Simulation of Gas‐Particle Partitioning of Semi‐Volatile n‐Alkanes and PAHs in Nanjing, China, and Denver, United States: Effects of Vapor Pressure and Surface Adsorption Estimation.

The concentration data of semi‐volatile n‐alkanes and PAHs in the gas phase and in PM2.5 were obtained from Nanjing and Denver. First, the gas‐particle partitioning coefficients of the target compounds were calculated and compared with the predictions based on the equilibrium absorptive partitioning theory. Although the vapor pressure (poL) estimation method was selected to improve the agreement between measured and predicted partitioning coefficients, n‐alkanes in Denver and PAHs in both cities exhibited stronger sorption to PM2.5 than predicted. By including the adsorption mechanism, the average logarithms and temporal variations of the partitioning coefficients were well simulated in both Nanjing and Denver. The partitioning of n‐alkanes in Nanjing can be primarily explained by the absorption mechanism, while the adsorption mechanism dominates that for n‐alkanes in Denver and PAHs in both cities. To obtain an optimal simulation, the poL of n‐alkanes and PAHs was estimated using the SPARC and SIMPOL—two group contribution methods, respectively, and the difference between desorption and vaporization enthalpies was set between 2 and 3 kcal mol−1 for n‐alkanes in Denver and PAHs in both cities. Therefore, the estimation of poL and surface adsorption should be carefully considered when parameterizing the gas‐particle partitioning of non‐polar organic compounds in future field and modeling studies. Plain Language Summary: The gas‐particle partitioning of semi‐volatile organic compounds is an important atmospheric process that can affect their fate and transport. The concentrations of selected n‐alkanes and PAHs significantly present in both the gas and particulate phases in Nanjing and Denver were obtained from our previous studies. When absorption in particulate organic matter (OM) was assumed as the dominant mechanism, the gas‐particle partitioning of n‐alkanes in Denver and PAHs in both cities cannot be well explained. When both absorption in the particulate OM phase and surface adsorption were considered, good agreement between measured and predicted partitioning coefficients could be obtained for all target compounds by selecting an appropriate method to estimate the vapor pressure (poL) and fixing the difference between desorption and vaporization enthalpies. In previous studies, the simulated partitioning coefficients differed by orders of magnitude from the measured values, partly due to the ignorance of poL estimation method selection and surface adsorption. Key Points: n‐Alkanes and PAHs can show stronger sorption to particulate matter (PM) than in the simulation based on the equilibrium absorptive partitioning theoryCombining absorption and adsorption mechanisms can improve the agreement between measured and predicted partitioning coefficientsThe sorption of n‐alkanes and PAHs on PM is less liquid‐like in Denver than in Nanjing [ABSTRACT FROM AUTHOR]