ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-559-2014 The transport of atmospheric NO<sub>x</sub> and HNO<sub>3</sub> over Cape Town Abiodun B. J. 1 Ojumu A. M. 2 Jenner S. 1 Ojumu T. V. 3 Climate Systems Analysis Group, Department of Environmental and Geographical Science, University of Cape Town, South Africa Department of Environmental and Agricultural Sciences, University of South Africa, South Africa Department of Chemical Engineering, Cape Peninsula University of Technology, South Africa 20 01 2014 14 2 559 575 Copyright: © 2014 B. J. Abiodun et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/14/559/2014/acp-14-559-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/559/2014/acp-14-559-2014.pdf

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<sub>x</sub> and HNO<sub>3</sub>) 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<sub>x</sub> and HNO<sub>3</sub> during the extreme pollution events. <br><br> The results show that extreme pollution events in Cape Town are associated with the lower level (surface – 850 hPa) transport of NO<sub>x</sub> from the Mpumalanga Highveld to Cape Town, and with a tongue of high concentration of HNO<sub>3</sub> 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<sub>x</sub> and HNO<sub>3</sub> 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.

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