Dry and wet removal of sulphur from the atmosphere

Dry deposition and removal in precipitation of SO₂ and of paniculate sulphate are considered in turn. Many assessments of the dry deposition of SO₂ to various surfaces give deposition velocities of about 0.8cm s⁻¹, although variations with season and weather conditions are important. The deposition velocity of the sulphate aerosol is probably about 0.l cm s⁻¹. Removal of SO₂ in rain is also a rather inefficient process and theoretical and experimental results suggest that the sulphur in precipitation results chiefly from the rainout of cloud condensation nuclei. The removal time constants for SO₂ and sulphate by moderate rain are probably of order 10⁻⁵ and 10⁻⁴s⁻¹ respectively. A much simplified model suggests that about a half of the SO₂ emitted to the atmosphere is removed by dry deposition, the remainder is oxidised to sulphate and removed in precipitation and the atmospheric residence time is about 5 days for sulphur. The method and climatological statistics for a more realistic treatment do not yet appear to be available.