The Distribution And Sources Of Organic Nitrogen In PM_2.5 In Shanghai, China

Organic nitrogen is an important nitrogen-containing substance in aerosols, which is ofgreat significance in terms of atmospheric nitrogen cycle and marine ecosystem. Previousstudies on atmospheric nitrogen mainly focus on inorganic nitrogen-containing components,and data on organic nitrogen is very limited. With the increasing amount of fuel consumption,fertilizer production and usage, organic nitrogen in fine particles in China is drawing moreand more concern. Organic nitrogen are becoming an important hotspot in environmentalresearch.In this study,113PM_2.5samples were collected from January2010to January2011attwo sampling sites, Baoshan and Xujiahui in Shanghai. Concentrations of total water-solublenitrogen and inorganic nitrogen （in NH4+and NO3-） were determined, and the concentration ofwater-soluble organic nitrogen （WSON） calculated; To help understanding the distributionand seasonal variation of WSON, concentrations of n-alkanes, polycyclic aromatichydrocarbons, and fatty acids were also measured. Correlation analysis of WSON withorganic components and inorganic ions were conducted to explore the sources and possibleformation mechanism of WSON in PM_2.5in Shanghai. Positive matrix factorization （PMF）was used to estimate the contributions of the sources to PM_2.5and WSON in Shanghai. Theresults showed that:（1） The average concentration of water-soluble nitrogen were6.22μg/m3in PM_2.5inShanghai,（Xujiahui:6.01μg/m3, Baoshan:6.42μg/m3）, much higher than other cities in thedeveloped countries of the world. The average concentration of water-soluble organicnitrogen in PM_2.5in Shanghai was0.73μg/m3, showing a distinct seasonal trend of higherconcentrations in winter and spring than autumn and summer.（2） Water-soluble organic nitrogen was significantly correlated with ambient temperature（R=0.303, P<0.01）, but not with relative humidity and wind speed. The correlation coefficient（R） between water-soluble inorganic nitrogen and temperature, relative humidity and windspeed were-0.485（P <0.01）,-0.336（P<0.01）,-0.451（P<0.01）, suggesting thatmeteorological conditions had stronger impact on inorganic nitrogen than organic nitrogen.（3） Seasonal variations of correlations between water-soluble organic nitrogen andn-alkanes was significant. Correlation between water-soluble organic nitrogen and n-alkanesin summer was significantly stronger than other seasons, but the correlations betweenwater-soluble organic nitrogen with individual n-alkanes with different carbon number weresimilar. Higher temperatures and stronger solar radiation in summer favored photochemicalreactions, leading to generation of organic nitrogen from secondary photochemical reactions of n-alkanes.（4） The average concentration of low molecular weight PAHs （LMW, MW≤228） was10.4ng m-3in PM_2.5in Shanghai, while25.1ng m-3for the high molecular weight PAHs（HMW, MW≥252）. Significant correlations between WSON and low molecular weightPAHs and Coronene were seen in summer, suggesting that low molecular weight PAHs werephotochemically reactive to form nitrogen-containing compounds in the atmosphere.（5） The concentrations of fatty acids were13.4⁶96.9ng/m3and28.3⁵84.3ng/m3inBaoshan and Xujiahui respectively, with the average concentrations of126.0ng/m3and117.1ng/m3. In summer, the correlation between WSON and> C22fatty acids （R=0.715） wasslightly higher than that of <C20fatty acids （R=0.688）. The correlation coefficient betweenWSON concentrations and C16/C18ratio was-0.606in summer, and the mean of C16/C18ratio was2.07±0.5, suggesting that PM_2.5in Shanghai was impacted by engine exhaust,kitchen emissions and resuspended dust in summer..（6） Strong correlations between WSON and NO₃^-、SO₄^2-、C₂O₄^2-、NH4were found insummer, suggesting that WSON was mainly secondary.（7） After detailed analysis of the composition of PM_2.5 in Shanghai,39species,including OC, EC, inorganic ions of F^-, Cl^-, NO₃^-, SO₄^2-, C₂O₄, NH₄⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺,primary organic compounds of C24^C31alkanes,<C25alkanes, hopanes, LMW and HMWPAHs, F16, F18fatty acids and>C22fatty acids, were selected to conduct the PMF analysis.Eight emission sources were resolved, and identified as secondary aerosols, biomass burning,gasoline vehicle emission, diesel vehicle emission, kitchen emission, industrial activities andcoal-combustion, semi-volatile components, and resuspended soils （transported particles）.（8） PMF results suggested that water-soluble organic nitrogen in PM_2.5in Shanghaicomes mainly from secondary aerosols （4.6%）, kitchen emissions （65%）, industrial activitiesand coal combustion （8%）, semi-volatile components （4%）, resuspended soil and transportedparticles （18.4%）.（9） Higher contribution of secondary aerosols and resuspended soil to WSON werefound in spring, coincide with the occurrence of dust intrusion and long-range aerosoltransportation. So transported aerosols from outside could be an important contributor toWSON in PM_2.5in Shanghai.