| Volatile Organic Compounds?VOCs?are important precursors of tropospheric ozone?O3?and secondary organic aerosols?SOA?and have important effects on environmental effects and human health.In July 2018,the State Council officially released the Three-Year Action Plan to Win the Blue Sky Defense War?hereinafter referred to as the Action Plan?in order to evaluate the impact of the Action Plan on VOCs in Beijing's ambient air before and after its implementation.In this study,using the Airmo VOC online analyzer produced by CHROMATOTEC,France.We observed the concentration levels of 59 VOCs in spring of Beijing urban area from 2017 to 2019,analyzed the characteristics of VOCs concentration and composition changes,identified the main active components and their source changes,and selected a typical pollution process for study each spring.The main conclusions are as follows:?1?The hourly average mass concentrations of TVOCs in spring of Beijing urban area during the observation periods of March to May 2017,2018,and 2019 were respectively2019?39.14±32.07?g/m3?>2017?34.36±19.96?g/m3?>2018?33.48±20.46?g/m3?.The increase in mass concentrations of VOCs in spring 2019 was primarily due to the increase in concentrations during the 00:00-06:00 period.And the mass concentration of NOx decreased year by year,as 2017?83.91?g/m3?>2018?70.37?g/m3?>2019?66.04?g/m3?.The inter-annual variation in VOCs was not significant compared to the decrease in NOx mass concentration.?2?The diurnal variation characterizations of VOCs and components during the three-year spring observation period indicated that diesel vehicle emissions during the early morning hours and gasoline vehicle emissions during the morning and evening traffic peaks were important sources of VOCs in Beijing's spring ambient air,while alkenes maxima occurred at the time of greatest temperature and light intensity,indicating that biological sources were also an important source of VOCs in Beijing's ambient air.?3?The mass concentrations of TVOCs and active VOCs were the lowest in 2018,but the O3 mass concentration?63.87?g/m3?was significantly higher than the other two seasons?53.65?g/m3 and 60.09?g/m3?.It can be seen that for Beijing only to weaken the mass concentration of VOCs alone or the NOx mass concentration is unable to effectively reduce the mass concentration of O3.The mass concentration of VOCs and NOx needs to be proportionately reduced to effectively reduce the O3 mass concentration in Beijing's ambient air.?4?The top 10 mass concentration VOCs species during the observation period were propane,ethane,n-butane,i-pentane and benzene,toluene.The key active species based on ozone formation potential?OFP?,the OH radical consumption rate(LOH)and secondary organic aerosol formation potential?SOAFP?were ethylene,propylene,benzene,toluene,xylenes,ethylbenzene and n-decane.?5?Positive Matrix Factorization?PMF?analysis showed that the main sources of VOCs were automotive exhaust sources and solvent usage sources in 2017 and 2019,while the main sources of VOCs were automotive exhaust sources and fossil fuel combustion sources in 2018.Based on the contribution rate of each source to OFP and SOAFP,solvent usage sources and automotive exhaust sources are the priority sources of VOCs to be controlled.?6?The trends of the components of VOCs in the heavy pollution process in 2017 and2018 and the moderate pollution process in 2019 were consistent with the changes in PM2.5,CO and NOx,indicating that their sources were relatively stable and their mass concentration changes were mainly influenced by meteorological conditions.Through the backward trajectory,potential source contribution function?PSCF?and concentration weighted trajectory?CWT?analysis showed that it can be determined that the urban area of Beijing was affected by external transmission as well as local emission sources during the heavy pollution process in 2017 and 2018 and the moderate pollution process in 2019.The potential source-areas of VOCs in Beijing were mainly concentrated in the southern and southeastern regions of Hebei province. |
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