Molecular compositions and stable carbon isotopes (δ13C) of PAHs in wintertime PM2.5 in urban Xi'an, China: Implications for source distribution and atmospheric oxidation

Abstract

Atmospheric polycyclic aromatic hydrocarbons (PAHs) are potentially carcinogenic and mutagenic to human beings, and thus have attracted significant attention in recent decades. Compound-specific stable carbon isotopes (δ13C) of PAHs are indicative for source identification, but have not been extensively applied in tracing atmospheric oxidation processes. In this study, we examined the molecular compositions and δ13C values of PAHs in wintertime PM2.5 in a highly polluted urban region in Norwest China. The average concentrations of total parent-PAHs (pPAHs) and oxygenated-PAHs (OPAHs) were measured as 70 ± 29 and 36 ± 17 ng m−3, respectively. The results obtained from the incremental lifetime cancer risk (ILCR) model suggested a high potential cancer risk of PAHs in the urban region. The temperoal variations of PAHs and carbonaceous fractions implied that the contribution of secondary organic compounds was limited during the severe pollution period (PM2.5 > 115 μg m−3). Both molecular diagnostic ratios and δ13C values indicated that the major emission sources of the pPAHs were coal and/or biomass burning. The δ13C value of fluorene became heavier as the OPAHs concentration increased, while the concentrations of fluorene was negatively correlated with OPAHs, indicating the secondary formation of OPAHs from fluorene oxidation in the cold season. Our results also confirmed that δ13C values of pPAHs are indeed indicative of atmospheric oxidation processes.