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 PM_2.5 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 (p^o_L) 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 PM_2.5 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 p^o_L 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 p^o_L and surface adsorption should be carefully considered when parameterizing the gas-particle partitioning of non-polar organic compounds in future field and modeling studies.