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Volume 16, issue 16
Atmos. Chem. Phys., 16, 10469-10487, 2016
https://doi.org/10.5194/acp-16-10469-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 10469-10487, 2016
https://doi.org/10.5194/acp-16-10469-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 19 Aug 2016

Research article | 19 Aug 2016

In situ observations of the isotopic composition of methane at the Cabauw tall tower site

Thomas Röckmann1,*, Simon Eyer2,*, Carina van der Veen1, Maria E. Popa1, Béla Tuzson2, Guillaume Monteil1,a, Sander Houweling1, Eliza Harris2, Dominik Brunner2, Hubertus Fischer6, Giulia Zazzeri3, David Lowry3, Euan G. Nisbet3, Willi A. Brand4, Jaroslav M. Necki5, Lukas Emmenegger2, and Joachim Mohn2 Thomas Röckmann et al.
  • 1Utrecht University, Institute for Marine and Atmospheric Research Utrecht, Utrecht, the Netherlands
  • 2Empa, Laboratory for Air Pollution/Environmental Technology, Dübendorf, Switzerland
  • 3Royal Holloway University of London, Department of Earth Sciences, Egham, UK
  • 4Max Planck Institute for Biogeochemistry, Jena, Germany
  • 5Environmental Physics Group, Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Krakow, Poland
  • 6University of Bern, Climate and Environmental Physics, Bern, Switzerland
  • anow at: Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden
  • *These authors contributed equally to this work.

Abstract. High-precision analyses of the isotopic composition of methane in ambient air can potentially be used to discriminate between different source categories. Due to the complexity of isotope ratio measurements, such analyses have generally been performed in the laboratory on air samples collected in the field. This poses a limitation on the temporal resolution at which the isotopic composition can be monitored with reasonable logistical effort. Here we present the performance of a dual isotope ratio mass spectrometric system (IRMS) and a quantum cascade laser absorption spectroscopy (QCLAS)-based technique for in situ analysis of the isotopic composition of methane under field conditions. Both systems were deployed at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands and performed in situ, high-frequency (approx. hourly) measurements for a period of more than 5 months. The IRMS and QCLAS instruments were in excellent agreement with a slight systematic offset of (+0.25±0.04)‰ for δ13C and (−4.3±0.4)‰ for δD. This was corrected for, yielding a combined dataset with more than 2500 measurements of both δ13C and δD. The high-precision and high-temporal-resolution dataset not only reveals the overwhelming contribution of isotopically depleted agricultural CH4 emissions from ruminants at the Cabauw site but also allows the identification of specific events with elevated contributions from more enriched sources such as natural gas and landfills. The final dataset was compared to model calculations using the global model TM5 and the mesoscale model FLEXPART-COSMO. The results of both models agree better with the measurements when the TNO-MACC emission inventory is used in the models than when the EDGAR inventory is used. This suggests that high-resolution isotope measurements have the potential to further constrain the methane budget when they are performed at multiple sites that are representative for the entire European domain.

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A dual isotope ratio mass spectrometric system (IRMS) and a quantum cascade laser absorption spectroscopy (QCLAS)-based technique were deployed at the Cabauw experimental site for atmospheric research (CESAR) in the Netherlands and performed in situ, high-frequency (approx. hourly) measurements for a period of more than 5 months, yielding a combined dataset with more than 2500 measurements of both δ13C and δD.
A dual isotope ratio mass spectrometric system (IRMS) and a quantum cascade laser absorption...
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