N 2O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison

Harris, Stephen J.; Liisberg, Jesper; Xia, Longlong; Wei, Jing; Zeyer, Kerstin; Yu, Longfei; Barthel, Matti; Wolf, Benjamin; Kelly, Bryce F. J.; Cendón, Dioni I.; Blunier, Thomas; Six, Johan; Mohn, Joachim

For the past two decades, the measurement of nitrous oxide (inline-formulaN2O) isotopocules – isotopically substituted molecules inline-formula14N15N16O, inline-formula15N14N16O and inline-formula14N14N18O of the main isotopic species inline-formula14N14N16O – has been a promising technique for understanding inline-formulaN2O production and consumption pathways. The coupling of non-cryogenic and tuneable light sources with different detection schemes, such as direct absorption quantum cascade laser absorption spectroscopy (QCLAS), cavity ring-down spectroscopy (CRDS) and off-axis integrated cavity output spectroscopy (OA-ICOS), has enabled the production of commercially available and field-deployable inline-formulaN2O isotopic analyzers. In contrast to traditional isotope-ratio mass spectrometry (IRMS), these instruments are inherently selective for position-specific inline-formula15N substitution and provide real-time data, with minimal or no sample pretreatment, which is highly attractive for process studies.

Here, we compared the performance of inline-formulaN2O isotope laser spectrometers with the three most common detection schemes: OA-ICOS (inline-formulaN2OIA-30e-EP, ABB – Los Gatos Research Inc.), CRDS (G5131-i, Picarro Inc.) and QCLAS (dual QCLAS and preconcentration, trace gas extractor (TREX)-mini QCLAS, Aerodyne Research Inc.). For each instrument, the precision, drift and repeatability of inline-formulaN2O mole fraction inline-formula[N2O] and isotope data were tested. The analyzers were then characterized for their dependence on inline-formula[N2O], gas matrix composition (inline-formulaO2, inline-formulaAr) and spectral interferences caused by inline-formulaH2O, inline-formulaCO2, inline-formulaCH4 and inline-formulaCO to develop analyzer-specific correction functions. Subsequently, a simulated two-end-member mixing experiment was used to compare the accuracy and repeatability of corrected and calibrated isotope measurements that could be acquired using the different laser spectrometers.

Our results show that inline-formulaN2O isotope laser spectrometer performance is governed by an interplay between instrumental precision, drift, matrix effects and spectral interferences. To retrieve compatible and accurate results, it is necessary to include appropriate reference materials following the identical treatment (IT) principle during every measurement. Remaining differences between sample and reference gas compositions have to be corrected by applying analyzer-specific correction algorithms. These matrix and trace gas correction equations vary considerably according to inline-formulaN2O mole fraction, complicating the procedure further. Thus, researchers should strive to minimize differences in composition between sample and reference gases. In closing, we provide a calibration workflow to guide researchers in the operation of inline-formulaN2O isotope laser spectrometers in order to acquire accurate inline-formulaN2O isotope analyses. We anticipate that this workflow will assist in applications where matrix and trace gas compositions vary considerably (e.g., laboratory incubations, inline-formulaN2O liberated from wastewater or groundwater), as well as extend topage2798 future analyzer models and instruments focusing on isotopic species of other molecules.

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Harris, Stephen J. / Liisberg, Jesper / Xia, Longlong / et al: N2O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison. 2020. Copernicus Publications.

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