An analysis of in situ measurements of ultrafine volatile aerosols in the plume near field of the DLR aircraft ATTAS using low (0.02 g/kg) and high (2.7 g/kg) fuel sulfur contents (FSCs) is presented. The observed growth of nanoparticles (diameter 5--10 nm) is reproduced in detail by a microphysical simulation with chemi-ion emissions of 2.6¿1017/kg fuel. Volatile aerosol dynamics is controlled by sulfuric acid (H2SO4) for high FSC, consistent with a S to H2SO4 conversion of 1.8%. The very high conversion for low FSC (55%) prescribed in the model to match the observations contradicts direct in situ measurements of H2SO4 and suggests that species other than H2SO4, likely exhaust hydrocarbons, control particle growth in such cases. ¿ 1998 American Geophysical Union