| dc.description.abstract | Mercury (Hg) is an extremely important element as it is ubiquitous, toxic, and considered a global pollutant. The atmospherically significant forms of Hg include inorganic gaseous elemental (Hg0), inorganic gaseous reactive (Hg2+), and inorganic particle-bound Hg (HgP). The former species, due to its stability and low reactivity has a long atmospheric lifetime. Due to these characteristics, it may be transported from where it was emitted to distant locations as far as the poles, hence why Hg is considered as a global pollutant. Mercury has one other form which is known as methylmercury. This species is formed after the deposition of Hg2+ and is considered as the most toxic form of environmental Hg. Due to their high reactivity and solubility, Hg2+ and HgP have short residence times (hours to days) and tend to deposited near their emission source. The recurrent oxidation and reduction conversions between Hg0 and Hg2+ is important in determining the atmospheric lifetime of Hg.
Mercury may be emitted by both natural and anthropogenic sources. Natural sources essentially reemit Hg that was previously deposited onto and into the Earth. The input that anthropogenic sources add increases this amount and essentially intensifies the global burden of this pollutant, as it cannot be removed from the environment. The leading anthropogenic source of Hg has been identified as coal-fired power plants, both annually and globally. In South Africa, most of these plants are located in the Highveld region – an area that is well-known for its anthropogenic activities. The state of Hg emitted by the power sector on the South African Highveld is, however, unclear. It is thus important to generate knowledge on Hg emissions in this region as it represents one of the most concentrated source regions of Hg in the world.
The state of Hg over the Highveld region was investigated by firstly characterising ambient Hg concentrations. This was done by analysing spatial, seasonal, and diurnal variation of total gaseous mercury (TGM) at Balfour (BF), Middelburg (MB), and Standerton (ST). Additionally, relationships between TGM and criteria pollutant concentrations were established. Concentrations ranged 0.40–28.72, 0.12–9.91, and 0.21–32.10 ng/m3 at BF, MB, and ST, respectively, with mean concentrations of 1.99 ± 0.94 ng/m3 (n = 6378), 1.04 ± 0.62 ng/m3 (n = 5473), and 1.25 ± 1.38 ng/m3 (n = 3568). Seasonally, concentrations at the sites varied as follows: summer>spring>winter>autumn; summer>winter>autumn>spring; and winter>spring>summer, respectively. Unfortunately, no data was captured during autumn at ST. The only pronounced diurnal pattern was observed at ST where local domestic combustion appears to have been the dominant source. The likelihood of the aforementioned is corroborated by the observed relationship between TGM and CO concentrations at this site. The penultimate part of the investigation involved estimating emissions from power plants using the bottom-up-approach. This entailed the estimation of emissions from 13 power plants during 2014 – 12 of which are located on the Highveld. This also involved an investigation on the influence of emission control devices and Hg-coal content on estimates. Emission control devices are installed in power plants to regulate emissions of particulate matter, SO2, and NOx, and indirectly Hg. The control devices used in South African power plants are fabric filters (FFs) and electrostatic precipitators (ESPs). Their Hg removal efficiencies vary from 89% (FFs), 36% (ESPs), and 62.5% where they are used collectively. Some power plants also install a flue-gas conditioning (FGC) system to enhance the performance of an ESP. It was found that if all power plants were to use FFs, the amount of emitted Hg would be substantially lower. This is an important finding as some power plants currently still use ESPs. Previous Hg emission studies in South Africa adopted Hg-coal content from literature. This study utilised Hg-coal content measured at each respective power plant. By comparing estimations using adopted and actual Hg-coal content values, it was found that estimates for 2014 were much lower when using actual values. This, therefore, highlights that actual Hg-coal content values should allow for more refined estimates.
In the final part of the investigation, the spatial distribution of the atmospherically significant forms of Hg was modelled with CALPUFF. Additionally, concurrent wet and dry deposition were simulated to investigate their effect on the spatial distribution of these species. As Hg0 is the only species that pose a threat to human health via the inhalation pathway, a health risk assessment was conducted. It was found the highest concentrations of the species were found in the centre of the domain where there is a cluster of power plants. Based on the results, the emissions from the power plants seem to be accumulating in an area of already high concentrations. Wet deposition of Hg2+ and HgP was observed to occur near the power plants with the highest removal occurring in the immediate vicinity of the modelled sources (<1 km). The species most affected by wet deposition was found to be Hg2+. The wet deposition of Hg0 was not modelled due to its low solubility and reactivity. The dry deposition of the species was highest in the centre of the domain with Hg0 the species most affected. The health risk assessment revealed that the exposed population in the Highveld region may be at risk to adverse health effects. The work done in this research may pave the way for prospective Hg studies over the South African Highveld. It also provides unique contributions to the broader area of knowledge regarding Hg in South Africa, as well as globally, and may subsequently enhance the understanding thereof | en_US |
