The perturbation of the sulfate surface area density in the tropopause region and the lower stratosphere by subsonic and supersonic aircraft fleets is examined. The background aerosol surface area, the conversion of fuel sulfur into new sulfate particles in aircraft plumes, and the plume mixing with ambient air control this perturbation. Results from an analytic expression for the surface area changes are presented which contains the dependences on these parameters and can be employed in large-scale atmospheric models. ¿ 1998 American Geophysical Union