Characteristics Of Atmospheric Semi And Intermediate Volatility Organic Compounds And Its Contribution To Secondary Organic Aerosols In Urban Chengdu In Winter

Fine particulate matter(PM_2.5)is an important problem facing the current development in China,in which secondary organic aerosol（SOA）is an important component of fine particulate matter.The absence of precursors and other reasons make it difficult to numerically close the external field observation and laboratory simulation of SOA.Semi and intermediate volatility organic compounds（S/IVOCs）are considered as important missing SOA precursors,capable of being present in both gas and particle phases,and able to react in the environment to form less volatile compounds.In this study,S/IVOCs samples in the actual atmosphere were collected in the winter season when the pollution was heavy,and non-targeted analysis of S/IVOCs in the actual atmosphere was performed using thermal desorption-total two-dimensional gas chromatography-mass spectrometry（TD-GC×GC-MS）to obtain the molecular characteristics of S/IVOCs components in the actual atmosphere,and to estimate the contribution of S/IVOCs to SOA based on the yield method,and the sources of S/IVOCs were resolved.The air quality in Chengdu during the smapling period was good or light polluted,and the concentration of gas pollutants is at a high level.The average mass concentration of PM_2.5(PM₁₀)was 86.1±30.0μg m^-3(108.1±36.2μg m^-3).On non-polluted days（air quality index,0<AQI≤100）,PM_2.5and PM₁₀ concentrations were 75%and 79%of those on polluted days（AQI>100）,and the decrease of CO,SO₂ and O₃ was not significant.There was a small increase in NO₂concentration.A total of 239 compounds were qualify and quantify,99 species in volatile organic compounds and 140 species semi and intermediate-volatility organic compounds,with an overall concentration of 15.31±7.01μg m^-3.In terms of volatility bins,the compounds were mostly concentrated in the more volatile bins,accounting for 87%of the overall concentrations.While in terms of polarity distribution,the two-dimensional retention times of the compounds were mostly 0.5-1.0s（P2）,1.5-2.0s（P3）and 2.5-3.0s（P4）,tooking up 85%of total mass concentartion.Based on the SOA production of 239 compounds identified by photochemical age estimation,the mean mass concentrations of SOA contributed by VOCs and S/IVOCs were 1.48±1.03μg m^-3 and 0.74±0.07μg m^-3,respectively.The difference in volatility-polarity of SOA production between polluted and non-polluted days was mainly in the more volatile bins,with no significant difference in the intermediate volatile and semi-volatility bins.The average contribution of VOCs to SOA was 1.48±1.03μg m^-3,the average contribution of S/IVOCs to SOA was 0.74±0.07μg m^-3,and the contribution of S/IVOCs was half of that of VOCs.But the mass concentration of S/IVOCs is only 20%of the mass concentration of VOCs.The difference of SOA yield in volatile-polarity between polluted and non-polluted days was mainly reflected in the region with high volatility,while the difference between intermediate volatility and semi-volatility was not significant.The sources of atmospheric S/IVOCs in urban areas of Chengdu were determined using the characteristic ratio method and backward trajectories.The mean n-alkane carbon dominance index（CPI）was 1.47,which belongs to the influence of anthropogenic source emissions,and the maximum carbon number(C_max)appeared in C₁₃,similar to the distribution of gasoline vehicle emissions.The mean value of toluene/benzene is 1.63±0.95,which is similar to the results of motor vehicle emissions;the mean value of aromatic hydrocarbons/CO is 0.42±0.29,which is easily influenced by motor vehicle emissions on polluted days,and the ratio has increased.The backward trajectory clustering indicates that the eastern part of Chengdu is an important source of S/IVOCs transport,and the potential source analysis shows that the inner Sichuan basin,western Chongqing and the border between Sichuan,Shaanxi and Gansu are important potential sources of S/IVOCs.