Airborne measurements of fine and ultrafine aircraft aerosol emissions are examined as a function of environmental conditions, fuel sulfur content, and aircraft type, to determine what factors influence the abundance of volatile particles and what the likely composition of these particles are. The data were obtained from aboard a small jet aircraft flying in near-field (<20 km aircraft separation) wakes generated by a variety of aircraft. All aircraft produced large numbers of particles relative to background concentrations, a majority of which evaporated at temperatures <200 ¿C. The volatile particles, except under humid conditions, were typically <20 nm in diameter, and their numbers varied both as a function of atmospheric conditions and plume age. Their apparent number-density dependence upon fuel sulfur content, and the fact that their inferred mass concentrations are less than the reported shortfall in gas phase sulfur budgets, strongly suggests they were primarily composed of condensed sulfur species. ¿ 1998 American Geophysical Union