The transport of pollutants over South Africa and atmospheric sulphur in Cape Town

Cape Town experiences unpleasant pollution (called the Brown Haze) in winter. Sulphur is a major constituent of this haze. To reduce sulphur pollution, and its negative impact on health in Cape Town, air quality management has focussed on identifying local sources and reducing the local emissions of sulphur. Meanwhile, the transport of sulphur pollutants from areas outside Cape Town can contribute to ambient sulphur concentrations. This work studies the transport of sulphur pollutants over South Africa and examines whether Cape Town is a net source of or sink for the pollutant. It shows the link between sulphur emissions on the Mpumalanga Highveld (the most polluted area in South Africa) and sulphur pollution in Cape Town. Two atmosphere chemistry-transport models (RegCM-Chem and WRF-Chem) were used for this study. The models were applied to simulate the atmospheric flow and chemistry transport over South Africa for two years (2001 and 2002), and the results were validated with observations within Cape Town. The models reproduced observed seasonal variability in atmospheric wind flow and sulphur concentrations over Cape Town. Results from both models show a transport of sulphur pollutants from the Mpumalanga Highveld to Cape Town. While the sulphur pollutants from the Mpumalanga Highveld are transported eastward (toward the Indian Ocean) at middle atmospheric levels, the pollutants are transported south-westward (towards Cape Town) at lower atmospheric levels. In addition, the pollutants are transported from the Mpumalanga Highveld to Cape Town, following the south coast of South Africa, in April. During an extreme sulphur pollution event in Cape Town, there is formation of either a col or a converging flow over the city. These features encourage the accumulation of sulphur over Cape Town. The sulphur flux budget analysis shows that Cape Town can be a net source of or sink for sulphur during an extreme pollution event. This study has application potential in developing policies to reduce sulphur pollution in Cape Town and in other areas of South Africa