Use of cosmogenic 35S to determine the rates of removal of atmospheric SO2

Abstract

GASEOUS sulphur dioxide supplied to the atmosphere is removed principally by three processes: direct scavenging in precipitation, oxidation to aerosol sulphate with subsequent deposition by vertical and horizontal precipitation, and 'dry' deposition, primarily on the surface of vegetation. The rates of these removal processes, which vary with environmental conditions, must be known in order to understand the fate of SO2 and the concentration and distribu-tion of aerosol sulphate1–3. The latter is thought to play a part in the heat balance of the lower troposphere4, and is thus relevant to the issue of global warming. Approaches to this problem using field observations5,6 have not given consistent or uncontested results. We report here the use of cosmogenic 35S (half-life 87.2 days) as a way of determining the time constants for oxidation, in-cloud scavenging and aerosol deposition. Our method involves determining 35S levels in gaseous SO2 , aerosol sulphate and precipitation. If these seasonally and regionally variable time constants can be applied to terrestrially produced SO2, 35S measurements could provide an independent method for studying the fate of SO2 in the atmosphere as a function of time and place.