The removal of methane in the atmosphere is dominated by oxidation by the hydroxyl radical (OH), with smaller contributions from chlorine (Cl, in troposphere and stratosphere), electronically excited oxygen atoms (O(1D), only stratosphere) and soil uptake [1]. The first stable reaction product of methane oxidation in the atmosphere is carbon monoxide (CO) and about 30% of the CO is produced from CH₄ oxidation. CO thus includes information on methane removal. Isotope measurements can amplify this information because CO produced from methane is very strongly depleted in 13 C compared to ambient CO. In fact, CH₄ oxidation is a main contributor to the seasonality of d 13C of CO [2-4]. The isotopic composition of CO is particularly sensitive to the removal of CH₄ by Cl, because of an extraordinarily strong isotope effect associated with this reaction [5]. The Cl pathway constitutes only a small fraction of present-day CH₄ removal, but it can have a disproportionate effect on its isotopic composition. The abundance of Cl is so low in the atmosphere that it cannot be measured directly but must be inferred from indirect measurements.

Precise quantification of CH₄ removal is of key importance for explaining the recent increase in atmospheric CH₄ and also as a basis for potential future implementation of enhanced removal techniques. However, this precise quantification remains a persistent problem and different approaches to estimate it lead to conflicting results (e.g. [6, 7]). This must be resolved, and additional information is needed to constrain CH₄ removal better. A key research question is whether the isotopic composition of CO can be used to better constrain CH₄ removal. Naturally, a key variable impacting success is the availability of reliable, high-precision measurement systems. To our knowledge, the analytical system at Utrecht University is the only system operating in routine mode for analysis of large sample numbers (>1000 per year) at present. The analytical system is reaching capacity limits. Being dependent on only one system presents a high risk for ongoing isotope monitoring programs (e.g. the SPARK Climate funded ISAMO project, which currently provides >80% of the samples) that depend on large sample throughput.

Quantitative evaluation of CO isotope measurements together with newly available model results can provide additional insight into CH₄ removal. This approach is presently developed at local, regional and global scales. The method is extremely sensitive for quantifying CH₄ removal by Cl [8, 9]. Routine measurement of CO isotopic composition can thus help quantifying methane removal. Of particular relevance is that d13C in CO may be one of the few, and most precise, methods to measure and verify the efficiency of future enhanced methane oxidation methods.