Toward Realistic Prognostic Modeling of the Methane Chemical Loss

Global modeling of the hydroxyl radical (OH) remains a significant challenge, pushing chemistry‐climate models to rely on idealized scenarios with methane (CH4 ) concentrations rather than emission fluxes. In this study, we employ an emission‐driven CH4 configuration in the Community Earth System Model Version 2.2 (CESM2.2) and demonstrate the effect of incorporating detailed Short‐Lived Halogen (SLH) chemistry representation on both emission‐ and concentration‐driven CH4 simulations in terms of global methane loss and overall chemical dynamics. The net impact of the updated SLH chemistry reduces ozone (O3 ) and hydroxyl radical (OH) in both hemispheres, resulting in higher abundance and longer lifetime of carbon monoxide (CO) and CH4 . Comparisons with NASA's Atmospheric Tomography (ATom) mission data show joint improvements in OH, O3 , CO and CH4 . Further evaluation against CO measurements from NASA's Measurement of the Pollution in The Troposphere (MOPITT), CH4 from JAXA's Greenhouse Gases Observing Satellite (GOSAT) confirms significant amelioration in modeled CO and CH4 , especially in the Northern Hemisphere during winter and spring, correcting a common wintertime underestimation. The annual tropospheric CH4 loss with OH is reduced from 573 to 504 TgCH4 yr 1 in 2017, resulting in an increase in lifetime of about 1.2 years, bringing it to approximately 10 years, which is well within the range of uncertainty in empirical estimates. In contrast, the estimated chlorine sink increases from 2 to around 15 TgCH4 yr 1 . Additionally, we find that the sensitivity of the CH4 ’s chemical loss to CO emission changes is underestimated in the prescribed CH4 simulations. Plain Language Summary Understanding tropospheric chemistry is essential to grasping how natural and human‐made emissions affect air pollution and climate. In this study, we used the short‐lived halogen version of a numerical model of atmospheric chemistry to include the latest knowledge on how halogens (chlorine, iodine, and bromine) and carbon monoxide (CO) influence the fate of methane emissions in the atmosphere. We find that adding halogen chemistry substantially reduces atmospheric oxidant levels such as ozone and hydroxyl radicals (OH), leading to a more realistic representation of atmospheric composition and oxidation processes controlling the fate of methane in the atmosphere.