We report measurements of resolved 12 CH 2 D 2 and 13 CH 3 D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve 12 CH 2 D 2 from 13 CH 3 D provides unprecedented insights into the origin and evolution of CH 4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Δ 12 CH 2 D 2 and Δ 13 CH 3 D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH 4/ H 2 D/H exchange, underscoring the importance of reliable thermometry based on the CH 4 molecules themselves. Where Δ 12 CH 2 D 2 and Δ 13 CH 3 D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis versus biological processes. Deficits in 12 CH 2 D 2 compared with equilibrium values in CH 4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in 13 CH 3 D that accompany the low 12 CH 2 D 2 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ 12 CH 2 D 2 values are a key tracer of microbial recycling.

We present a theoretical model to investigate the potential of 13CH3D and 12CH2D2, the doubly substituted mass‐18 isotopologues of methane, as tools for tracking atmospheric methane sources and sinks. We use electronic structure methods to estimate kinetic isotope fractionations associated with the major sink reactions of methane in air (reactions with OH and Cl radicals) and combine literature data with reconnaissance measurements of the relative abundances of 13CH3D and 12CH2D2 to estimate the compositions of the largest atmospheric sources. This model atmospheric budget is investigated with a simplified box model in which we explore both steady state and dynamical (nonsteady state) conditions triggered by changes in emission or sink fluxes. The steady state model predicts that sink reactions will generate a marked (>100‰) clumped isotope excess in atmospheric Δ12CH2D2 relative to the net source composition. 12CH2D2 measurements may thus be useful for tracing both atmospheric source and sink fluxes. The effect of sinks on Δ13CH3D is much less pronounced, indicating that 13CH3D in air will give a more focused picture of the source composition.

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Methane clumped isotopologues are thought to be able to better differentiate methane of different origins, indicate certain thermodynamic equilibrium and kinetic processes, and are expected to provide further constraints on the global methane budget. Methane emitted by vehicle exhaust less because it is thought to be a minor part of global methane emissions. However, considering the large number and the high frequency use of internal combustion engines, the possibility that a large proportion of emissions come from a small proportion of episodic high-emitting engines, and the potentially unique isotopic composition, knowing about vehicle methane emissions is of interestWe collected exhaust gas samples from gasoline cars and mower (spark ignition, SI), diesel buses (compression ignition, CI), and wooding burning (WB). The vehicles cover different brands, model years, stroke cycles, ignition methods, operating temperatures, and catalytic converter conditions. The concentration data suggest that older cars emit high amounts of methane, while new cars can also emit gases with methane concentrations several times higher than air. Diesel buses are cleaner in terms of methane emissions, probably because CI allows higher air to fuel. We measured methane isotopes of these samples, as well as air samples from the Washington DC Beltway during morning rush hour.Exhaust methane from most spark ignition engines is around 0‰ for both δ 13 CH 3 D and δ 12 CH 2 D 2 . These clumped isotopologue signals are consistent with high temperature equilibration. The elevated methane concentration of the Beltway samples (1.15-1.2 times atmospheric concentration) requires air to be mixed with one or more endmember(s). One explanation that fits the relationships between δ 13 C, δD, δ 13 CH 3 D, and δ 12 CH 2 D 2 signals would be mixing with a natural gas component such as Marcellus that leaks from municipal pipeline. However, an equally plausible explanation is mixing with a component that is predominantly exhaust methane with a small biogenic contribution. It could be important to consider the contributions from car exhaust in addition to those from natural gas to explain the observed fluctuations in methane concentrations and isotopic data in urban areas.