| dc.description.abstract | To date, most source apportionment studies in South Africa, as well as globally, have focused on determining the chemical composition of aerosols/particulate matter (PM) or precipitation, with little consideration of the gaseous phase. This can be attributed to the methodologies available to perform source apportionment studies, which rely on the statistical analysis of large datasets with detailed assessments of the chemical composition of these types of atmospheric samples. However, a recent study performed by Chiloane et al. (2017) developed and utilised a method through which sources of equivalent black carbon (eBC) at the Elandsfontein atmospheric monitoring site in the Mpumalanga Highveld were identified by assessing the chemical composition of plumes measured at a receptor site. In this method, sources were identified by considering the air mass history associated with each plume and investigating the co-incidental appearances of peaks of typical atmospheric pollutant species measured at Elandsfontein. Therefore, plumes could be related to specific sources in this region.
This method was further developed and used to determine the predominant sources of hydrogen sulphide (H2S), sulphur dioxide (SO2), nitrogen oxide (NO) and nitrogen dioxide (NO2) at Elandsfontein. An algorithm for this novel source quantification method introduced in this study for these atmospheric gaseous species was compiled, which could also be used in other studies. The Mpumalanga Highveld is a highly industrialised area in South Africa, with anthropogenic sources of atmospheric pollutants, including coal-fired power stations, as well as large petrochemical and pyrometallurgical smelters, while emissions from urban areas (e.g. household combustion, waste burning and traffic) are also well documented. Furthermore, the Mpumalanga Highveld has been included in the Highveld Priority Area as declared by the South African government, which is a region with high ambient concentrations of atmospheric pollutants that necessitate intervention. Measurements were conducted for two years at Elandsfontein in the European Integrated Project on Aerosol Cloud Climate and Air Quality Interactions.
The receptor method developed to enable the identification of sources and quantification of their contribution was applied to atmospheric concentrations of these trace gas levels above their baseline levels. The results indicate that emissions from urban areas on the Mpumalanga Highveld, associated with sources in towns, as well as semi- and informal settlements (e.g. ineffective household combustion of low-grade coal, sewage wastewater treatment facilities, landfills, small industries and traffic) contributed most to atmospheric H2S levels in excess of baseline concentrations (34.6%), followed by pyrometallurgical smelters (19.8%) and a petrochemical operation near Secunda (17.9%). The Johannesburg-Pretoria (Jhb-Pta) megacity, coal-fired power stations, burning coal dumps and cattle feedlots contributed 10.9%, 4.7%, 3.8% and 0.4%, respectively, to H2S concentrations in excess of the baseline levels. The contribution from urban areas is even more pronounced if the contributions from urban areas and the Jhb-Pta megacity on atmospheric H2S levels are combined. The largest contributor to SO2 in excess of baseline levels was determined to be high-stack emissions associated with coal-fired power stations (62.8%), while the pyrometallurgical sector made the
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second largest contribution (23.4%) and the third largest contribution was from the petrochemical sector (7.2%). Urban sources, the Jhb-Pta megacity and burning coal dumps contributed 4.6%, 1.1% and 0.4%, respectively, to atmospheric SO2 concentrations determined at Elandsfontein. Only three high-stack sources were determined for NO with coal-fired power stations contributing the majority of measured NO (85.9%). NO is typically associated with fresher plumes with a large contribution originating from coal-fired power stations in the proximity of Elandsfontein. Coal-fired power stations also made the largest contribution to atmospheric NO2 concentrations measured at Elandsfontein (59.4%), while the second and third largest contributions were from pyrometallurgical smelters (15%) and petrochemical operations (10.4%). Urban sources, traffic emissions, the Jhb-Pta and burning coal dumps contributed 6.1%, 5.7%, 1.6% and 0.3%, respectively. Although the contributions of these low-level emissions to NO2 are relatively low, it is not insignificant, as reflected by the temporal patterns.
In addition to the new method introduced in this study to quantify sources of atmospheric gaseous species, a novel use of satellite data was also developed through which burning coal dumps could be identified and their impacts on concentrations of atmospheric pollutants could be determined. Satellite fire radiative power (FRP) data from the Moderate Resolution Imaging Spectro-Radiometer (MODIS) was used to determine the locations and durations of spontaneously combusted coal within the Mpumalanga Highveld. It was concluded that five mining sites, all opencast mines situated on old board-and-pillar mines, were prone to spontaneous combustion. Furthermore, a relatively well-defined seasonal pattern in the combustions was also observed, with combustion events being more prevalent during the winter months. Considering the active burning periods of the areas where spontaneous combustion was recorded, it was noted that burning coal dumps could be a major source of atmospheric pollutants on the Mpumalanga Highveld.
Mean concentrations of 3.1 ppbv, 11.5 ppbv, 2.7 ppbv and 12.5 ppbv were determined for H2S, SO2, NO and NO2, respectively, for the entire sampling period. Well-defined seasonal and diurnal patterns indicated that low-level emission sources likely made the most significant contribution to the ambient H2S concentrations, whereas high-stack emissions were the dominant emission source for SO2, NO and NO2. However, the impact of high-stack emissions on H2S was also evident, while low-level emissions also contributed to ambient NO2 concentrations, which supports the source results obtained through the application of the novel method. | en_US |
