Spatial and temporal distribution of trace elements in aerosols in the Vaal Triangle
Kleynhans, Engela Helena
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The Vaal Triangle, largely an industrialized area and a so-called "air pollution hot spot" in South Africa, was declared as the first air pollution priority area in South Africa on the 21st of April 2006. In such an industrial and highly populated area, concentrations of trace elements and particulate matter are expected to exceed concentration levels that are safe for the environment and for the population. There is very little existing data on trace element concentrations in this region, therefore the purpose of this study was to monitor concentrations of certain harmful trace metals (such as Cr, V, Fe, Ni, and Pb), as well as the total concentration trace elements in the PM2.5 and PM10 particle fractions. Three towns in close proximity to one another were selected for sampling, namely Sasolburg, Vanderbijlpark and Vereeniging. The samples were collected using MiniVol Portable Air Samplers (product of Airmetrics) and teflon filters. The MiniVol samplers were set to an air-flow rate of 5 litres per minute. The sampling time was twenty four hours per sample, collected over three days during a winter season (July 2006) and a summer season (March 2007). Background sampling was conducted at Botsalano game reserve in August 2007. This was essential as a reference for this study, due to the paucity of data on trace metal concentrations in South Africa and data on the trace metal content of various source emissions in the Vaal Triangle. The samples were analysed using Inductive Coupled Plasma - Mass Spectrometry (ICP-MS) and Scanning Electron Microscopy (SEM). The comparison of the two analytical techniques showed that ICP-MS is the preferred method to determine the concentrations of trace metals in ambient particulate matter. ICP-MS was used to determine the concentrations of all elements from Li to U for each sample. The average of these concentrations at each site was higher during the winter sampling period than the summer sampling period. This could possibly be attributed to the higher atmospheric stability during the winter and the increase in rainfall during the summer. Another possible reason could be the higher occurrence of field fires and residential combustion (biomass and coal) during the winter. Fe concentrations were higher than most other elements and could possibly be attributed to the activities of numerous steel manufacturing industries in the Vaal Triangle. However, Cr, Ni and Co concentrations were higher during the summer and could be attributed to the influence of local metallurgical industries. Fe was the most abundant trace element, followed by Zn and Mn. The average Fe concentrations were approximately 1.009 μg.m-3 (PM2.5) and 1.499 μg.m-3 (PM10) during the winter, and approximately 0.775 μg.m-3 (PM2.5) and 1.071 μg.m-3 (PM10) during the summer. The average Fe concentrations in the background samples were approximately 0.001 μg.m-3 (PM2.5) and 0.097 μg.m-3 (PM10). The values in the Vaal Triangle are significantly higher compared to the background concentrations at Botsalano, but it is much lower than values reported for the Rustenburg area. The most significant observation that could be made from the SEM/EDS results was that carbonaceous particles were the dominant species present and the percentages were higher during the winter months, possibly due to the elevated occurrence of residential biomass burning and residential coal combustion to produce heat, especially in the low-income residential areas. Further research should be conducted to get clear seasonal trends and annual average concentrations.
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