Abstract
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 (NOx 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 NOx 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 NOx 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 NOx 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.
Highlights
Accumulation of atmospheric mono-nitrogen oxides (NOx) and its derivatives (i.e. HNO3) may have severe impacts on climate, environment, and human health
This study shows that Cape Town can be a sink for the NOx and HNO3 during extreme pollution events and suggests that the accumulation of pollutants transported from other areas may contribute to the air pollution in Cape Town
The present results suggest that the amount of HNO3 transported from the Highveld pollutants southward may be substantial, and given that the winds are weaker at lower level than at upper level, and the pollutant concentrations are higher at low level than at upper level, it is important to have a better understanding of pollutants’ transport at low level, especially over South Africa
Summary
Accumulation of atmospheric mono-nitrogen oxides (NOx) and its derivatives (i.e. HNO3) may have severe impacts on climate, environment, and human health. Reaction of NOx and ammonia with other substances generates particles and nitric acid (HNO3). In particular, can penetrate deeply into sensitive parts of the human lungs and cause respiratory diseases, such as emphysema and bronchitis (Yang and Omaye, 2009). They can aggravate existing heart disease (Stern et al, 1988). The complexity of nutrient cycling in ecosystems may cause the long-term impact of nitric acid to take decades to become apparent (Fields, 2004).
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