Observational Study Of Atmospheric Benzene Series In Beijing Urban Area And Background Areas

Volatile organic compounds(VOCs)are important gaseous pollutants in the air.VOCs can generate ozone(O3)and secondary organic aerosol(SOA)through photochemical reactions,and thus have an important influence on the ecological environment and human health.BTEX are important components of atmospheric VOCs.In recent years,the concentration of nitrogen oxides(NOx)in urban areas of China has decreased,but the concentration of O3 has continued to rise.The variation and role of VOCs are not clear.Under this background,the concentrations of BTEX in Beijing urban area and background area were obtained through online monitoring,and their variation characteristics and influencing factors were analyzed.The generation potentials of BTEX to O3 and SOA were calculated,and their environmental significance was discussed.From July 2018 to May 2020,the average mixing ratio of BTEX in urban area was 8.50±12.17 ppb.From January to December,2018,the average mixing ratio of BTEX in the background area was 0.60±0.65 ppb.Among BTEX,the highest concentration is toluene both in the urban area(3.54±5.17 ppb)and in the background area(0.33±0.34 ppb).The diurnal variations of BTEX mixing ratios in urban and background areas both showed high values at night and low values in the daytime with the lowest appeared during 13:00-15:00.The low values in daytime were mainly affected by photochemical removal and better convection diffusion in daytime.There is a significant difference in BTEX concentrations between the period from July 2018 to February 2019 and from June 2019 to May 2020.In terms of seasonal variation,in the former period,it showed highest value in autumn(7.11±5.74 ppb),then winter(4.31±4.99 ppb),summer(2.88±2.04 ppb),while in the latter period,it showed highest value in summer(17.15±14.50 ppb),then in autumn(13.63±12.66 ppb),winter(9.88±18.85 ppb),and spring(1.73±1.80 ppb).During the COVID-19 epidemic period,urban BTEX concentrations decreased significantly.The concentrations of BTEX in the background area were generally low,with highest value in spring(0.89±0.29 ppb),then in winter(0.73±0.33 ppb),autumn(0.52±0.19 ppb),and summer(0.48±0.17 ppb).The source of BTEX was analyzed using the ratio of benzene to toluene(B/T)method.The main influencing factors of BTEX are mainly affected by coal burning(B/T=1.37)and solvent or traffic sources(B/T=0.47)in summer,by traffic(B/T=0.50)and solvent sources(B/T=0.23)in autumn,by coal combustion(B/T=1.08)and traffic sources(B/T=0.44)in winter,and by coal combustion(B/T=0.8)and traffic sources(B/T=0.47)in spring.The sources of BTEX in urban areas were relatively complex with seasonal differences.In the background area,BTEX were mainly affected by biomass combustion with B/T values of 0.82,0.85,1.07 and 1.47 in spring,summer,fall,and winter,respectively.The ratio of m/p-xylene to ethylbenzene(X/E)was used to study the aging degree of air mass.Small X/E value indicates a higher aging air mass,being mainly affected by regional transport.On the contrary,it is mainly affected by local emission.During July 2018 to February 2019,BTEX in summer(X/E=1.27)and winter(X/E=2.04)is mainly affected by fresh emission sources,but in autumn(X/E=0.63)by aging air masses.During June,2019 to May,2020,the air mass aging degree was the highest in summer(X/E=2.44),then in spring(X/E=1.29),autumn(X/E=0.22)and winter(X/E=0.29).Further classifying the X/E ratios according to the B/T values in summer 2018,the X/E value of 1.20 for coal burning was close to the X/E value of 1.29 for solvent source,indicating that the urban area was still affected by local coal burning source even under the strict policy of coal ban.The air mass aging degree in spring(X/E=0.82)and summer(X/E=0.85)were higher than that in winter(X/E=1.07)and autumn(X/E=1.47)in the background area.The ozone forming potentials(OFP)of BTEX components were calculated by the maximum incremental reactivity method.Toluene was the most component to contribute for the ozone forming potential both in urban and background area,followed by m/p-xylene.The SOA forming potentials of BTEX components were calculated by fractional aerosol coefficient method.It was found that benzene and toluene contributed the most to SOA.The results show that the sources and influencing factors of BTEX in urban areas are very complex,and it is necessary to strengthen the supervision,especially on toluene,which has high concentration and the most important contribution to O3 and SOA forming potentials.