Importance of emissions-driven simulation of methane for representation of key atmospheric oxidants

Abstract. Methane (CH₄) is a major greenhouse gas influencing both climate and atmospheric chemistry through its interactions with hydroxyl radicals (OH) and ozone (O₃). Using the FRSGC/UCI chemical transport model, we compare a new emission-driven simulation with configurations where CH₄ mixing ratios are prescribed at the surface or throughout the troposphere. The emission-driven simulation better reproduces the observed spatiotemporal characteristics of CH₄ and captures stronger positive trends over major source regions. With a global CH₄ source of 602 Tg∙yr^-1 we find sinks of 515 Tg∙yr^-1 from tropospheric chemical loss and ~36 Tg∙yr^-1 each from soil uptake and stratospheric loss. The impact of CH₄ treatment on global oxidants is modest, with tropospheric O₃ and OH burdens 0.6 % and 0.05 % higher in the emission-driven simulation, respectively. The simulated CH₄–OH feedback is insensitive to the choice of CH₄ configuration, with conventional feedback factors ranging from 1.403 to 1.409 across the three configurations. However, there is a pronounced influence on the longer-term response to emission perturbations. A 20 % reduction in surface NO emissions produces immediate decreases in O₃ and OH, but the emission-driven simulation additionally captures the persistent response in CH₄ burden, which peaks at 21.5 Tg in the following year and decays with a perturbation lifetime of 12.5 years. The elevated CH₄ enhances O₃, partially offsetting its initial decrease. These results demonstrate that an emission-driven treatment provides a more complete framework for quantifying long-term CH₄–OH–O₃ responses to emission changes and assessing their impacts on atmospheric chemistry and climate.