The influence of ozone precursor emissions from four world regions on tropospheric composition and radiative climate forcing

Ozone (O 3 ) precursor emissions influence regional and global climate and air quality through changes in tropospheric O 3 and oxidants, which also influence methane (CH 4 ) and sulfate aerosols (SO 4 2− ). We examine changes in the tropospheric composition of O 3 , CH 4 , SO 4 2− and global net radiative forcing (RF) for 20% reductions in global CH 4 burden and in anthropogenic O 3 precursor emissions (NO x , NMVOC, and CO) from four regions (East Asia, Europe and Northern Africa, North America, and South Asia) using the Task Force on Hemispheric Transport of Air Pollution Source‐Receptor global chemical transport model (CTM) simulations, assessing uncertainty (mean ± 1 standard deviation) across multiple CTMs. We evaluate steady state O 3 responses, including long‐term feedbacks via CH 4 . With a radiative transfer model that includes greenhouse gases and the aerosol direct effect, we find that regional NO x reductions produce global, annually averaged positive net RFs (0.2 ± 0.6 to 1.7 ± 2 mWm −2 /Tg N yr −1 ), with some variation among models. Negative net RFs result from reductions in global CH 4 (−162.6 ± 2 mWm −2 for a change from 1760 to 1408 ppbv CH 4 ) and regional NMVOC (−0.4 ± 0.2 to −0.7 ± 0.2 mWm −2 /Tg C yr −1 ) and CO emissions (−0.13 ± 0.02 to −0.15 ± 0.02 mWm −2 /Tg CO yr −1 ). Including the effect of O 3 on CO 2 uptake by vegetation likely makes these net RFs more negative by −1.9 to −5.2 mWm −2 /Tg N yr −1 , −0.2 to −0.7 mWm −2 /Tg C yr −1 , and −0.02 to −0.05 mWm −2 /Tg CO yr −1 . Net RF impacts reflect the distribution of concentration changes, where RF is affected locally by changes in SO 4 2− , regionally to hemispherically by O 3 , and globally by CH 4 . Global annual average SO 4 2− responses to oxidant changes range from 0.4 ± 2.6 to −1.9 ± 1.3 Gg for NO x reductions, 0.1 ± 1.2 to −0.9 ± 0.8 Gg for NMVOC reductions, and −0.09 ± 0.5 to −0.9 ± 0.8 Gg for CO reductions, suggesting additional research is needed. The 100‐year global warming potentials (GWP 100 ) are calculated for the global CH 4 reduction (20.9 ± 3.7 without stratospheric O 3 or water vapor, 24.2 ± 4.2 including those components), and for the regional NO x , NMVOC, and CO reductions (−18.7 ± 25.9 to −1.9 ± 8.7 for NO x , 4.8 ± 1.7 to 8.3 ± 1.9 for NMVOC, and 1.5 ± 0.4 to 1.7 ± 0.5 for CO). Variation in GWP 100 for NO x , NMVOC, and CO suggests that regionally specific GWPs may be necessary and could support the inclusion of O 3 precursors in future policies that address air quality and climate change simultaneously. Both global net RF and GWP 100 are more sensitive to NO x and NMVOC reductions from South Asia than the other three regions.