Headline: Consideration of methane emissions in the modelling of ozone concentrations in chemical transport models. Final report

Methane is a powerful greenhouse gas as well as a precursor of ground-level ozone, a pollutant which is damaging to the health of both humans and ecosystems. Methane concentrations are currently about 3 times higher than in pre-industrial times, and are still rising. Accurate assessment of the consequences requires high quality modelling tools along with good emission data. We systematically examine the treatment of methane in a representative selection of widely-used models: three regional-scale models and one global-scale model, with a special focus on how this treatment of methane influences the production of ozone. There is still considerable uncertainty about global methane emissions, especially from natural sources. None of the models studied here rely exclusively on methane emission data, but rather adjust their modelled methane concentrations to be consistent with observations. Due to the relatively long lifetime of ozone in the troposphere, all models must include information about ozone production from methane at the global scale. Based on existing model datasets, we quantify the contribution of methane to annual-average surface ozone in Germany at 20 μg/m³, with only 3.2 μg/m³ of this due to oxidation of methane in the European region. Future work should focus on intercomparison of alternative methods for source attribution of ozone, including the contribution of different ozone precursors to policy-relevant exposure metrics. Large inter-model differences remain in the simulation of ground-level ozone. A better understanding of these differences is still required for more accurate simulation of ground-level ozone.