A three-dimensional (3-D) regional chemistry transport model covering Europe, North America, and the North Atlantic Flight Corridor (NAFC) has been used to study the impact of NOx emissions from subsonic aircraft when plume processes were taken into account. An aircraft plume model has been used to quantify the chemical conversion of NOx to reservoir species. With the turbulent conditions used, the plume was spread to scales resolved by the 3-D model after 15 hours. The conversion of NOx was calculated as a function of time of emission, latitude, and background concentration of ozone and NOx under summer conditions. The conversion of NOx in the plume showed a strong variation with time of emissions. In some cases, more than 50% of the emitted NOx was converted to reservoir species after 15 hours at midlatitudes. The results from the plume model were used to modify the aircraft emissions used as inputs to the 3-D regional model. When the aircraft NOx emissions were instantaneously mixed over the model grid area, a standard procedure in regional and global models, monthly averaged NOx increases of up to 70 parts per trillion (ppt) were calculated over the midwest United States and over England in July 1998. The highest ozone increase was calculated northeast of Scandinavia (2.7 parts per billion (ppb)). When plume processes were taken into account, the corresponding monthly averaged NOx contribution from aircraft was reduced by ~40% or less. Ozone was lowered by up to ~30% within the NAFC, while the peak over Scandinavia was reduced by ~20%. ¿ 2000 American Geophysical Union