Net radiative forcing due to changes in regional emissions of tropospheric ozone precursors

The global distribution of tropospheric ozone (O 3 ) depends on the emission of precursors, chemistry, and transport. For small perturbations to emissions, the global radiative forcing resulting from changes in O 3 can be expressed as a sum of forcings from emission changes in different regions. Tropospheric O 3 is considered in present climate policies only through the inclusion of indirect effect of CH 4 on radiative forcing through its impact on O 3 concentrations. The short‐lived O 3 precursors (NO x , CO, and NMHCs) are not directly included in the Kyoto Protocol or any similar climate mitigation agreement. In this study, we quantify the global radiative forcing resulting from a marginal reduction (10%) in anthropogenic emissions of NO x alone from nine geographic regions and a combined marginal reduction in NO x , CO, and NMHCs emissions from three regions. We simulate, using the global chemistry transport model MOZART‐2, the change in the distribution of global O 3 resulting from these emission reductions. In addition to the short‐term reduction in O 3 , these emission reductions also increase CH 4 concentrations (by decreasing OH); this increase in CH 4 in turn counteracts part of the initial reduction in O 3 concentrations. We calculate the global radiative forcing resulting from the regional emission reductions, accounting for changes in both O 3 and CH 4 . Our results show that changes in O 3 production and resulting distribution depend strongly on the geographical location of the reduction in precursor emissions. We find that the global O 3 distribution and radiative forcing are most sensitive to changes in precursor emissions from tropical regions and least sensitive to changes from midlatitude and high‐latitude regions. Changes in CH 4 and O 3 concentrations resulting from NO x emission reductions alone produce offsetting changes in radiative forcing, leaving a small positive residual forcing (warming) for all regions. In contrast, for combined reductions of anthropogenic emissions of NO x , CO, and NMHCs, changes in O 3 and CH 4 concentrations result in a net negative radiative forcing (cooling). Thus we conclude that simultaneous reductions of CO, NMHCs, and NO x lead to a net reduction in radiative forcing due to resulting changes in tropospheric O 3 and CH 4 while reductions in NO x emissions alone do not.