Atmospheric Gas-particle Partition And Precipitation Scavenging Of Polycyclic Aromatic Hydrocarbons At Mount Hengshan

Atmospheric gaseous and particulate as well as the rain samples were collected to study the gas-particle partitioning of PAHs and the scavenging ratios of atmospheric PAHs by the wet deposition at Mt. Hengshan, a high elevation site with1269m A.S.L. This study will present the background concentrations of measured PAHs at such a high elevation, which will be useful to the researchers of home and abroad in this field. What’s more, it will be of great significance to further investigate the long range transport of PAHs in China.A total of41fine particle,48total suspended particle,39gaseous, and34rain samples were collected at the summit of Mt. Hengshan during March to May in2009to study the concentrations of PAHs in the atmosphere and precipitation, the partitioning between gaseous and particulate phases of PAHs, the scavenging ratios of atmospheric PAHs by rain, as well as the sources of PAHs in this area.A total concentration of15USEPA PAHs measured in this area varied from2.67to137.00ng/m3in the ambient, with an average of24.10ng/m3. It was lower than those reported for urban and rural sites in China, but higher than that of the high altitude sites abroad. It was dominated by the3rings of PAHs which have lower molecular weights but higher volatility.Junge—Pankow model, Harner—Bidleman (Koa) model, as well as Ksoot-air model (Ksa) considering the adsorption by soot were used to predicted the partitioning coefficient Kpof PAH compounds, and were compared with the measured Kpvalues. A linear relationship between log Kp and log PL0(PL0is the compound’s subcooled liquid-vapor pressure, Pa) were found (R2=0.92), but the slope of-0.35was steeper than the expected value of-1. In the present study, the variation of temperature and the artifact during the sampling, and the non-exchangeability of PAHs on the particulate phase, etc. were recognized to associate to the steeper slope.The fraction of organic matter of20%in the atmospheric particles were estimated for the prediction of log Kp using the Koa model, and an additional adsorption of5%black carbons was considered by Ksoot-air model. Comparison of the predicted Kp by the two models and the measured values, Koa model underpredicts the Kp by a factor of1-2log units. The Ksa model could well predict PHE, PYR, CHR and FLA, pointing to the importance of soot carbon for the long range transport of aerosol-associated PAHs. However, ANT and BaA were not well predicted by the Ksa model, which were likely due to their degradation via OH radical and photo degradation.The average concentration of total PAHs was536.42ng/1in rain water, and was dominated by2-4rings PAHs, with the two highest concentrations PHE and FLU. The concentrations of PAHs in the filtrate were observed higher than the filter particles. It was lower than those reported for urban and rural sites in China, but higher than that of the high altitude sites abroad.Rain samples and gas/particulate phase samples before rain were selected to investigate precipitation scavenging of PAHs. Total scavenging ratios ranged from104to105for PAHs, lower than other reports in India and Singapore, which was possibly due to the cleaner atmosphere at Mt. Hengshan.The particle and gas scavenging ratios observed ranged from103to106and from104to105, respectively, and the gas scavenging was the dominant removal process, accounting for45.9-96.8%for PAHs. While the predicted gas scavenging ratios by a comprehensive precipitation scavenging model were1-3orders of magnitude lower than the observed. The particle scavenging was the dominant process accounting for42.05-99.01%in terms of the particle contribution to the total scavenging, much higher than that for gas scavenging. The difference between the observed and predicted would be likely to due to the submicron particles existing in the filtrate.Diagnostic Ratio and principal components analysis (PCA) were used to do the source identification. Domestic coal and biomass combustion were recognized the main sources of PAHs in this area. Three factors were identified as pyrolytic sources, coal combustion and coking industry. However, the results of PCA would be better interpreted to be associated with the environmental behaviors, e.g. deposition and degradation during the transport process of PAHs. More effort should be given to the source idendification of PAHs at Mt. Hengshan using the advantaged model, e.g. PCA-MLR, PMF and CMB.