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

Abstract

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. Copyright 2008 by the American Geophysical Union.