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

[摘要] The methane chemical sink estimated by atmospheric chemistry models (bottom-up method) is significantly larger than estimates based onmethyl chloroform (MCF) inversions (top-down method). The difference ispartly attributable to large uncertainties in hydroxyl radical (OH)concentrations simulated by the atmospheric chemistry models used to derivethe bottom-up estimates. In this study, we propose a new approach based onOH precursor observations and a chemical box model. This approachcontributes to improving the 3D distributions of tropospheric OH radicals obtained from atmospheric chemistry models and reconcilingbottom-up and top-down estimates of the chemical loss of atmosphericmethane. By constraining simulated OH precursors with observations, theglobal mean tropospheric column-averaged air-mass-weighted OH concentration([OH] trop-M ) is ∼ 10 × 10 5  molec. cm −3 (which is 2×10 5  molec. cm −3 lower than the original model-simulated global [OH] trop-M ) and agrees with thatobtained by the top-down method based on MCF inversions. With 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 in the range of the latest top-down estimate of the Global Carbon Project (GCP) (459–516 Tg yr −1 ), contrary to the bottom-up estimates that use 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, andoverestimation of nitrogen dioxide. Our results highlight that constrainingthe model-simulated OH fields with available OH precursor observations canhelp improve bottom-up estimates of the global methane sink.

[学科分类] 医学（综合）