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Abstract: Six unique OH-reaction products of naphthalene and phenanthrene were detected and quantified in ambient PM2.5 collected in Seoul, Korea between 2006 and 2007. The range of annual average concentrations of six reaction products, 2-formylcinnamaldehyde, phthalic acid, phthalide, dibenzopyranone, 9-fluorenone, and 1,2-naphthalic anhydride extended from 2.45 to 49.9 ng m−3 and all of these values were higher than the average concentration of single particulate PAH compounds in Seoul, Korea. The seasonal pattern of six reaction products generally showed higher concentrations in winter months than in summer months. This indicates that the formation of these compounds by atmospheric photochemical reactions is significant both in winter and summer and that enhanced primary sources and higher particle to gas partitioning activity due to lower ambient temperature may contribute to the high concentrations of these compounds in the winter months in Seoul, Korea. The high correlation of the formation of 2-formylcinnamladehyde and dibenzopyranone with the estimated SOC concentration, suggests that the formation of SOA from gas phase PAH reactions in the real atmospheres is significant. Also, we suggest that (E)-2-formylcinnamaldehyde and dibenzopyranone could be used as unique indicators for the atmospheric oxidative reactions of naphthalene and phenanthrene, respectively. [Copyright &y& Elsevier]