ABSTRACT

The objective of this study is to clarify the effect of photooxidation on δ¹³C of benzo(a)pyrene (BaP) and benzo(e)pyrene (BeP) in the atmosphere. They are structural isomers but are known to differ significantly in terms of photooxidation rate. The results of literature reviews and measurements revealed that the concentration ratio of BaP and BeP (BaP/BeP) is almost constant independent of the fuel type and emission source (almost constant at 2.1), while δ¹³C values for the two PAHs are almost equal. On the other hand, the results of ultraviolet irradiation experiments indicated that the concentration of BaP was reduced by photooxidation while δ¹³C of the remaining BaP increased due to isotopic fractionation. In contrast, BeP was not photooxidized and there was no change in δ¹³C. These results lead to the expectation that BaP/BeP and the difference in δ¹³C between BaP and BeP (δ¹³C_BaP – δ¹³C_BeP) would decrease and increase, respectively, from the corresponding values at the time of emissions (BaP/BeP: ∼2.1; δ¹³C_BaP – δ¹³C_BeP: ∼0) with the progress of photooxidation. This anticipated relationship was close to the results observed over Matsue, which is located on the Japan Sea coast, strongly suggesting there is a change in δ¹³C of BaP due to photooxidation. In addition, the observed results revealed that BaP/BeP and δ¹³C_BaP – δ¹³C_BeP levels during the winter, when photooxidation is less likely to occur compared to the summer, were similar to the levels during the summer. This supports the hypothesis that in winter, BaP emitted in the Asian continent was photooxidized during the long-range transport process before reaching the atmosphere over Matsue.