Coupling of wet scavenging of sulphur to clouds in a numerical weather prediction model

The in-cloud conversion of SO₂ to sulphate is an important part of the modelling of sulphur dispersion. The description of this process requires a coupling to meteorological processes in particular clouds and precipitation. In this study, the description of bulk wet scavenging is coupled to clouds and precipitation in a weather prediction model. Results are shown for summer, autumn and winter episodes of regional scale wet sulphur deposition over Scandinavia. We find that the liquid-phase formation of sulphate is in all cases comparable to the uptake of sulphate through cloud condensation nuclei. Sub-cloud scavenging by precipitation is small. The formation of sulphate through liquid reaction between SO₂ and ozone is small for the summer simulation where the hydrogen peroxide concentrations was chosen equal to 1 ppb, but important for the winter case with large-scale stratiform precipitation and hydrogen peroxide level of 0.25 ppb. The study indicates that the wet deposition is particularly sensitive to the initial hydrogen peroxide concentrations and subgrid mixing of hydrogen peroxide. Comparison with surface measurements show realistic patterns of sulphur concentrations in rain, and that most concentrations are within a factor of two of the measurements.