Characteristics And Source Apportionment Of Water-soluble Organic Nitrogen Pollution In PM_2.5 In The Northern Suburbs Of Nanjing

Recently,China has frequently suffered severe haze pollution,the PM_2.5pollution has become a major environmental problem in China.Water-soluble organic nitrogen(WSON),which is one of the important components of secondary organic aerosols,has become a hot topic in atmospheric environmental research.It is proved to be impritant on the nitrogen cycle,atmospheric visibility,global climate change and human health.Previous studies mainly focused on inorganic nitrogen,and little has been reported on the chemical composition and sources of water-soluble organic nitrogen,In China,fuel combustion,chemical fertilizer production and use,biomass combustion have a great contribution to organic nitrogen emissions.In this study,PM_2.5samples were collected by PM_2.5high flow sampler from June 2017 to May 2018 in the northern suburbs of Nanjing,and a total of 260 valid samples were obtained.The carbonaceous fraction,inorganic ions and absorbance properties of the samples were determined by organic carbon/elemental carbon(OC/EC)analyzer,total organic carbon/nitrogen analyzer(TOC/TN),ion chromatography(IC)and ultraviolet-visible spectrophotometer(UV-3600).Then the seasonal variation of organic nitrogen and its influence factors were discussed.At the same time,the mass spectra of WSON were measured by offline aerosol mass spectrometer(SP-AMS).Combined with the positive matrix factorization(PMF)analysis the sources of WSON were discussed in Nanjing.Based on the above studies,the following main conclusions were drawn:(1)The measured components(WSIIs,OM and EC)accounted for 83.4%of the total mass of PM_2.5,and the mass contribution of inorganic ions was the highest,which was twice as high as OM(22.6%).The trend of components(expect for EC)was winter>spring>autumn>summer.(2)The concentration WSTN was 10.9?g m^-3and the concentration of WSOM was 2.2?g m^-3.The seasonal variations showed the highest in winter and lowest in summer.The contribution of WSON in WSTN was highest in winter accounted for25.1%,followed by spring(23.8%),and the lowest was in summer.The average contribution was 19.7%.(3)There was a certain correlation between WSON and PM_2.5and its components,indicating the multi-sources of water-soluble organic nitrogen;significant correlation with WSTN,indicating that it was relatively stable in water-soluble total nitrogen;high correlation with OM In EC showed that WSON came from mixed primary and secondary sources;the correlation with SOM(r=0.57)and with WSOM(r=0.56)was better than with POM(r=0.26)and with WIOM(r=0.37),indicating that secondary generation was more important than primary emission for the formation of WSON in PM_2.5;correlation analysis with inorganic salts showed that secondary conversion and biomass combustion may be the main source of WSON in PM_2.5in the northern suburbs of Nanjing.(4)Quantitative analysis of WSON samples by SP-AMS showed that oxygen-free ON ions(C_xH_yN_p⁺)accounted for 91.2%of the total WSON,while oxygen-containing ON ions(C_xH_yO_zN_p⁺)contributed very little,accounting for only8.8%,indicating that amines dominate and there are other oxidizing ON compounds.(5)The PMF analysis identified four sources for WSON,which called HOA,OA associated with traffic emissions,BBOA,biomass combustion and two secondary sources of OA,namely low oxidation OA(LO-OOA)and high oxidation OA(MO-OOA).The proportion of primary sources is 39.5%,and the proportion of secondary sources is 60.5%.In winter,the contribution of MO-OOA?ON was higher(56.9%),while in summer,the contribution of LO-OOA?ON was significant(40%),indicating that the liquid phase formation was more important for WSON in winter,while photochemical oxidation was more important in summer.