Construction And Application Of Denitrifier Meathod For Determination Of Nitrogen And And Oxygen Stable Isotopes In Trace Nitrate

With the rapid development of industry and agriculture,environmental contamination related nitrate is more and more serious,and nitrate is one of the important forms of bioavailable nitrogen,then stable isotope is an important method for researching the active nitrogen cycle and nitrate pollution problems in ecosystems.In the past 30 years,the methods for the determination and analysis ofδ¹⁵N、δ¹⁸O andΔ¹⁷O of nitrate have been continuously improved and developed,providing a new approach for the analysis of nitrate sources,oxidation generation mechanism and so on in atmosphere,water,biology,soil and polar snow and ice.The quantification for different formation pathways of nitrate must rely on the measurement of theΔ¹⁷O（NO₃^-）.However,due to the limitations of sample size and experimental conditions,stable isotopes analysis especiallyΔ¹⁷O analysis in trace nitrate are more difficult and only a few laboratories in the world have test capabilities.Domestic laboratories began to conduct tentative study for the determination ofΔ¹⁷O in trace nitrate,but these studies still require a large amount of sample and then large-scale determinations and applications ofΔ¹⁷O（NO₃^-）in atmospheric aerosol,surface water,snow,ice and other environmental samples cannot be carried out.Therefore,there is still little such research and it is currently unknown about the sources and oxidation pathways of atmospheric nitrate samples,etc.Based on the above,this study constructed a set of system method called as denitrifier meathod for the determination of nitrate nitrogen and oxygen isotopes,which is suitable for polar ice,fresh water,sea water and atmospheric aerosol.It has many advantages over other methods,such as simple steps,high conversion efficiency,short test time and high analysis precision.And finally,the method was applied to the determination of oxygen stable isotopes of nitrate in atmospheric aerosol samples for exploring different formation pathways of nitrate.This study mainly obtains the following results:（1）Construct the platform of denitrifier meathod for determination of nitrogen and oxygen stable isotopes in trace nitrate.A set of 6×10 lattice sample denitrification treatment system for 20.0m L and 50.0m L headspace sample bottles designation and construction in the study area.It can purge 60 samples at a time,and the purge gas flow can be adjusted and controlled,which can not only save time but also minimize the blank in sample pretreatment.Design and set up the injection system in the study area,we made the double-layer injection needle and the corresponding injection tray with 54holes and 27 holes,which met the requirements of a variety of sample size measurements and matched with the automatic injection device.Design and construct purification and enrichment system of trace N₂O in the study area.It fully realized the enrichment and purification of samples in double cold trap.The Gas-BenchⅡfor gas introduced into device was transformed to realize counter blowing process and sample enrichment through the twelve-way valve switch,transformation of baking for chromatographic column and gas into the mass spectrometer through the four-way valve switch and on-line transition of measuringδ¹⁵N,δ¹⁸O（N₂O measurement mode）andδ¹⁷O（O₂measurement mode）by the eight-way valve switch.（2）Present the parameter scheme of important measuring conditions in the measuring system:Under the N₂O determination mode,when 20.0m L and 50.0m L headspace sample bottles are used to measure respectively,the referenced flow rate of He1 purging sample gas purged from the headspace sample bottles are about 40m L/min and 32m L/min,respectively,the referenced flow rate of He3 purging sample gas purged from the liquid nitrogen cold trap2 is about 3m L/min and the separation temperature of chromatographic column is 25℃.Under the O₂ determination mode,when 20.0m L and50.0m L headspace sample bottles are used to measure respectively,the referenced flow rate of He1purging sample gas purged from the headspace sample bottles are also about40m L/min and 32m L/min,respectively,the referenced flow rate of He3 purging sample gas purged from the liquid nitrogen cold trap2 is about 4m L/min,the separation temperature of chromatographic column is 40℃and the decomposition temperature is600℃.The referenced flow rate of He2 in the backblowing process in the whole system is about 10m L/min.