Dry deposition of sulphur dioxide on wheat

ABOUT 6 million tonnes of sulphur dioxide (SO2) are emitted annually into the atmosphere over Britain. Atmospheric concentrations and the quantity of SO2 exported are determined by the rates of emission, dispersal and removal. Removal as sulphate by rainfall over Britain accounts for about 15% only of annual emission1. As long ago as 1950 Meetham2 suggested that turbulent transfer was an important process for removing SO2 molecules at the Earth's surface, a process usually referred to as 'dry deposition' even when the surface is wet. Measurements of the dry deposition rates on short grass and water have not been made until recently3,4. From limited evidence for short grass Garland et al.1 suggested that dry deposition over Britain may remove 40% of the SO2 emitted annually. The surface factors which may control dry deposition rates have not yet been established, however, nor have dry deposition rates on field crops been measured, although it has been suggested that the sulphur supplied in this way may avoid deficiences on some British soils5. Here we report the results of an extensive series of measurements of dry deposition rates on a field of winter wheat and point to factors which may limit the deposition rate