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

Yuanhong Zhao (1) , Marielle Saunois (1, 2) , Philippe Bousquet (1, 2) , Xin Lin (1) , Antoine Berchet (1, 2) , Michaela I. Hegglin (3) , Josep G. Canadell (4) , Robert B. Jackson (5) , Edward J. Dlugokencky (6) , Ray L. Langenfelds (4) , Michel Ramonet (1, 7) , Doug Worthy (8) , Bo Zheng (1)

1 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
2 SATINV - Modélisation INVerse pour les mesures atmosphériques et SATellitaires
3 Department of Meteorology [Reading]
4 CSIRO - Commonwealth Scientific and Industrial Research Organisation [Canberra]
5 Stanford Woods Institute for the Environment
6 ESRL - NOAA Earth System Research Laboratory
7 ICOS-RAMCES - ICOS-RAMCES
8 ECCC - Environment and Climate Change Canada

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.

Domaines: Physique Atmosphérique et Océanique [physics.ao-ph]

Yuanhong Zhao, Marielle Saunois, Philippe Bousquet, Xin Lin, Antoine Berchet, et al.. Influences of hydroxyl radicals (OH) on top-down estimates of the global and regional methane budgets. Atmospheric Chemistry and Physics, 2020, 20 (15), pp.9525-9546. ⟨10.5194/acp-20-9525-2020⟩. ⟨hal-02971159⟩