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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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Atmos. Meas. Tech., 9, 263-280, 2016
https://doi.org/10.5194/amt-9-263-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
27 Jan 2016
Real-time analysis of δ13C- and δD-CH4 in ambient air with laser spectroscopy: method development and first intercomparison results
S. Eyer1, B. Tuzson1, M. E. Popa2, C. van der Veen2, T. Röckmann2, M. Rothe3, W. A. Brand3, R. Fisher4, D. Lowry4, E. G. Nisbet4, M. S. Brennwald5, E. Harris1, C. Zellweger1, L. Emmenegger1, H. Fischer6, and J. Mohn1 1Empa, Laboratory for Air Pollution & Environmental Technology, Dübendorf, Switzerland
2Utrecht University (UU), Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht, the Netherlands
3Max Planck Institute (MPI) for Biogeochemistry, Jena, Germany
4Royal Holloway University of London (RHUL), Department of Earth Sciences, Egham, UK
5Eawag, Water Resources and Drinking Water, Dübendorf, Switzerland
6University of Bern, Climate and Environmental Physics, Bern, Switzerland
Abstract. In situ and simultaneous measurement of the three most abundant isotopologues of methane using mid-infrared laser absorption spectroscopy is demonstrated. A field-deployable, autonomous platform is realized by coupling a compact quantum cascade laser absorption spectrometer (QCLAS) to a preconcentration unit, called trace gas extractor (TREX). This unit enhances CH4 mole fractions by a factor of up to 500 above ambient levels and quantitatively separates interfering trace gases such as N2O and CO2. The analytical precision of the QCLAS isotope measurement on the preconcentrated (750 ppm, parts-per-million, µmole mole−1) methane is 0.1 and 0.5 ‰ for δ13C- and δD-CH4 at 10 min averaging time.

Based on repeated measurements of compressed air during a 2-week intercomparison campaign, the repeatability of the TREX–QCLAS was determined to be 0.19 and 1.9 ‰ for δ13C and δD-CH4, respectively. In this intercomparison campaign the new in situ technique is compared to isotope-ratio mass spectrometry (IRMS) based on glass flask and bag sampling and real time CH4 isotope analysis by two commercially available laser spectrometers. Both laser-based analyzers were limited to methane mole fraction and δ13C-CH4 analysis, and only one of them, a cavity ring down spectrometer, was capable to deliver meaningful data for the isotopic composition. After correcting for scale offsets, the average difference between TREX–QCLAS data and bag/flask sampling–IRMS values are within the extended WMO compatibility goals of 0.2 and 5 ‰ for δ13C- and δD-CH4, respectively. This also displays the potential to improve the interlaboratory compatibility based on the analysis of a reference air sample with accurately determined isotopic composition.


Citation: Eyer, S., Tuzson, B., Popa, M. E., van der Veen, C., Röckmann, T., Rothe, M., Brand, W. A., Fisher, R., Lowry, D., Nisbet, E. G., Brennwald, M. S., Harris, E., Zellweger, C., Emmenegger, L., Fischer, H., and Mohn, J.: Real-time analysis of δ13C- and δD-CH4 in ambient air with laser spectroscopy: method development and first intercomparison results, Atmos. Meas. Tech., 9, 263-280, https://doi.org/10.5194/amt-9-263-2016, 2016.
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Short summary
We present a newly developed field-deployable, autonomous platform simultaneously measuring the three most abundant isotopologues of methane using mid-infrared laser absorption spectroscopy. The instrument consists of a compact quantum cascade laser absorption spectrometer (QCLAS) coupled to a preconcentration unit, called TRace gas EXtractor (TREX). The performance of this new in situ technique was investigated during a 2-week measurement campaign and compared to other techniques.
We present a newly developed field-deployable, autonomous platform simultaneously measuring the...
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