Modelled transport and deposition of sulphur over Southern Africa

Abstract

Ambient SO2 concentrations and atmospheric deposition of sulphur resulting from emissions on the industrialised highveld region of South Africa are estimated using the multi-scale atmospheric transport and chemistry (MATCH) modelling system, developed at the Swedish Meteorological and Hydrological Institute (SMHI), and compared with an inferential model driven by measured input quantities. Modelled SO, concentrations on the central highveld mostly range between 10 and 50 ppb, exceeding 50 ppb in source areas. Dry deposition rates for sulphur exhibit a similar spatial pattern to the ambient SO2 concentrations and both are consistent with synoptic-scale transport patterns. Maximum dry deposition rates for sulphur of more than 10 kg S ha(-1)a(-1) occur over the central highveld with a well-defined gradient decreasing away from the source region. Despite the significant differences in modelling approaches, the estimates of dry deposition provided by MATCH are in reasonable agreement with those of the inferential model. The maximum modelled wet deposition rates occur over the South African highveld and its periphery and range between 1 and 5 kg S ha(-1)a(-1) and compare favourably with measurements from an acid rain network. Wet deposition generally exceeds dry deposition on the highveld and the adjacent areas except in the central highveld source region. Over the drier western half of South Africa MATCH-modelled dry and wet deposition rates are again similar and are less that 1 kg S ha(-1)a(-1). Wet deposition exceeds dry in the higher rainfall regions along the south and east coasts of South Africa.