. In order to replace the SO₄ removed from the atmosphere by each precipitation event, SO₂ is required to oxidize at an average rate (over several days) of 0.2–2.0% per hour.