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 CH4 on radiative forcing through its impact on O-3 concentrations. The short-lived O-3 precursors (NOx, 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 NOx alone from nine geographic regions and a combined marginal reduction in NOx, 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 CH4 concentrations (by decreasing OH); this increase in CH4 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 CH4. 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 CH4 and O-3 concentrations resulting from NOx 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 NOx, CO, and NMHCs, changes in O-3 and CH4 concentrations result in a net negative radiative forcing (cooling). Thus we conclude that simultaneous reductions of CO, NMHCs, and NOx lead to a net reduction in radiative forcing due to resulting changes in tropospheric O-3 and CH4 while reductions in NOx emissions alone do not.