31 Aug 2022
31 Aug 2022
Status: this preprint is currently under review for the journal ACP.

Reconciling the bottom-up and top-down estimates of the methane chemical sink using multiple observations

Yuanhong Zhao1,2, Marielle Saunois2, Philippe Bousquet2, Xin Lin2, Michaela I. Hegglin3,4, Josep G. Canadell5, Robert B. Jackson6, and Bo Zheng7 Yuanhong Zhao et al.
  • 1College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao, 266100, People's Republic of China
  • 2Laboratoire des Sciences du Climat et de l'Environnement, LSCE-IPSL (CEA-CNRS-UVSQ), UniversitéParis-Saclay, 91191 Gif-sur-Yvette, France
  • 3Institute of Energy and Climate Research – Stratosphere (IEK-7), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
  • 4Department of Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, United Kingdom
  • 5Global Carbon Project, CSIRO Oceans and Atmosphere, Canberra, Australian Capital Territory 2601, Australia
  • 6Earth System Science Department, Woods Institute for the Environment, and Precourt Institute for Energy, Stanford University, Stanford, CA 94305, USA
  • 7Institute of Environment and Ecology, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China

Abstract. The methane chemical sink estimated by atmospheric chemistry models (bottom-up method) is significantly larger than estimates based on methyl-chloroform (MCF) inversions (top-down method). The difference is partly attributable to large uncertainties in hydroxyl radical (OH) concentrations simulated by the atmospheric chemistry models used to derive the bottom-up estimates. In this study, we propose a new approach based on OH precursor observations and a chemical box model. This approach improves the 3-dimensional distributions of tropospheric OH radicals obtained from atmospheric chemistry models and reconciles the bottom-up and top-down estimates of the methane sink due to chemical loss. By constraining the model simulated OH precursors with observations, the global tropospheric mean OH concentration ([OH]trop-M) is ~10×105 molec cm−3 (which is 2×105 molec cm−3 lower than the original model-simulated global [OH]trop-M) and agrees with that obtained by the top-down method based on MCF inversions. With the OH constrained by precursor observations, the methane chemical loss is 471–508 Tg yr−1 averaged from 2000 to 2009. The new adjusted estimate is more consistent with the top-down estimates in the recent global methane budget by the Global Carbon Project (GCP) (459–516 Tg yr−1) than the bottom-up estimates using the original model-simulated OH fields (577–612 Tg yr−1). The overestimation of global [OH]trop-M and methane chemical loss simulated by the atmospheric chemistry models is caused primarily by the models’ underestimation of carbon monoxide and total ozone column, and overestimation of nitrogen dioxide. Our results highlight that constraining the model simulated OH fields with available OH precursor observations can help improve the bottom-up estimated methane sink.

Yuanhong Zhao et al.

Status: open (until 22 Oct 2022)

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Yuanhong Zhao et al.

Yuanhong Zhao et al.


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Short summary
The large uncertainties in OH simulated by atmospheric chemistry models hinder accurate estimates of CH4 chemical loss through the bottom-up method. This study presents a new approach based on OH precursor observations and a chemical box model, to improve the tropospheric OH distributions simulated by atmospheric chemistry models. Through this approach, both the global OH burden and the corresponding methane chemical loss reach consistency with the top-down method based on MCF inversions.