The formation and evolution of aerosols in jet aircraft plumes at high altitude are simulated using a detailed aerosol microphysics model that explicitly resolves particle size and composition. Two approaches are used to simulate the microphysics: a standard, or classical, approach in which new particle formation initially occurs via homogeneous binary nucleation, followed by condensation and coagulation; and a kinetic approach in which the entire course of particle evolution is calculated as a unified collisional mechanism. In the latter approach chemiions generated in the engine combustors can affect molecular cluster growth and ultrafine particle aggregation. Simulations with both approaches reveal that large numbers of volatile ultrafine sulfuric acid particles are generated in the near field behind the engines. The concentrations and subsequent evolution of these volatile particles are sensitive to the initial sulfuric acid vapor concentration, the plume dilution rate, and microphysical factors, especially the chemiion abundance, which is brought out through a sensitivity analysis. By contrasting predictions against available field data, it is demonstrated that the kinetic model provides a more realistic representation of particle formation and growth and hence their observable properties than the classical model. Moreover, the effects of chemiions during the early evolution of the plume are found to be crucial to the evolution of the largest volatile aerosols, which may play a role as cloud condensation nuclei. Major sources of uncertainty in the plume aerosol models are noted. ¿ 1998 American Geophysical Union