We consider the effects on aircraft plume microphysics of ions generated by chemiionization processes within the engine combustors. Ions provide centers around which molecular clusters rapidly coalesce, thus promoting the formation of electrically charged sulfuric acid/water aerosols. The resulting charged micro-particles exhibit enhanced growth due to condensation and coagulation aided by electrostatic effects. Simulations with a plume microphysics code show that volatile particles observed behind aircraft may be explained by such processes, as long as initial ion concentrations in the exhaust exceed ~108/cm3. This analysis also suggests that the primary emissions of sulfuric acid (plus sulfur trioxide) should amount to at least 20--30% of the fuel sulfur to explain the observed number of volatile particles >9 nm in diameter. Ionized plume simulations reveal a distinct bimodal aerosol distribution, in which an ion mode constitutes the larger activated volatile sulfuric acid particles, while a smaller neutral mode comprises the residual slowly-growing neutral molecular clusters formed in the highly supersaturated region of the plume. ¿ 1997 American Geophysical Union