Chemical Characteristics,Optical Properties And Source Apportionment Of Atmospheric Brown Carbon Based On Offline AMS Technology

Organic aerosols(OA)constitute a large fraction of atmospheric Aerosols.Recently,a number of studies indicate that a portion of OA,termed“Brown Carbon(Br C)”can absorb lights in the ultraviolet–visible(UV–Vis)range.Br C has attracted much attention due to its potential impacts on atmospheric chemical reactions,earth's radiation balanceand human health.However,the light absorption properties,sources and chemical composition of Br C are still not well understood,hindering accurate estimations of its climate effects and radiation forcing.Thus,in this work,we first assessed the ability for characterization and source apportionment of atmospheric water-soluble OA(WSOA)by offline aerosol mass spectrometry(AMS)analysis.We found the results obtained by the offline-AMS technique compared well with those from simultaneous online Aerosol Chemical Speciation Monitor(ACSM)measurements.The water-solubility of OA factors was determined by comparing the source apportionment results of offline AMS analysis and online ACSM results.The fossil fuel?related OA(FFOA)has the lowest water-solubility of 0.17 among all resolved OA factors in this study,consisted with previous studies.Biomass burning OA(BBOA)has a wide range of water-solubility(on average 1.07),is likely caused by its high oxygen-to-carbon(O/C)ratio.The water-solubility of less-oxidized OA(LO-OOA,0.95)is much higher than the more-oxidized OA(MO-OOA,0.39).We found that LO-OOA showed much-enhanced contribution with high relative humidity(RH),while photochemical process played a more important role in the formation of MO-OOA.These findings may help to explain the the different water-solubility of these two secondary organic aerosol(SOA)factors.We further applied offline AMS technology to filter samples collected over long time scales.By combining AMS-WSOA high resolution mass spectra data with light absorption data by ultraviolet-visible spectroscopy measurements,we investigated the relationship between the optical and chemical characteristics of Br C in Nanjing and Yangzhou.Sources of WSOA,which were determined by positive matrix factorization(PMF)analysis,were correlated to the Br C light absorbance to identify the major sources of Br C.A multiple linear regression model was applied to determine the mass absorption efficiency(MAE)values of different WSOA factors and the potential contribution of WSOA factors to the aerosol light absorbance.We found that winter samples had the strongest light absorption coefficient(Abs₃₆₅)and mass absorption efficiency(MAE₃₆₅)at 365 nm among four seasons in Nanjing.The MAE₃₆₅showed no clear dependences on average oxidation states(OS_c)during spring,summer and fall,but decreased against the OSc during winter,indicating chemical aging may lead to photo-bleaching of Br C in winter.Moreover,positive responses of Abs₃₆₅ to nitrogen-to-carbon(N/C)ratios were found during all seasons,indicating nitrogen-containing organics can be important Br C chromophores.A multiple linear regression model was applied to apportion Abs₃₆₅ to contributions of different factors resolved from positive matrix factorization on WSOA,and obtained MAE₃₆₅ values of different WSOA factors.The MAE₃₆₅ values of FFOA,BBOA,LO-OOA and MO-OOA were 0.41 m² g^-1,0.6 m² g^-1,0.04 m² g^-1 and 0.85 m² g^-1respectively.We found that Br C was mainly affected by biomass burning during(29.93%)during the whole year,and such influence was more evident during summer(44.56%).The contribution of MO-OOA was significant(49.39%),and a much higher contribution of MO-OOA to Br C was found during winter(63.2%)than that insummer(23.74%).The LO-OOA factor dominated water-soluble OA mass(45.22%),yet its light absorptivity was negligible and contributed only 3.56%of Abs₃₆₅.Potential source areas of Br C were further discussed to improve our understanding of Br C sources in this region.Recent studies demonstrate that the water-insoluble Br C might have greater light-absorbing abilities than the water-soluble Br C.We also found that the light absorption coefficient at 365 nm of methanol-soluble organics(Abs_365,MSOC)and water-soluble organics(Abs_365,WSOC)were 13.50±7.03 Mm^-1 and 6.08±4.30 Mm^-1,respectively in Yangzhou.Mass absorption efficiency at 365 nm(MAE₃₆₅)of methanol-soluble Br C(1.12±0.35 m²g^-1C)was also higher than water-soluble Br C(0.75±0.29 m²g^-1C).The Abs₃₆₅ values of both water and methanol extracts showed tighter correlations with high molecular weight polycyclic aromatic hydrocarbons(PAHs)than low/median molecular weight PAHs.In addition,the water-soluble Br C exhibited a tighter correlation with PAHs than methanol-soluble Br C.The results might be owing to that the water-soluble Br C components and PAHs have similar sources,or possibly a considerable fraction of Br C chromophores were oxygenated products of PAHs.For water-soluble Br C,both Abs₃₆₅and MAE₃₆₅ generally decreased with increases of its O/C ratios and OS_c,indicative of photo-bleaching upon chemical ageing.Positive responses of Abs_365,WSOCand MAE_365,WSOC to nitrogen-to-carbon(N/C)ratios and water-soluble organic nitrogen(WSON)contents reveal that ON species are important Br C chromophores.The MAE₃₆₅ values of traffic-related OA(HOA),cooking-related OA(COA),BBOA,LO-OOA and MO-OOA were 1.46 m² g^-1,0.36 m² g^-1,0.77 m² g^-1,0.85 m² g^-1and 0.11m² g^-1 respectively.Overall,primary OA sources including traffic(18.9%),biomass burning(23.7%),and cooking-related OA(10.5%)together dominated the Abs_365,WSOC despite their total mass contribution was only about one third(31.9%).The largest single contributor of Abs_365,WSOC,however,was the less-oxidized secondary OA(33.8%);The more-oxidized secondary OA factor dominated water-soluble OA mass(50.8%),yet its light absorptivity was weakest and contributed only 13.1%to Br C absorption.