The impact of gaseous degradation on the equilibrium state of gas/particle partitioning of semi-volatile organic compounds
Abstract. The partitioning of semi-volatile organic compounds (SVOCs) between gas and particle phases plays a crucial role in their long-range transport and health risk assessment. However, the accurate predicting of the gas/particle (G/P) partitioning quotient (KP') remains a challenge, especially for the light molecular weight (LMW) SVOCs due to their upward deviation from the equilibrium state. Based on the diurnal study of concentrations and KP' values for methylated polycyclic aromatic hydrocarbons (Me-PAHs), it was found that the diurnal variations of methylated naphthalenes (Me-Naps, one type of LMW SVOCs) were different from other Me-PAHs, that KP' values during daytime were higher than that during nighttime, and the regression lines of log KP' versus log KOA (octanol-air partitioning coefficient) for daytime and nighttime were non-overlap. It was found that the higher gaseous degradation of Me-Naps during daytime than that during nighttime should be responsible for their special diurnal variation of KP', which provided a new explanation for the non-equilibrium behavior of KP' of LMW SVOCs. Moreover, the influence of gaseous degradation on the deviation of KP' from the equilibrium state was deeply studied based on a theoretical model considering particulate proportion in emission (ϕ0). It was found that the deviation occurred when ϕ0FGR (FGR, degradation flux of gas phase) cannot be ignored when compared with FGP (flux from gas phase to particle phase). It can be concluded that the deviation was not only related to the gaseous degradation rate (kdeg), but also related to ϕ0. Furthermore, an amplification of KP' ranging from 1 to 8.4 times under different ϕ0 (0 to 1) in the temperature range of −50 to 50 °C was estimated based on the individual degradation rates of Me-Naps and three LMW PAHs. In summary, it can be concluded that the influence of gaseous degradation should also be considered for the G/P partitioning models of SVOCs, especially for the LMW SVOCs, which provided new insights into the related fields.
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