Optical Properties Of Organic Carbon In Atmospheric Particles In The Northern Suburbs Of Nanjing

The strong absorption of solar radiation by organic carbon（OC）in atmospheric particulates has an important impact on the balance of atmospheric radiation and climate change.Among them,A fraction of OC can absorb lights at the wavelengths of near-ultraviolet（UV）and visible ranges,with absorption increasing sharply towards shorter wavelengths,which is referred to as brown carbon（Br C）.Herein,two types of OC extracted from one-year PM_2.5samples（June 2017–May 2018 in Nanjing）,i.e.methanol-extracted organic carbon（MSOC）and ultrapure water-extracted organic carbon（WSOC）,were obtained to investigate distinct optical properties of atmospheric Br C.The extraction efficiency of Br C was as high as 91%in methanol solution,and the corresponding light absorption coefficient（Abs）of MSOC at 365 4.84±2.97 Mm^–1),indicating that the water-insoluble compounds were the most significant contributor to the light absorption of Br C.The seasonal variations of Abs_365-WSOC and Abs_365-MSOC were followed the sequence of winter>spring>autumn>summer,due to the dominated emissions from fossil fuel combustion and biomass burning in the cooling seasons.The mass absorption efficiency of MSOC(MAE_365-MSOC,0.93±0.26 m²g^-1)is slightly higher than that of absorption efficiency of Br C in winter is higher.The wavelength dependence（AAE）shows that the light absorption of water-insoluble Br C has a relatively low wavelength dependence.The average fluorescence intensity of OC shows the seasonal trend of winter>autumn>spring>summer.Fluorescent index（FI）,biological index（BIX）and humification index（HIX）together show that the humification and aromatic degree of local fluorescent substances are high,and the microbial source is complex.Additionally,four fluorescent chromophores in WSOC and MSOC,containing three humic-like chromophores（C1,C2,C3）and one protein-like chromophore（C4）were obtained by parallel factor analysis（PARAFAC）.Humic-like substances are the major contribution of WSOC fluorescent chromophores.C1 may be related to the photodegradation of macromolecules,and C3 is more aromatic than C1,which may be related to the condensation reaction that produces more unsaturated substances.C4 is a tryptophan-like component.Among the fluorescent chromophores of MSOC,C1,C2 and C3are humus like chromophores,C1 contains more conjugated molecules,C2 may come from photochemical products,and the oxidation degree of C4 is low.The contribution of protein-like chromophore（C3）in MSOC is important and higher than that of WSOC,indicating that protein-like substances are more soluble in methanol.Whether it is WSOC or MSOC,the larger seasonal difference contribution between different fluorophore components indicates that the OC fluorescent chromophore source has a greater seasonal dependence.Considering the correlation between primary organic carbon（POC）and secondary organic carbon（SOC）,it was found that the optical characteristics of water-soluble OC in Nanjing are mainly from secondary and primary sources（i.e.,traffic emissions,biomass combustion）.Besides,OC dissolved in methanol may come from emissions from biomass combustion and fossil fuel combustion.