Deposition and transformation rates of sulphur oxides during atmospheric transport over the Atlantic

The dry deposition of sulphur dioxide to land and water surfaces

The direct absorption of sulphur dioxide has been measured in the field by the concentration gradient method and by using 35S as a tracer. The mean deposition velocities (the ratios of deposition flux to concentration) for grass, soil and water were close to 1 cm s -1 No concentration gradient could be detected over coniferous forest indicating an upper limit of 2cm s -1 on the deposition velocity. Radioactive measurements suggest that the deposition velocity for a dry forest canopy varies from 0.1 to 0.6cm s_1. Laboratory experiments showed that the rate of deposition to soil increased with soil pH. Deposition to calcareous soil, water and short grass was largely controlled by the rate of turbulent mixing, while diffusion into the leaf or the rate of sorption controlled deposition to taller vegetation.