ON THE MECHANISMS CONTROLLING THE FORMATION AND PROPERTIES OF VOLATILE PARTICLES IN AIRCRAFT WAKES

New observations taken in aircraft wakes, including the DLR ATTAS, pro vide strong constraints on models of aircraft plume aerosols. Using a comprehensive microphysics code, we have performed sensitivity studies to identify the key microphysical mechanisms acting in such plumes. A nalysis of these simulations reveals that the largest volatile plume p articles-those most likely to contribute to the background abundance o f condensation nuclei-are dominated by ion-mode particles when chemiio ns are included. Moreover, such modeling demonstrates that standard tr eatments of plume microphysics-in the absence of Chemiions-fails to ex plain field measurements. The principal factor controlling the populat ion of ultrafine plume particles is the number of chemiions emitted by the aircraft engines. Since the ions are a byproduct of the combustio n itself, and their abundance in the exhaust stream is controlled by i on-ion recombination, the initial ion concentrations-and so the eventu al emission indices for ion-mode particles-are expected to be relative ly invariant. Our results indicate that reductions in fuel sulfur cont ent, while not likely to lower the total number of volatile particles emitted, would decrease the size of the ion-mode particles in fresh ai rcraft wakes, reducing their atmospheric lifetimes and potential envir onmental effects.