Articles | Volume 9, issue 8
Atmos. Meas. Tech., 9, 3717–3737, 2016
Atmos. Meas. Tech., 9, 3717–3737, 2016

Research article 12 Aug 2016

Research article | 12 Aug 2016

Development and evaluation of a suite of isotope reference gases for methane in air

Peter Sperlich1,2, Nelly A. M. Uitslag1,3, Jürgen M. Richter1, Michael Rothe1, Heike Geilmann1, Carina van der Veen4, Thomas Röckmann4, Thomas Blunier5, and Willi A. Brand1 Peter Sperlich et al.
  • 1Max Planck Institute for Biogeochemistry (MPI-BGC), Jena, Germany
  • 2National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand
  • 3Centre for Isotope Research (CIO), Energy and Sustainability Research Institute Groningen (ESRIG), University of Groningen, Groningen, the Netherlands
  • 4Institute of Marine and Atmospheric Science in Utrecht (IMAU), Utrecht, the Netherlands
  • 5Centre for Ice and Climate (CIC), Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

Abstract. Measurements from multiple laboratories have to be related to unifying and traceable reference material in order to be comparable. However, such fundamental reference materials are not available for isotope ratios in atmospheric methane, which led to misinterpretations of combined data sets in the past. We developed a method to produce a suite of synthetic CH4-in-air standard gases that can be used to unify methane isotope ratio measurements of laboratories in the atmospheric monitoring community. Therefore, we calibrated a suite of pure methane gases of different methanogenic origin against international referencing materials that define the VSMOW (Vienna Standard Mean Ocean Water) and VPDB (Vienna Pee Dee Belemnite) isotope scales. The isotope ratios of our pure methane gases range between −320 and +40 ‰ for δ2H–CH4 and between −70 and −40 ‰ for δ13C–CH4, enveloping the isotope ratios of tropospheric methane (about −85 and −47 ‰ for δ2H–CH4 and δ13C–CH4 respectively). Estimated uncertainties, including the full traceability chain, are < 1.5 ‰ and < 0.2 ‰ for δ2H and δ13C calibrations respectively. Aliquots of the calibrated pure methane gases have been diluted with methane-free air to atmospheric methane levels and filled into 5 L glass flasks. The synthetic CH4-in-air standards comprise atmospheric oxygen/nitrogen ratios as well as argon, krypton and nitrous oxide mole fractions to prevent gas-specific measurement artefacts. The resulting synthetic CH4-in-air standards are referred to as JRAS-M16 (Jena Reference Air Set – Methane 2016) and will be available to the atmospheric monitoring community. JRAS-M16 may be used as unifying isotope scale anchor for isotope ratio measurements in atmospheric methane, so that data sets can be merged into a consistent global data frame.

Short summary
Isotope measurements in atmospheric CH4 are performed since more than 3 decades. However, standard gases to harmonize global measurements are not available to this day. We designed two methods to calibrate a suite of 8 CH4 gases with a wide range in isotopic composition to the VPDB and VSMOW scales with high precision and accuracy. Synthetic air mixtures with ~2 ppm of calibrated CH4 can be provided to the community by the ISOLAB of the Max Planck Institute for Biogeochemistry in Jena, Germany.