Fractionation of stable carbon isotopes during UV degradation of toluene: implications for the quantitative characterization of volatile organic compounds.

Volatile organic compounds (VOCs) are major atmospheric pollutants commonly derived from the fossil fuel combustion. The concentration of VOCs in the atmosphere and its dynamics have widely been used to evaluate their source, formation processes, residence time, and photochemical reactions involved in the atmosphere. However, little is known about the effect of UV degradation of VOCs during their transport from the source to the study area, which always reduces accuracy in the understanding of VOCs' characteristics in the atmosphere. In the present study, we investigated the fractionation of carbon isotopes (13C/12C) of toluene (methyl benzene) during UV degradation (254 nm UV-C), a common VOC, in the atmosphere. The observed fractionation (α = 0.9935) confirms that UV-C photodegradation enriches 13C in residual toluene, but applying this result to quantify sources or degradation in the atmosphere requires further constraints and assumptions. This correlation thus can be useful for the quantitative illustration of the environmental behavior of toluene (e.g., excretion sources, transfer, UV degradation, deposition) in the atmosphere and biosphere.