| In recent years,the in-depth of advancement of urbanization,the rapid development of modern industry and transportation have led to the continuous increase in energy demand and the increasing consumption of various energy sources.During the utilization of energy,especially the utilization of fossil energy such as coal and petroleum,a large amount of pollutants,such as sulfur dioxide,nitrogen oxides,aerosol particles and volatile organic compounds(VOCs),are emitted into the environment,which could result in the air pollution occurring frequently and human health threatening.VOCs are a typical pollutant produced in the process of energy utilization.Some VOCs have the characteristics of carcinogenicity,teratogenicity and mutagenicity,and also could increase the concentration of ozone and PM2.5 as the precursor,which could lead the environmental and health effect.Due to the high negative effects on health and environment,it is necessary to study environmental behavior and emission sources of VOCs.In this research,the compositional characteristic and the carbon/hydrogen isotopic composition of VOCs were studied both in the utilization process of main energy(coal chemical industry,coal consumption,vehicle exhaust,solvent evaporation and gasoline volatilization)and ambient atmosphere.The primary conclusions derived from this research have been followed:1.Emission characteristics of VOCs during energy utilization(1)Coal-to-liquids is an important way of coal conversion.The five VOCs exhaust ports,namely sewage pretreatment waste gas,sewage biochemical treatment waste gas,thermoelectric engineering waste gas,intermediate tank area oil and gas recovery exhaust gas,and automobile oil and gas recovery device exhaust gas,was studied as the major VOCs emission source during the process of coal-to-liquids.The highest level of VOCs groups emitted from the five exhaust ports in the coal-to-liquids process were all alkanes,with mass percentages ranging accounting for 61.66%to 87.49%,followed by alkene.Alkane contributed the higher ozone generation potential(OFP)than other VOCs groups in automobile oil and gas recovery device exhaust gas,with the contribution about 63.06%,while alkene contributed the higher OFP than other VOCs groups in the other four VOCs exhaust ports,with the contribution about 53.77%～77.35%.(2)The dominant group of VOCs in coal combustion was alkene,accounting for 60.11%of the total VOCs mass concentration,with the most abundant compounds of 1-butene(41.72%).The proportion of alkanes(17.88%)and aromatic hydrocarbons(19.48%)in the total VOCs were almost equivalent.Compared with alkanes,alkynes and aromatic hydrocarbons,the OFP contributed by alkene was the highest,with the contribution of 87.82%,followed by aromatic hydrocarbons.(3)The dominant group of VOCs in solvent evaporation was aromatic hydrocarbons.The proportion of 1,2,4-trimethylbenzene was the highest(29.71%),followed by m,p-xylene(21.85%).The mass percentage of 9 compounds was greater than 1%,accounting for 88.91%of total VOCs.2.δ13C and δD isotope characteristics of typical VOCs in the process of energy utilization(1)For vehicle exhaust and gasoline volatilization,the δ13C values of typical VOCs(volatile aromatic compounds)ranged from-27.58‰ to-25.50‰ and-30.90‰ to-28.02‰,respectively.The average δ13C value of volatile aromatic compounds in vehicle exhaust indicated the enrichment of 13C compared with gasoline volatilization,especially of benzene(5.05‰).This characteristic can be used to identify the volatile aromatic compounds emission from vehicle exhaust in the ambient air.The average δD value of benzene in vehicle exhaust was poor with D compared to gasoline.Benzene in vehicle exhaust may be generated by a small molecule unsaturated hydrocarbon produced from pyrolysis of large molecules in gasoline,and stable hydrogen isotopes can be used in the study of pollutant transformation process.(2)For coal combustion,the δ13C values of volatile aromatic compounds ranged from-25.14‰～-23.44‰,which showed 13C enrichment of volatile aromatic compounds emission from coal combustion compared with other sources.δD values of volatile aromatic compounds ranged from-168.30‰～-142.33‰,indicating D poverty of volatile aromatic compounds emission from coal combustion compared with other sources.(3)For solvent evaporation,the δ13C values of volatile aromatic compounds ranged from-27.67%～-32.13‰.δD values of volatile aromatic compounds ranged from-147.00%～-121.08‰.3.Component characteristics of VOCs in ambient airDuring the observation period,the concentration of VOCs of ambient air in Taiyuan city was 127.46μg·m3,and the percentage of alkanes and aromatic hydrocarbons accounted for 44.44%and 45.71%of the total VOCs,respectively.The average concentration of VOCs was 49.73 μg·m-3 in summertime and 205.19 μg·m-3 in wintertime.The peak and valley values appeared in periods of 7:00-9:00 in summertime and 9:00-10:00 in wintertime,which showed VOCs diurnal variation was obviously affected by vehicle emission and photochemical reaction.4.Stable hydrocarbon isotopic composition of typical VOCs in ambient airThe δ13C values of volatile aromatic compounds in ambient air of three sampling site in Taiyuan,namely SL,TY and XD,were-24.8‰ to-26.2‰,-24.8‰ to-26.9‰ and-23.9‰ to-25.4‰,respectively.The average δ13C value of XD site indicated 13C enrichment compared with SL and TY.The δD values of volatile aromatic compounds in ambient air of SL and XD were-131.66‰ to-115.09‰ and-131.91‰to-74.85‰.This study was the first time reported the stable hydrocarbon isotope values of volatile aromatic compounds in ambient air in different areas of Taiyuan City,and analyzed the distribution characteristics of isotopes.The results could provide data support for further source analysis of volatile aromatic compounds.5.Source apportionment of VOCs in Taiyuan(1)Ratios of benzene/toluene was higher than 1 in Taiyuan,indicating that VOCs were derived from coal consumption.The result of source apportionment by positive matrix factorization(PMF)model shows that,the sources of coal combustion,vehicle emission,evaporation,LPG and biomass emission,contributed 28.10%,27.41%,22.90%,14.90 and 6.69%,respectively.(2)Isosource-based quantitative calculation,stable carbon isotope characteristics of volatile aromatic compounds in the various emission sources and ambient air was conducted to determine the contribution of various emission sources in the three sampling sites.The results shows that the mean contribution of coal combustion,vehicle exhaust,solvent evaporation,and gasoline volatilization to volatile aromatic compounds in the ambient air were 62.3%,22.0%,7.2%and 8.5%in SL;46.0%,30.6%,10.8%and 12.6%in TY;74.1%,15.3%,4.8%and 5.8%in the XD,respectively.Coal combustion and vehicle exhaust were the major sources of volatile aromatic compounds in ambient air in Taiyuan.(3)Based on the stable hydrogen isotope characteristics of volatile aromatic compounds in the various emission sources and ambient air,the linear fitting method of mass balance equation was used to analyze the source of volatile aromatic compounds.Coal combustion and vehicle exhaust source were the major sources of pollution in the ambient air in Taiyuan.The results of different source apportionment method all showed that the main sources of VOCs in Taiyuan were coal combustion and vehicle exhaust.Therefore,to reduce the usage of coal and control the pollutant emission of motor vehicles is an important way to reduce the VOCs pollutants in the ambient air in Taiyuan city. |
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