Influences of hydroxyl radicals (OH) on top-down estimates of the global and regional methane budgets

Abstract

The hydroxyl radical (OH), which is the dominant sink of methane (CH 4 ), plays a key role in closing the global methane budget. Current top-down estimates of the global and regional CH 4 budget using 3D models usually apply prescribed OH fields and attribute model-observation mismatches almost exclusively to CH 4 emissions, leaving the uncertainties due to prescribed OH fields less quantified. Here, using a variational Bayesian inversion framework and the 3D chemical transport model LMDz, combined with 10 different OH fields derived from chemistry-climate models (Chemistry-Climate Model Initiative, or CCMI, experiment), we evaluate the influence of OH burden, spatial distribution, and temporal variations on the global and regional CH 4 budget. The global tropospheric mean CH 4 - reaction-weighted [OH] ([OH]GM-CH 4 ) ranges 10.3-16:3× 10 5 molec cm -3 across 10 OH fields during the early 2000s, resulting in inversion-based global CH 4 emissions between 518 and 757 Tg yr -1 . The uncertainties in CH 4 inversions induced by the different OH fields are similar to the CH 4 emission range estimated by previous bottom-up syntheses and larger than the range reported by the top-down studies. The uncertainties in emissions induced by OH are largest over South America, corresponding to large inter-model differences of [OH] in this region. From the early to the late 2000s, the optimized CH 4 emissions increased by 22-6 Tg yr -1 (17-30 Tg yr -1 ), of which 25 % (on average) offsets the 0.7 %(on average) increase in OH burden. If the CCMI models represent the OH trend properly over the 2000s, our results show that a higher increasing trend of CH 4 emissions is needed to match the CH 4 observations compared to the CH 4 emission trend derived using constant OH. This study strengthens the importance of reaching a better representation of OH burden and of OH spatial and temporal distributions to reduce the uncertainties in the global and regional CH 4 budgets.