Study On The Characteristics Of Atmospheric VOCs And Ozone Formation In Summer In Taiyuan

At present,the pollution of fine particulate matter(PM_2.5)in key areas of China has been effectively controlled,whereas the concentration of ozone（O₃）showed increasing trend and O₃ pollution has become an important environmental problem.Taiyuan is an important energy and heavy industry base in China,O₃ levels in Taiyuan have been rising year by year.The number of days with O₃ as the primary pollutant increased from 35 days in 2015 to 100days in 2018.Untill now,the characteristics and sensitivity of O₃ and its precursors（volatile organic compounds（VOCs）and nitrogen oxides（NO_x））in Taiyuan have not been reported in detail.In this study,real-time monitoring of O₃ and NO_x and 3-hour off-line measurement of VOCs during a 10-day intensive campaign were conducted at four sites（SL,TY,JY and XD）in the urban area of Taiyuan in July 2019.The spatial and temporal distribution of O₃ and its precursors（VOCs and NO_x）,and meteorological impacts on O₃ formation were analyzed.The sensitivity of O₃formation was identified.The sources of VOCs and their contribution to the ozone formation potential（OFP）were discussed.The impacts of regional transport and local emissions of VOCs sources to OFP were further evaluate.The main results are as follows:1.During the sampling period,the average concentration of O₃ in Taiyuan was 65.41 ppbv,and the difference of the average concentration of O₃ at four sites were small.During the sampling period,the maximum hourly concentration of O₃ during the O₃ attainment period（hourly averaged concentration<103 ppbv,EP1）was 97.56 ppbv,and the maximum hourly concentration during the O₃ exceed period（hourly averaged concentration>103ppbv,EP2）was 131.74 ppbv.The peak of O₃ concentration appeared at15:00-18:00,and the peaks of VOCs and NO₂ occurred at 3:00-9:00.Higher temperature,lower relative humidity,weaker winds and local photochemical reaction were to the main reasons for the occurrence of O₃ pollution days.2.The average concentration of TVOCs in Taiyuan was 14.79 ppbv,with the highest concentration of TVOCs at TY site（17.20 ppbv）,followed by XD（16.34 ppbv）,JY（14.66 ppbv）and SL（10.97 ppbv）.Ethane（26%）,ethylene（10%）,acetylene（9%）,propane（9%）and benzene（6%）were the top five compounds in TVOCs,higher concentrations of C₂ species and benzene indicated that atmospheric VOCs in Taiyuan was mainly related to coal combustion.3.The analysis results of the maximum incremental reaction（MIR）and the propylene equivalent concentration（PE）found that the top 10 VOCs contributing to O₃ included 6 alkenes and 3 aromatics.Among the 10 top VOCs species,the contribution of isoprene was the highest（32-38%）.The results of VOC/NO_x ratio indicated that the formation of O₃ was under co-controlled by both VOCs and NO_x during the period of 12:00-18:00.4.The results of PMF model analysis showed that coal or biomass combustion（33%）,coking sources（28%）,vehicular emissions（14%）,solvent usage（10%）,industrial processes（8%）and biological sources（7%）were the main sources of VOCs in Taiyuan.Combined with wind direction and wind speed,it was found that coal or biomass combustion,industrial processes and vehicular emissions were caused by local emissions.In addition to local emissions,the high concentrations of coking sources and solvent usage were mainly concentrated in the southwest industrial zone of Taiyuan.5.The backward trajectory analysis by TrajStat found that higher concentration of O₃ in air masses from local（70%）and southern areas（22.5%）during EP2.The local（32%）and southern（30%）coking sources were the main sources of OFP during EP2.The diurnal variations of the contribution of VOCs sources to OFP showed that the higher values of the contribution of coking sources to OFP appeared between 6:00-12:00.