Size analyses of Ames wire impactor aircraft exhaust aerosol samples resulted in a continuous particle size distribution between 0.01 &mgr;m and 1.0 &mgr;m particle radius. The two orders of magnitude size range covered by the measurements correspond to 6--7 orders of magnitude particle concentration. The data determined a nucleation mode, composed of aircraft-emitted sulfuric acid aerosol, that contributed between 62% and 85% to the total aerosol surface area and between 31% and 34% to its volume. Soot aerosol contributed 0.3% to the surface area and 0.07% to the volume of sulfuric acid aerosol. Emission indices were: EIH2SO4=0.09 g/kgFUEL and (0.2--0.5) g/kgFUEL (for 75 ppmm and 675 ppmm fuel-S, respectively), 1.7E-40.01=1.7E14 and (7.7E14 and 5.2E15) particles/kgFUEL (for 75 and 675 ppmm fuel-S, respectively). The sulfur (gas) to sulfuric acid (particle) conversion efficiency was between 10% and 37%. ¿ 1998 American Geophysical Union