主要人为排放源中氮氧化物 15N自然丰度的测定
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土壤与农业可持续发展国家重点实验室分析技术方法研究项目(Y412010010)资助


Determination of 15N Natural Abundance in Nitrogen Oxides from Major Anthropogenic Emission Sources
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Supported by the Analysis Method Research Program of State Key Laboratory of Soil and Sustainable Agriculture (No.Y412010010)

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    摘要:

    氮氧化物(NOx=NO+NO2)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(NOx)转化为氧化亚氮(N2O),测定气体中NOx15N自然丰度,即氮稳定同位素比值(δ15N-NOx)。利用酸性氧化剂将NOx氧化为硝酸(NO3-),并通过三氯化钒(VCl3)和叠氮钠(NaN3)的共同作用将其转化为N2O,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-NOx。该方法的测量精度可达0.09‰,所需样品量仅为7 μg(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。

    Abstract:

    【Objective】Nitrogen oxides (NOx = NO and NO2) are now a critical kind of trace component that calls for strict control, for they threaten the environment and human health. As NOx 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 15N 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 NOx emissions 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 NOx into nitrate (NO3-) with hydrogen peroxide (H2O2) in an acid condition. The resultant NO3- was then converted into N2O under the joint effect of vanadium (III) chloride (VCl3) and sodium azide (NaN3) in a strong acid condition. The newly formed N2O 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 NOx 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 NOx 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 isotope is inhibited due to incomplete absorption. As conversion of NOx into NO3- takes place in the absorption solution, it is possible to use standard nitrate solutions to simulate the absorption solutions that contained NOx standard gas sample. Usgs32 and usgs34 are mixed at different volume ratios to prepare standard reference solutions different in stable isotope ratio. After the NO3- in these solutions is converted into N2O using the above-described method and δ15N’s in the two are correlated one by one, a conversion curve between NO3- and N2O in acquired. This curve is equal to the one between NOx and N2O in value. Along with increasing concentration of NO3- 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 and then 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 δ15N Air of the NOx 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 NOx 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 NOx.

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王 曦,遆超普,罗永霞,孙晓丽.主要人为排放源中氮氧化物 15N自然丰度的测定[J].土壤学报,2016,53(6):1552-1562. DOI:10.11766/trxb201604180064 WANG Xi, TI Chaopu, LUO Yongxia, SUN Xiaoli. Determination of 15N Natural Abundance in Nitrogen Oxides from Major Anthropogenic Emission Sources[J]. Acta Pedologica Sinica,2016,53(6):1552-1562.

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  • 收稿日期:2016-02-02
  • 最后修改日期:2016-07-20
  • 录用日期:2016-07-21
  • 在线发布日期: 2016-08-30
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