Characteristic Analysis Of PM_2.5 In Typical Cities In The Eastern Fenwei Plain And Numerical Simulation Study Of Its Sources

In recent years,air pollution has caused widespread attention.The study on air pollution was mainly concentrated in megacities,but the problem of air quality in smaller cities is also urgent to be solved.With its unique energy structure,industrial distribution and topographical characteristics,the Fenwei Plain had become another hot area of air pollution after the Beijing-Tianjin-Hebei area,the Yangtze River Delta,the Pearl River Delta and the Sichuan Basin.Sanmenxia City is a small city in the Eastern Fenwei Plain,which is located in the important area of the junction of Henan Province,Shanxi Province and Shanxi Province.With the complex terrain,the air pollution of Sanmenxia city deserved in-depth analysis,which can provide scientific basis for Sanmenxia City to establish heavy pollution control and regional joint prevention and control.In this paper,the pollution characteristics of Sanmenxia City,a typical city in the east of Fenwei plain,were analyzed by using the hourly data of pollutant concentration and meteorological elements.Based on this,a daily variation model of pollutant concentration in Sanmenxia City was established by combining the emission source data and WRF model.Then,the FLEXPART-WRF model was used to analyze the horizontal and vertical transport characteristics of Sanmenxia City in different years,different seasons and different pollution levels.Finally,combining the observation data such as aerosol extinction coefficient,vertical wind and PM_2.5chemical composition and numerical models such as WRF model,FLEXPART-WRF model and NAQPMS model,the pollution characteristics of Sanmenxia City during the wintertime field campaign from December 2018 to January 2019 were analyzed.There were three heavy pollution during the wintertime field campaign,the formation,development,dissipation and pollution characteristics of each heavy pollution were analyzed based on the weather,local circulation characteristics and an tracer-tagged module of NAQPMS.The research shows:(1)Since the Fenwei plain was included in the key monitoring area in 2018,the air quality of Sanmenxia City had improved,the average PM_2.5concentration of Sanmenxia City was 56.9?g/m³(standard deviation was 67.4?g/m³),and the air pollution was still serious.Sanmenxia is located in the canyon region between two northeast-southwestern mountains.Industry,power plants and other elevated pollution sources are dominant sources,and industry of Sanmenxia City is concentrated in the west of the city.Combined with the dynamic evolution of the boundary layer and regional transmission and other factors,the daily variation characteristics of PM_2.5,PM₁₀and SO₂in Sanmenxia City during heavy pollution were a cycle of daytime-maximum and nighttime minimum,and the peak value appears at 12:00-14:00.And the pollution was heaviest when the dominant wind direction was westerly during heavy pollution period.(2)Large-scale atmospheric circulations,local circulation and other meteorological conditions,as well as the difference of air quality in the source area,would affect the particle transport process.During heavy pollution,Sanmenxia City mainly had four transport paths:East,northwest,West and south.During the heavy pollution in autumn and winter,the particles with horizontal transport distance r?100 km were dominant,during the heavy pollution in autumn and winter,Sanmenxia City was greatly affected by the surrounding areas.During the spring heavy pollution period,the proportion was the largest in the range of 500<r?1000 km away from Sanmenxia City,that is to say,long-distance transport had a great impact on heavy pollution in spring.In the main particles transport paths during heavy pollution,except for the transport from the northwest in spring,the vertical transport height of particles was relatively low,mainly moving within the boundary layer,heavy pollution generally occured in the stable boundary layer.(3)During the wintertime field campaign from December 2018 to January 2019,SO₄^2-,NO₃^-,NH₄⁺and OC accounted for more in PM_2.5,of which NO₃^-accounted for the largest proportion,accounting for 37.4%.The daily variation characteristics of PM_2.5,PM₁₀,and SO₂were similarly,which were high in daytime and low at night.NOR and SOR were higher and positively correlated with PM_2.5concentration.It showed that NOx,SO₂,NH₃,carbonaceous aerosols and their secondary chemical reaction from biomass burning,industry,power plant,residents and traffic sources were the important reasons relative to atmospheric chemistry for the growth of PM_2.5concentration in Sanmenxia City.During the wintertime field campaign,the horizontal and vertical wind speed were relatively low,while during the heavy pollution,the east wind was dominant,and the vertical wind presented obvious updraft at the stage of pollutant concentration decreasing.When the large-scale atmospheric circulation was stable,the local circulation was dominated by downslope wind at night,while the mountain-crossing wind and zonal wind were dominant during the day.There were three main transport paths of East,North and West during the wintertime field campaign.The PM_2.5source apportionment revealed by an on-line tracer-tagged of NAQPMS showed that the main regional sources of Sanmenxia City were Shanxi Province,Sanmenxia City and Shaanxi Province.The contribution to PM_2.5concentration was 41%,25%and 19%respectively.The contribution of residential emissions,industrial emissions,power plant emissions and vehicle emissions to PM_2.5concentration was 63%,19%,7%and 12%,respectively.(4)Three heavy pollution cases occurred in Sanmexia City during the wintertime field campaign,and the pollution characteristics of three cases had similarities and characteristics.The commonality was that the chemical genesis was basically the same,the high value of aerosol extinction coefficient appeared in the entire interval below 1.5km,and the three source regions that contributed the most to PM_2.5in Sanmenxia were Yuncheng City,Sanmenxia City,and Weinan City.The characteristics were due to the different weather patterns and local circulations during each heavy pollution,resulting in the three cases of the formation,development,dissipation process and the contribution of each source area to Sanmenxia PM_2.5significantly different.