THE FORMATION AND EVOLUTION OF AEROSOLS IN STRATOSPHERIC AIRCRAFT PLUMES - NUMERICAL SIMULATIONS AND COMPARISONS WITH OBSERVATIONS

The formation and evolution of aerosols in jet aircraft plumes at high altitude are simulated using a detailed aerosol microphysics model th at explicitly resolves particle size and composition. Two approaches a re used to simulate the microphysics: a standard, or ''classical,'' ap proach in which new particle formation initially occurs via homogeneou s binary nucleation, followed by condensation and coagulation; and a ' 'kinetic'' approach in which the entire course of particle evolution i s calculated as a unified collisional mechanism. In the latter approac h chemiions generated in the engine combustors can affect molecular cl uster growth and ultrafine particle aggregation. Simulations with both approaches reveal that large numbers of volatile ultrafine sulfuric a cid particles are generated in the near field behind the engines. The concentrations and subsequent evolution of these ''volatile'' particle s are sensitive to the initial sulfuric acid vapor concentration, the plume dilution rate, and microphysical factors, especially the chemiio n abundance, which is brought out through a sensitivity analysis. By c ontrasting predictions against available field data, it is demonstrate d that the kinetic model provides a more realistic representation of p article formation and growth and hence their observable properties tha n the classical model. Moreover, the effects of chemiions during the e arly evolution of the plume are found to be crucial to the evolution o f the largest volatile aerosols, which may play a role as cloud conden sation nuclei. Major sources of uncertainty in the plume aerosol model s are noted.