Chemical (C, N, S, black carbon, soot and char) and stable carbon isotope composition of street dusts from a major West African metropolis: Implications for source apportionment and exposure.

Street dust is a major source of pollution and exposure of residents of West Africa to toxic chemicals. There is however, limited knowledge about the chemical composition and sources of street dust in urban areas of sub-Saharan Africa. The total carbon (TC), nitrogen (TN), sulfur (TS) and the stable carbon isotope ratios (δ13C) contents of street dust sampled from 25 sites distributed across Kumasi (a metropolis in Ghana with a population of ca. 2 million) were determined. In addition, black carbon (BC) and their subunits (soot and char) in these samples were also determined. The concentrations of TC, TN and TS in the dusts were 5-71 mg g-1, 0.3-4.3 mg g-1 and 0.2-1.4 mg g-1, respectively. The concentrations of TC, TN and TS were higher than at the background site of the metropolis by a factor of 5.1 (range: 1.7-12), 3.9 (1.1-13) and 2.8 (0.7-5), respectively. The BC, char and soot concentrations in these samples averaged 1.6 mg g-1 (0.13-4.4), 1.2 mg g-1 (0.08-3.7) and 0.36 mg g-1 (0.05-1.5), respectively. The concentrations of BC, char and soot in the street dust were higher than in the background location by factors of 5 (range: 0.8-13), 6 (0.7-17) and 3 (0.5-12), respectively. The TC, TN, TS, BC, soot and char concentrations were positively correlated with each other and with polycyclic aromatic compounds (PAHs, oxygenated PAHs and azaarenes from a previous study), indicating their common origin and fate. The δ13C values ranged from -27 to -24 [‰], with more polluted sites being more depleted in 13C. Based on the chemical composition of the street dusts, the 25 sites could be clustered into four groups by hierarchical cluster analysis which reflect areas of varying anthropogenic influence and, accordingly, exposure to hazardous chemicals.