The transport of atmospheric NO_x and HNO₃ over Cape Town

Abstract. Cape Town, the most popular tourist city in Africa, usually experiences air pollution with unpleasant odour in winter. Previous studies have associated the pollution with local emission of pollutants within the city. The present study examines the transport of atmospheric pollutants (NO_x and HNO₃) over South Africa and shows how the transport of pollutants from the Mpumalanga Highveld, a major South African industrial area, may contribute to the pollution in Cape Town. The study analysed observation data (2001–2008) from the Cape Town air-quality network and simulation data (2001–2004) from a regional climate model (RegCM) over southern Africa. The simulation accounts for the influence of complex topography, atmospheric conditions, and atmospheric chemistry on emission and transport of pollutants over southern Africa. Flux budget analysis was used to examine whether Cape Town is a source or sink for NO_x and HNO₃ during the extreme pollution events.

The results show that extreme pollution events in Cape Town are associated with the lower level (surface – 850 hPa) transport of NO_x from the Mpumalanga Highveld to Cape Town, and with a tongue of high concentration of HNO₃ that extends from the Mpumalanga Highveld to Cape Town along the south coast of South Africa. The prevailing atmospheric conditions during the extreme pollution events feature an upper-level (700 hPa) anticyclone over South Africa and a lower-level col over Cape Town. The anticyclone induces a strong subsidence motion, which prevents vertical mixing of the pollutants and caps high concentration of pollutants close to the surface as they are transported from the Mpumalanga Highveld toward Cape Town. The col accumulates the pollutants over the city. This study shows that Cape Town can be a sink for the NO_x and HNO₃ during extreme pollution events and suggests that the accumulation of pollutants transported from other areas (e.g. the Mpumalanga Highveld) may contribute to the air pollution in Cape Town.