Abstract
Air quality monitoring has been lacking in the rural and western North West Province. Here ambient sulphur dioxide (SO2), nitrogen dioxide (NO2) and ozone (O3) concentrations, monitored with passive samplers at 10 sites, are presented. Widespread SO2 and NO2 problems weren’t observed. However, regular O3 standard limit exceedances are likely across the province. Increased SO2 and NO2 concentrations in the colder and drier months were evident. Inversion layer trapping of low-level emissions during the colder months and open biomass burning in the drier months increased ground level pollutant concentrations. Wet deposition of SO2 and NO2, and enhanced SO2 conversion to particulate sulphate, result in lower wet season concentrations. O3 concentrations were lower from May to July and higher from August to March. Three phenomena contributed to this. Firstly, shorter daylight hours (less photochemistry) and secondly, lower biogenic volatile organic compound (O3 precursors) concentrations during the colder months. Thirdly, the late winter/early spring open biomass burning peak lead to elevated carbon monoxide (CO) concentration (also an O3 precursor). Spatial patterns indicated higher SO2 concentrations in the west, due mainly to industrial emissions. The NO2 spatial map indicated two areas of higher concentration, i.e. Bapong in the east due mainly to industrial emissions, and Taung with its higher population density. The O3 spatial map was almost the inverse of NO2. The lower O3 and higher NO2 around Taung showed that O3 is titrated there. Additionally, the results indicate that non-point source emissions of NO2 are high enough to results in exceedances of the O3 standard limit. Overlay back trajectory maps showed that sites in the east are more frequently impacted by pollution transported from the Mpumalanga Highveld, Vaal Triangle and the Johannesburg-Pretoria megacity if compared to the west. Conversely, cleaner air masses impact the west more than sites in the east.
Highlights
Anthropogenic activities are increasing the ambient tropospheric concentrations of inorganic gaseous pollutants, which include nitrogen dioxide (NO2), sulphur dioxide (SO2) and ozone (O3)
In order to contextualise the results, the statistical spread of SO2, NO2 and O3 concentrations measured at each site, over both sampling campaigns are presented in Figures 2a, b and c, respectively
The Bapong median/mean concentrations were lower than the 3.80 ppb reported for Marikana (Venter et al 2012) located close to several smelters in the North West Province, and within the concentration range reported for the Mpumalanga Highveld (2.80 to 13.30 ppb according to Martins et al 2007, Lourens et al 2011, Laakso et al 2012)
Summary
Anthropogenic activities are increasing the ambient tropospheric concentrations of inorganic gaseous pollutants, which include nitrogen dioxide (NO2), sulphur dioxide (SO2) and ozone (O3). These species are globally considered as important pollutants and are criteria pollutants according to the South African National Environment Management: Air Quality Act of 2004 (Government Gazette 2005). Human health issues associated with NO2 and nitrogen oxide (NO) (combined referred to as NOx), as well as SO2 include irritation of the respiratory system, which can cause breathing difficulties. Exceedances of the South African NO2 and SO2 ambient air quality standard limits have been reported for numerous locations/areas (e.g. DEA 2010). In South Africa it has been indicated that these species contribute significantly to the acidity of atmospheric particulate matter (Venter et al 2018), CLEAN AIR JOURNAL Volume 31, No 1, 2021
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