Observation-based Research On Atmospheric Oxidation Mechanism Of Volatile Organic Compounds And Causes Of Ozone Pollution In Yulin City

With the China's air pollution prevention and control work pushing forward,the concentration of PM_2.5 in China has decreased significantly since 2013.However,the environmental O₃ concentration shows an increasing trend in recent years.The causes of O₃ pollution in Beijing-Tianjin-Hebei and its surrounding areas,Fenwei plain,Yangtze River Delta,Pearl River Delta and Chengdu-Chongqing areas have attracted more and more attention.Yulin city is an important energy base and one of the largest coal chemical industry bases.The Loess Plateau and Mu Us Desert are distributed in this area,the research on its air pollutants mainly focused on particulate matter in the past and it is considered as one of the sources of spring dust in northern China.With the rapid development of coal chemical industry,petrochemical industry and transportation industry,the emissions of volatile organic compounds(VOCs)and nitrogen oxides(NOx)are increasing in recent years.As a result,O₃ pollution has become the most important factor affecting the local air quality in recent years.In this study,Yulin city of Shaanxi Province was taken as the research object,analyzed the current situation of environmental O₃ pollution,VOCs atmospheric oxidation analysis,characteristics of NOx pollution the origin of O₃ pollution based on observation.The main conclusions are as follows:(1)From 2017 to 2019,the O₃ concentration in Yulin City showed an overall upward trend,the O₃ concentration increased from 72 ppbv to 75 ppbv,and the O₃pollution days also increased from 26 days to 34 days.The seasonal variation of O₃was obvious,and the O₃ concentrations in summer and spring were higher than that in autumn and winter.O₃ pollution events occurred mainly from May to August.The correlation between O₃ concentration and other atmospheric conventional pollutants concentration in summer was greater than that in other seasons.O₃ and PM_2.5concentrations showed the same rising phenomenon in summer,which indicated that the formation of secondary aerosol caused by photochemical action in summer was also an important factor of PM_2.5,and the two had good homology,which could effectively control precursors in summer.At the same time,the emission of PM_2.5 can be controlled synergistically.(2)From July 7 to August 10,2019,the enhance observation of O₃ and VOCs showed that the concentration of VOCs ranged from 5.75 ppbv to 90.85 ppbv and the concentration of alkanes in VOCs was the highest,and the atmospheric reactivity of alkene,oxygenated VOCs and hydrocarbons was the highest.The key VOCs species were ethanol,2-hexanone,isopentane,ethylene and 1-butene.Positive matrix factor analysis was used to analyze the source of VOCs.The result showed that diesel exhaust(20.6%)and industrial sources(17.0%)are the key sources which effect environmental O₃ concentration in Yulin city.The sensitivity of O₃ generation was analyzed by using the observation-based model.It was found that Yulin city was in the control area of VOCs,the relative incremental reactivity of anthropogenic VOCs(0.76%/%)was the highest.The relative incremental reactivity of each source was analyzed.It was found that the O₃ generation was more sensitive to vehicle and industrial emission.(3)From 2018 to 2020,the average annual concentration of NO₂ in the ambient air of Yulin city is 41?g/m³,42?g/m³ and 37?g/m³ respectively,showing a downward trend as a whole,and the days exceeding the standard are mostly from October to march of the next year.The NOx emission of Shaanxi Gansu Ningxia Inner Mongolia Shanxi border area is high as a whole.The local high emission of Yulin city causes the local NO₂ concentration to be relatively high.At the same time,the air mass transport from the eastern,southern Shanxi and northern regions with high NOx emission and high NO₂concentration has an important contribution to the NOx pollution of Yulin city.The NOx emission of Yulin city also has an important impact on the surrounding areas,especially in the downwind of Yan'an City.(4)Through the analysis of meteorological factors of O₃ exceeding the standard from May to August in 2017-2019,it is found that when the temperature is higher than 25?,the humidity is less than 60%,the wind speed is less than 3m/s and the southerly wind,O₃ pollution was easy to be caused.The backward trajectory model was used to analyze the pollutant air mass transmission when the O₃ pollution event occured.The results show that,from the perspective of interannual variation,the pollution air mass transmission from the Fen Wei plain was mainly affected in 2017and 2019,and the pollution air mass transmission from Ordos,Yan'an and Fen Wei plain was mainly affected in 2018.(5)The influences of different emission reduction scenarios on O₃concentration were analyzed through the observation-based model.From the emission reduction ratio of anthropogenic VOCs and NOx,when the emission reduction ratio is greater than 1:1,O₃ concentration can be reduced.During the intensive observation period,the average concentration of maximum daily average 8h O₃(MDA8 O₃)on the O₃ pollution day was about 5%higher than the national secondary standard,and the maximum concentration of MDA8 O₃ was about 40%higher than the national secondary standard.Therefore,four O₃ concentration reduction targets were set,which were 5%,10%,30%and 40%,respectively.When the reduction target is 5%and 10%,it is expected to be achieved through the existing precursor emission reduction means and technologies.However,when the reduction targets of O₃ are 30%and 40%,VOCs will be reduced by at least 60%and 80%,respectively.