Effect of photooxidation on δ13C of benzo(a)pyrene and benzo(e)pyrene in the atmosphere

The objective of this study is to clarify the effect of photooxidation on delta(13)C 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 delta(13)C 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 delta(13)C of the remaining BaP increased due to isotopic fractionation. In contrast, BeP was not photooxidized and there was no change in delta(13)C. These results lead to the expectation that BaP/BeP and the difference in delta(13)C between BaP and BeP (delta(13)C(BaP)-delta(13)C(BeP)) would decrease and increase, respectively, from the corresponding values at the time of emissions (BaP/BeP: similar to2.1; delta(13)C(BaP)-delta(13)C(BeP): similar to0) 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 delta(13)C of BaP due to photooxidation. In addition, the observed results revealed that BaP/BeP and delta(13)C(BaP)-delta(13)C(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.