Assessing atmospheric trace gas concentrations in rural areas of the North West Province
Anthropogenic activities are increasing the ambient atmospheric concentrations of inorganic gaseous pollutants, which include nitrogen dioxide (NO₂), sulphur dioxide (SO₂), and ozone (O₃). These species were also included as criteria pollutants according to the National Environment Management: Air Quality Act. Depending on the concentration and exposure periods, these gasses could cause direct and indirect adverse impacts on the environmental, human health and climate. To date, no compliance monitoring (and research monitoring) of the afore-mentioned species have been conducted in many rural areas of the North West Province, especially the western portion of the province. The Atmospheric Chemistry Research Group (ACRG) at the North-West University (NWU) was contracted by the Department: Rural, Environment and Agriculture Development (READ) of the North West Provincial (NWP) Government to measure SO₂, NO₂ and O₃ ambient concentrations at 10 sites in the North West Province. The site measurement sites were selected in collaboration with READ. Monthly average concentrations were determined by using passive samplers developed by the ACRG. Passive samplers are ideal for this study considering that they are small, lightweight, silent and do not require electricity, field calibration nor a technician to function. Overall the results indicated that there is not wide spread SO₂ and NO₂ pollution problems in rural areas of the North West Province. Obviously, industrialised areas and/or larger cities were not considered in this study. However, it was evident that widespread exceedances of the 8-hrs. moving average standard limit for O₃ is likely across the North West Province. Seasonal patterns proved that for SO₂ and NO₂ household combustion for space heating that occurs more frequently in the colder months, as well as open biomass burning that occurs more frequently in the drier months are regional relevant sources. Additionally, enhanced trapping of low-level emissions during the colder months by a low-level thermal inversion layer(s) lead to increased concentrations of pollutants at ground level. Furthermore, increased wet deposition of both SO₂ (as sulphate, SO₄²⁻) and NO₂ (as nitrate, NO₃-), as well as enhanced conversion of SO₂ to particulate SO₄²⁻- that occur during the wet season when the relative humidity (RH) is higher, result in lower gaseous concentrations during the warmer/wetter months. O₃ concentrations were lowest during the colder months of May to July and higher in the period August to December, as well as January to March. Three phenomena contribute to this observed O₃ season pattern. Firstly, the colder months have shorter daylight hours, hence less time for photochemical formation of O₃. Secondly, biogenic volatile organic compound (BVOC) emissions are lower during the colder months. VOCs are important within the context of O₃ formation, since the alkylperoxy radical (ROO•) that form during the oxidation of VOCs convert NO to NO₂, from which O₃ is formed. Thirdly, the peak in open biomass burning in southern Africa during late winter and early spring (typically August to mid-October) also lead to a peak in carbon monoxide (CO) concentrations). The oxidation of CO results in the formation of the hydroperoxy radical (HOO•), which similar to the ROO• radical enhance conversion of NO to NO₂. Spatial patterns proved that higher SO₂ concentrations were evident in the western North West Province, due mainly to industrial emission. The NO₂ spatial concentrations map indicated two areas of higher concentration, i.e. the extreme east near Bapong and the area around Taung where population density was higher. This proved that two major sources of NO₂, i.e. industrial emissions in the eastern North West Province and vehicle emissions in more rural areas, are important. The O₃ concentration spatial map exhibited almost the inverse spatial trend than the NO₂ map. Particularly the lower O₃ measured around the Taung area was of interest. This low O₃ concentration area, associated with higher NO₂, prove that O₃ is being titrated here. The spatial map also proved that although significant industrial NO₂ emissions do not occur in the western North West Province, non-point source emission (e.g. vehicle emission, household combustion) emits enough NO₂ to results in regional exceedances of the O₃ ambient AQ standard limit. Overlay back trajectory maps proved that regional air mass movement patterns also played a contributing role in the observed pollutant concentrations in the North West Province. Sites in the eastern North West Province are more impacted by pollution transported from the Mpumalanga Highveld, Vaal Triangle and the JHB-Pta megacity if compared to sites in the western North West Province. Clean air masses, arriving from the west and southwest SA coast, also impact the western North West Province more than sites in the east.