A11I-0174
Real-time analysis of δ13C- and δD-CH4 in ambient air with laser spectroscopy: Method development and intercomparison

Monday, 14 December 2015
Poster Hall (Moscone South)
Simon Eyer1, Joachim Mohn2, Thomas Röckmann3, Elena Popa3, David Lowry4, Euan G Nisbet4, Rebecca Elizabeth Fisher5, Matthias Stefan Brennwald6, Hubertus Fischer7, Lukas Emmenegger8, Béla Tuzson1 and Christoph Zellweger2, (1)Empa, Laboratory for Air Pollution & Environmental Technology, Duebendorf, Switzerland, (2)Empa, Laboratory for Air Pollution / Environmental Technology, Dübendorf, Switzerland, (3)Utrecht University, Utrecht, Netherlands, (4)Royal Holloway University of London, Egham, United Kingdom, (5)Royal Holloway, Univ London, Egham, United Kingdom, (6)EAWAG, Dübendorf, Switzerland, (7)University of Bern, Bern, Switzerland, (8)Swiss Federal Institute for Materials Science and Technology, Dubendorf, Switzerland
Abstract:
Methane (CH4) is the second most important anthropogenically emitted greenhouse gas after carbon dioxide (CO2). Its mole fraction has increased from around 722 ppb in pre-industrial times to 1824 ppb in 2013 and the anthropogenic fraction is estimated to be 60 % of the total emissions. A promising approach to improve the understanding of the CH4 budget is the use of isotopologues to distinguish between various CH4 source processes.

In the presented study 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, mmole/mole) methane is 0.1‰ and 0.5 ‰ for δ13C- and δD-CH4 at 10 min averaging time. [1]

Based on replicate measurements of compressed air during a two-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 IRMS based on glass flask and bag sampling and real time CH4 isotope analysis by two commercially available laser spectrometers (Figure). 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. Thus, the intercomparison also reveals the need for reference air samples with accurately determined isotopic composition of CH4 to further improve the interlaboratory compatibility. [1]

References:

[1] S. Eyer et al. (2015) submitted to AMTD

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