Assessment of atmospheric trace metals and water soluble ionic species at two regional background sites
Venter, Andrew Derick
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In this study, atmospheric trace metals and water soluble ionic species were investigated. Five research articles are presented. In the first two articles specific trace metals species, i.e. mercury and hexavalent chromium, which are of particular importance within the South African context, are considered. Thereafter two articles on general trace metal concentrations and water soluble inorganic ionic concentrations measured at a regional background site are presented. In the last article trace metal and water soluble ionic concentrations, together with many other species/parameters determined in the plume of a typical South African braai are discussed as a case study. In article one, the continuous high-resolution gaseous elemental mercury (GEM) data from the Cape Point Global Atmosphere Watch (CPT GAW) station between 2007 and 2011 were evaluated with different statistical analysis techniques. GEM data were evaluated by cluster analysis and the results indicated that two clusters, separated at 0.904 ngm-3, existed. The two-cluster solution was investigated by means of backtrajectory analysis to determine the air mass history. The net result indicated that not all low GEM concentrations are of marine origin, and similarly, not all high GEM concentrations have a terrestrial origin. Equations were developed by means of multilinear regression (MLR) analysis that allowed for the estimation and/or prediction of atmospheric GEM concentrations from other atmospheric parameters measured at the CPT GAW station. These equations also provided insight into the relation and interaction of GEM with other atmospheric parameters. Both measured and MLR calculated data confirm a decline in GEM concentrations at CPT GAW over the period evaluated. In article two, hexavalent chromium, Cr(VI), was investigated and the regional atmospheric pollution of Cr(VI) from the ferrochromium and other related industries located in the western Bushveld Complex (wBC) of South Africa was determined. Particulate matter was sampled for an entire calendar year at a regional background site, which is situated downwind of the wBC on the dominant anti-cyclonic recirculation route of air mass over the South African interior. Results indicated that Cr(VI) concentrations in air masses that had passed over the regional background were below the detection limit of the analytical technique applied. However, Cr(VI) in air masses that had passed over the wBC were elevated and had a median concentration of 4.6 ngm-3. The majority of Cr(VI) was found to be in the finer size fraction (PM2.5), which could be explained by the properties of Cr(VI)-containing PM being emitted by the sources in the wBC and the atmospheric lifetimes of different PM size fractions. The results also indicated that it is possible that not only pyrometallurgical sources in the wBC, but also other combustion sources outside the wBC contributed to the observed atmospheric Cr(VI) concentrations. In article three, aerosol sampling was performed at Welgegund in South Africa, which is a regionally representative background site. PM1, PM1-2.5 and PM2.5-10 samples were collected for thirteen months and 32 atmospheric trace metal species were detected. Atmospheric Fe had the highest concentrations in all three size fractions, while Ca was the second most abundant species. Cr and Na concentrations were the third and fourth most abundant species respectively. Trace metal concentrations determined at Welgegund were compared to levels thereof in the wBC. Similar trace metals were detected and both indicated that Fe was the most abundant species. However, concentrations of trace metal species in the wBC were significantly higher compared to levels thereof at Welgegund. With the exception of Ni, none of the trace metals measured at Welgegund exceeded local and international standard limit values. No distinct seasonal pattern was observed in the PM2.5-10 size fraction, while the PM1 and PM1-2.5 size fractions indicated elevated trace metal concentrations coinciding with the end of the dry season, which could partially be attributed to decreased wet removal and increases in wind generation of particulates. Principal Component Factor Analysis (PCFA) analysis was successfully applied and revealed three meaningful factors in the PM1 size fraction, i.e. fly ash, pyrometallurgical-related and crustal. No meaningful factors were determined for the PM1-2.5 and PM2.5-10 size fractions. Pollution roses confirmed this impact of wind-blown dust on trace metal concentrations, while the influence of industrial activities was also substantiated. In article four, PM1, PM1-2.5 and PM2.5-10 samples were collected for thirteen months at Welgegund and analysed in order to determine the concentrations of the major inorganic ionic species. Results indicated that SO4 2- concentrations in the PM1 size fraction were significantly higher compared to the other species in all three size fractions. SO4 2- and NH4 + dominated the PM1 size fraction, while SO4 2- and NO3 - were the predominant species in the PM1-2.5 and PM2.5-10 size fractions. SO4 2- had the highest contribution in the two smaller size fractions, while NO3 - had the highest contribution in the PM2.5-10 size fraction. SO4 2- levels could be attributed to the impacts of aged air masses passing over source regions, while marine air masses were considered to be the major source of NO3 -. The reaction of SO4 2- with gas-phase NH3 was considered to be the major source of NH4 + in the PM1 size fraction. The PM at Welgegund was determined to be acidic, mainly due to excess concentrations of SO4 2-. Comparison of Welgegund inorganic ion measurements to measurements thereof at Marikana indicated that the concentrations of almost all the inorganic ion species were higher at Marikana. At Welgegund PM1 and PM1-2.5 fractions revealed a seasonal pattern with higher inorganic ion concentrations measured from May – September. Higher concentrations could be attributed to decreased wet removal of these species, since these months coincide with the dry season in this part of South Africa. Increases in pollutants concentrations due to more pronounced inversion layers trapping pollutants near the surface, as well as increases household combustion and wild fires during these months were also considered to contribute to elevated levels of inorganic ions. Back trajectory analysis of each of the sampling months was also performed, which revealed higher concentrations of inorganic ionic species corresponding to air mass movements over source regions. In article five, a case study, a comprehensive analysis of atmospheric gaseous and aerosol species within a plume originating from a typical South African braai (barbeque) at Welgegund was conducted. Five distinct phases were identified during the braai. The highest trace metal concentrations were associated with species typically present in ash. High Pb concentrations were detected. SO4 2–, Ca2+ and Mg2+ were the dominant water-soluble species present in the aerosols. The largest number of organic aerosol compounds was in thePM1-2.5 fraction, which also had the highest semiquantified concentration. It was indicated that PM10 concentrations were problematic during the meat grilling phase. From a climatic point of view, a relatively high single scattering albedo (ωo) indicated a cooling aerosol direct effect, while periods with lower ωo coincided with peak black carbon (BC) emissions. SO2, NOx and CO increased significantly, while O3 did not notably change. Aromatic and alkane volatile organic compounds were determined, and high benzene levels were observed. The results indicated that a recreational braai does not pose significant health risks. However, the longer exposure periods that are experienced by occupational vendors will significantly increase health risks.