Optical Properties And Source Apportionment Of Atmospheric Carbonaceous Aerosols In Xi’an

Light-absorbing carbonaceous aerosols are the main particulate light-absorbing substances in the atmosphere,which significantly affect the energy-radiation balance of the Earth’s climate system.In recent years,the western cities of China have been suffering from carbonaceous aerosol pollution.In the thesis,a one-year（2018）PM_2.5filter membrane sampling was carried out with Xi’an atmospheric light-absorbing carbonaceous aerosol.The light absorption coefficients of black carbon（BC）and brown carbon（Br C）were obtained based on the binary model.The characteristics and influencing factors of seasonal changes were discussed.The source of carbonaceous aerosol light absorption was analyzed using the receptor model.The causes of pollution events,as well as the sources and affecting factors of carbonaceous aerosol light absorption,were analyzed based on chemical components,meteorological data.This may provide decision-making suggestions related to carbon emission reduction for agencies.The main conclusions are shown as follows:（1）The total absorption coefficient（Abs）of carbonaceous aerosols decreased gradually with the increase of wavelength（405 nm,445 nm,635 nm,780 nm,808 nm and 980 nm）.The average value of Abs₄₀₅/Abs₉₈₀ was 6.0,indicating the presence of light absorptions of other substances besides BC in the short wavelength.Meanwhile,there was obvious seasonal variation characteristics of Abs,the highest value appeared in spring,followed by winter and autumn.The annual mean values of the optical absorption intensities of Br C and BC at 405 nm(Abs _{Br C,405} and Abs _BC,405,respectively)were 24.5±28.5 Mm^-1 and 19.5±22.9 Mm^-1,respectively,and their contributions to the total absorption coefficient were 56.3%and 43.7%,respectively,indicating that Br C had strong light absorption at short wavelengths.From the perspective of different seasons,the values of Abs _BC in autumn and winter were significantly higher than those in spring and summer,indicating that BC had a greater impact on light absorption in autumn and winter,which may be caued by the increase of biomass burning and coal burning activities.The lowest Abs _BC in summer was associated with emission source intensity,high mixed layer height,and frequent rainfall.Abs _{Br C} was lower in summer and autumn and higher in spring and winter.（2）The power-law relationships between Abs and wavelength show high correlations（R²=0.95～0.97）with apparent increases in Abs at shorter wavelengths.The light Absorption Angstroms（AAEs）of carbonaceous aerosols were significantly higher than 1.0 in different seasons,indicating that the influence of Br C existed throughout the year.The AAEs of Br C(AAE_{Br C})were similar in spring（3.5）,summer（3.4）,and autumn（3.4）.The AAE_{Br C}（3.0）in winter was about 12.4%lower than that in other seasons,indicating that the chemical composition of Br C in winter was different from other seasons,which may be related to the more complex emission sources in winter.The mass absorption cross section of BC(MAC_BC)showed a decreasing trend with the increase of wavelength.The MAC_BC in autumn and winter both were higher than those in spring and summer.（3）Based on the PMF receptor model,the sources of Abs mainly included coal combustion,biomass burning,traffic-related sources,and mineral dust sources.Biomass burning and traffic-related sources were the main contributors to Abs₄₀₅ in spring,accounting for 37.7%and 38.1%,respectively.Traffic-related sources and coal combustion dominated in autumn and winter,accounting for about 77%of Abs₄₀₅.Traffic-related sources had the greatest impact on Abs of different wavelengths in four seasons,especially in summer（51.3%）and autumn（48.9%）.Mineral dust sources contributed less to Abs₄₀₅,accounting for 1～11%of Abs.The sources that affected the light absorption of BC are traffic-related sources,coal combustion,biomass burning and mineral dust sources.The sources that affected the light absorption of Br C were traffic sources,coal combustion,biomass burning,mineral dust sources and secondary sources.In the four seasons,the light absorption of BC was mainly affected by traffic-related sources;the light absorption of Br C was mainly affected by traffic-related sources in spring and winter,coal combustion in summer,and both traffic-related and biomass combustion in autumn.The MAC_BC of each source was higher than that of MAC_{Br C},indicating that the light absorption intensity of BC from different sources was significantly higher than that of Br C.Comparing the contribution of the same source to the absorption coefficients of BC and Br C,it can be seen that the traffic source had a greater impact on the absorbance of BC,while biomass burning and coal combustion had a greater impact on the absorbance of Br C.（4）The light absorption coefficients of carbonaceous aerosols and their sources in different seasons had basically the same trend with the increase of AQI index.Through the analysis of pollution events in autumn and winter（AQI≥100）,it was found that the traffic sources in event 1,event 2,event 3,and event 6 contributed the most to the light absorption of BC,while in event 4,the biomass burning contributed the most to the light absorption of BC.In contrast,the traffic sources in event 1,event2,event 3,and event 4 contributed the most to the light absorption of Br C,while in event 6,the biomass burning contributed the most to the light absorption of Br C.The carbonaceous aerosol absorption coefficients showed different correlations with relative humidity and oxidants in different pollution events,indicating that liquid-phase chemistry and photochemical reactions had different effects in pollution events.