Study On Atmospheric Fine Particles In Sichuan Basin City Based On Full-mode And Single Particle Method

In order to deeply understand the characteristics of urban air pollution in Sichuan Basin,this study comprehensively analyzed the chemical composition,morphology,source and interannual evolution of atmospheric fine particles in Chengdu,the capital of Sichuan Province,by using the research methods of typical full sample（membrane sampling）and single particle（transmission electron microscope X-ray energy spectrometer,TEM-EDS）.Firstly,based on the membrane sampling method,various physical and chemical characteristics of atmospheric fine particles in the winter of 2017 and 2020 were compared,and the important impact of the implementation of the"three-year plan on defending the blue sky"on local air pollution was evaluated;Secondly,combined with membrane sampling and TEM-EDS method,a typical pollution process in the winter of 2020 was deeply analyzed,and the key factors caused heavy pollution were identified;Thirdly,the TEM-EDS method was used to compare the atmospheric fine particles in Chengdu and Emeishan,which systematically presented the differences of particle physicochemical characteristics in different types of regions in Sichuan Basin.The results showed that:From the winter of 2017 to the winter of 2020,the atmospheric PM_2.5 in Chengdu decreased from 153.2±60.6μg/m³ to 97.5±40.4μg/m³,a decrease of 36.4%;Due to the impact of emission reduction measures,various gaseous pollutants（SO₂,NO₂,CO and O₃）have also decreased to varying degrees,with decreased of 47.8%,27.6%,29.4%and 24.5%respectively.In terms of chemical components,the mass concentrations of OC and various types of water-soluble inorganic ions（WSIIs）decreased after the"three-year plan on defending the blue sky"and EC increased;Different from the change trend of mass concentration,the contribution of OC to PM_2.5 was not different in the two periods,accounting for 19.9%and 19.8%respectively;The contribution of WSIIs to PM_2.5increased from 37.9%to 43.3%;Among all kinds of WSIIs,the secondary conversion of ions was obvious in the winter of 2020,and the contribution of mobile sources was further enhanced.In terms of source analysis,the contribution sources of atmospheric PM_2.5 in the two periods were mainly secondary sources,traffic sources,combustion sources and dust sources.In terms of the distribution of potential source areas,Chengdu and its surrounding areas,southeast region of Sichuan and northeast region of Sichuan had a significant impact on the atmospheric environment of Chengdu in the winter of 2017,while the distribution of potential source areas decreased significantly in the winter of 2020,mainly concentrated in Chengdu and its surrounding areas and southeast region of Sichuan.In terms of the evolution characteristics of various factors with pollution,with the aggravation of pollution,the mass concentrations of OC,SOC and all kinds of WSIIs showed a continuous upward trend in the two periods,while EC increased first and then decreased;In terms of contribution proportion,OC,NO₃^-,Cl^-and K⁺contributed greatly to the aggravation of pollution in winter 2017,while in 2020,they were mainly affected by the contribution of OC and NO₃^-.TEM-EDS was used to study the atmospheric single particle aerosol in Chengdu.Chengdu atmospheric single particle aerosols mainly included particles in the form of external mixed（OM particles,S-rich particles,soot particles,mineral particles and fly ash/metal particles）and particles in the form of internal mixed particles（OM-S particles,OM-soot particles,OM-mineral particles,OM-fly ash/metal particles,OM-S-fly ash/metal,OM-S-mineral particles and OM-S-soot particles）.In addition,two different analysis methods were used to study a typical pollution process in the winter of 2020.It was found that from"mild pollution day"to"moderate pollution day",the aggravation of pollution was dominated by carbon components,the mass concentrations of OC and SOC increased by 1.8 times and 4.5 times respectively,while the mass concentration of EC and its proportion in PM_2.5 showed a downward trend;In this process,except for OM-S particles,the proportion of other particles increased in varying degrees,especially OM-soot particles,soot particles and OM particles.From"moderate pollution days"to"severe pollution days",the aggravation of pollution was dominated by WSIIs.among them,the ratio of SO₄^2-and NO₃^-to WSIIs increased significantly,increasing by 79.7%and 73.2%respectively,and the proportion of other ions increased by 34.3～63.8%.At the same time,the proportion of mixed particles in this process decreased significantly.In the process of pollution dissipation,except EC,the mass concentration of other chemical components decreased in varying degrees;At the same time,the proportion of mixed particles increased.By comparing the composition of particles in Chengdu and Emeishan,it was found that the contribution of sulfur-containing particles in Chengdu was higher than that in Emeishan,while the proportion of carbon particles in Emeishan was higher than that in Chengdu.Affected by rainfall,the proportion of sulfur-containing particles and particles with coarser particle size in the atmospheric environment of Emeishan City decreased significantly,while the proportion of soot and OM-soot particles increased during the rainfall period.