Assessing SO2, NO2 and O3 in rural areas of the North West Province

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.