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| Titel |
Real-time analysis of δ¹³C- and δD-CH4 by high precision laser spectroscopy |
| VerfasserIn |
Simon Eyer, Lukas Emmenegger, Béla Tuzson, Hubertus Fischer, Joachim Mohn |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250096634
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| Publikation (Nr.) |
 EGU/EGU2014-12146.pdf |
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| Zusammenfassung |
| Methane (CH4) is the most important non-CO2 greenhouse gas (GHG) contributing 18% to
total radiative forcing. Anthropogenic sources (e.g. ruminants, landfills) contribute 60% to
total emissions and led to an increase in its atmospheric mixing ratio from 700 ppb in
pre-industrial times to 1819 ± 1 ppb in 2012 [1]. Analysis of the most abundant methane
isotopologues 12CH4, 13CH4 and 12CH3D can be used to disentangle the various source/sink
processes [2] and to develop target oriented reduction strategies. High precision isotopic
analysis of CH4 can be accomplished by isotope-ratio mass-spectrometry (IRMS) [2] and
more recently by mid-infrared laser-based spectroscopic techniques. For high precision
measurements in ambient air, however, both techniques rely on preconcentration of the target
gas [3].
In an on-going project, we developed a fully-automated, field-deployable CH4
preconcentration unit coupled to a dual quantum cascade laser absorption spectrometer
(QCLAS) for real-time analysis of CH4 isotopologues. The core part of the rack-mounted
(19 inch) device is a highly-efficient adsorbent trap attached to a motorized linear
drive system and enclosed in a vacuum chamber. Thereby, the adsorbent trap can
be decoupled from the Stirling cooler during desorption for fast desorption and
optimal heat management. A wide variety of adsorbents, including: HayeSep D,
molecular sieves as well as the novel metal-organic frameworks and carbon nanotubes
were characterized regarding their surface area, isosteric enthalpy of adsorption and
selectivity for methane over nitrogen. The most promising candidates were tested on the
preconcentration device and a preconcentration by a factor > 500 was obtained. Furthermore
analytical interferants (e.g. N2O, CO2) are separated by step-wise desorption of trace
gases.
A QCL absorption spectrometer previously described by Tuzson et al. (2010) for CH4
flux measurements was modified to obtain a platform for high precision and simultaneous
analysis of CH4 isotopologues. The infrared radiation emitted by the two cw-QC laser
sources are combined and coupled into a 0.5 L astigmatic multipass absorption cell with
an optical path length of 76 m. An Allan variance minimum of the isotope ratio
time-series of 0.1 ofor δ13C-CH4 and 0.3 ofor δD-CH4 has been achieved using
300 s integration time. First experiments of the developed analytical technique
demonstrate its potential with respect to field-applicability and temporal resolving
power.
References:
[1] WMO, Greenhouse Gas Bulletin No. 9, 2013, WMO GAW, pp. 4.
[2] H. Fischer, M. Behrens, M. Bock, U. Richter, J. Schmitt, L. Loulergue, J.
Chappellaz, R. Spahni, T. Blunier, M. Leuenberger and T. F. Stocker, Nature, 2008, 452,
864-867.
[3] J. Mohn, B. Tuzson, A. Manninen, N. Yoshida, S. Toyoda, W. A. Brand, and L.
Emmenegger, Atmos. Meas. Tech., 2012, 5, 1601–1609.
[4] Tuzson, B., Hiller, R. V., Zeyer, K., Eugster, W., Neftel, A., Ammann, C., and L.
Emmenegger, Atmos. Meas. Tech., 2010, 3,1519–1531. |
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