主要人为排放源中氮氧化物^(15)N自然丰度的测定

Determination of ^(15)N Natural Abundance in Nitrogen Oxides from Major Anthropogenic Emission Sources

氮氧化物(NO_x=NO+NO_2)因其对环境和人类健康的威胁,成为大气中需要严格控制的一种痕量组分。首次运用化学转化法,通过将主要人为排放源废气中的氮氧化物(NO_x)转化为氧化亚氮(N_2O),测定气体中NO_x的15N自然丰度,即氮稳定同位素比值(δ15N-NO_x)。利用酸性氧化剂将NO_x氧化为硝酸(NO_3^-),并通过三氯化钒(VCl_3)和叠氮钠(NaN_3)的共同作用将其转化为N_2O,利用带有自动预浓缩装置的稳定同位素比值质谱计(PT-IRMS)测定δ15N-NO_x。该方法的测量精度可达0.09‰,所需样品量仅为7μg(以氮计),对火力发电站、机动交通工具等人为排放源均适用,测量精度和准确度可满足NOx相关研究的需求。

【Objective】 Nitrogen oxides（NO_x = NO and NO_2）are now a critical kind of trace component that calls for strict control,for they threaten the environment and human health. As NO_x has its unique isotope signature that varies with its source,the technique of stable isotope analysis can be used to investigate its origin and pathways. This study is to introduce a new method,chemical conversion process,for determination of 15 N natural abundance（δ15N）of the nitrogen oxides from major anthropogenic sources. With the aid of this method,it is feasible for us to gradually understand the problem caused by NO_xemissions and hence to design appropriate relevant strategies to manage it.【Method】Based mainly on the technique of isotopic analysis of nitrous oxide（N2O）,the chemical conversion method is first to oxidize NO_x into nitrate（NO_3^-）withhydrogen peroxide（H2O2）in an acid condition.The resultant NO_3^- was then converted into N_2O under the joint effect of vanadium（Ⅲ）chloride（VCl3）and sodium azide（Na N3）in a strongacid condition.The newly formed N_2O is trapped in an automatic cryogenic system and then diverted into a gas chromatographic column for analysis. The isolated nitrous oxide is analyzed in a stable isotope mass spectrometer under the continuous flowing mode for determination of δ15N,which is then converted into that of the initial NO_x using an equation. While testing this method,some other tests were carried out to optimize the experimental conditions for this method. After these tests,eventually NO_x in gas samples of different sources can be trapped completely in the absorption solution,with absorption rate reaching as high as 99%. At this time,fractionation of stable isotopeis inhibited due to incomplete absorption. As conversion of NO_xinto NO_3^- takes place in the absorption solution,it is possible to use standard nitrate solutions to simulate the absorption solutions that contained NO_x standard gas sample. Usgs32 and usgs34 are mixed at different volume ratios to prepare standard reference solutions different in stable isotope ratio. After the NO_3^- in these solutionsis convertedinto N_2O using the above-described method and δ15N＇s in the two are correlated one by one,a conversion curve between NO_3^- and N_2O in acquired. This curve is equal to the one between NO_xand N_2O in value. Along with increasing concentration of NO_3^- in the solution,the curve rises in gradient,tending towards 0.5 in theoretical value. This phenomenon appears as a result of blank in experiment. When the conversion curve is collated through blank correction,the phenomenon disappears andthen gradient of the curve approaches 0.5,the theoretic value.【Result】Determination using this method may reach 0.09‰ in precision. Moreover,the minimal amount of a sample for the test is only 7 μg in terms of nitrogen. Gas samples from different sources,including vehicles and power plants,can be analyzed using this method. It is found that δ15NAir of the NO_x in vehicle exhaust is-3.6‰ on average,ranging from-17.4‰ to ＋4.0‰;and that in exhaust from power plants,＋21.9‰,on average,ranging from ＋11.9‰ to ＋29.5‰,which are in good agreement with the findings in previous researches.【Conclusion】This method can be used for determining stable isotope ratio of NO_x from a variety of anthropogenic sources,including power plants and vehicles,and is good enough in precision and accuracy to meet the needs of the researches on NO_x.