Constructing an Atmospheric Methane Budget Using 13CH3D and CH2D2 in Sources and Sinks

Abstract:

We develop a theoretical model using relative abundances and fractionations of ¹³CH₃D and CH₂D₂, the doubly substituted mass-18 isotopologues of methane, to quantitatively track the sources and the sinks of atmospheric methane. The goal is a better determination of the methane budget in the atmosphere. Different methane sources have different isotope ratios because of variations in substrates, formation reactions, and temperatures. Isotope ratio measurements will provide useful constraints on source components and sink processes. However, bulk isotope ratios alone are unlikely to be diagnostic because of mixing of sources. Using recently published budgets (Whiticar and Schaefer 2007) and estimates of equilibration temperatures of various methane sources (Stolper et al., 2014; Wang et al., 2015), including an assumption that biogenic methane sources are near-stochastic (Wang et al., 2015), we estimated the abundances in air of singly- and doubly-substituted isotopologues in terms of both bulk ratios and deviations from the stochastic distributions of multiply-substituted species. δ¹³CH₃D and δCH₂D₂ for the total atmospheric sources are predicted to be -493‰ and -330‰, whereas Δ¹³CH₃D, and ΔCH₂D₂, enrichments relative to stochastic, are predicted to be +4.7‰ and +21.5‰. The composition of atmospheric methane will also be influenced by sink reactions. The main sink reactions with OH• and Cl• have been modeled with first-principles transition state theory, using simplified corrections for tunneling (Wigner 1932). Our model predicts that the main sink reactions in the atmosphere generate distinct signatures of lower Δ¹³CH₃D and ΔCH₂D₂ relative to the source composition, while at the same time increasing δ¹³CH₃D and δCH₂D₂. Measurements of both Δ¹³CH₃D and ΔCH₂D₂ are now possible with the new large-geometry gas-source mass spectrometer at UCLA permitting testing of these predictions.