Global distribution of sulphate in the troposphere: A three-dimensional model study

The tropospheric distributions of DMS, sulphur dioxide and sulphate have been studied in a global 3-D chemical tracer model (CTM) which is based on the NASA-GISS CTM and further developed to study chemical active compounds in the troposphere. The chemical scheme of the sulphur compounds, which has a simplified parameterisation of heterogeneous oxidation, seems to give a good agreement with observations, especially over polluted areas. Considerable amounts of sulphur is predicted to be transported into the Arctic in winter, also in good agreement with observations. The model gives high values of sulphate in the upper troposphere north of 20°N due to slow loss at these levels, a feature which is supported by the few measurements available. Discrepancies with observations over marine background areas may be due to the simplified treatment of the heterogenous oxidation, but may also indicate problems regarding the DMS-SO 2-SO 2- 4 oxidation path as modelled in this work. The results obtained point to the need for more measurements of the emissions and studies of the chemical conversion of natural sulphur compounds to SO 2 and sulphate.