Molecular Composition Of Biomass Burning Organic Aerosols By Ultrahigh Resolution Mass Spectrometry

Dissolved organic matter（DOM）is the main carrier of the global carbon cycle.Hence,it is necessary to analyze the chemical composition of DOM at the molecular level for better understanding the intrinsic connection between the mechanisms of DOM molecular interactions and the causes of environmental pollution.With ultrahigh resolution and accuracy,Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR MS）,has become one of the most powerful technical method to accomplish this.Solid-phase extraction（SPE）is a traditional pretreatment procedure widely used for DOM desalination and enrichment prior to the FT-ICR MS analysis,and the extracts are usually acidified to p H=2 with hydrochloric acid（HCl）before passing through the cartridge in order to enhance extraction efficiency during the process.However,little is known about the effects of acidification on the integrity and molecular composition of DOM.Here,the differences in molecular compositions between acidified and nonacidified DOM samples of soil,seawater and atmospheric aerosol were performed by FT-ICR MS.The results showed that the quantity and intensity of aromatic compounds with high oxygen content（e.g.,polyphenols,tannin-like and highly oxygenated organic compounds）were greatly enhanced after acidification,while highly saturated compounds（lipid-like and aliphatic/peptide-like）were absent.The underlying reason is the variation of solubility and hydrolysis of DOM under acidic conditions.The atmosphere is also one of the main environmental media in which DOM exists,and DOM is an important part of atmospheric aerosols.The organic matters of atmospheric aerosols that absorb radiation in the near ultraviolet and visible regions are collectively referred to as brown carbon（Br C）.Biomass burning is one of the major sources of Br C,and the study of its molecular composition will be of great significance for understanding the exact source,formation mechanism and physical and chemical properties of Br C.FT-ICR MS,as an effective method for studying the molecular composition of organic matter,can get the best analysis results by combining with a variety of ion sources.Electrospray（ESI）,a soft ionization method,is widely used to study organics in complex systems.And the negative ESI-FT-ICR MS was adopted as the main method in previous studies on biomass burning organic aerosols（BBOA）which have a large number of hydroxyl,carboxyl and phenolic groups.However,it is worth noting that some lower polar compounds with light absorption cannot be detected using negative ion mode.Hence,four representative BBOA samples,which are firewood,cow dung,corn stalks and cotton stalks,were selected and analyzed by ESI-FT-ICR MS in both positive and negative modes in this study.The result showed that compounds with unsaturated functional groups such as benzene rings and phenols were more likely to be ionized and detected in the negative mode,while some aliphatic compounds with lower polarity were easier to be ionized in positive mode.This study can provide a reference for fully characterizing the molecular composition of BBOA.China is a large agricultural country with a vast territory,and the types of crops and the environmental pollution status in different areas are not the same.It is extremely important to study the characteristics of BBOA from different areas for understanding the causes of air pollution.In this study,the molecular composition of nitrogen-containing organic matter was analyzed by ESI-FT-ICR MS,and the optical properties were investigated by using ultraviolet-visible（UV-vis）spectroscopy and excitation-emission matrix（EEM）spectroscopy.The results showed that E₂/E₃ of the biomass burning organic aerosols from southern cities was higher than that from northern cities in China.Compared with herbaceous plants,the nitrogen-containing organic molecules of woody plants had higher O/C and lower H/C,and there were more highly oxygenated organic compounds（HOC）and nitrophenol compounds.By studying the relationship between optical properties and chemical molecular composition of BBOA,it was found that the proportion of compounds that aromaticity index(AI_mod)≥0.5 and nitrophenol compounds in them were consistent with their optical properties and contribution to atmospheric light absorption.These results provide theoretical basis in favor of the study the source and formation mechanism of Br C.Meanwhile,this study also provides useful information for investigating the causes of environmental pollution in autumn and winter.In addition,analyzing the optical properties in combination with chemical composition characteristics of BBOA can contribute to the research of the influence mechanism of biomass burning on atmospheric light absorption.