MEASUREMENTS OF THE KINETIC ISOTOPE EFFECT IN O(1D) REACTIONS OF THE CLUMPED ISOTOPOLOGUE OF METHANE CH2D2 USING CAVITY RING-DOWN SPECTROSCOPY

Abstract:

As the most abundant atmospheric hydrocarbon and a potent greenhouse gas, methane plays a major role in the chemistry of Earth’s atmosphere. Observations of methane isotopologues in the atmosphere can provide insights into methane chemistry and aid in distinguishing between anthropogenic and biogenic methane sources. However, source identification requires understanding the prior history of the atmospheric oxidation of a given air parcel. Isotope distributions in air that has been processed in the lower stratosphere will exhibit kinetic isotope effects (KIEs) of reactions with O(¹D). Our recent experiments on reactions with ¹³CH₄ disagree with currently accepted values. Clumped (doubly-substituted) isotopologues of methane can offer additional constraints, but KIEs of their reactions are poorly characterized.

In this work, the KIE of the reaction of O(¹D) with the doubly-substituted methane isotopologue CH₂D₂ KIE was measured over a wide temperature range. Depletions in enriched samples of mixed methane isotopologues (CH₂D₂ and CH₄) by O(¹D) were determined using the high sensitivity method frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) in a dual wavelength near-IR laser system. Reactions were carried out in a temperature–controlled static cell reactor; O(¹D) was generated by the pulsed flash photolysis of a precursor N₂O. The high sensitivity of FS-CRDS allows us to measure KIEs under conditions of small depletions, minimizing the impact of secondary and wall chemistry.