Interest in understanding why methane (CH₄) is rising is driven by the fact that CH₄ is a potent greenhouse molecule with an ~10 yr lifetime and the fact that that measures taken to temper or reverse the march of CH₄ to higher levels have the potential to provide time for development of climate change solutions. Critical insight into why CH₄ is rising is provided by isotopic measurements of atmospheric CH₄, which in turn are evaluated using models that include information about kinetic isotope effects (KIE); KIE associated with major sink reactions of CH₄ with the ▪OH radical, the ▪Cl radical, and O(1 D), describe the lifetimes of isotopically-substituted species (isotopologues), including 12CH₄, 12CH₃ D, and the ‘clumped’ isotopologues 13CH₃ D and 12CH₂ D₂ . Further insight into why CH₄ is rising will be obtained as newer and better measurement techniques are developed; high mass resolution instruments, for instance, now reach precisions for D/H approaching the 0.1 ‰ level. These are some of the reasons high- quality calibrations of KIE are of scientific and societal value. We want to understand and curb the rise of CH₄, and we do not want KIE to be a limiting factor.

High-quality calibrations of KIE for reaction of CH₄ with ▪OH, ▪Cl, and O(1 D) will allow models to predict (and extract) accurate information about the CH₄ cycle, including source and sink strength, source inventories, and changing natural and anthropic drivers (including those resulting from possible future policy, mitigation, or removal) throughout the atmosphere and around the globe. The proposed work aims to improve constraints on KIE using ab initio calculations with the key goals to:

1. Refine constraints on the transition states (TS) and their immediate potential energy surface (PES) for reaction of CH₄ with ▪OH, ▪Cl, and O(1 D);
2. Calibrate frequencies for each of the TS isotopologues (including those that can be presently measured at high precision and others);
3. Derive KIE starting with transition state theory (TST) with tunneling corrections;
4. Evaluate the derived KIE relative to existing determinations of KIE and identify any preliminary implications that changes in KIE may have for understanding of the atmospheric cycling of CH₄.

The KIE will be evaluated by comparison with known reaction rates and convergence with other calibrations.

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If successful, the proposed work would provide calibration of KIE for three relevant atmospheric sink reactions for CH₄ to:

1. Advance understanding of the mismatch between experimental and theoretical KIE for isotopologues;
2. Improve process-model constraints using atmospheric CH₄ isotopologues to apportion atmospheric sinks;
3. Calibrate CH₄ inventories and efforts to reduce CH₄ on a global scale;
4. Inform models to evaluate global distribution of sinks in, for instance, N-S global transects that measure CH₄ isotopologues; and
5. Apply isotopologues in studies of the atmospheric CH₄ cycle in both the troposphere and stratosphere.

A goal of this research is to produce high quality theoretical calibrations of KIE with fundamental physical chemical underpinnings that allow extrapolation to a range of conditions relevant in the natural world, to isotopologues, and to enable accurate modeling of the atmospheric CH₄ cycle.