Spatial and temporal assessment of pollutants in the Highveld Priority Area, South Africa
Lourens, Alexandra Susanna Maritz
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One of the major concerns that has been facing South Africa in recent years is the amount of gaseous pollutants emitted into the atmosphere. The Mpumalanga province is largely an industrialised area, which contributes approximately 83% to the country's total coal production. Eastern Gauteng and western Mpumalanga were identified as an air pollution hotspot and therefore declared the Mpumalanga Highveld Priority Area (HPA) on 4 May 2007. Diverse anthropogenic activities in this region result in high levels of organic aromates, such as benzene, toluene, ethylbenzene, and xylene (BTEX), as well as nitrogen dioxide (N02), sulphur dioxide (S02) and ozone (03). The monitoring of inorganic gases with active and passive samplers is well established in South Africa. In contrast, very little data exists for volatile organic compounds (VOCs), which necessitates the measurement of these gases. Therefore, the primary aim of this research project is to determine the spatial and temporal distribution of BTEX, N02, S02, and 03 in the HPA. A vertical assessment of BTEX was also conducted, in order to compare the upper atmospheric concentrations with the ground-level measurements. Lastly, all data were compared to the current legislation, in order to establish compliance. Eight sites, within a grid of 600 km2, were selected for the sampling. Passive sampling for the selected criteria pollutants was conducted on a monthly basis for a period of one year. The ground-level concentrations of BTEX were measured with Tenax TA adsorbent tubes, while N02, S02 and 03 were measured with passive samplers developed and used by the Atmospheric Chemistry Research Group of the North-West University. Vertical BTEX profiles were obtained with a Cessna 182 during a two-day winter and a two-day summer field campaign utilising 6L TO-14 canisters and absorbent tubes. Flights were undertaken at two altitudes, 500 ft and 1 500 ft above ground level (AGL), and on three flight paths. VOC samples were analysed, by using a gas chromatograph attached to a mass spectrometer for detection. Adsorbent tube samples were introduced into the system by a thermo desorber, while the canister had a slight modified inlet. Sulphurdioxide and 03 analyses were done on an Ion Chromatograph, while a UV-visible Spectrophotometer was used for N02 analysis. The spatial distribution of N02, S02, and BTEX indicated an increase towards the western parts of the HPA. This can be attributed to the prevailing north-easterly winds, as well as the increased industrial activity in the western parts. The rural areas such as Balfour and Delmas were influenced by the industrial activities in the surrounding areas. The temporal distribution of the inorganic gaseous species N02, S02, and 03 indicated seasonal trends. The N02 and S02 peaked during winter because of meteorological conditions that trap and recirculate the air mass, as well as increased household and biomass combustion. The 03 peak during spring could be explain by the CO peak, which is probably the most important 03 precursor species in South African conditions. The CO peak occurred due to increased veldt fires during the dry season. No seasonal trend was observed for BTEX. From the vertical BTEX assessment, it was clear that no significant difference exists between the upper atmospheric concentrations and the ground-level measurement. A good comparison between the canisters and adsorbent tubes was found. All the measured species were below their national- and proposed standards. However, the higher levels of S02, N02, and BTEX, measured in the western parts of the HPA, require attention. Witbank was the most polluted municipal area that was monitored.