Black carbon in soils from different land use areas of Shanghai,China: Level,sources and relationship with polycyclic aromatic hydrocarbons

Black carbon (BC) in soils plays a key role of carrying hydrophobic pollutants like polycyclic aromatic hydrocarbons (PAHs). However, little is known about the spatial distribution, sources of BC and its relationship with PAHs in urban soils. We studied BC, total organic carbon (TOC) and PAHs concurrently in 77 soils collected from downtown area, suburban and rural area and industrial area of Shanghai, China. BC was determined by both chemical oxidation (dichromate oxidation, BC_Cr) and chemo-thermal oxidation (CTO-375, BC_CTO). BC sources were identified qualitatively by BC/TOC concentration ratios and BC-cogenerated high molecular weight (HMW) PAH isomer ratios and quantitatively by principal component analysis followed by multiple linear regression (PCA-MLR). Results showed that BC_Cr concentration (4.65 g/kg on average) was significantly higher than BC_CTO (1.91 g/kg on average) in Shanghai soils. BC_Cr concentrations in industrial area were significantly higher than those in other two. Stronger correlation was found between PAHs and TOC, BC_Cr than that between PAHs and BC_CTO, which indicates the possibility of PAHs being carried by charcoal and other organic matters thus negating its exclusive dependence on soot. Charcoal was therefore suggested to be taken into account in studies of BC and its sorption of PAHs. BC/TOC ratios showed a mixed source of biomass burning and fossil fuel combustion. PCA scores of BC-cogenerated HMW PAHs isomer ratios in potential sources and soil samples clearly demonstrated that sources of BC in urban soils may fall into two categories: coal and biomass combustion, and traffic (oil combustion and tire wear). PCA-MLR of HMW PAHs concentrations in soil samples indicated that coal and oil combustion had the largest contribution to BC in urban soils while tire wear and biomass combustion were important in downtown and rural area, respectively, which indicated they were main sources of HMW PAHs and presumably of BC.