Re-evaluating the NO&lt;sub&gt;2&lt;/sub&gt; hotspot over the South African Highveld

Alexandra S.M Lourens,Klaus-Peter Heue,Steffen Beirle,Timothy M Butler,Pieter G Van Zyl,Johan P Beukes,Thomas K Wagner,Mark G Lawrence,Jacobus J Pienaar,Gerhardus D Fourie

doi:10.4102/sajs.v108i11/12.1146

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

Alexandra S.M Lourens, Klaus-Peter Heue + Show 8 more

Open Access

https://doi.org/10.4102/sajs.v108i11/12.1146

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Abstract

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.

Highlights

Satellite-based instruments such as the Global Ozone Monitoring Experiment (GOME 1 & 2),[1] the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY)[2] and the Ozone Monitoring Instrument (OMI)[3] have been used to quantify trends of various atmospheric trace gas concentrations on a global scale.[4]
One prominent NO2 hotspot seen in global maps from satellite retrievals is over the South African Highveld
The megacity diurnal cycles were characteristic of urban areas dominated by traffic emissions, with maximum atmospheric NO2 levels during peak traffic periods in the early morning and late afternoon (Figure 3)

Summary

Introduction

Satellite-based instruments such as the Global Ozone Monitoring Experiment (GOME 1 & 2),[1] the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY)[2] and the Ozone Monitoring Instrument (OMI)[3] have been used to quantify trends of various atmospheric trace gas concentrations on a global scale.[4]. The tropospheric NO2 column density of this area (shown in Figure 1) is comparable to that observed for central and northern Europe, eastern North America and south-east Asia.[9] In addition to the well-known source of NO2 – 11 coal-fired power stations – there are other widespread area sources, such as road transport, biomass burning and human settlement, as well as other large point sources including metallurgical, mining and petrochemical operations.[11] A total of 90% of the industrial NOx emissions in South Africa originate from the Highveld hotspot.[12] In a recent assessment of global emissions, South African emissions amounted to 420 kt/year of nitrogen, which constituted 1.6% of global anthropogenic NO2 emissions.[13]

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