We report airborne measurements of total and nonvolatile (at T<290 ¿C) fine and ultrafine aerosol emission indices (EI's) generated by a variety of jet aircraft. The data were obtained using an instrumented jet aircraft, flown repeatedly through aircraft wakes. These aircraft were observed to produce 0.5-10¿1015 nonvolatile particles kg-1 of fuel burned. Their numbers varied as a function of aircraft type, age, and engine operating parameters, but less with atmospheric conditions. Large numbers of volatile aerosols were measured in all cases. Volatile EI's ranged from 0.1 to 40¿1016 kg-1 fuel burned. The observed soot emissions are estimated to have only a minor impact on atmospheric aerosol loading, but a future fleet producing such high concentrations of implied sulfate aerosols could perturb cloud formation and heterogeneous chemical processes in the upper troposphere/lower stratosphere. ¿ 1998 American Geophysical Union