（3）Reliability of the established system verification in the study area.Under the N₂O measurement mode,the SD values ofδ¹⁵N andδ¹⁸O obtained of different time batches USGS34,USGS35 and IAEA-NO-3 are all less than the given standard deviation(σ(δ¹⁵N)≤0.2‰,σ(δ¹⁸O)≤0.6‰).Forδ¹⁵N,the pooled standard deviation（1σ_p）is 0.2‰（USGS34,n=30）,0.2‰（USGS35,n=30）and 0.2‰（IAEA-NO-3,n=30）.Forδ¹⁸O,the 1σ_p is 0.3‰（USGS34,n=30）,0.4‰（USGS35,n=30）and 0.3‰（IAEA-NO-3,n=30）,respectively.Under the O₂ measurement mode,the SD values ofΔ¹⁷O correction values of different time batchs USGS34 and USGS35 are in the range of0.56～0.73‰and the SD values ofδ¹⁸O correction values of different time batchs USGS34 and USGS35 are in the range of 0.21～0.89‰,the 1σ_p are 0.7‰(USGS34Δ¹⁷O,n=30)and 0.6‰(USGS35Δ¹⁷O,n=30),respectively.The results are no obvious fluctuation.At the same time,theΔ¹⁷O（NO₃^-）andδ¹⁸O（NO₃^-）values of two batch real samples determined by the constructed systematic methods in our laboratory are consistent with the results measured in Brown University laboratory.In addition,the t-test of paired samples shows that there is no significant difference about the determination of the two batches of samples in the two laboratories,which indicates that the constructed method is reliable.In terms of optimizing the determination process,enriching efficiency,shortening the sample transformation time and producing almost no pollutants to the environment,the method is worth promoting.（4）Measure NO₃^-concentration,Δ¹⁷O（NO₃^-）andδ¹⁸O（NO₃^-）of Antarctic trace nitrate atmosphere samples and high levels of Shanghai nitrate atmospheric samples using Ion Chromatograph and building system and analysis method.We studied the time distribution features of NO₃^-concentration,Δ¹⁷O（NO₃^-）andδ¹⁸O（NO₃^-）of Antarctic atmosphere samples,the time distribution features of water-soluble ions andΔ¹⁷O（NO3^-）of Shanghai atmosphere samples and its relationship with environmental factors.Also,on the basis ofΔ¹⁷O（NO3^-）,we eatimated possible contribution percentage of different nitrate formation pathways of Shanghai.The results show that the different seasonalΔ¹⁷O（NO3^-）values of the Antarctic atmospheric samples with trace nitrate are higher and its range and mean value are 27.16～42.85‰and 34.39±4.40‰,respectively.The seasonal variation characteristics of NO₃^-concentration andΔ¹⁷O（NO₃^-）are spring>winter>summer>fall and winter>spring>summer influenced by light intensity,respectively.Light intensity results in nitrate formation mainly by O₃oxidation and part of nitrate caused by stratospheric inputs during the winter and nitrate formation mainly by HO_x,RO_x and H₂O₂and part of nitrate released from the snow during the summer.TheΔ¹⁷O（NO3^-）of the Shanghai winter atmosphere TSP samples ranges from28.64～38.62‰and its average value is 33.20±2.51‰.Also,the results show thatΔ¹⁷O（NO3^-）has a significant positive correlation relationship with PM_2.5 and NO3^-,but significant negative correlation relationship with relative humidity and temperature.The calculation based onΔ¹⁷O（NO3^-）shows that atmospheric nitrate is mainly dominated by night reactions of NO₃+HC,N₂O₅+H₂O and N₂O₅+Cl^-with about 78～95%possible contribution percentage,and the daytime reactions of NO₂+OH and NO₂+H₂O may contribute as much as 5～22%to nitrate formation in Shanghai.We also find that the exceed ratio of NO₂ concentration is as high as 30.00%,and in contrast to the O₃concentration change trend,which can indicate that O₃ controled the circulation of NO_xand atmospheric environment is mainly NO₂ pollution during the period of observation in Shanghai.Although Antarctica and Shanghai have different atmospheres environments,both the atmospheric nitrate formation in winter are dominated by O₃.Alao,we speculate that the higherΔ¹⁷O（NO3^-）of winter atmosphere in Antarctic may be influenced by the stratosphere denitrification,while theΔ¹⁷O（NO3^-）of winter atmosphere in Shanghai may be affected by the stratosphere denitrification weakly.