This problem statement was submitted as part of a research funding application which was awarded by Spark.

A combination of high-quality internally-consistent calibration of kinetic isotope effects (KIE) for atmospheric CH4 reactions with ▪OH, ▪Cl & O(1D), and of fractionations accompanying loss to soils is needed to invert isotopic observations for source apportionment. Using information from the greatest number of isotopologues, including 12CH4, 13CH4, 12CH3D, and ‘ clumped’ isotopologues 13CH3D and 12CH2D2 will provide the most complete understanding atmospheric CH4 cycling and of future engineering solutions to reduce CH4 levels. Existing experimental calibrations for KIE for sink reactions are incomplete. They do not exist for all measured atmospheric isotopologues. Ab initio calibrations which provide information about all isotopologues have not yet converged on same KIE as measured in experiments. Calibrations of the soil sink reactions are not known for all measurable CH4 isotopologues. A need therefore exists for better calibrations of the suite of sinks for CH4 isotopologues.

This research will provide/improve calibrations of atmospheric CH4 isotopologue sinks using a combination of control experiments (for soils) and ab initio calculations for KIE of ▪OH, ▪Cl & O(1D). Success (and impact on the inversions) will be evaluated by experiment/theory comparisons. The current 1-3‰ level experiment-theory mismatch of KIE for 13CH4 + ▪OH contributes >10% uncertainty to fossil/microbial source apportionment of global CH4. Similarly KIE for 13CH4 +▪Cl (Whitehill, 2017) contribute at ~1/2 this due to a > 30‰ mismatch. In the case of +▪Cl, however, an alternative calibration using semi-classical transition state at higher (CCSD) level theory (Barker, 2012) more closely match the experimental KIE, suggesting e-correlation interaction treatment of CCSD is important, and calibrations for other reactions will also benefit. Revisiting the 13CH4 + ▪OH reaction and adding 13CH4 + O(1D), if similar to 13CH4 +▪Cl will reduce inversion model uncertainties from >10% to % level.

By providing full internally-consistent calibration of atmospheric CH4 isotopologue sinks, this work will lay the foundation for process-model inversions of global data that better constrain source apportionment and that can be used to monitor future engineering solutions for methane removal. Many constraints needed to do this do not yet exist, and of those that do exist, some add considerable uncertainty to the models (as described above). For deliverables, this work will reduce this part of model uncertainty, if successful from >10% to the % level, and add isotopologue constraints that reduce the number of free variables and allow for more complete representation of the atmospheric CH4 cycle and apportioning its sources. Furthermore, ab initio calibrations of KIE allow constraints on related atmospheric CH4 isotopologues (e.g., 14CH4 and its oxidation products). This work addresses Track 1.4 – toward improved representations of atmospheric CH4 sink mechanisms into relevant models.