Origin of atmospheric polycyclic aromatic hydrocarbons (PAHs) in Chinese cities solved by compound-specific stable carbon isotopic analyses

We report measurement of molecular and carbon isotopic compositions of atmospheric polycyclic aromatic hydrocarbons (PAHs) in three Chinese cities, Beijing, Chongqing, and Hangzhou. Precision of δ¹³C measurement by GC/C/ IRMS ranged from 0.4 to 0.7‰, and accuracy ranged from 0.0 to 0.4‰. δ¹³C of atmospheric PAHs ranged from -22.9 to -27.2‰ for Chongqing, from -24.2 to -26.8‰ for Hangzhou, from -21.1 to -26.1‰ for Beijing summer and from -22.5 to -25.7‰ for Beijing winter. Atmospheric PAHs of both Chongqing and Hangzhou become more depleted in ¹³C with increasing PAH molecular weight. On the other hand, atmospheric PAHs in Beijing become more enriched in ¹³C with increasing PAH molecular weight. The difference of isotopic signature between "Beijing" and "Chongqing + Hangzhou" is statistically significant (t-test, P < 0.05). The isotopic trends of PAHs in Chongqing and Hangzhou are similar to that obtained from fluidized-bed coal combustion particles, whereas the isotopic trend of PAHs in Beijing air is similar to that obtained from automotive exhaust particles. According to this observation, it appears that automotive exhausts significantly affect PAH loading to Beijing air and that coal combustion is the main cause of PAH existence in Chongqing and Hangzhou air. While the isotopic signatures can classify the three Chinese cities into two groups, the molecular profiles do not show a clear trend among them. This can be explained by the following description: carbon isotopic signatures of PAHs are expected to have source- and/or combustion field-specific values and patterns mainly, whereas molecular compositions of PAHs depend on various factors such as source material, burning temperature, air/fuel ratio, and environmental alteration during transport.