Role of Atmospheric Chemistry in Monitoring Changes in the CH4 Budget, Investigating the Role of Chemical Feedback

Modeling the hydroxyl radical (OH) at the global scale has been found to be extremely challenging and led the chemistry-climate models to use idealized scenarios with prescribed fields of methane (CH₄) concentration instead of emission fluxes. Here, we evaluate the impact of an improved halogen chemistry representation, the Short-Lived Halogen (SLH), in emission- and concentration-driven methane simulations using the Community Atmosphere Model with chemistry (CAM-chem) in the Community Earth System Model Version 2.2 (CESM2.2).

Comparison with the NASA Atmospheric Tomography (ATom) mission shows improvement for most species, despite a reversal of the ozone (O₃) bias from a Northern Hemisphere overestimation to an underestimation following the SLH implementation. Evaluation against the NASA Measurement of the Pollution in the Troposphere (MOPITT) XCO retrievals, the JAXA Greenhouse Gases Observing Satellite (GOSAT) XCH4 retrievals, and both XCO and XCH4 from the Network for Detection of Atmospheric Composition Change (NDACC) show remarkable amelioration.

The net impact of the updated chemistry reduces O₃, hydrogen peroxides (H₂O₂), nitric acid (HNO₃), and hydroxyl radical (OH), which increases the carbon monoxide (CO) and CH₄ abundance and lifetime. The SLH-induced weakening of CH₄ tropospheric chemical sink after 4 years of simulations (2014 to 2017) is dependent on the chemistry representation with a reduction of around 40 TgCH₄ yr^-1 in emission-driven runs and 55 TgCH₄ yr^-1 in concentration-driven runs. The tropospheric chemical sink is less sensitive in the CH4 emissions driven runs and rather translates to a higher annual growth rate of CH₄ concentration. This demonstrates that the widely adopted surface concentration prescription approach can lead to biased estimates of CH₄ sink, as well as the model's sensitivity to a change in tropospheric chemistry representation (e.g., SLH).