Re-evaluating the NO<sub>2</sub> hotspot over the South African Highveld

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Globally, numerous pollution hotspots have been identified using satellite-based instruments. One of these hotspots is the prominent NO 2 hotspot over the South African Highveld. The tropospheric NO 2 column density of this area is comparable to that observed for central and northern Europe, eastern North America and south-east Asia. The most well-known pollution source in this area is a large array of coal-fired power stations. Upon closer inspection, long-term means of satellite observations also show a smaller area, approximately 100 km west of the Highveld hotspot, with a seemingly less substantial NO 2 column density. This area correlates with the geographical location of the Johannesburg–Pretoria conurbation or megacity, one of the 40 largest metropolitan areas in the world. Ground-based measurements indicate that NO 2 concentrations in the megacity have diurnal peaks in the early morning and late afternoon, which coincide with peak traffic hours and domestic combustion. During these times, NO 2 concentrations in the megacity are higher than those in the Highveld hotspot. These diurnal NO 2 peaks in the megacity have generally been overlooked by satellite observations because the satellites have fixed local overpass times that do not coincide with these peak periods. Consequently, the importance of NO 2 over the megacity has been underestimated. We examined the diurnal cycles of NO 2 ground-based measurements for the two areas – the megacity and the Highveld hotspot – and compared them with the satellite-based NO 2 observations. Results show that the Highveld hotspot is accompanied by a second hotspot over the megacity, which is of significance for the more than 10 million people living in this megacity.