A box model representative for a mesoscale volume and three different plume models are used to estimate the chemical conversion of exhaust species of a subsonic aircraft at cruise altitude. Clearly deviating results have been obtained for instantaneous mixing of the exhaust in a box and gradual dispersion of a plume. The effect of varying daytime of release as well as the impact of changing dispersion time is studied with emphasis on the aircraft-induced O3 production. Effective emission indices are calculated which enable a correction for expanding plume effects in global or mesoscale models. ¿ 1998 American Geophysical Union