Tracing The Formaiton Mechanism And The Source Composition Of Nitrate Using Stable Nitrogen And Oxygen Isotopes (?¹⁵N,?¹⁸O And ?¹⁷O) In Atmospheric Particulates In Yangtze River Delta

Nitrate is one of the most important components in aerosols in China,which affects particulates formation,climate change and human health.Millions of tons of NO_x from natural sources and anthropogenic sources are emitted into the atmosphere every year in the world.NO_x emission was not effectively controlled by China's emission reduction policies,which resulted in higher contribution of atmospheric nitrate to haze formation and the explosive growth of particulate matter.NO_x cycle and the nitrate production is critical in the atmospheric chemistry and closely related to the concentration of O₃ and OH peroxy,the main oxidants in the atmosphere.Therefore,studying the nitrate formation mechanism and the NO_x sources is crucial for the future studies on the atmospheric oxidation capacity and the regional air quality.Stable nitrogen and oxygen isotopes of atmospheric nitrate(?¹⁵N,?¹⁸O and?¹⁷O)are powerful in tracing nitrate formation pathways as well as the NO_x sources.However,due to the immature development of triple oxygen isotope(¹⁶O,¹⁷O and¹⁸O)measurement in China,the application of stable isotopes on tracing the formation mechanism and NO_x sources are limited,especially for the analysis of diurnal variation of nitrate formation mechanism.In that case,we accomplished the method modification(denitrification bacteria method combined with the N₂O thermal decomposition)for the detection of nitrogen(¹⁵N)and triple oxygen isotopes in atmospheric nitrate.High-time-resolved aerosol collection(3h/sample)during a severe haze in winter and regular-time-resolved aerosols collection(24h/sample)throughout the year were conducted in Nanjing and Hangzhou,two megacities in Yangtze River Delta Region.The measurement of multiple isotopes(?¹⁵NO₃^-,?¹⁸NO₃^-and?¹⁷NO₃^-)were used for the analysis of relative importance of nitrate formation pathways and the NO_x sources.The main results are as follows:The pretreatment using denitrification bacteria was optimized and the detecting system was modified in this study.After that,the isotope precision were better than 0.5‰(for the detection of?¹⁵N and?¹⁸O)for the measurement of nitrate with 0.8?g N at N₂O mode.The isotope precision were better than 0.5‰and 0.3‰(for the detection of?¹⁵N and?¹⁷O,respectively)for the measurement of nitrate with 0.8?g N at and N₂-O₂ mode,and the testing time could be shorten to 13 min for each single run.Compared with the high-time-resolved aerosols,RACM(Regional Atmospheric Chemistry Mechanism)showed better results in simulating the nitrate formation pathways in regular-time-resolved aerosols.In Nanjing and Hangzhou,NO₂+OH and N₂O₅ hydrolysis pathways were dominant and the contribution of DMS/HC(Dimethyl sulfide,DMS and/or Hydrocarbon,HC)pathway was demonstrated to be negligible.The?¹⁷NO₃^-(23.4‰to 39.3‰)was lower during the day and higher at night during winter in Nanjing.The?¹⁷NO₃^-in haze days only represented the average contribution of nitrate production due to the buffering effect of cumulative nitrate on?¹⁷NO₃^-in haze days.The cleansing effect during the precipitation for restting the atmospheric nitrate chemistry was observed in this study,and the sudden decrease of?¹⁷NO₃^-revealed the great importance of NO₂+H₂O pathway after the precipitation.52±25%nitrate was produced through NO₂+OH pathway.The dominant nitrate formation pathway during the day and at night was NO₂+OH pathway(57±8%)and N₂O₅ hydrolysis pathway(66±24%),respectively.The?¹⁷NO₃^-(20.0‰to 37.9‰)and?¹⁵NO₃^-(-2.9‰to 19.4‰)was reported to have higher values in winter and lower values in summer in Hangzhou.The average contribution of N₂O₅ hydrolysis pathway was 59±29%during the sampling period.The greatest contribution of NO₂+OH pathway and N₂O₅ hydrolysis pathway were in summer(64±27%)and spring(74±22%),respectively.The equilibrium isotopic fractionation in NO₂-HNO₃ conversion was the most important fraction(69%?97%)of nitrogen isotope fractionation in nitrate production.The discrepancy of?¹⁵N between NO_x and NO₂ showed greatest impact on the NO_x source apportionment in summer NO_x in Hangzhou was mainly emitted from traffic sources(27±16%).NO_x emitted from coal combustion was the most important fraction(>30%)in winter,the contribution of NO_x from soil emission was relatively important in summer(>30%).The mixing ratio and the composition of NO_x sources during the sampling period was highly affected by the policies for the air quality control during the G20 summit in2016.