13CH3D kinetic isotope effects for methane oxidation by OH - predicting the "clumped" isotopic signature of atmospheric methane

Abstract:

Methane is a significant long-lived greenhouse gas, but the tropospheric methane budget is not entirely constrained. "Clumped" isotopologues of methane, including 13CH₃D, can provide additional constraints on the atmospheric methane cycle. Interpretation of these novel isotope tracers requires an understanding of the "clumped" isotopic signature of various methane sources, as well as the kinetic isotope effects of the methane sink reactions.

We performed a series of photochemical experiments to measure the isotopic fractionation during the CH₄+OH reaction. Experiments were carried out in a 100 L quartz photochemical reactor. Photolysis of ozone (O₃) in the presence of water (H₂O) was used to produce OH radicals. Experiments were performed in a helium bath gas. Fourier transform infrared spectroscopy (FTIR) was used to monitor reaction progress. At various intervals during the reaction, methane was sampled from the cell and analyzed for isotope ratios by tunable infrared laser direct absorption spectroscopy (TILDAS). By simultaneously measuring four different isotopologues of methane (¹²CH₄,¹²CH₃D, ¹³CH₄, ¹³CH₃D), we were able to constrain the kinetic isotope effects for ¹²CH₃D, ¹³CH_4, and the doubly-substitued isotopologue ¹³CH₃D. These results are combined with published clumped isotope data from different methane sources to model the Δ¹³CH₃D (i.e. deviation from "stochastic" distribution of isotopes) of tropospheric methane and its sensitivity to different sources.

The Δ¹³CH₃D value of tropospheric methane does not strongly depend upon isotope fractionation during the OH sink reaction. Rather, the Δ¹³CH₃D value of tropospheric methane reflects a mixing of different source signatures. Due to nonlinearity in mixing of Δ¹³CH₃D, the Δ¹³CH₃D value of tropospheric methane will be larger than the weighted average of the Δ¹³CH₃D value of the sources. A first order interpretation of variations in the Δ¹³CH₃D value of tropospheric methane is that it reflects changes in the relative contribution from microbial (lighter δD, δ¹³C) versus non-microbial (heavier δD, δ¹³C) sources.