Characterization And Causes Analysis Of Ground-level Ozone In A Typical City In Fen River Basin—jinzhong

In recent years,the situation of ambient ozone（O₃）pollution in urban areas of China has become prominent.O₃has become one of the main pollutants affecting the ambient air quality.Prevention and control of O₃pollution has become the key to improving air quality.Accurate understanding of the characteristics of O₃pollution and its causes is an important basis for formulating effective O₃pollution control strategies in urban areas.At present,most of the studies focus on key areas such as Beijing-Tianjin-Hebei and surrounding areas,the Yangtze River Delta,the Pearl River Delta,and Chengdu-Chongqing.There are few reports about the urban areas of the Fen River Basin.This paper selects Jinzhong,a representative city in the Fenhe River Basin as the research object,and comprehensively uses observation-based simulation and machine learning to clarify the temporal and spatial variation characteristics of O₃pollution and its impact on air quality in Jinzhong from 2017 to 2020.On the basis of,the cause analysis of typical O₃pollution process was carried out,and the relative importance of meteorological factors,precursors and other conventional pollutants in the process was quantitatively evaluated by using the random forest algorithm in machine learning.The main conclusions are as follows:（1）Although the evaluation value of O₃in Jinzhong has been decreasing year by year from 2017 to 2020,the evaluation value of O₃in 2020 has not yet reached the standard（91.9 ppbv,or 196μg/m³）.During the four years,when the O₃pollution process occurred in autumn,the average of the daily hourly average of O₃,the average of the 90^thpercentile of the daily hourly average,and the average of the 10^thpercentile of the daily hourly average,the average value and the daily maximum 8-hour average showed an increasing trend year by year;and when the O₃pollution process occurred in spring and summer,the above averages showed a downward trend year by year.In Jinzhong,there was an obvious O₃weekend effect in 2017 and 2020,and a negative weekend effect in 2018 and 2019.The spatial distribution of O₃concentration in Jinzhong is characterized by the fact that the concentration at the north-south direction is higher than that at the east-west direction.When O₃pollution occurred in Jinzhong,the hourly average wind speed was 0.65 m/s,the high-frequency wind directions were east and east-northeast,the hourly average air pressure was 915.6 hpa,the hourly average temperature was 22.6°C,and the hourly average relative humidity was 68.6%.（2）The analysis of the O₃pollution process at different sites in Jinzhong at the same time period showed that the O₃pollution at two sites（Nandu Community and Yuci No.3 Middle School）in Jinzhong from July 20-26,2020（Episodes Pollution 28,EP28）.Most of the generation is in the coordinated control area of volatile organic compounds（VOCs）and nitrogen oxides（NO_x）,and they are more sensitive to anthropogenic VOCs.The air mass transport mainly comes from east-southeast,which has a great potential contribution to O₃generation.The analysis of the O₃pollution process at the same site in Jinzhong in different periods shows that the three periods of Nandu Community[（April 27-30,2020,Episodes Pollution 24,EP24）,（June 3-14,2020,Episodes Pollution 25,EP25）,（July 14-16,2020,Episodes Pollution 27,EP27）]O₃generation is mostly in the collaborative control area of VOCs and NO_x,and both are more sensitive to anthropogenic VOCs.During EP24,the air mass mainly came from Shaanxi,Inner Mongolia,have a small potential contribution to O₃generation.During EP25,air mass transport mainly comes from Shandong,Henan,Anhui,and has a greater potential contribution to O₃generation.During EP27,air mass transport mainly comes from the southwest direction of Shanxi Province,It has a large potential contribution to O₃generation.The analysis of key VOCs species shows that the main active VOCs species generated by O₃in Jinzhong are mainly olefins,aromatic hydrocarbons and aldehydes and ketones.Positive Matrix Factorization（PMF）results show that from May to July 2020,the main source of VOCs in O₃pollution process was process source（44.4%）,followed by oil volatilization（11.6%）and plant source（11.6%）),and show that from July 20 to 26,2020,the sources of VOCs in the O₃pollution process were mainly fossil fuel combustion,plant sources and solvent volatilization sources.（3）VOCs,meteorological and NO_xare the key influencing factors in the process of O₃generation in Jinzhong.The relative importance（RI）is the largest for VOCs（RI is 48.4%）,followed by meteorological（RI is 26.2%）and NO_x（RI is 217.0%）,SO₂（RI is 2.4%）,CO（RI is 2.2%）,PM_2.5（RI is 2.0%）,PM₁₀（RI is 1.7%）.The influence of VOCs on the generation of O₃is the greatest in spring and winter,the most important is the effect of aromatic hydrocarbons in summer,and the largest effect of oxygenated VOCs in autumn.The relative importance of NO_xon O₃showed the characteristics of spring>summer>autumn>winter,accounting for 29%,26.5%,24.75%and 19.75%of the total NO_xRI,respectively.Meteorological factors have the greatest influence on O₃generation,accounting for about 50%of all meteorological factors.Besides temperature,the secondary key meteorological factors are relative humidity in spring and summer,wind direction in autumn,and atmospheric pressure in winter.Based on the above research results,it is suggested that Jinzhong should strengthen the control of O₃pollution in autumn;Jinzhong should focus on controlling the emission of VOCs in the local west-southwest direction,namely Jiexiu,Lingshi and other places,and reduce olefins,aromatic hydrocarbons and aldehydes and ketones.When the east-southeast air mass occurs,the control of O₃precursors should be strengthened;and the establishment of a regional joint prevention and control mechanism for air pollution in the east-southeast direction（ie,with Hebei,Henan,Anhui,Shandong and other places）should be actively promoted.