3.1. Changes in Surface O3 Metrics and Net Radiative Forcing
 Figure 1 shows the response of surface ozone (O3srf) to 20% decreases in anthropogenic precursor emissions. We show the daily maximum 8-hour O3 averaged over the “O3 season,” which we define for each grid cell individually as the consecutive three-month period with highest average 8-hr. O3 in the base simulation. The reduction in O3srf due to the decrease in CH4 emissions is widespread globally, with the largest effects (>1 ppbv) in North Africa, the Middle East, and Europe (due to greater down-welling from the free troposphere and availability of NOx) in agreement with previous results using different models or meteorology [Fiore et al., 2002; West and Fiore, 2005; West et al., 2006]. In contrast, the responses to changes in NOx, NMVOCs and CO are concentrated near source regions, with changes in background concentrations occurring mainly in the temperate Northern Hemisphere. In some locations (e.g., Northern Russia), NOx reductions increase the modeled O3srf due to the decreased local destruction of O3 by reaction with fresh NO emissions.
Figure 1. The change in 8-hr. daily maximum surface O3 concentrations, averaged over the “O3 season” (the three-month period with highest O3 in each grid cell), due to 20% reductions in global anthropogenic emissions of O3 precursors. Results are shown at steady state for (a) NMVOCs, (b) CH4, (c) CO, and (d) NOx, and short-term responses are shown for (e) CO and (f) NOx. The short-term response for NMVOCs is nearly identical to the steady-state response.
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 Figure 2 shows the changes in CH4 and O3 concentrations resulting from reductions in emissions of each precursor. The 20% CH4 reduction yields the greatest decrease in both the tropospheric O3 burden (−6.7 Tg O3; Figure 2b) and the global annual average O3srf (−0.61 ppbv; Figure 2c). Our estimated reduction of 0.12 Tg O3 per Tg yr−1 change in CH4 emissions is within the range of other values in the literature of 0.09 to 0.19 [Prather et al., 2001; Fiore et al., 2002; Shindell et al., 2005; West et al., 2006]. Changes in the O3 burden also compare well for NOx (0.29 Tg O3 per Tg yr−1 (as NO2), compared to 0.25 to 0.47 from other studies) [Fiore et al., 2002; Shindell et al., 2005; Naik et al., 2005]. Finally, results for the CO (0.013 Tg O3 per Tg yr−1) and NMVOC reductions (0.12 Tg O3 per Tg C yr−1) agree well with those reported by Fiore et al.  (0.014 and 0.14, respectively) (see auxiliary material).
Figure 2. Effects of sustained 20% reductions in global anthropogenic emissions of four O3 precursors on metrics of (a) CH4 concentration, (b) O3 burden, (c) surface O3 concentration, (d) radiative forcing (RF), and (e) ΔRFnet/ΔO3srf. Steady-state results are shown with solid symbols; open symbols indicate short-term results (the CH4 perturbation only causes a steady-state change). Base simulation values are shown by the vertical lines (see auxiliary material). Air quality ΔO3srf metrics include the population-weighted 8-hr. O3, annually averaged globally and averaged over the three-month O3 season globally and regionally (all 8-hr. metrics shown are population-weighted). Radiative forcing indicates the global annual mean forcing at the tropopause.
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 Figure 2c also includes changes in O3srf metrics relevant for air quality and human health, including the population-weighted daily maximum 8-hr. O3, averaged over the whole year and over the O3 season. The 20% NOx reduction yields the greatest decrease in the population-weighted O3srf metrics, as the ΔO3srf is concentrated in populous source regions.
