Results from a coupled microphysical-chemical-dynamical trajectory box model have been compared to recent in situ observations of particles generated in the wake of aircraft. Sulfur emissions mainly cause the formation of ultrafine volatile particles in young aircraft plumes (mean number radius ~5 nm). Volatile particle numbers range between 1016 and 1017 per kg fuel burnt for average to high fuel sulfur levels, exceeding typical soot emission indices by a factor of 10--100. Model results come into closer agreement with observations when chemi-ions from fuel combustion are included in the aerosol dynamics. Ice particles (mean number radius 1 μm) crystals. Contrails with larger crystals would also form without soot and sulfur emissions. The lifecycle of cirrus clouds can be modified by exhaust aerosols. ¿ 1998 American Geophysical Union