Molecular Composition And Seasonal Variation Of Organic Nitrogen Compounds In Atmospheric Aerosols In Beijing

The composition of organic nitrogen compounds（ONC）in PM_2.5 is very complex.Many organic nitrogen compounds have negative effects for environment and human health,Although ONC in aerosols play a significance role for haze formation,climate and health effect,limited compositional information of individual compounds is known.This study used liquid chromatography combined with high resolution Orbitrap mass spectrometry（LC-Orbitrap MS）to characterize the chemical composition and seasonal variation of ONC in the atmospheric PM_2.5 extracts from April 2016 to February 2017 in a suburban sampling station in Beijing.A total of 26 class species of ONC were detected by Orbitrap MS with negative-ion electronspray ionization（ESI）,namely N₁（the class species with one nitrogen atom in the molecules）,N₂,N₁O₁-N₁O₉,N₂O₁-N₂O₉,and N₁O₄S₁-N₁O₉S₁.These compounds should be pyrrolic nitrogen compounds,organonitrates,nitro-organosulfates,and nitrophenols.A total of 22 class species were detected with positive-ion ESI,including N₁,N₂,N₁O₁-N₁O₁₀,and N₂O₁-N₂O₁₀.Most of these compounds should be pyridinic compounds.A total of 14 class species were detected by atmospheric pressure photo-ionization（APPI）,including N₁,N₂,N₁O₁-N₁O₆,and N₂O₁-N₂O₆.These compounds mostly are oxygen and nitrogen-containing polycyclic aromatic hydrocarbons.Nitro compounds in water extracts were characterized by LC-MS and nitrophenols and nitro-organosulfates were found as the major nitro compounds in the PM_2.5.Tandem mass spectrometry provided solid evidence for the structural speculation of nitrophenols and nitro-organosulfates.Compared to other seasons,N₁O₇S₁,N₁O₈S₁ nitro-organosulfates in winter has higher molecular condensation and wider carbon number distribution range,which can be explained as the increase in emissions of precursors such as SO₂,NO_X,and aromatics in winter.Compared with the other three seasons,the carbon number and DBE range of N₁O₃ and N₂O₅ compounds in the spring were wider,indicating that the spring has a wide range of sources.The most abundant N₁O₃ compounds in the four seasons are all C₆H₄NO₃,and the two most abundant N₂O₅ compounds are the same,which are C₆H₃N₂O₅ and C₇H₅N₂O₅,respectively,indicating that a certain source or several sources always dominates in N₁O₃ and N₂O₅ compounds in the four seasons.Eight nitrophenol and eight dinitrophenol molecules were quantitatively analyzed in water extracts.The concentrations were 78.2,33.1,82.6,and 302.4 ng/m³ in spring,summer,autumn and winter,respectively.The concentration of winter is 10 times of summer,showing a significant seasonal change.The concentration of ONC in different seasons was analyzed by ion chromatography and elemental analysis.The results showed that the contribution of water soluble organic nitrogen to total nitrogen was 20%,25%,26%,and 38%in spring,summer,autumn and winter,respectively.The contribution of water soluble organic nitrogen to total nitrogen and the concentration of nitrophenols increased significantly in winter.It is closely related to the increase of fossil fuel combustion emissions in winter,with obvious anthropogenic characteristics and quadraticity.The nitrogen heterocyclic compounds in dichloromethane extracts in winter are dominated by basic nitrogen.The most abundant species are N₁ and N₂ compounds.The N₁ is mainly quinoline and pyridine compounds containing C₀-C₉ and C₂-C₁₁ alkyl side chains,respectively.The N₂ is mainly imidazole and benzimidazole compounds containing C₃-C₁₀ and C₀-C₈ alkyl side chains,respectively.