Impacts Of Sources And Atmospheric Processes On The Molecular Composition And Light-absorption Properties Of Organic Aerosols

Atmospheric organic matter（OM）constitutes an important fraction of aerosol particles,which has substantial effects on climate,environment,and human health.Brown carbon（Br C）,a kind of light-absorbing organic aerosols in OM,has attracted much attention because of its remarkable light-absorbing ability in near ultraviolet and visible（UV-vis）wavelengths of sunlight.Due to the strong light absorption ability of Br C,it affects global radiation balance and climate changes.Therefore,it is necessary to carry out extensive and in-depth research on the light-absorption properties,sources,and molecular composition of Br C.Understanding the effects of carbon sources and environmental conditions on the molecular composition and light absorption properties of Br C is not only conducive to control Br C emissions accurately,but also helps to understand the biogeochemical processes of Br C and reduce the uncertainty in global radiation forcing estimation of previous climate models.However,current knowledges on the sources,molecular composition,and light-absorption properties of Br C,as well as the linkages between the three at molecular level are insufficient.To investigate above issues,dissolved organic matter（DOM）in PM_2.5 in the Pearl River Delta（PRD）were characterized by UV-vis spectroscopy,chemical tracers,radioisotopes（carbon,lead,beryllium）,and Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR-MS）,as well as models and statistical methods analysis.（1）Atmospheric PM_2.5samples from May 2016 to April 2017 were collected at Heshan station（Pearl River Delta regional station）,and 28 of them were selected to study the light absorption properties of polarity-dependent DOM;moreover,we also explored the characteristics of molecular composition and sources for different polar components which affect light absorption properties of each carbon fraction.Our results showed that water-soluble organic carbon（WSOC）and water-insoluble organic carbon organic carbon（WISOC）accounted for 50%and 43%of the total DOM carbon,respectively.WSOC had relatively strong light absorption capability,contributing 60%of total light absorption at 365 nm(Abs₃₆₅),while WISOC only accounted for about 40%of total Abs₃₆₅.Humic-like substance（HULIS）and medium polarity（MP）are the main light-absorbing components in WSOC and WISOC,respectively.HULIS,MP and low polar fraction（LP）contributed 46±17%,30±7%and 7±3%to the total Abs₃₆₅ of total extracts,respectively.The carbon mass absorption efficient(MAE₃₆₅)of HULIS,MP and LP decreased with the polarity,with mean values are 1.6±0.4 m²·g^-1C,1.2±0.3 m²·g^-1C and 0.8±0.4 m²·g^-1C,respectively.¹⁴C analysis showed the proportion of non-fossil carbon decreased（HULIS:66±2%,MP:52±2%and LP:36±3%,respectively）with the decrease of polarity for the components.The molecular characteristics of average degree of oxidation,unsaturation of per carbon decreased with the decrease of the polarity.Based on the carbon oxidation state diagram and the chemical tracer-based correlation analysis,biomass burning has different effects on the molecular composition of polarity-dependent Br C:the major type of potential Br C in HULIS and MP are oxidized biomass burning organic aerosols（BBOA）,but the potential Br C chromophores in LP are mainly associated with primary BBOA.Seasonal variation showed that the carbon content and light absorption capability of each fraction presented an increasing trend in winter,which were positively correlated with the proportion of non-fossil carbon and the concentration of levoglucosan,implying that biomass burning had a significant effect on Br C absorption of all polar fractions.Combined with the FT-ICR-MS analysis,our results indicated that elevated BBOA emissions in winter in the PRD led to the increase of the relative abundance of light-absorbing nitrogen-containing compounds in each carbon fractions,which eventually result to the increase of Br C absorption.（2）In order to quantitatively determine the sources of DOM and dissolved-Br C in PM_2.5 accurately,55 representative PM_2.5 samples were collected in Guangzhou from July 2017 to June 2018 to analyze the chemical composition,light-absorption properties,double carbon isotope and organic molecular markers.Source apportionments of DOM and dissolved-Br C in PM_2.5in Guangzhou were performed by using radiocarbon isotope(¹⁴C)results constrained positive matrix factorization（PMF）method coupled with multiple linear regression（MLR）analysis.Results showed that both primary emission sources and secondary sources accounted for 50%of carbon to DOM.Primary emission sources included fossil fuel combustion（FF,32%）and biomass burning（BB,18%）;secondary sources include secondary nitrate formation process（NT,20%）,the mixing-source of photochemical process of anthropogenic and biogenic volatile organic compounds and waste combustion（PW-SOA,7%）,and mixing-source related to the organosulfates formations from isoprene and fatty acid under sulfate condition（ISO+OS,22%）.The contributions of sources to dissolved-Br C were quite different from that to DOM,FF and BB accounted for 34%and 27%of total dissolved-Br C absorption at365 nm,respectively,the total mean light absorption contributed by secondary sources was 39%.The Abs₃₆₅ contributed by FF changed insignificantly and was dominated in summer,whereas the Abs₃₆₅ from BB and NT increased and contributed larger fractions in the winter monsoon period.Based on the ²¹⁰Pb and ⁷Be（atmospheric transport tracers）as well as air mass backward trajectory analysis,our result indicated that the increase of Abs₃₆₅ of DOM in winter may be related to the regional transport of BBOA.According to the variations of Abs₃₆₅ value for near-surface transport and upper-atmosphere transport process which evaluated by ⁷Be/(⁷Be+n²¹⁰Pb)ratio,we further confirmed that the pollutants with high absorption capability mainly affect the light-absorption of Br C in Guangzhou through near-surface transport processes.Based on the positive correlations between ²¹⁰Pb and Abs₃₆₅ and non-fossil DOM in the winter monsoon period,we estimated that the contribution of invasive Br C to total absorption during the period of elevated Br C was approximately 50%.