The impact of gaseous degradation on the gas-particle partitioning of methylated polycyclic aromatic hydrocarbons

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 prediction of the gas-particle (G-P) partitioning quotient ( K P ' ) remains a challenge, especially for the light-molecular-weight (LMW) SVOCs due to their upward deviation from equilibrium state. In this study, the phenomenon with the influence of gaseous degradation on G-P partitioning was observed. Based on the diurnal study of concentrations and K P ' values for methylated polycyclic aromatic hydrocarbons (Me-PAHs), it was found that the K P ' values of methylated naphthalenes (Me-Naps; one type of LMW SVOC) during the daytime were higher than during the nighttime, and the regression lines of log K P ' versus log K OA (octanol-air partitioning coefficient) for daytime and nighttime were non-overlapping, which were different from other Me-PAHs. Compared with other diurnal influencing factors, the higher gaseous degradation of Me-Naps in the daytime than in the nighttime should partially explain their special diurnal variation in K P ' , which provided a new explanation for the non-equilibrium behavior of K P ' for LMW SVOCs. Moreover, the influence of gaseous degradation on the deviation of K P ' from equilibrium state was deeply studied based on the steady-state G-P partitioning model considering particulate proportion in emission ( phi 0 ). The increasing times of K P ' influenced by the gaseous degradation deviated from equilibrium state can be calculated by 1 + 13.2 phi 0 x k deg ( k deg , gaseous degradation rate). The increase in K P ' along with the increase in k deg proved that higher gaseous degradation in the daytime could increase the K P ' value. Furthermore, an amplification in K P ' ranging from 1.11 to 5.58 times (90 % confidence interval: 1.01 to 14.4) under different phi 0 values (0 to 1) in the temperature range of - 50 to 50 degrees C was estimated by the Monte Carlo analysis. In summary, it can be concluded that the influence of gaseous degradation should also be considered in the G-P partitioning models of SVOCs, especially for the LMW SVOCs, which provided new insights into the related fields.