Understanding the atmospheric circulations that lead to high particulate matter concentrations on the west coast of Namibia

Abstract

Atmospheric circulations play a significant role in determining the extent and impact of local and regional air pollution. The Erongo Region, located in the western part of Namibia, falls within the west coast arid zone of southern Africa, and is characterised by low rainfall, extreme temperatures and unique climatic factors influencing the natural environment and biodiversity. Episodic dust storms, associated with easterly wind conditions, are common during austral autumn and winter months. During these events, dust is transported westwards over long distances across the Namibian continent towards the Atlantic Ocean. During 2017, such easterly wind conditions appeared to occur earlier and more frequently than in previous years. Of interest is that high PM10 concentrations (particulate matter with aerodynamic diameters of less than or equal to 10 micron) measured at the coastal towns of Swakopmund and Walvis Bay in the Erongo Region during 2017 were found to also coincide with south-westerly to north-westerly winds from the ocean during prevailing easterly wind events. In this study, the easterly wind events that occurred on 19 March 2017 and 6 July 2017 were assessed to investigate how local-scale coastal atmospheric circulation changes could have developed from the easterly wind conditions, and how such development could have contributed to wind direction deviations and the high PM10 concentrations measured at Swakopmund and Walvis Bay. It was found that in addition to the westward transport of PM10 from inland sources during easterly wind events, higher coastal concentrations of PM10 can also develop as a result of north-easterly / south-westerly wind conversion lines and the cyclonic circulation enhancement associated with easterly wind induced coastal troughs and coastal lows.