Characteristics And Optical Properties Of Brown Carbon In PM_2.5 In Chongqing

Brown carbon?BrC?has been recognized as a group of colored organic compounds with light absorption that increases sharply from the visible?Vis?to the ultraviolet?UV?range.BrC has profound effects on air quality,radiant forcing budget and visibility.Chongqing is a mountainous city located in southwestern China,where the Yangtze River flows through and forms a long river-reservoir.The area is climate-sensitive and suffering from air pollution.The characteristics of BrC in Chongqing is critical for accurate estimations of air quality and radiative balance in this climate-sensitive region.Based on the online and offline aerosol analyses of brown carbon,the characteristics of BrC in Chongqing has been systematically investigated in this study.In this study,PM_2.5 filter samples were collected from December 2015 to July 2017 at four typical sites in Chongqing.Then we focused on the spatial and seasonal variations of BrC concentrations and light absorption,the sources of BrC and impact of BrC light absorption,and the fractional contribution of BrC relative to BC on radiation.Moreover,the light absorption of BrC during winter 2016 in Chongqing and Chengdu was analyzed to further investigated the light absorption of BrC during winter in Chongqing.Finally,based on the concentration and light absorption of BrC measured via online analyzers during winter 2018 at Changshou site in Chongqing,the driving forces of BrC pollution have been investigated.The average concentration of BrC during winter in Chongqing was 2.42±1.54?g m^-3 and about 4.0 times than during summer(0.60±0.21?g m^-3).Similar to the seasonal variation of BrC,higher contributions of BC to organic carbon?OC?and PM_2.5 were found both in winter than summer.The spatial distribution of BrC concentrations was obvious,and significantly higher BrC concentrations were observed at the urban sites than rural site.According to the relationships between BrC and aerosol chemical components and the air mass trajectory analyses,we found that BrC was mainly affected by the secondary formation and regional transport during summer.The results of PMF and OpenAir analysis indicated that biomass burning was one of the major sources,followed by secondary formation and coal combustion.Additionally,a higher contribution of biomass burning to BrC was found at the rural site?40.3%?than that in urban sites?33.4%?.The average light absorption coefficient of BrC at 405 nm(Abs_405,BrC),its contributions to aerosol light absorption and mass absorption efficiency in winter were 2.7±8.8 Mm^-1,27.8±7.0%and 0.9±0.4 m² g^-1 respectively,about 9.8,2.6 and3.0 times those in summer respectively.Spatially,the average Abs_405,BrC at the urban sites(16.2±12.2 Mm^-1)was 80%higher than that at the rural site(9.2±5.4 Mm^-1),while its contributions to aerosol light absorption and the absorption?ngstr?m exponents at 405⁹80 nm(AAE_405-980)at the urban sites?22.3±6.2 and 1.3±0.1,respectively?were significantly lower than those at the rural site?32.3±6.3%and 1.6±0.2,respectively??p<0.001?.This result might be related to biomass burning at the rural site.The relationship between BrC and aerosol chemical components indicated that the optical absorption of BrC aerosol was affected by the BrC emitted from biomass burning and produced from the secondary formation?containing C=C or C=N bonds?.Compared with Chengdu,higher values of Abs_405,BrC,MAE_405,BrC,OC and AAE_405-980 during the pollution period in winter in Chongqing were related to biomass burning.The fractional contribution of radiation absorbed by BrC relative to BC in the wavelengths of 405⁹80 nm and 405⁴45 nm were 30.6±6.5%and 63.6±14.6%in winter,about 2.6 and 2.7 times those in summer.Additionally,a significant contribution of radiation absorbed by BrC relative to BC was found in the wavelength range of 405⁴45 nm,which should be related to the strong absorption of BrC at shorter wavelengths.A significant contribution of radiation absorbed by BrC relative to BC at short wavelengths might affect the radiative balance during winter.Therefore,the effects of BrC absorption needs to be considered for more accurate estimations of air quality and radiative balance.The average concentration of BrC during winter 2018 at Changshou site was1.10±0.49?g m^-3,with a clear diurnal trend with relatively high levels at nighttime.PMF results suggest that BrC pollution was mainly driven by the primary organic aerosols produced from biomass burning?BBOA?during winter.The average of Abs_BrC at 370 nm(Abs_370,BrC)and its contributions to aerosol light absorption were59.2±31.2 Mm^-1 and 35.5±8.2%respectively.The result is about 2.8-12.6 times and1.7-4.7 times that at long wavelengths respectively,indicating more light absorption of BrC was observed at short wavelength.The diurnal variations at short wavelengths exhibited more obvious diurnal variations with relatively high levels at nighttime,possibly being associated with the BrC emitted from biomass burning at nighttime.According to the source apportionment of organic aerosol using PMF analysis,we estimated the contributions of different OA components to Abs_BrC at each wavelength using a multiple linear regression model.Then,we found that the contributions of POA to Abs_BrC was higher than SOA at each wavelength.The contribution of BBOA to Abs_BrC decreased at longer wavelengths,while the contributions of SVOOA,LVOOA,HOA and CCOA to Abs_BrC all increased at longer wavelengths.Compared with other OA components,the diurnal cycles of the light absorption coefficient of BBOA(Abs_370,BBOA)and BrC(Abs_370,BrC)at 370 nm share similar patterns?r=0.67?.Moreover,AAE of BrC could be increased by the BBOA absorption,indicating that Abs_BrC at short wavelengths was mainly driven by by BBOA.