Volatile organic compounds (VOCs) are important precursors of both ozone and secondary organic aerosols in the atmosphere, some of which are carcinogenic, teratogenic, or mutagenic. VOCs in ambient air originate from many sources, including vehicle exhausts, gasoline evaporation, solvent use, natural gas emissions, and industrial processes, and undergo intricate chemical reactions in the atmosphere. To develop efficient air pollution remediation strategies, it is important to clearly identify

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... e emission sources and elucidate the reaction mechanisms in the atmosphere. Recently, Kikuchi and Kawashima (2013) 1 and Kawashima and Murakami (2014) 2 used gas chromatography/thermal conversion (combustion)/isotope ratio mass spectrometry coupled with a thermal desorption instrument (TD-GC/IRMS) to measure hydrogen (δ 2 H) and carbon (δ 13 C) stable isotope compositions of atmospheric VOCs. In these studies, larger δ 2 H differences between sources were found in comparison to the corresponding δ 13 C. Therefore, determining δ 2 H values of VOCs in ambient air is potentially useful in identifying VOC sources and their reactive behaviors in the atmosphere. However, to elucidate the sources and behavior of atmospheric VOCs more accurately, isotopic fractionation during atmospheric reaction must be characterised. Although some studies on hydrogen and carbon isotope fractionation during photochemical reactions have been reported 3,4 , additional studies of photochemical reactions under different wavelengths and irradiation times should be conducted. In this study, we determined the δ 2 H values of the atmospheric VOCs undergoing various irradiation conditions.