（3）Based on the works of source apportionment of dissolved Br C in PM_2.5 samples in Guangzhou,in order to understand the relationship between molecular composition and light-absorption of DOM,and how carbon sources and environmental factors affect the molecular composition and light-absorption properties of DOM,we characterized the molecular composition of DOM by using negative electrospray ionization（ESI–）FT-ICR-MS,and then carried out statistical analysis and modeling performance combining use FT-ICR-MS data,online meteorological data and chemical tracers.Our results showed that CHON and sulfur-containing compounds were the main from the elemental groups of DOM in PM_2.5,accounting for 28%（41%）and 44%（33%）of total relative contents（number）,respectively,and CHO compounds accounted for 28%（26%）of total relative contents（number）.Form the view of molecular structures,saturated aliphatic and peptide compounds account for 62%of the total relative contents,followed by unsaturated/phenolic compounds,accounting for 28%;polyphenols and condensed aromatic compounds only contributed 6%and 3%to the total relative contents,respectively.The relative abundance of CHON compounds and aromatics（polyphenols and condensed aromatics）increased about 12%and 1 times from summer to winter,respectively,and showed positive correlations with each other,indicating that the increases in relative contents of CHON compounds accompanied by the elevated aromatics emissions in winter.Principal component analysis（PCA）showed that light-absorption of DOM were positively correlated with the average degree of unsaturation,aromaticity,N contents,but negatively correlated with the relative contents of aliphatic compounds.Rank correlation analysis showed that the compounds with positive correlation between relative abundance and Abs₃₆₅ were mainly polyphenols,condensed aromatics and highly-oxygenated unsaturated/phenolic compounds,of which CHON compounds accounted for more than 68%of total number.Seventeen nitrogen-containing compounds were identified by random forest model,which can be used to indicate the variations of dissolved-Br C in PM_2.5 in Guangzhou.Based on non-metric multidimensional scaling（NMDS）and decision tree model analysis,the molecular distribution of Br C is mainly driven by meteorological conditions and human activities.Biomass burning and OH radical had great impact on the variations of Br C absorption.The elevated biomass burning emissions and the secondary nitrogen-chemistry will lead to the enrichment of polyphenols and condensed aromatics,as well as some highly-oxygenated unsaturated/phenolic compounds,resulting in light-absorption enhancement.However,the increase of atmospheric oxidation（OH radical concentration）,solar radiation and relative humidity will result in the photolysis or photobleaching of Br C compound,lead to the transformation of Br C to more stable fatty compounds.（4）Considering nitrogen-containing compounds has relationship with the light-absorption of DOM,in order to understand the molecular distribution of nitrogen-containing organic compounds and how it varies with the environmental conditions,we characterized the nitrogen-containing organic compounds in PM_2.5 both by using positive and negative ESI（ESI+and ESI–）FT-ICR-MS analysis.Our results showed that the number and relative content of the detected CHON compounds（CHON–）with O/N≥2（or O/N≥3）were over than 80%,and they were mainly distributed around O/N=6,indicating that CHON–compounds were mainly composed of highly-oxygenated organic nitrates.The CHON+compounds detected in ESI+have large number of common formulas with CHON–,probably indicating they were amino acids or oxygen-containing polymers.Among CHON+compounds,protonated CHON+compounds（CHON-H）accounted for 90±9%total relative contents,while sodium ionized CHON+（CHON-Na）compounds accounted for only 10±9%.The relative contents of CHON-H increased in winter,which may be related to biomass combustion;while the relative contents of CHON-Na showed higher values in summer,which may be related to the secondary reactions of biogenic sources.In addition,relatively low contents of CHN compounds were detected in ESI+mode.76%of the CHN compounds have aromatic equivalence index（Xc）more than 2.5,which may be attributed to reduced nitrogen-containing compounds with phenyl,such as amines,nitrogen heterocyclic compounds,alkaloids.The relative abundance of 1N compounds was higher than that of 2N compounds.The relative contents of a small number of CHN compounds with 2N atoms,which were also detected in BBOA samples,was positively correlated with the concentration of levoglucosan,indicating they were probably emitted from biomass burning.NMDS analysis showed that the molecular distribution of nitrogen compounds was affected by many factors,such as biomass burning,secondary nitrogen chemical process,meteorological conditions,biogenic VOCs,and liquid water content.Patterns in molecular profiles showed that the relative abundance of condensed aromatics,polyphenols,unsaturated/phenolic compounds,and part of saturated aliphatic CHON compounds increased with the elevated biomass burning emissions and secondary nitrogen-chemistry processes.The increase of OH,temperature,RH and MSR will lead to the structural transformation of nitrogenous compounds to lipids and protein/amino sugars,making them relatively stable.