We examine the response of Arctic gas and aerosol concentrations to perturbations in pollutant emissions from Europe, East and South Asia, and North America using results from a coordinated model intercomparison. These sensitivities to regional emissions (mixing ratio change per unit emission) vary widely across models and species. Intermodel differences are systematic, however, so that the relative importance of different regions is robust. North America contributes the most to Arctic ozone pollution. For aerosols and CO, European emissions dominate at the Arctic surface but East Asian emissions become progressively more important with altitude, and are dominant in the upper troposphere. Sensitivities show strong seasonality: surface sensitivities typically maximize during boreal winter for European and during spring for East Asian and North American emissions. Mid-tropospheric sensitivities, however, nearly always maximize during spring or summer for all regions. Deposition of black carbon (BC) onto Greenland is most sensitive to North American emissions. North America and Europe each contribute ~40% of total BC deposition to Greenland, with ~20% from East Asia. Elsewhere in the Arctic, both sensitivity and total BC deposition are dominated by European emissions. Model diversity for aerosols is especially large, resulting primarily from differences in aerosol physical and chemical processing (including removal). Comparison of modeled aerosol concentrations with observations Chem. Phys. Discuss.: 6 May 2008 Revised: 5 August 2008 – Accepted: 15 August 2008 – Published: 10 September 2008 Abstract. We examine the response of Arctic gas and aerosol concentrations to perturbations in pollutant emissions from Europe, East and South Asia, and North America using re- sults from a coordinated model intercomparison. These sen- sitivities to regional emissions (mixing ratio change per unit emission) vary widely across models and species. Intermodel differences are systematic, however, so that the relative im- portance of different regions is robust. North America con- tributes the most to Arctic ozone pollution. For aerosols Correspondence to: D. T. North America and Europe each contribute ∼40% of total BC deposition to Greenland, with ∼20% from East Asia. Elsewhere in the Arctic, both sensitivity and total BC deposition are dominated by Euro- Published by Copernicus Publications on behalf of the European Geosciences Union. 5354 Multi-model assessment of pollution transport to the Arctic: D. T. Shindell et al. pean emissions. Model diversity for aerosols is especially large, resulting primarily from differences in aerosol physi- cal and chemical processing (including removal). Compari- son of modeled aerosol concentrations with observations in- dicates problems in the models, and perhaps, interpretation of the measurements. Differences in me- teorology were present, however, as models were driven by data from several reanalysis centers, or in some cases mete- orology was internally-generated based on prescribed 2001 ocean surface conditions. Additionally, some models directly prescribed meteorology while others used linear relaxation towards meteorological fields. Note that the North Atlantic Oscillation index was weakly negative during 2001, while the broader Arctic Oscillation index showed a stronger nega- tive value during winter, with weak positive values for most Arctic North America Europe East Asia South Asia Fig. 1. The Arctic and the four source regions (shaded) used in this study. In the middle troposphere, the sensitivities to emissions from Europe and North America are usually comparable, sensitivities to East Asian emissions are somewhat less, and sensitivities to South Asian emissions are quite small outside of summer (probably because of the greater distance to the Arctic from this region, Fig. 1). The relative importance of regional NOx emission changes to Arctic ozone is less dependent upon altitude, with the largest contribution from North America at all levels. The results are consistent looking at either the multi-model mean or me- dian values. These are generally quite similar for CO and ozone, while the median is typically lower than the mean for the aerosols, but the relative importance of different regions is almost unchanged between these two statistics (Table 3). no longer normalizing by emissions) leads to larger fractional variances across the models. This is especially so for East Asia, where including the intermodel variation in emissions nearly triples the fractional variance of the Arctic response at the surface and middle troposphere across models. The effects are smaller for emissions from Europe or North America at these levels, where emissions variations add ∼5–13% to the fractional variance, a com- parable range to that from transport (8–14%) and oxidation (6–11%) variations among models. Emissions uncertainties from South Asia have an even smaller impact than those from Europe or North America, barely changing the frac- tional variance. Thus the intermodel variation in the influence of source region CO emissions on the Arctic surface and mid- troposphere is dominated by emissions for East Asia, by transport and oxidation for South Asia, and all three terms (transport, oxidation and emissions) play comparable roles for Europe and North America. In the upper troposphere, the intermodel fractional variations are dominated by oxida- tion differences, whose importance gradually increases with altitude. Phys., 8, 5353–5372, 2008 www.atmos-chem-phys.net/8/5353/2008/ Multi-model assessment of pollution transport to the Arctic: D. T. Shindell et al. 5365 a long-lived species such as CO. This is especially true for quality-screened models, in which case fractional variations in the mid and lower troposphere are 22% or less for CO sensitivity to emissions from Europe, and 13% or less for emissions from East Asia or North America.