Light Absorption Properties And Source Analysis Of Brown Carbon Aerosols In Typical Cities In The Yangtze River Delta

Light-absorbing organic carbon（brown carbon,BrC）of atmospheric aerosols can directly impact the Earth’s radiative balance by absorbing solar radiation.In this study,two identical samplers installed with double quartz filters were used for PM_2.5 sampling side by side at a suburban site in northern Nanjing.Collocated sampling was conducted every sixth day during daytime and nighttime from September 2018 to September 2019.Particulate matter was collected on a quartz filter（Q_f）with a backup quartz filter（Q_b）behind for estimating and adjusting sampling artifacts.The analysis of gravimetric mass,water-soluble inorganic ions（WSIIs）,organic carbon（OC）,elemental carbon（EC）,water-soluble OC（WSOC）,water-soluble total nitrogen（WSTN）,and water-soluble organic nitrogen（WSON）were performed on both Q_f and Q_b.To evaluate the light-absorbing properties of BrC with light absorption coefficient(Abs₃₆₅),mass absorption efficiency at 365 nm(MAE₃₆₅),and absorption Angstrom exponent（A）,water and methanol extracts of Q_f and Q_b samples were measured using ultraviolet/visible spectrometer.The measurement error of chemical components and light-absorbing properties of BrC were estimated according to the analytical results of collocated samples.The paired difference(SD_diff)and the average relative percent difference（ARPD,%）were used to reflect absolute and relative error,respectively.The final concentrations of target components and light-absorbing properties of BrC in PM_2.5 were calculated based on the averages of collocated Q_f-Q_b data,and the associated error was determined by calculating their ARPD.Positive Matrix Factorization（PMF）was used to explore the sources of BrC light absorption incorporating mass concentration data of bulk species and organic molecular markers.The main results were as follows:Due to the high mass loadings on Q_f,the measurement error（ARPD=5.28%～9.40%）of gravimetric mass and dominant species(NH₄⁺,NO₃^-,SO₄^2-,OC,WSOC and WSTN)are lower than those from previous work.The measurement error（9.83%～12.2%）of light-absorbing properties of WSOC(Abs₃₆₅,w,MAE₃₆₅,w,and AWSOC)on Q_f are much lower than those of methanol extractable OC（MEOC,14.7%～23.0%）,because the amount of filters used for extracting WSOC was 8.3 times that of MEOC.The concentrations of target components,Abs₃₆₅,w and Abs₃₆₅,m on Q_b are only 3.02%～22.4%,13.8%and 18.1%of those on Q_f,which leads to higher measurement error on Q_b（25.0%～47.3%,52.6% and 55.0%）.The relative error（ARPD）ranges of bulk components concentrations and light-absorbing properties of BrC in PM_2.5 are 5.62%～13.1% and 14.0%～27.9%,respectively.A fixed error fraction（10%） was applied for PM_2.5 components in a number of source apportionment studies.The results in this work suggest that selecting this value will underestimate the measurement error of Ca²⁺（23.6%）,Mg²⁺（29.7%）,WSON（17.1%）,and Abs₃₆₅（19.3%and 27.9%）.Moreover,the average Abs₃₆₅,w and Abs₃₆₅,m during the sampling period were 3.01±2.02 Mm^-1 and 6.39±4.67 Mm^-1,respectively.Both Abs₃₆₅,w and Abs₃₆₅,m exhibited diel variations with higher levels at night than those of daytime,and seasonal variations with greater values during winter than those during summer.This can be attributed to the variations of source emissions,meteorological characteristics（e.g.,boundary layer height,etc.）and "photobleaching effects".The annual average value of Abs₃₆₅,m/Abs₃₆₅,w was evidently larger than that of MEOC/WSOC（1.47±0.32,p<0.01）.The MAE₃₆₅ value of water-insoluble fraction of MEOC(3.24±7.10 m²·g^-1)was 4.7 and 2.5 times higher than those of MAE₃₆₅,w(0.68±0.29 m²·g^-1)and MAE₃₆₅,m(1.02±0.58 m²·g^-1),respectively.These results indicate that water-insoluble organic components dominate BrC absorption.In this study,a 7-factor solution was finally determined for the PMF analysis based on compositional data of PM_2.5.The factors were identified as fossil fuel combustion,on-road traffic emission,volatilization of fuel,secondary inorganic salt formation,secondary organic aerosol formation,biomass burning,and combustion and volatilization of lubricants.The results suggest that biomass burning is the main source of BrC for the study location,which had the highest contribution to Abs₃₆₅,w（42.8%）and Abs₃₆₅,m（58.6%）.This conclusion is consistent with those in previous work.