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

Reducing methane (CH4) 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 CH4 emissions over a range of anthropogenic emissions from 0-430 Tg CH4 a(-1) (0.11-0.16 Tg tropospheric O-3 or similar to 11-15 ppt global mean surface O-3 decrease per Tg a(-1) CH4 reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH4 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 CH4 controls would offset the positive climate forcing from CH4 and O-3 that would otherwise occur (from increases in NOx and CH4 emissions in the baseline scenario) and improve O-3 air quality. We estimate that anthropogenic CH4 contributes 0.7 Wm(-2) to climate forcing and similar to 4 ppb to surface O-3 in 2030 under the baseline scenario. Although the response of surface O-3 to CH4 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 NOx-saturated. In the model, CH4 oxidation within the boundary layer (below similar to 2.5 km) contributes more to surface O-3 than CH4 oxidation in the free troposphere. In NOx-saturated regions, the surface O-3 sensitivity to CH4 can be twice that of the global mean, with > 70% of this sensitivity resulting from boundary layer oxidation of CH4. Accurately representing the NOx distribution is thus crucial for quantifying the O-3 sensitivity to CH4.