Molecular Characterization And Origins Of Organic Nitrogen And Sulfur Compounds In Atmospheric Wet Deposition By FT-ICR MS

Wet deposition is the most effective scavenging mechanism for the atmospheric particulate matter.Organic nitrogen-containing and organic sulfur-containing substances in the atmosphere provide as a vital source of the nitrogen and sulfur of the surface-earth system with the link by wet deposition.The deposition of organic nitrogen and organic sulfur directly affects the balance of nitrogen and sulfur in water and soil,which might pose a potential threat to the biodiversity of entire ecosystems.For now,the research studies mainly focus on the concentrations,sources,deposition fluxes,and the influence on the ecosystems of inorganic nitrogen and sulfur in atmospheric aerosols and precipitations.The organic part is generally focused on the total amount of organic nitrogen or sulfur,some low molecular weight organic nitrogen compounds(such as polycyclic aromatic hydrocarbons,amines,nitro phenols,etc.)and some low molecular weight reductive or volatile organic sulfur(COS,DMS and CS2).However,the molecular composition,seasonal variation,and the scavenging mechanism of the organic nitrogen and organic sulfur in natural organic matter(NOM)are merely studied.The electrospray ionization(ESI)coupled to Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS)with ultrahigh resolution and accuracy,has made remarkable achievements in the molecular characterization of complex matrix of medicines,proteomics,metabolomics and pharmacological fields.FT-ICR MS can detect the m/z of different ions without separate the ions in the samples.FT-ICR MS can be connected with other detectors in series for further structural analysis.In addition,FT-ICR MS can acquire high quality accuracy with internal calibration.The application of FT-ICR MS could elucidate the molecular composition,seasonal variation,and the scavenging mechanism of the organic nitrogen and organic sulfur substances in wet deposition.In addition,these knowledges could broaden our understanding of organic matter in wet deposition,and build a scientific foundation for exploring the scavenging mechanism of atmospheric particulate matter.In this study,FT-ICR MS was used to elucidate the molecular composition of organic nitrogen and sulfur substances of precipitation samples collected from different regions and different seasons.Main conclusions of this study are briefly descripted as follows:(1)The molecular composition of organic nitrogen and sulfur substances in snow samples from Beijing,were compared with those collected in Nyingchi,Tibet at different altitudes.Nyingchi is regarded as relatively clean area,while Beijing is typical urban area.The excitation emission matrix fluorescence spectroscopy(EEM)results indicated that protein peak T is the main fluorescent group in the snow samples from Nyingchi area,while hulis-like peak A has the highest signal in the snow sample from Beijing.The molecular formula numbers of organic nitrogen-containing molecules(CHON)in snow samples of Nyingchi are higher than those in Beijing snow sample,whereas the molecular formula numbers of organic sulfur-containing molecules(CHOS)present an opposite trend.The aliphatic/peptide-like molecules in the snow samples from Nyingchi area contain a lot of CHON molecules with O/N ratio ≥ 9,indicating the great contribution of biogenic aerosols in the atmosphere.In contrast,there are many CHOS molecules with oxygen/sulfur ratio(O/S)≥9 and mass/charge ratio(m/z)≥ 500 Da in the snow sample from Beijing,indicating that organic sulfur molecules produced by anthropogenic sources(such as fossil fuel combustion)contribute more in urban areas.(2)Four snow samples from one precipitation event are collected in the megacities of Beijing,Tianjin,Shijiazhuang and Taiyuan in North China.Detailed organic nitrogen composition and the possible scavenging mechanisms of CHON were analyzed.The elemental O/N ratio(≥ 3),together with the preference in the negative ionization mode,indicates that CHON in snow were largely in the oxidized form(as organic nitrates,-ONO2).This study assumed that scavenging of particle-phase and gasphase organic nitrates might be significant sources of CHON in precipitation.Gasphase oxidation process and particle-phase hydrolysis process,at a molecular level,were used to trace the potential precursors of CHON.Results show that more than half of the snow CHON molecules may be related to the oxidized and hydrolyzed processes of atmospheric organics.Potential formation processes of atmospheric organics on a molecular lever provide a new concept to better understand the sources,scavenging mechanisms of organic nitrogen species in the atmosphere.(3)By extending the modified oxygen(O*)and redefined aromaticity index(AI*)from previous studies,we provide a new classification of organic sulfur species in the atmosphere.Typical known organosulfates detected in smog-chamber experiments(secondary organic aerosol formation)and anionic surfactants(emitted directly from anthropogenic sources)are used to verify the validity of this new classification.Many of these typical known CHOS are detected in the four snow samples of one precipitation event from North China,indicating the scavenging effect of snow on these CHOS molecules.In addition,high mass CHOS(≥ 500 Da),with oxygen to sulfur ratio varying from 10 to 16,contribute more in polluted regions than in clean areas.(4)Precipitation samples in 2018 collected from Tianjin,a typical city area,were used to study the scavenging effects on atmospheric organic nitrogen and sulfur pollutants of precipitation in different seasons.The dissolved organic carbon(DOC)content of precipitation samples in Tianjin presents a nonlinear coupling relationship with precipitation amount,and the DOC content decreases with the increase of precipitation amount.Among the four seasons,the precipitation samples in autumn have the highest ultraviolet-visible absorbance.It should be noted that the precipitation samples in spring have a slope bulge between 260-320 nm,which may be affected by nucleotides,RNA and DNA.The molecular formula numbers of CHON in winter and autumn precipitation samples are higher than those in spring and summer precipitation samples.Majority of the CHON molecules are identified as lignins-like,aliphatic/peptides-like,and lipids-like molecules.CHON molecules with O/N ≥ 3contributed nearly 93%-99% of the total CHON molecules in each precipitation sample,indicating that the CHON in precipitation samples were largely as organic nitrates.Biomass burning organic aerosol(BBOA)and hydrocarbon-like organic aerosol(HOA)might be important sources of CHON in precipitation samples.CHOS molecules in Tianjin precipitation samples are largely identified as aliphatic/peptides-like,ligninslike,and lipids-like molecules.High mass CHOS(≥ 500 Da),with oxygen to sulfur ratio varying from 9 to 14,contribute more in summer precipitation samples than samples from other three seasons,which might be come from aqueous organic sulfate catalyzed by acidic sulfate in aerosol.Known organosulfates(secondary organic aerosol formation)and anionic surfactants(emitted directly from anthropogenic sources),which are in medium and low mass(< 500 Da),were detected in Tianjin seasonal precipitation samples.(5)Two kinds of natural organic matter(NOM)standard purchased from International Humic Substances Society(IHSS)were selected to represent for the molecular composition database of IHSS-NOM.In addition,all of the precipitation samples in this study were sorted into five subgroups(databases)based on different collection sites(relatively clean area or typical urban area),different precipitation forms(in rain or in snow),different seasons.The molecular composition and differences of organic nitrogen and sulfur molecules among six different organic matter databases were compared and analyzed,which could provide a basis for the traceability of organic nitrogen and sulfur molecules in wet deposition samples and atmospheric environment in the future.CHON and CHOS molecules in five precipitation databases were largely identified as lipids-like,aliphatic/peptides-like and lignins-like molecules,whereas those molecules in IHSS-NOM database were largely identified as liginins-like,tannins-like/ highly oxygenated compounds(HOC).Biomass burning organic aerosol(BBOA)and hydrocarbon-like organic aerosol(HOA)might be important sources of CHON and CHOS molecules in five precipitation databases.In contrast,the organic molecules in IHSS-NOM database were much more functionalization(addition of polar function groups)and fragmentation(cleavage of C-C bonds).