Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality

Reducing methane (CH 4 ) emissions is an attractive option for jointly addressing climate and ozone (O 3 ) air quality goals. With multidecadal full‐chemistry transient simulations in the MOZART‐2 tropospheric chemistry model, we show that tropospheric O 3 responds approximately linearly to changes in CH 4 emissions over a range of anthropogenic emissions from 0–430 Tg CH 4 a −1 (0.11–0.16 Tg tropospheric O 3 or ∼11–15 ppt global mean surface O 3 decrease per Tg a −1 CH 4 reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH 4 emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005–2030) transient simulations, we demonstrate that cost‐effective CH 4 controls would offset the positive climate forcing from CH 4 and O 3 that would otherwise occur (from increases in NO x and CH 4 emissions in the baseline scenario) and improve O 3 air quality. We estimate that anthropogenic CH 4 contributes 0.7 Wm −2 to climate forcing and ∼4 ppb to surface O 3 in 2030 under the baseline scenario. Although the response of surface O 3 to CH 4 is relatively uniform spatially compared to that from other O 3 precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O 3 formation regime is NO x ‐saturated. In the model, CH 4 oxidation within the boundary layer (below ∼2.5 km) contributes more to surface O 3 than CH 4 oxidation in the free troposphere. In NO x ‐saturated regions, the surface O 3 sensitivity to CH 4 can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH 4 . Accurately representing the NO x distribution is thus crucial for quantifying the O 3 sensitivity to CH 4 .