Fast-response measurements of particle size distributions were made for the first time in the near-field plume of a Boeing 737--300 aircraft burning fuel with fuel sulfur (S) contents (FSCs) of 56 and 2.6 ppmm, as well as in fresh and dissipating contrails from the same aircraft, using nine particle counters operating in parallel. Nonsoot particles were present in high concentrations, with number maxima at diameters ≤3 nm. From these and ancillary measurements we determined the apparent emission index, EI*, or amount produced per kilogram of fuel burned, for particle number, surface, and volume, and the value of &eegr;*, the apparent fraction of fuel S found in the particulate phase in the plume assuming the particles were composed of sulfuric acid and water. All of these parameters were functions of the age of the plume since emission, FSC, and presence or absence of contrail. The measurements support the use of values of &eegr;* of <10% in numerical models of the effects of the current aircraft fleet on the atmosphere, suggest that non-S species become important contributors to particulate mass at very low FSCs, and place significant constraints on numerical models of plume microphysical processes. ¿ 2000 American Geophysical Union