A technique for determining in‐cloud formation of SO₄

A technique based on SO 4 /Se ratio is developed to determine in‐cloud formation of SO 4 from the oxidation of SO 2 . Whereas nucleation scavenging is the only significant source of Se in cloud water, both nucleation scavenging and dissolution of SO 2 with subsequent oxidation are significant sources of SO 4 in cloud water. Based on the assumption that no fractionation occurs between Se and SO 4 aerosols during nucleation scavenging, it is shown that a simultaneous measurement of SO 4 and Se in cloud water and ambient air permits a direct determination of in‐cloud SO 4 formation. The data to support this assumption are provided. The results from two cloud events during July and August, 1987 show that in‐cloud formation ranged between 8 and 28% in ten hourly samples.