Analysis Of Molecular Composition Of Nitrogen-containing Organic Compounds In PM₁ In Shanghai By HPLC-Orbitrap High Resolution Mass Spectrometry

More and more attentions have been paid to the atmospheric fine particles and haze in Shanghai and the Yangtze River Delta area.Sub-micron particles?PM₁?can scatter sunlight efficiently and thus is the most important pollutants causing the occurrence of haze.Meanwhile,PM₁ is more hazardous to human health than the coarse particles.Nitrogen-containing organic compounds?NOCs?is an important component of the fine particles,which has important impacts on human health and ecosystem.But available studies on NOCs in sub-micron particles in our country are very limited.Therefore,systematical study on the concentration and composition of the nitrogen-containing organic compounds,and the investigation of the factors influencing the formation of NOCs are of great importance to the study of the formation mechanism of atmospheric haze and to the formulation of air pollution control policies.In this study,PM₁ samples were collected on the rooftop of a teaching building in the campus of Shanghai University from June 2017 to May 2018.The concentrations of water-soluble organic nitrogen?WSON?were measured,and the distribution characteristics of WSON were investigated.Source apportionment of WSON in PM₁ in Shanghai were fulfilled using PMF.The qualitative and semi quantitative analysis of the molecular composition of NOCs in PM₁ was carried out by high-pressure liquid chromatography tandem orbitrap high-resolution mass spectrometry?HPLC Orbitrap MS?.The following conclusions are drawn:The annual average concentration of WSON in PM₁ in Shanghai was 0.40?g N/m³,accounting for about 13%of the water-soluble total nitrogen?WSTN?.The concentration of WSON was the highest in winter,followed by spring and summer,and the lowest in autumn.While the contribution of WSON to WSTN was higher in summer and lower in winter.Significant correlations were found between WSON and NH₄⁺,SO₄^2-,K⁺,NO₃^-,but not between WSON and Ca²⁺,indicating that secondary generation and biomass burning were the important sources of WSON.The results of PMF analysis showed that secondary formation and biomass burning contributed 48%of the WSON,20%from the conversion of biogenic VOCs,21%from kitchen and vehicle emissions,and 11%from coal combustion.Significant seasonal variations for the sources of WSON were found.About 45%of the WSON would be from photochemical transformation of biogenic VOCs in summer.In ESI+mode,1742?5173 compounds were detected by the HPLC Orbitrap MS,among which 63%?74%were NOCs.CHON+compounds accounted for 39%of the compound numbers and 31%of the peak areas,and CHN+compounds accounted for7%of the compound numbers and 26%of the peak areas.In ESI-mode,320?2781compounds were detected,and NOCs?mainly CHON-and CHONS-compounds?accounted for 12%?36%.More NOCs were detected in ESI+mode,suggesting that a large part of the water-soluble nitrogen-containing organic compounds were reductive in nature.When biomass burning activity was strong,more CHN compounds with higher intensity were observed,and many of them were derivatives of amine,indicating that the number and intensity of CHN compounds can be used to indicate the intensity of biomass burning.CHON compounds can be detected in both positive and negative ionization modes,and more CHON compounds were detected in the positive ion mode.Amine and nitrile compounds from biomass burning were the important CHON+compounds.In polluted days,more organic nitrates could be found in the category of CHON compounds,and also more aromatic NOCs,which were important components of light-absorbing brown carbon,can be detected.Quantitative analysis results showed that the concentration of 3-nitrophenol was0.96?8.73 ng/m³.3-nitrophenol was significantly correlated with levoglucan?r=0.657?,suggesting that 3-nitrophenol can be used as a molecular tracer of the secondary organic aerosols from biomass burning.The average concentration of C₁₀H₁₇NO₇S?pinenediol mononitroxy sulfate?in June,July,August of 2017 and March 2018 was 8.68 ng/m³,33.72 ng/m³,12.89 ng/m³ and 13.40 ng/m³ respectively.C₁₀H₁₇NO₇S was significantly positively correlated with SO₄^2-?r=0.709?and MBTCA,the oxidation product of pinene?r=0.512?,and can be used as a tracer for the secondary organic aerosols produced by the oxidation of biogenic VOCs.