A trajectory box model describing the expansion and cooling of jet aircraft plumes at cruise altitude in the wake regime is presented. The forcing along the trajectory takes into account turbulent entrainment rates for the distinct dynamical regimes (jet, vortex, and dispersion), with special emphasis on the proper treatment of jet mixing. The model is used to study the plume evolution, especially the overall plume dilution history, behind a subsonic B 747 and a planned supersonic airliner under cruising conditions. The accuracy with which microphysically relevant quantities like plume temperature and water saturation ratio can be predicted with the box model is discussed by comparing the results to spatially resolved, two-dimensional model calculations of jet mixing. As an application, the gas phase chemical conversion of NO into HNO2, of NO2 into HNO3, and of SO2 into H2SO4 taking place in the early jet regime is investigated using a simplified plume chemistry. Maximum conversion rates and their sensitivity to variations of the OH exit plane concentration are examined by means of analytical expressions. ¿ American Geophysical Union 1995