A three-dimensional (3-D) global chemistry transport model (CTM) has been used to study the impact of NOx emissions from subsonic aircraft on the NOx and ozone concentrations in the troposphere. Plume scale processes have been included by a dispersion model which modifies the NOx emissions to include formation of nitrogen reservoir species and nonlinear chemical effects on ozone production and loss within the plume regime. These modifications were varied as a function of time of day of emissions, latitude, season, temperature, atmospheric turbulence and background concentrations of NOx and ozone. With a global emission rate of 0.7 Tg(N) yr-1 from aircraft and including plume scale effects in the 3-D CTM, we estimate a maximum ozone perturbation at 250 hPa from 5.5 to 10.5 ppbv north of 40¿N in May. In January the maximum is lower (3--4 ppbv) and more confined to a zonal band between 35 and 50¿N. During July the influence of vertical mixing is found to reduce the maximum at midlatitudes, and the largest perturbations (6.5--8 ppbv) are found in the polar region. The use of modified aircraft emissions had a significant influence on the estimated NOx and ozone increases, and the largest effects were found in May. Compared with an unmodified NOx emission from aircraft, the NOx perturbations at cruise altitude decreased by 25--35% over the eastern U.S., the North Atlantic Ocean, and Europe. The corresponding decrease in the ozone perturbation was 15--18% at northern middle and high latitudes.