Analysis Of Elemental Composition And Seasonal Variation Of Nitrogen-Containing Organic Compounds In PM_2.5 In Shanghai Based On High Resolution Mass Spectrometry

In recent years,haze?low visibility?occurred frequently in China,with prolonged duration and expanded coverage in many cases.PM_2.5.5 pollution has become a serious environmental problem,and is drawing more and more attention in China.Nitrogen-Containing Organic Compounds?NOC?is an important component of PM_2.5,which affects regional air quality and human health,and has become a hot topic in recent years.However,there are few reports on the composition of NOC in China.Studying the composition characteristics of NOC in aerosols in Shanghai can provide references and suggestions for the government to formulate air pollution control measures.The research results will also have important value for expanding and improving the existing air quality model.In this study,water-soluble organic compounds in PM_2.5.5 samples collected at two sampling sites in Qingpu and Pudong in Shanghai from June 2016 to December2016 were analyzed using ultra-high resolution mass spectrometer?Orbitrap-MS?.The molecular composition and seasonal variation of nitrogen-containing organic compounds in PM_2.5.5 were studied.The main conclusions are as follows:During the sampling period in 2016,the average concentrations of water-soluble organic nitrogen?WSON?in PM_2.5.5 in summer,autumn and winter at Pudong were 0.88,1.01 and 2.25 ugN/m³,respectively.While the average concentrations of WSON at Qingpu were 0.85,0.91 and 0.97 ugN/m³,respectively.The concentration of WSON was the highest in winter and the lowest in summer,but the contribution of WSON to the water-soluble total nitrogen?WSTN?was the lowest in winter and the highest in summer and autumn.Significant positive correlations between WSON concentration and major inorganic ions such as NO₃^-,SO₄^2-,NH₄⁺,K⁺,WSOC,HuLis-C and other organic components in PM_2.5.5 were found.There is also a significant positive correlation between particle acidity and WSON/WSTN ratio,indicating that secondary formation was the important source of WSON in PM_2.5.5 in Shanghai,and primary sources such as biomass combustion also had significant contributions.High resolution mass spectrometry analysis showed that more than 60%of the thousands of water-soluble organic compounds detected in PM_2.5.5 in Shanghai were nitrogen-containing organic compounds,mainly CHON and CHONS compounds,and the average molecular weight of nitrogen-containing organic compounds was higher in winter than that in summer.The proportion and intensity of CHONS in summer at Qingpu were higher than those at the Pudong sampling site,and also higher than those in autumn and winter at Qingpu.The emission of sulfur-and nitrogen-containing VOCs from the blooming microbial in Dianshan Lake in summer may be the important reason.Analysis of the molecular composition of CHN-containing organic components showed that CHN-containing compounds with 2 and 3 N atoms accounted for the vast majority of the total CHN-containing compounds.The seasonal distribution of relative intensity of CHN species shows that the relative intensity of CHN species was higher in autumn than that in winter and summer,and the relative intensity of CHN species at Qingpu was higher than that at Pudong in autumn.Biomass combustion may be the main source of CHN species.The O/N atomic ratio of most CHON compounds is greater than 2,indicating that most CHON compounds may be nitrogen-containing organic compounds with-NO₂ or-ONO₂ structure formed by secondary oxidation.From the VK diagram of CHON compounds,it can be found that the overall oxidation state of CHON species in summer was higher than that in winter and autumn.The O/S ratio of most CHONS compounds is more than 4,indicating that the structure of CHONS compounds may contain-SO₄ group,and the intensity of CHONS compounds was proportional to the sulfate content,indicating that most of the CHONS compounds might be nitrogen-containing organic sulfates.