Methane plays a major role in the chemistry of Earth’s atmosphere as the most abundant hydrocarbon and a potent greenhouse gas. Methane isotopologue abundances can provide insights into methane chemistry and help distinguish between anthropogenic and biogenic methane sources. However, source identification requires understanding the history of a given air parcel’s the atmospheric oxidation. Air exposed to lower stratospheric oxidation will exhibit kinetic isotope effects (KIEs) of reactions with O(1D), while methane oxidized in the troposphere will exhibit KIEs due to reactions with OH. Our recent experiments on reactions with 13CH4 disagree with currently accepted values, potentially resolving discrepancies in some atmospheric models of methane cycling. Clumped (doubly-substituted) methane isotopologues can offer additional model constraints, but the KIEs of these reactions are poorly studied. In this work, the KIE of the reaction of O(1D) and OH with the doubly-substituted methane isotopologue CH2D2 was measured over a wide temperature range. Depletions in enriched samples of mixed methane isotopologues (CH2D2 and CH4 by O(1D)) were determined using a dual-wavelength near-IR laser system using highly sensitive frequency-stabilized cavity ring-down spectroscopy (FS-CRDS). Reactions were carried out in a temperature–controlled static cell reactor; O(1D) was generated by the pulsed flash photolysis of the precursor N2O, and OH produced through subsequent reaction with H2O or H2. The high sensitivity of FS-CRDS allows us to measure KIEs with low reactant depletion, minimizing the impact of secondary and wall chemistry.

Kinetic isotope effect temperature-dependence measurements in O(1D) reactions of methane isotopologues using cavity ring-down spectroscopy

ACS Spring Meeting 2021

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