Effects of Methane and Hydroxyl Radical Chemistry on Decadal Methane Emissions Estimates

The rise of atmospheric methane experienced a brief pause between 2000 and 2007 with a subsequent resumption in growth continuing to the present day. Questions on both the commencement and termination of this stabilization remain open due to uncertainty in methane sources and its main sink, reaction with the hydroxyl radical (OH). In the atmosphere, methane undergoes oxidation reactions and thus, its lifetime depends on the concentration of hydroxyl radicals. Methyl Chloroform has been used to constrain global hydroxyl concentrations and infer methane lifetimes. Here, we report on results using a two-hemispheres box model that includes dynamic chemistry of methane, carbon monoxide, and the hydroxyl radical. We will discuss the impact of critical assumptions on methane flux inversions: 1) Constant OH concentrations and 2) Constant OH sources with interactive chemistry assuming constant or variable CO, 3) variable OH sources, 4) variations in OH inter-hemispheric gradients. Hemispheric sources are calculated from a nonlinear, stochastic Bayesian inversion constrained by data from methane (NOAA), carbon monoxide (NOAA), and Methyl chloroform (NOAA, GAGE/AGAGE) observations. We perform sensitivity experiments by calculating the impact that including each aspect of the chemical system has on the emissions estimates. Based on our results, we find that methane estimates are highly sensitive to the inter-hemispheric hydroxyl gradient and uncertainties of carbon monoxide emissions.