A mesoscale chemistry transport model is coupled to a numerical weather prediction model (NWP) for Europe and the North Atlantic. It is applied to show that air traffic emissions significantly increase the concentrations of NOx as well as the net chemical formation rate of ozone over both the Atlantic Ocean and over central Europe. The time period studied was July 1--10, 1991, a period characterized by a high-pressure ridge over North Europe. In convection the vertical gradient of NOx is strongly reduced because the midtropospheric concentrations increase. In the updraft region of convective plumes, chemical ozone formation up to 1.8 ppbv/h is calculated in the upper troposphere, and this is about a factor 50 higher than the average for 10.5 km over continental Europe during the 10-day period. The high production rates are not sustained for more than a few hours, and it would be difficult to identify the effect of the process in a local measurement of the ozone concentration. The calculations indicate that 10--25 molecules of O3 (0.03--0.05 ppb/h) are generated on the average per NOx molecule emitted from aircraft (2--3 ppt/h). On convective days over regions with significant surface sources of NOx, ozone formation in the upper troposphere due to NOx brought there by convection, outweighs the role of aircraft NOx emissions. The net chemical formation rate of ozone across the model domain depends on the boundary concentrations of NOx and O3, but the change in the upper troposphere in the net chemical formation rate of ozone induced by aircraft NOx emissions is quite independent of the concentrations of NOx or O3. ¿ American Geophysical Union 1996