Sources,Light Absorption And Molecular Characteristics Of Brown Carbon In Atmospheric Fine Particles At Nanjing,China

Atmospheric brown carbon(BrC)affects global radiation balance due to its strong light absorption at the ultraviolet wavelength.As a kind of light-absorbing organic aerosols,few studies characterize atmospheric BrC mass concentrations.The light absorption of BrC change with different sources and atmospheric secondary processes,however,the molecular structure determines the light absorption capacity of BrC.Current researches focus on the light absorption properties and formation mechanism of BrC at the molecular level.Humic-like substances(HULIS)are recognized as important components of BrC which are a mixture of highly polar compounds,strongly absorbing light at the short wavelength.In this study,we investigated the sources,light absorption properties and molecular characteristics of BrC at Nanjing,a typical city in the Yangtze River Delta,identified important BrC chromophores and quantified their light absorption contributions.(1)we established a two-wavelength method(658 and 405 nm)applying in the Sunset thermal–optical carbon analyzer and obtained the mass concentrations of OC and EC at 658 nm and 405 nm.Since BrC absorbs light at 405 nm more effectively than 658 nm,we defined the enhanced concentrations(d EC=EC_{405 nm}-EC_{658 nm})and gave the possibility of providing an indicator of BrC.Based on a 1-year observation,the results showed that the diurnal patterns of OC and EC appeared bimodal distributions,affected by the traffic emissions.Similar diurnal variations of d EC/OC and OC/EC,SOC/OC ratios were found which showed unimodal distributions.In addition,the d EC/OC increased when the OC/EC ratios increased,indicating strong biomass burning or secondary sources contributions to d EC.The high concentrations of OC and EC were found under high relative humidity,while the d EC concentrations did not strongly depend on temperature and relative humidity,indicating the formation mechanism of d EC was more complex.The wind rose and receptor model results showed that strong local emissions were found for OC and EC,however,d EC was significantly affected by regional or long-range transport.We analyzed two biomass burning events and found that the mass concentrations of d EC increased significantly during biomass burning periods.The back trajectories and MODIS fire information further supported that biomass burning from regional transport was the main source of d EC.d EC can be an indicator of biomass burning on biomass burning days and reflects the mass concentrations of atmospheric BrC.(2)We quantified emission source contributions to the mass concentrations and light absorption of water soluble HULIS from 2017 to 2018 using laboratory chemical and optical analysis techniques,the radiocarbon analysis(?¹⁴C)and the positive matrix factorization(PMF)model.The results showed that the HULIS light absorption coefficient at 365 nm(Abs₃₆₅)averagely accounted for 71±19%of that of water soluble organic carbon(WSOC),suggesting that HULIS are the main light-absorbing components of WSOC.The radiative transfer model results showed the direct radiative forcing caused by HULIS at the top of tropopause was 0.21W m^-2,accounting for 27%of the total light absorbing carbon species.The good correlations between HULIS with levoglucosan,K⁺and water soluble secondary ions suggested the influence of biomass burning and secondary sources on HULIS formation.High concentrations of HULIS and secondary tracers were found in present of high relative humidity,indicating the significant influence of relative humidity on secondary HULIS formation.The PMF results showed that the industrial and traffic emissions contributions to HULIS mass concentrations were 39%and 41%in spring and autumn,respectively,highlighting the fossil fuel combustions contributions to HULIS in urban area.Biomass burning significantly contributed to the HULIS mass concentrations(39%)in winter which was affected by a biomass burning event.Strongest biogenic secondary organic aerosols(SOA)contribution(32%)was found in summer due to the wet scavenging effect of rainfall.The ¹⁴C results showed that the averaged non-fossil contributions of HULIS were 39%and 36%in summer and winter,respectively,coinciding with the PMF results.The multiple linear regression(MLR)model results showed the similar contributions of biomass burning and fossil fuel combustions to Abs₃₆₅and HULIS mass concentrations.However,the anthropogenic SOA contributions to Abs₃₆₅ were higher than those to HULIS mass concentrations in the four seasons,suggesting the stronger light absorption capacity of anthropogenic SOA than that of biogenic SOA.(3)Based on the Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR-MS)technique,the molecular composition and formation mechanism of HULIS during heavy haze periods(winter)and clean periods(summer)were investigated.The results showed the proportions of the tannins-like species and carbohydrates-like species were higher in summer,indicating stronger biogenic secondary organic aerosols(SOA)formation in summer.The higher proportions of condensed aromatics increased in winter suggested the increasing of anthropogenic emissions in winter.More high oxygen-containing macromolecular oligomers were detected in CHO compounds in summer,indicating stronger biogenic SOA formation in summer.Phenols,flavonoids and the polycyclic aromatic hydrocarbons(PAHs)presented higher relative intensities in winter,which were closely related to biomass burning and fossil fuel combustion.The CHON compounds were mainly composed of organonitrates or nitro compounds,which presented significant higher relative intensities in winter,suggesting that the high NO_xconditions in winter promoted the formation of organonitrates in winter.In addition,the formation of nitrophenols were promoted influenced by biomass burning in winter.However,the nitrogen heterocyclic aromatic compounds showed high relative intensities in summer,which were produced from atmospheric secondary processes in present of ammonia.The sulfur-containing compounds were mainly composed of organosulfates(OSs).The long-chain alkane derived OSs widely consisted in summer and winter samples,indicating the stable traffic emission contributions to HULIS at the study site.The formation of isoprene derived OSs was significant in summer of which the relative intensities increased with the increasing of sulfate concentrations in summer.Stronger aromatic hydrocarbon derived OSs formation were found in winter,indicating the increasing anthropogenic emissions in winter such as coal combustion.The relative intensities of OSs increased with the increasing of SO₂ concentrations and aerosol liquid water content in winter,suggesting the liquid phase formation of OSs in the presence of SO₂ or heterogeneous formation of OSs through the reactions between SO₂ and the organic aerosols in winter.(4)The formulas with double bond equivalent between 0.5C and 0.9C were identified as the potential BrC chromophores in HULIS to establish the relationship between the molecular composition and the light absorption of BrC.The results showed that the nitrogen-containing compounds were the main potential BrC chromophores after which were CHO compounds.Good correlations between mass absorption efficiency(MAE₃₆₅)with the proportion of condensed aromatics in CHO and CHON compounds and the aromatic index of CHO and CHON compounds were found,suggesting the aromaticity dominate the light absorption capacity of BrC.Phenols,nitrophenols and aromatic hydrocarbon derivatives were the main components of the BrC formulas which showed good correlations with MAE₃₆₅,suggesting they are the main light-absorbing chromophores.Further investigating the influence of different sources on BrC light absorption and the results showed that the BrC formulas which showed good correlations with levoglucosan and anthropogenic SOA tracers were strong light-absorbing chromophores,indicating significant contributions of biomass burning and anthropogenic SOA to the light absorption of BrC.The random forest model results showed that the contributions of the top 20 BrC formulas to MAE₃₆₅ range from 0.73%to 3.36%of which were all aromatic compounds,including phenols and nitrophenols which were closely related to biomass burning.