Comparisons of gas-PM2.5 and gas-TSP partitioning of non-polar organic molecular makers in northern Nanjing, China

Abstract

The gas-particle (G-P) partitioning of semi-volatile organic compounds has been commonly investigated by simultaneously measuring their concentrations in the gas and particle phases, but the effects of coarse particles were not well known. In this study, collocated gas-PM_2.5 and gas-TSP samples were collected in the suburbs of Nanjing, a typical megacity in the Yangtze River Delta in China, and analyzed for non-polar organic molecular markers (NP-OMMs), including n-alkanes, polycyclic aromatic hydrocarbons (PAHs), oxygenated PAHs (oxy-PAHs), and steranes and hopanes. Except for low-volatility n-alkanes (vapor pressure < 10⁻¹⁰ atm), all other groups of NP-OMMs in paired PM_2.5 and TSP samples had comparable median concentrations, and the concentration time series of PAHs, oxy-PAHs, and steranes and hopanes were strongly correlated (r > 0.80, p < 0.01). Based on the collocated measurements of gaseous samples, the relative uncertainties for the concentrations of grouped n-alkanes and oxy-PAHs (17.3 %–27.7 %) are greater than for the PAHs (12.4 %–21.3 %). The mean particulate fractions of the grouped NP-OMMs were almost identical between gas-PM_2.5 and gas-TSP samples, indicating a weak influence of coarse particles on the distribution of NP-OMMs in the gas and particle phases in Nanjing. Whenever the gas-PM_2.5 or gas-TSP data were used to validate the prediction of G-P partitioning coefficients, the partitioning of gaseous n-alkanes and PAHs into the particle phase was majorly driven by absorption and adsorption mechanisms, respectively. When coarse particles were considered for the prediction of the G-P partitioning of PAHs, less liquid-like sorption had to be assumed.