Abstract
Abstract. We report results from a worldwide interlaboratory comparison of samples among laboratories that measure (or measured) stable carbon and hydrogen isotope ratios of atmospheric CH4 (δ13C-CH4 and δD-CH4). The offsets among the laboratories are larger than the measurement reproducibility of individual laboratories. To disentangle plausible measurement offsets, we evaluated and critically assessed a large number of intercomparison results, some of which have been documented previously in the literature. The results indicate significant offsets of δ13C-CH4 and δD-CH4 measurements among data sets reported from different laboratories; the differences among laboratories at modern atmospheric CH4 level spread over ranges of 0.5 ‰ for δ13C-CH4 and 13 ‰ for δD-CH4. The intercomparison results summarized in this study may be of help in future attempts to harmonize δ13C-CH4 and δD-CH4 data sets from different laboratories in order to jointly incorporate them into modelling studies. However, establishing a merged data set, which includes δ13C-CH4 and δD-CH4 data from multiple laboratories with desirable compatibility, is still challenging due to differences among laboratories in instrument settings, correction methods, traceability to reference materials and long-term data management. Further efforts are needed to identify causes of the interlaboratory measurement offsets and to decrease those to move towards the best use of available δ13C-CH4 and δD-CH4 data sets.
Highlights
Methane (CH4) is an important anthropogenic and natural greenhouse gas, and it has a large role in atmospheric chemistry through its reaction with the hydroxyl radical
The Institute for Marine and Atmospheric research Utrecht (IMAU) δ13C-CH4 standard scale is based on a set of assigned values for 13 firn air samples measured at Max Planck Institute for Chemistry (MPIC; Bräunlich et al, 2001) and they are referenced to a CO2 gas produced from NBS-19 (Röckmann, 1998; Bergamaschi et al, 2000)
The results indicated measurement offsets among laboratories, which range from −0.2 to +0.3 ‰ with respect to the NIWA dual-inlet isotope ratio mass spectrometry (DI-IRMS) measurement for δ13C-CH4 and up to −13 ‰ with respect to the IMAU measurement for δD-CH4
Summary
Methane (CH4) is an important anthropogenic and natural greenhouse gas, and it has a large role in atmospheric chemistry through its reaction with the hydroxyl radical. It is clear that both traceability to the standard scales and interlaboratory comparisons (intercomparisons) are indispensable for combined use of δ13C-CH4 and δD-CH4 data from different laboratories. Many such intercomparisons have already been made, either on an ad hoc basis or on a more organized scale. We link the intercomparison results through a survey of previously published intercomparisons and provide the current best estimates of measurement offsets among data sets from different laboratories We summarize the current status and briefly discuss possible causes of the measurement offsets as well as remaining issues that should be kept in mind when combining the use of currently existing data sets of isotopic composition of CH4 Offset RM 8563 & Oztech Gas R20, R10, R12, R12, current cor- with respect RM 8564
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