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o estimate the impact of emissions by road, aircraft and ship traffic on ozone and OH of the present-day atmosphere seven different atmospheric chemistry models simulated the atmospheric composition of the year 2003. Based on newly developed global emission inventories for road, maritime and aircraft emission data sets each model performed a series of five simulations: A base scenario using the full set of emissions, three sensitivity studies with each individual sector of transport reduced by 5% and one simulation with all traffic related emissions reduced by 5%. The approach minimizes non-linearities in atmospheric chemical effects and are later scaled to 100%. The global annual mean impact of ship emissions on ozone in the boundary layer leads to an increase of ozone of 1.2%, followed by road (0.87%) and aircraft emissions (0.3%). In the upper troposphere between 200¿300 hPa both road and ship traffic affect ozone by 1.1%, whereas aircraft emissions contribute 0.9%. However, the sensitivity of ozone formation per NOx molecule emitted is highest for aircraft exhausts. The local maximum effect of the summed traffic emissions on the ozone column predicted by the models is 4.0 DU and occurs over the northern subtropical Atlantic. The impact of traffic emissions on total ozone in the Southern Hemisphere is approximately half of the northern hemispheric perturbation. Below 800 hPa both ozone and OH respond most sensitively to ship emissions in the marine boundary layer over the Atlantic, where the effect can exceed 10% (zonal mean) which is 80% of the total traffic induced ozone perturbation. In the Southern Hemisphere ship emissions contribute relatively strongly to the total ozone perturbation by 60%¿80% throughout the year (equivalent to 1¿1.5 ppbv). Road emissions have the strongest impact on ozone in the continental boundary layer and the free troposphere in summer. They also affect the upper troposphere particularly during northern summer associated with strong convection in mid latitudes. Ozone perturbations due to road traffic show the strongest seasonal cycle in the northern troposphere, and can even change sign in the continental boundary layer during winter. The OH concentration in the boundary layer is most strongly affected by ship emissions, which has a significant influence on the lifetime of many trace gases including methane. Methane lifetime changes due to ship emissions amount to 4.1%, followed by road (1.6%) and air traffic (1.0%)., JRC.H.2-Air and Climate