| The Chengyu urban agglomeration located in Sichuan Basin with large area and population is a key area for joint prevention and control of air pollution in China.It is also a high concentrations area for atmospheric fine particulate matter(PM2.5)and ozone（O3）,especially in Chengdu,a mega-city adjacent to the eastern foot of the Qinghai-Tibet Plateau,is most prominent.Extinction caused by atmospheric fine particulate matter through scattering and absorption of visible light is a key factor affecting urban atmospheric visibility and air quality.Particulate matter extinction is closely related to its chemical composition,particle size distribution,morphological characteristics and environmental humidity.Therefore,taking Chengdu as a typical city,it is of great significance to study the chemical composition of PM2.5 and its extinction contribution for understanding the environmental impact of regional pollution in the basin.To explore the main reasons for the lack of improvement of urban visibility in this study,the PM2.5 concentration and its chemical component characteristics,extinction coefficient variation characteristics in Chengdu were systematically investigated based on continuous observation data from the Chengdu Urban Atmospheric Super Observatory from 2018-2019.The mass scattering coefficients of key chemical species Organic matter（OM）,NH4NO3,and（NH4）2SO4 in PM2.5 were fitted by a multiple linear regression method（maximum likelihood estimation）,the main sources of contribution to the extinction coefficient by season were discussed.In addition,the variation characteristics of extinction coefficients were analyzed when PM2.5 and O3 combined pollution occurred in the spring of 2018.The main conclusions obtained are as follows:The PM2.5 mass concentration at the observation site in 2018 and 2019 were 45.7μg/m3and 42.6μg/m3,respectively,of which sulfate,nitrate,ammonium(SO42-,NO3-,NH4+,SNA)and carbonaceous components（OC,EC）accounting for a total of nearly 65%.Compared to2018,NO3-concentration increased by 17.1%in 2019,becoming the largest proportion of water-soluble inorganic ions in PM2.5.The mean values of atmospheric extinction coefficient for 2018 and 2019 were 275.1 and 275.4 Mm-1,respectively,and the visibility were 25.2 km and 23.4 km,respectively.With the decrease of PM2.5 mass concentration,yet the visibility has not improved accordingly.It was found that at the same PM2.5 concentrations,as the contribution of NO3-to PM2.5(fNO-3)increased from<0.1 to>0.3,the atmospheric visibility gradually decreased.Therefore,it was inferred that NO3-concentration increase was the leading factor for the failure to improve urban visibility simultaneously.Based on the chemical composition of PM2.5,the mass scattering efficiencies of NH4NO3,（NH4）2SO4 and OM,which contribute most prominently to the extinction coefficient,were obtained by maximum likelihood estimation and log-transformed residual of 3.83,3.80,and 7.42 m2/g,respectively.It was found that high relative humidity conditions promote the production of NO3-through heterogeneous reactions and increase the hygroscopicity of NH4NO3,which can further significantly enhance the contribution of NH4NO3 to extinction coefficient.Five main prominent factors contributing to PM2.5 extinction coefficient were traffice emission,secondary particulate matter（nitrate and sulfate）,industrial emission and dust obtained from positive matrix factorization analysis.Nitrate in secondary particulate matter contributed the largest to extinction coefficient in winter,while traffice emission contribute the most to the extinction coefficient in other seasons.In the case of PM2.5 and O3 combined pollution in spring,the mass of secondary pollution components SNA and secondary organic carbon increased significantly,which increased by 1.0 and 1.3 times respectively compared with the clean day,and OM became the largest extinction contributor（32.2%）,followed by NH4NO3 and（NH4）2SO4,contributing 22.8%and 20.5%,respectively. |
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