Changes And Source Tracking Of PM_2.5 Chemical Components In The Sichuan Basin-eastern Margin Of The Qinghai-Tibet Plateau

Sichuan Basin is one of the regions with the most serious haze pollution in China.In order to fully understand the characteristics of PM_2.5 pollution and chemical composition changes in the Sichuan Basin-the eastern margin of the Qinghai-Tibet Plateau.In this study,membrane samples were collected continuously day and night in the spring,summer and winter of 2017in Wenjiang,Jianyang,Chengdu and Xinjin of Sichuan Basin,in the famous mountains of the basin plateau transition zone and in Litang of the eastern margin of the Qinghai-Tibet Plateau.The collected samples were analyzed in the laboratory by Thermal/Optical carbon analyzer and ion chromatograph,and the temporal and spatial distribution characteristics of carbonaceous components and water-soluble ion components in PM_2.5 at different sites in the Sichuan Basin-Eastern Edge of the Qinghai-Tibet Plateau were discussed.And the source and the contribution of secondary pollutants such as SO₄^2-,NO₃^-and NH₄⁺(the three are collectively referred to as SNA).In addition,the FLEXPART-WRF model was used to trace the source of a heavy pollution process in the Sichuan Basin.The main conclusions are as follows:(1)During the observation period in 2017,the Total Carbon(TC)concentration of the famous mountain sites in the plateau transition zone in spring and summer accounted for 19.6%and 32.4%of PM_2.5,respectively.The proportion of TC concentration in PM_2.5in spring,summer,and winter at the Litang site in the eastern margin of the Qinghai-Tibet Plateau is23.7%,42.9%,and 38.9%,respectively.In the basin,the TC concentration of Wenjiang site in spring,Jianyang site in winter,Chengdu site in summer,and Xinjin site in spring accounted for23.5%,18.9%,27.6%,and 19.1%of the PM_2.5 mass concentration,respectively.The carbon components of the Sichuan Basin,the transitional zone between the basin and the plateau,and the eastern margin of the Qinghai-Tibet Plateau all contribute to PM_2.5.During the spring observation period,changes in the average mass concentration of organic carbon(OC),elemental carbon(EC)and PM_2.5 all showed the characteristics of basin city>basin suburbs>basin and plateau transition zone>eastern margin of the plateau,indicating that the Sichuan Basin Air pollution is much heavier than the eastern edge of the Qinghai-Tibet Plateau.Through the study of the correlation between OC and EC,the OC and EC in the basin,plateau-to-plateau transition zone,and Qinghai-Tibet plateau area have similar sources of OC and EC.In addition,the Litang area on the eastern margin of the Qinghai-Tibet Plateau is significantly more affected by direct combustion sources than cities,suburbs,and the transition zone between the plateau and the basin in the Sichuan Basin.Positive Matrix Factorization(PMF)source analysis results show that the contribution of motor vehicle exhaust gas to the PM_2.5 carbonaceous aerosol in basin urban sites,basin-plateau transition zone sites,and sites on the eastern edge of the Qinghai-Tibet Plateau is higher than that of coal and biomass combustion.(2)The Sichuan Basin,the plateau-basin transition zone,and the eastern margin of the Qinghai-Tibet Plateau all have the highest PM_2.5 in winter,with the highest concentration in Chengdu,and changes in meteorological conditions and human-made emissions play an important role in PM_2.5 pollution.The PM_2.5 pollution of stations in the basin and the stations in the transition zone between the basin and the plateau is much higher than that in the eastern margin of the Qinghai-Tibet Plateau.The water-soluble ions in the basin and in the transition zone of the basin mainly exist in the form of SO₄^2-,NO₃^-and NH₄⁺(SNA),and the NH₄⁺in Mingshan and Xinjin stations in spring mainly exists in the form of(NH₄)₂SO₄ and NH₄NO₃.In the spring of Wenjiang site,NH₄⁺exists in the form of(NH₄)₂SO₄ and NH₄NO₃,but also some NH₄HSO₄.In winter,all Jianyang NH₄⁺is combined with SO₄^2-and NO₃^-,and there is still some NH₄Cl.According to the SO₄^2-/NO₃^-value,Wenjiang,Mingshan and Litang are mainly fixed sources in spring,and mobile sources in Jianyang in winter.Principal component analysis shows that the secondary transformation has the greatest contribution to water-soluble ions in cities,suburbs,and the transition zone between the plateau and the basin in the basin.The contribution of the secondary transformation reached 49.2%,while the PM_2.5 in the Litang site on the eastern margin of the Qinghai-Tibet Plateau The water-soluble ions mainly come from the dust,and the contribution of the dust is 46.4%.(3)In the city of Jianyang in the western Sichuan Basin,PM_2.5,carbonaceous components and water-soluble ions in winter show basically the same changing trends over time.During the heavy pollution of PM_2.5 in the winter of 2017,the average relative humidity was 79.1%,and the average wind speed was 1.3m/s.During the light pollution period,the relative humidity and wind speed were 65.4%and 1.2m/s,respectively.Lower wind speed is conducive to pollution.The accumulation of substances and higher humidity are conducive to the generation of new particles and the growth of PM_2.5 moisture absorption.The proportions of SNA and TC in PM_2.5in heavily polluted days are 32.57%and 15.40%,respectively.SNA and TC accounted for 26.28%and 21.86%of PM_2.5 in lightly polluted days,respectively.Clean world SNA and TC account for 26.25%and 22.89%of PM_2.5,respectively.High humidity environment is more conducive to the formation of secondary pollutants such as SNA.Secondary pollutants such as SNA are important for heavy PM_2.5 pollution.contribution.Based on the FLEXPART-WRF simulation,the analysis of potential sources during the heavy pollution period in the Sichuan Basin shows that the Jianyang site in the hinterland of the Sichuan Basin mainly comes from local sources;the pollution from the famous mountain sites in the transitional area of the plateau and the basin basically comes from the transmission in the Chengdu and Meishan basins;At the Kangding site on the east slope of the plateau,air pollution mainly comes from the local area,followed by transmission from the basin and the transitional area between the basin and the plateau;the Litang site on the east edge of the Qinghai-Tibet Plateau is basically not affected by pollution in the basin.