 In the United States, O3srf responds strongly to the 20% NOx reduction, reflecting high regional NOx emissions and NOx-sensitive chemistry due to the high emissions of biogenic NMVOCs in the eastern US. In Europe, O3srf is particularly sensitive to changes in CH4. The NMVOC emission reduction causes rather small changes in O3srf, in part because of large biogenic NMVOC emissions (the 20% anthropogenic reduction is only 1.0% of total NMVOC emissions). Note that the global model likely underestimates the effects of changes in NMVOC emissions on O3 air quality in urban areas, particularly for the population-weighted metrics, due to its coarse resolution and lack of detail for highly reactive VOCs. For NOx decreases, global models may either under- or overestimate O3srf changes in urban regions [Liang and Jacobson, 2000; Karamchandani et al., 2002]. Local sensitivity to changes in emissions of NMVOCs and NOx is best determined using a local or regional model which has been tested for local conditions.
 Figure 2c also shows a notable difference between the short-term and steady-state ΔO3srf metrics, due to long-term changes in CH4. The 20% NOx reduction causes long-term increases in the population-weighted O3srf metrics of about 0.2 ppbv, which counteract the short-term O3srf decreases (1.3 to 3.4 ppbv) by 6 to 14%. For the CO reduction, the short-term ΔO3srf (0.4 to 0.5 ppbv) is amplified by about 0.1 ppbv at steady state (a 16–21% increase). For NMVOCs, the additional long-term O3srf decrease is small (about 0.01 ppbv). These long-term changes in O3srf are globally widespread, following the spatial pattern of ΔO3srf from the CH4 perturbation (Figure 1).
 Changes in net radiative forcing (RFnet) are shown in Figure 2d. The largest negative RFnet results from the CH4 reduction, with 76% of the ΔRFnet from the decrease in CH4 itself, and the remainder from the decrease in O3. While reducing each precursor causes a negative forcing due to O3, the NOx reduction causes a larger positive CH4 forcing, producing a positive global average ΔRFnet. This agrees with previous results showing that the ΔRFnet is positive for surface NOx emission reductions in all world regions [Fuglestvedt et al., 1999; Berntsen et al., 2005; Naik et al., 2005] (see auxiliary material for a comparison with other RFnet estimates).
 Changes in ozone precursor emissions also affect the concentrations of aerosols, through changes in oxidant chemistry. We find that the CH4 reduction decreases the tropospheric burden of sulfate aerosol by 0.23%, in agreement with previous results [West et al., 2006]. This decrease results from a competition between decreased heterogeneous sulfate production by hydrogen peroxide (−0.9 Tg yr−1, or −1.1%) and increased gas-phase production by OH (+0.55 Tg yr−1, or +2.5%). Decreases in other precursor emissions are likewise estimated to decrease the global sulfate burden: −0.16% when decreasing CO, −0.14% for NMVOCs, and −0.06% for NOx. The global mean positive radiative forcing by the direct effect resulting from each of these decreases in sulfate are estimated to be less than ∼0.002 W m−2 (see auxiliary material), smaller than the forcings in Figure 2 but potentially important for local forcing [Unger et al., 2006].
3.2. Change in RFnet Per Unit Change in O3srf Metrics
 We consider the ratio ΔRFnet/ΔO3srf as an indicator of the effects on radiative forcing of actions to decrease metrics of O3 air quality by a given amount. High positive values indicate that actions to reduce O3srf by one unit cause a large negative radiative forcing. In Figure 2e, the CH4 emission reduction causes the largest decrease in RFnet per unit ΔO3srf, mainly due to the decreased forcing from CH4 itself. Reductions in NOx increase RFnet, causing a negative ΔRFnet/ΔO3srf. Abatement of CO emissions leads to a greater ΔRFnet/ΔO3srf than for NMVOCs, as the ratio of the reduction in CH4 to O3 is greater for CO. We find that the order of ΔRFnet/ΔO3srf (CH4, CO, NMVOCs, NOx) is the same for all ΔO3srf metrics considered, and we expect this order to be robust over model uncertainties as well. If ΔRFnet were evaluated after a short time period (<10 yr), however, the change in short-term O3 forcing would dominate, and the order of the precursors would likely differ.