Determination and Source Apportionment of Selected Heavy Metals in Aerosol Samples Collected from Sebele

Abstract

Bush fires and dust in the dry winter months establish moderately high background levels of aerosols. Emissions into the atmosphere by copper mines, coal mines and vehicular emissions are all possible sources of air pollution with heavy metals. Although there has been assessment and monitoring of heavy metals and their air pollution in other countries such as Brazil, Europe and America, there is little documentation about concentration of heavy metals in aerosols in Botswana. A total of 63 Aerosol samples were collected at the Botswana College of Agriculture and were analysed for concentrations of Al, Co, Cu, Fe, Pb, Mn, Ni and Zn in filtrate using a Flame Atomic Absorption Spectrometer (Varian SpectrAA 220 FS). Statistical receptor models were applied to investigate potential sources of the studied metals. Data exhibits enhanced enrichments of Zn (EF_g = 76), Al (EF_g = 14391), Co (EF_g = 19), Cu (EF_g = 5), Pb (EF_g = 3) and Ni (EF_g = 2), which was attributed to contributions from non-crustal sources, whereas Mn and Fe (EF_g < 2) were attributed to crustal origin, such as airborne dust. Data was subjected to factor analysis (FA) twice. From FA-1, two Principal Components (PC) were revealed. PC-1 showed high positive loadings of Pb, Zn, Ni and Al, whereas PC-2 had loadings for Fe and Mn. On the other hand, FA-2 had three principal components. PC-1 from FA-2 showed strong loadings for Cu, Fe and Mn. FA-2 had strong loadings of Pb, Al and Zn, whereas loadings for PC-3 were comprised of Ni and Co. The difference between loadings of FA-1 and FA-2 suggested possibilities of mixed origins of the studied metals. Finally, air mass back-trajectory analysis showed that during the sampling period, there were only 5 cluster groups that represented significantly different transport pathways of aerosol samples, where only Zn and Ni mean concentrations revealed dependence on geographical origin of aerosol samples. Results of the analysed concentrations of Al, Co, Cu, Fe, Pb, Mn, Ni and Zn in aerosol samples showed that the presence of Fe and Mn in the atmosphere in the ambient air of Sebele is mainly due to contributions from lithogenic sources. Pb, Ni, Co, Al and Zn exist because of anthropogenic sources, whereas, Cu, exist because of mixed origins. Trajectory analysis further shows that the anthropogenic Zn and Ni could also be present in the air sampled due to air mass transportation from distant sources.