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

Abstract

The global distribution of tropospheric ozone (O 3) depends on the emission of precorsors, 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 (wartning) 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. Copyright 2005 by the American Geophysical Union.