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Abstract

Measurements made by multiple analytical facilities can only be comparable if they are related to a unifying and traceable reference. However, reference materials that fulfil these fundamental requirements are unavailable for the analysis of 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 standard gases that can be used to unify methane isotope ratio measurements of laboratories in the atmospheric monitoring community. We calibrated a suite of pure methane gases of different methanogenic origin against 20 international referencing materials that define the VSMOW and VPDB 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 −90 ‰ and −47 ‰ for δ2H-CH4 and δ13C-CH4, respectively). We estimate combined uncertainties for our δ2H and δ13C calibrations of <1.5 ‰ and <0.2 ‰, respectively. Aliquots of the calibrated pure methane gases have been diluted with methane-free air to atmospheric methane levels and filled 25 into 5-L glass flasks. These synthetic gas mixtures comprise atmospheric oxygen/nitrogen ratios as well as appropriate argon, krypton and nitrous oxide mole fractions to prevent gas-specific measurement artefacts. The resulting synthetic atmospheric reference gases will be available to the atmospheric monitoring community. This will provide unifying isotope scale anchors for isotope ratio measurements of atmospheric methane so that data sets can be merged into a consistent global data frame.