Measurements of ultrafine particles in the near field of the DLR research aircraft ATTAS using low (0.02 g/kg fuel) and high (2.7 g/kg) fuel sulfur contents (FSCs) are presented. Soot emissions of ~1015/kg show no significant dependence on FSC. Strong evidence is found that ~1/3 of the soot particles must be involved in ice nucleation in contrails, in addition to freezing of newly formed volatile particles. In the absence of contrails, numbers of volatile particles with diameters D>5 nm reach ~1017/kg for high FSC, and still reach ~1016/kg for low FSC. A clear contribution of H2SO4 to volatile particle growth is observed. If growth is exclusively linked to H2SO4, the S to H2SO4 conversion efficiency increases with decreasing FSC. Depletion of ultrafine particles is observed in contrails, very likely due to scavenging by contrail ice crystals. ¿ 1998 American Geophysical Union