THE ROLE OF IONS IN THE FORMATION AND EVOLUTION OF PARTICLES IN AIRCRAFT PLUMES

We consider the effects on aircraft plume microphysics of ions generat ed by chemiionization processes within the engine combustors. Ions pro vide centers around which molecular clusters rapidly coalesce, thus pr omoting the formation of electrically charged sulfuric acid/water aero sols. The resulting charged micro-particles exhibit enhanced growth du e to condensation and coagulation aided by electrostatic effects. Simu lations with a plume microphysics code show that volatile particles ob served behind aircraft may be explained by such processes, as long as initial ion concentrations in the exhaust exceed similar to 10(8)/cm(3 ). This analysis also suggests that the primary emissions of sulfuric acid (plus sulfur trioxide) should amount to at least 20-30% of the fu el sulfur to explain the observed number of volatile particles >9 nm i n diameter. Ionized plume simulations reveal a distinct bimodal aeroso l distribution, in which an ''ion'' mode constitutes the larger ''acti vated'' 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.