Multi - model Estimates of Intercontinental Source - Receptor Relationships for Ozone 1 Pollution

60 61 A thorough understanding of the surface O 3 response over a " receptor " region to 62 emission changes over a foreign " source " region is needed to evaluate the potential gains 63 from an international approach to abate ozone (O 3) pollution. We apply an ensemble of 64 21 global and hemispheric chemical transport models to estimate the average surface O 3 65 response over East Asia (EA), Europe (EU), North America (NA) and South Asia (SA) to 66 20% decreases in anthropogenic emissions of the O 3 precursors, NO x , NMVOC, and CO 67 (individually and combined), from each of these regions. The ensemble mean O 3 68 matches observed surface concentrations throughout the year over EU but overestimates 69 them by >10 ppb during summer and early fall over NA and EA. The sum of the O 3 70 responses to NO x , CO, and NMVOC perturbations is approximately equal to that with all 71 three precursors reduced together, and we focus here on the response to combined 72 reductions. The most robust annual mean responses across the models are that surface O 3 73 concentrations over EU are most influenced by anthropogenic emissions reductions over 74 NA (ensemble mean: 0.4 ppb; inter-model range: 0.2-0.5 ppb), and that emissions 75 reductions over SA have the smallest effect on the other receptor regions (< 0.2 ppb). 76 Emission reductions in the three foreign regions decrease annual mean surface O 3 over 77 the Asian regions by 0.2-0.4 ppb each, and reductions in EA and EU decrease annual 78 mean O 3 by 0.2 ppb each over NA, although the relative impacts vary by model. 79 Applying these ensemble annual mean results to changes in anthropogenic emissions 80 from 1996 to 2002, we estimate a Northern Hemispheric increase in background surface 81 O 3 of about 0.1 ppb yr-1 , at the low end of the 0.1-0.5 ppb yr-1 derived from observations. 82 The ensemble mean surface O 3 response to emission reductions in the 3 foreign regions 83 combined is largest in spring and late fall (~0.8 ppb in all regions). We define an " import 84 sensitivity " to gauge the relative contribution of foreign versus " domestic " (i.e., over the 85 source region itself) emission changes for each region. This ratio ranges from 0.5 to 1.1 86 during the months …