Pollution Characteristics,Scavenging And Source Analysis Of PAHs In Precipitation In Taiyuan City

Organic/inorganic pollutants in the atmosphere could be transported over a long distance or removed by atmospheric dry and wet deposition.Polycyclic aromatic hydrocarbons(PAHs)could be tranferred into other environmental medium by wet deposition,and threaten the ecosystem and human health.In this study,precipitation samples were collected from 2012 to 2016 in Taiyuan.After that the pollution levels,scavenging efficiency and sources of 15 PAHs were analysed,as well as the removal mechanism.The main results were as follows:(1)The concentrations of PAHs in wet deposition in Taiyuan ranged from1844.50 to 4268.05 ng/L,with an average of 2495.37 ng/L.The particulate PAHs contributed 54.60-77.67%.The total PAHs,dissolved PAHs and particulate PAHs in precipitation samples were all dominated by 3-4 rings PAHs.Wet deposition flux of PAHs in Taiyuan was 1761.15-4738.96 ng/m~2-d,with an average of 3054.42 ng/m~2-d.Wet deposition flux of PAHs was mainly accumulated in the particulate phase,accounting for 54.60-77.67%of the total wet deposition flux.(2)As a whole,the annual variation of the wet deposition flux of PAHs showed a declining trend from 2013 to 2015,while rebounded in 2016,but the value in 2016 was still lower than that of 2013.The high wet deposition flux in2013 was mainly because of the use of heavy machinery and coal-bitumens for road paving,while that in 2016 was because of large-scale construction activities.Wet deposition flux of PAHs had no significant change from March to May,while increased with the enhancement of precipitation from June to October,and held steady from November to February in the next year.The wet deposition flux of PAHs in spring was comparable to that in autumn,while higher than that in winter and lower than that in summer.(3)Scavenging efficiency of total PAHs in Taiyuan in 2016 ranged from1.37×10~4 to 2.27×10~5.Scavenging efficiency of particulate phase was dominated,accounting for 74.25%of total scavenging efficiency.With the increase of PAHs ring number,the contribution of particulate phase scavenging to the total showed a decrease trend,because the low-ring PAHs are easy to adsorb on the coarse particles,and the precipitation has a significant effect on the removal of coarse particles.Larger fraction of high-ring PAHs was removed by gas phase than low-ring PAHs,which may be related to the pore of the filter membrane(0.45μm).(4)The theoretical scavenging efficiency of gas phase PAHs was5.18×10~2-3.64×10~5,with an average of 6.05×10~4.The scavenging efficiency of PAHs in the dissolution process was higher than that in the adsorption process.The measured scavenging efficiency was higher than the theoretical scavenging efficiency,indicating that the supersaturation of the PAHs in the dissolved phase had an influence on the scavenging efficiency.The concentration of the aerosols,the concentrations of OC and EC in the aerosols,the meteorological parameters and precipitation intensity all could affect the measured scavenging efficiency.(5)The results based on the analysis of diagnostic ratios showed that the PAHs in precipitation in Taiyuan were mainly from the combustion of coal,biomass and oil,while a small part originated from petroleum sources.Source apportionment by Positive Matrix Factorization(PMF)model showed that the sources of PAHs in precipitation were coal combustion,motor vehicle emissions,coking sources,biomass burning and petroleum sources.The cluster analysis of backward trajectories from 2012 to 2016 showed that air masses from south and northeast contributed 55%and 27%,respectively.In conclusion,coal combustion and coking were the major sources of the PAHs in the 5-year precipitation in Taiyuan,and the contribution of petroleum source was smaller.