Study On The Absorption Characteristics, Sources And Evolution Of Atmospheric Brown Carbon In Nanjing Based On Online Observatio

Brown carbon（BrC）is a class of light-absorbing organic aerosols in the atmosphere that play important roles in visibility,climate,and human health.Despite its significance,our understanding of the optical properties,sources,and atmospheric evolution of BrC remains limited due to observational constraints and data scarcity,resulting in large uncertainties in assessing the environmental and climate effects of BrC aerosols.Therefore,further observational studies of the physicochemical properties and evolution mechanisms of BrC are of great scientific importance for validating and improving BrC simulations in the atmosphere.In this study,we conducted field observations at two sites（representing suburban and urban environments）in the typical city of Nanjing in the Yangtze River Delta region of China using a high-time-resolution online BrC observation system.Our main research objectives and results are as follows:First,the seasonal variations in the optical properties of BrC was investigated.The BrC absorption coefficient showed a clear seasonal pattern,with higher mean values in spring(8.90 Mm^-1)and winter(8.31 Mm^-1)and lower values in summer(6.70 Mm^-1)and autumn(6.75 Mm^-1).Among the four seasons,BrC contributed the most to the total aerosol absorption coefficient in winter（24.9%）and the least in summer（19.3%）.The BrC absorption coefficient decreased with increasing wavelength,with seasonal（?）ngstrom exponent（AAE）values of 1.25,1.29,1.20,and 1.32,respectively.Primary BrC(BrC_pri)and secondary BrC(BrC_sec)were quantitatively separated by using the minimum correlation method（MRS）.BrC_pri showed similar seasonal trends in the absorption coefficient and contribution to total BrC absorption coefficient as the overall BrC,with the highest contribution in winter（69.10%）,indicating the importance of primary emission sources.The contribution of BrCsec to total BrC absorption coefficient was 44.15%in summer,higher than in other seasons（30.90%–40.02%）,indicating the importance of secondary formation processes in summer.The spectral dependence of BrC_pri was generally stronger than that of BrC_sec,and the contribution of BrC_pri decreased with increasing wavelength.Secondly,the source of atmospheric brown carbon（BrC）in Nanjing was investigated.By analyzing the correlation between BrC and gas pollutants,as well as the diurnal variations of primary BrC(BrC_pri)and secondary BrC(BrC_sec),it was found that coal and biomass burning significantly influenced BrC in suburban areas during winter,whereas transportation emissions had a larger impact on BrC in other seasons and were similarly influential on BrC in both urban and suburban areas during summer.The aqueous process was the main generation pathway for BrC during winter,while gas-phase photochemistry was the main pathway during summer.However,urban summer BrC may also be affected by other secondary sources.Furthermore,we quantitatively analyzed the sources of BrC at urban monitoring sites using high-resolution aerosol mass spectrometry.We utilized positive marrix factorization（PMF）to identify the major sources of urban BrC,which include HOA（traffic-related hydrocarbon-like OA）,COA（cooking-related OA）,LO-OOA（less oxidized oxygenated OA）,MD-OOA（medium oxidizing organic aerosol）,MO-OOA（high oxidizing organic aerosol）,and LSOA（local secondary organic aerosol）.By employing a multiple linear regression（MLR）model,we quantified the absorption cross-section values and contributions of different sources.HOA and LO-OOA were found to be the main contributors to BrC,accounting for 26.4%and 42.6%of BrC absorption at 370nm,respectively,and together they accounted for most of the BrC absorption.Analysis of meteorological conditions revealed that the horizontal wind speed has some influence on BrC,but the impact of the boundary layer height was relatively greater,indicating that the atmospheric vertical diffusion and dilution ability had an important effect on the content of atmospheric brown carbon.Backward trajectory analysis was performed to investigate the impact of long-range transport on BrC in different seasons.We found that BrC_sec from sea sources had a significant impact on BrC during spring,summer,and autumn,while short-range transport of air masses was the primary influence on BrC during winter.Potential source（PSCF）analysis revealed that local emissions were the main source of BrC during spring and summer,whereas local emissions in the surrounding areas of the monitoring sites were the primary source during fall.During winter,the area of local emissions was more concentrated and widespread due to increased combustion activity.Finally,the analysis of typical evolution processes of BrC revealed that short-term emission reduction measures such as coal and biomass burning reduced the post-Chinese New Year BrC emissions by 40%,indicating their significant impact on BrC.Secondary generation,including aqueous process and gas-phase photochemistry,were the main pathway for BrC formation,with varying degrees of influence at different times.Further analysis of BrC sources during different pollution periods revealed that HOA and MO-OOA were the main sources of BrC during light pollution periods,with their respective contributions to light absorption reaching 44.57%and 29.03%.LO-OOA was the main source of BrC during heavy pollution periods,with its contribution to light absorption reaching 67.31%.Therefore,it was the primary source of BrC during pollution periods.In addition,during this period,the concentration of O₃ in the atmosphere increased,promoting the generation of LO-OOA and enhancing the light absorption capacity of BrC.