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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 12 Feb 2020

Submitted as: research article | 12 Feb 2020

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This preprint is currently under review for the journal ACP.

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

Yuanhong Zhao1, Marielle Saunois1, Philippe Bousquet1, Xin Lin1,a, Antoine Berchet1, Michaela I. Hegglin2, Josep G. Canadell3, Robert B. Jackson4, Edward J. Dlugokencky5, Ray L. Langenfelds6, Michel Ramonet1, Doug Worthy7, and Bo Zheng1 Yuanhong Zhao et al.
  • 1Laboratoire des Sciences du Climat et de l'Environnement, LSCE-IPSL (CEA-CNRS-UVSQ), Université Paris-Saclay, 91191 Gif-sur-Yvette, France
  • 2Department of Meteorology, University of Reading, Reading, RG6 6LA, UK
  • 3Global Carbon Project, CSIRO Oceans and Atmosphere, Canberra, Australian Capital Territory 2601, Australia
  • 4Earth System Science Department, Woods Institute for the Environment, and Precourt Institute for Energy, Stanford University, Stanford, CA 94305, USA
  • 5NOAA ESRL, 325 Broadway, Boulder, CO 80305, USA
  • 6Climate Science Centre, CSIRO Oceans and Atmosphere, Aspendale, Victoria 3195, Australia
  • 7Environment and Climate Change Canada, Toronto, M3H 5T4, Canada
  • anow at: Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA

Abstract. The hydroxyl radical (OH), which is the dominant sink of methane (CH4), plays a key role to close the global methane budget. Previous research that assessed the impact of OH changes on the CH4 budget mostly relied on box modeling inversions with a very simplified atmospheric transport and no representation of the heterogeneous spatial distribution of OH radicals. Here using a variational Bayesian inversion framework and a 3D chemical transport model, LMDz, combined with 10 different OH fields derived from chemistry-climate models (CCMI experiment), we evaluate the influence of OH burden, spatial distribution, and temporal variations on the global CH4 budget. The global tropospheric mean CH4-reaction-weighted [OH] ([OH]GM-CH4) ranges 10.3–16.3 × 105 molec cm−3 across 10 OH fields during the early 2000s, resulting in inversion-based global CH4 emissions between 518 and 757 Tg yr−1. The uncertainties in CH4 inversions induced by the different OH fields are comparable to, or even larger than the uncertainty typically given by bottom-up and top-down estimates. Based on the LMDz inversions, we estimate that a 1 %-increase in OH burden leads to an increase of 4 Tg yr−1 in the estimate of global methane emissions, which is about 25 % smaller than what is estimated by box-models. 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 CH4 emissions increased by 21.9 ± 5.7 Tg yr−1 (16.6–30.0 Tg yr−1), of which ~ 25 % (on average) is contributed by −0.5 to +1.8 % 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 CH4 emissions is needed to match the CH4 observations compared to the CH4 emission trend derived using constant OH. This study strengthens the importance to reach a better representation of OH burden and of OH spatial and temporal distributions to reduce the uncertainties on the global CH4 budget.

Yuanhong Zhao et al.

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Yuanhong Zhao et al.

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Publications Copernicus
Short summary
The hydroxyl radical (OH), which is the dominant sink of methane (CH4), plays a key role to close the global methane budget. This study quantifies how the uncertainties in hydroxyl radical can influence to the top-down estimation of CH4 emissions based on 4D Bayesian inversions with different OH fields and the same surface observations. We show that uncertainties in CH4 emissions driven by different OH fields are comparable to the uncertainties given by current bottom-up and top-down estimations.
The hydroxyl radical (OH), which is the dominant sink of methane (CH4), plays a key role to...