| Cities are the most densely populated area,and the pollution problems in city are increasingly concerned,especially organic pollutants such as polycyclic aromatic hydrocarbons(PAHs).PAHs are a class of typical organic pollutants,which are semi-volatile and highly toxic,and have carcinogenic,teratogenic,and mutagenic effects.In this paper,58 surface dust samples and 62 topsoil samples were collected from Xi'an city.The physiochemical parameters such as pH value,electrical conductivity(EC),total organic carbon(TOC),low frequency susceptibility,high frequency susceptibility,frequency-depended susceptibility,clay,silt and sand of top soil and surface dust were measured by pH meter,conductivity meter,TOC analyzer,magnetic susceptibility meter and laser size analyzer.And the concentrations of 16 priority PAHs of United State Environmental Protection Agency(U.S.EPA)including Nap,Acy,Ac,Flu,Phe,Ant,Fla,Pyr,BaA,Chy,BbF,BkF,BaP,InP,DBA and BghiP were analyzed by gas chromatography with flame ionization detector(GC-FID).Combined with GIS spatial analysis,ratios and multivariable statistics,the spatial distribution characteristics and environmental sources were studied.The pollution level and ecological risk of PAHs in surface dust and topsoil were assessed by means of single pollution index,Nemero comprehensive pollution index,and risk quotient.In addition,the health risk of human exposure to PAHs in surface dust and topsoil was evaluated according to the exposure assessment model of U.S.Department of Energy Risk Assessment Information System.The main results obtained were as follows:(1)The concentration analysis showed that U.S.EPA 16 priority PAHs in surface dust in Xi'an were all detected.In topsoil in Xi'an,the detectable frequencies of Nap,Chy,BbF,BkF,BaP,InP,DBA,and BghiP were 85%,98%,97%,87%,82%,85%,97%,and 94%,respectively;other PAHs(Acy,Ace,Flu,Phe,Ant,Fla,Pyr and BaA)were detected in all urban soil samples.The concentrations of individual PAHs in surface soil varied between 14.69 and 6370.48 ?g·kg-1;the sum of 16 PAHs(?16PAHs)ranged from 5039.67 to 47738.50 ?ig·kg-1,with an average of 13845.82 ?g·kg-1.The content of individual PAHs in topsoil in Xi'an was between ND?1897.63 ?g·kg-1,the total concentration of PAHs(?16PAHs)ranged from 390.62 to 10652.76 ?g·kg-I with an average of 2052.63 ?g-kg-1.Compared with other cities at home and abroad,the contents of PAHs in surface dust and topsoil were at a relatively high level.(2)The correlation between PAHs and physiochemical parameters showed that?16PAH was positively correlated with low and high frequency susceptibilities in surface dust and topsoil samples of Xi'an city.And the frequency-depended susceptibility was negatively correlated with the ?16PAHs in topsoil,but irrelative with?16PAHs in surface dust samples.In addition,there is no obvious correlation between PAHs and other physiochemical parameters(e.g.pH,EC,TOC,particle size)in surface dust and topsoil samples.(3)The ratios of 2-ring,3-ring,4-ring,5-ring and 6-ring PAHs to ?16PAHs in surface dust samples of Xi'an were 1.74%,19.81%,39.44%,23.95%and 15.07%,respectively,which in the topsoil samples were 0.82%,20.08%,41.28%,5 18.79%and 19.03%,respectively.Therefore,4-ring PAHs was the largest proportion in surface dust and topsoil in Xi'an.Meanwhile,the highest proportion of individual PAHs was Fla in surface dust and topsoil,the lowest proportion was Ace and Nap,respectively.(4)The average concentration of ?16PAHs in surface dust of Xi'an decreased in order of industrial areas>educational areas>the traffic areas>mixed business and traffic areas>residential areas>parks.The content of ?16PAHs in topsoil in educational areas was highest,the concentration of ?16PAHs were medium in traffic areas,industrial areas,mixed business and traffic areas,and the values in residential areas and parks were relatively low.In terms of PAHs in different ring roads,the?16PAHs content in surface dust increased from city center to the third ring roads.The highest content of ?16PAHs in topsoil was found in the third ring roads,and the lowest in the second ring roads.Spatial analysis based on GIS showed that the content of?16PAHs in surface dust in the eastern suburbs,the western suburbs and the second part of the surface dust were obviously high,the main urban areas and the southeast corner of Xi'an were relatively low;?16PAHs in topsoil were relatively high in the north second ring road and the north third ring road,and relatively low in main urban areas and southern suburbs of Xi'an(5)The characteristic ratios and multivariate statistical analyses showed that PAHs in surface dust of Xi'an were originated mainly from the burning of fossil fuels and coal,the variance contribution rate of diesel combustion and gasoline combustion were 36.07%and 32.31%,respectively,and the contribution rate of coal combustion was 23.40%.Unlike the surface dust,the combustions of grass,wood and coal combustion as well as oil were the main resources of PAHs in topsoil.The variance contribution rate of coal combustion was 49.97%,and the rate of gasoline combustion and diesel combustion were 31.17%and 12.54%,respectively.(6)The results of pollution evaluation showed that Chy,BbF,BaP,InP and DBA in surface dust were moderate pollution levels and other PAHs were low pollution levels.However,16 individual PAHs in topsoil presented low pollution levels.The comprehensive pollution degrees of ?16PAHs in surface dust and topsoil were moderate level and no pollution,respectively.The ecological risk of PAHs in surface dust in Xi'an as well as its different functional areas was high ecological risk levels.PAHs in topsoil showed middle ecological risk.In general,the pollution level and ecological risk of PAHs in surface dust were higher than those in topsoil.(7)Health risk assessment showed that the ingestion via mouth was the most important pathway of children and adults exposure to PAHs in urban surface dust and topsoil for.And there were no obvious non-carcinogenic health hazards of human exposure to PAHs in surface dust and topsoil.The total carcinogenic risk of human exposure to 7 carcinogenic PAHs in surface dust and topsoil were 2.51×10-5 and 7.25×10-6,which were within the acceptable range of 10-6 to 10-4.However,the carcinogenic risk levels of BaA,BbF,BaP,InP,and DBA in surface dust as well as BaA and DBA intopsoil were higher than the lower standard limit of 10-6.Thus,more attention should be paid for them. |
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