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

Ozone (O₃) precursor emissions influence regional and global climate and air quality through changes in tropospheric O₃ and oxidants, which also influence methane (CH₄) and sulfate aerosols (SO₄²⁻). We examine changes in the tropospheric composition of O₃, CH₄, SO₄²⁻ and global net radiative forcing (RF) for 20% reductions in global CH₄ burden and in anthropogenic O₃ precursor emissions (NOx, 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 O₃ responses, including long-term feedbacks via CH₄. With a radiative transfer model that includes greenhouse gases and the aerosol direct effect, we find that regional NOx 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 CH₄ (−162.6 ± 2 mWm⁻² for a change from 1760 to 1408 ppbv CH₄) 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 O₃ on CO₂ 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 SO₄²⁻, regionally to hemispherically by O₃, and globally by CH₄. Global annual average SO₄²⁻ responses to oxidant changes range from 0.4 ± 2.6 to −1.9 ± 1.3 Gg for NOx 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 (GWP₁₀₀) are calculated for the global CH₄ reduction (20.9 ± 3.7 without stratospheric O₃ or water vapor, 24.2 ± 4.2 including those components), and for the regional NOx, NMVOC, and CO reductions (−18.7 ± 25.9 to −1.9 ± 8.7 for NOx, 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 GWP₁₀₀ for NOx, NMVOC, and CO suggests that regionally specific GWPs may be necessary and could support the inclusion of O₃ precursors in future policies that address air quality and climate change simultaneously. Both global net RF and GWP₁₀₀ are more sensitive to NOx and NMVOC reductions from South Asia than the other three regions.