Size, morphology and microstructure of Palas soot, Diesel soot and of Diesel soot/ammonium sulfate mixtures were studied by transmission electron microscopy (TEM). The diameter of the primary particles derived from TEM is 6.6 +/- 1.7 nm for Palas soot and 22.6 +/- 6.0 nm for Diesel soot. Palas soot predominantly consists of amorphous carbon. In a few cases, nanocrystalline graphite with domain sizes on the order of I nm were observed. Primary particles of Diesel soot always show an onion-shell structure of nanocrystalline graphite with domain sizes between 2-3 nm. Fractal properties of 37 Diesel soot agglomerates were determined from TEM images by two different techniques. The average fractal dimension of Diesel soot derived from TEM is 1.70 +/- 0.13. TEM further showed that the initially external mixture of Diesel soot and ammonium sulfate developed with time in a significant degree of internal mixing. A second independent approach to determine the fractal properties of soot is based on computer simulations of the aerosol dynamics. A good reproduction of the time evolution of mass and number concentrations and of the mobility size distribution was achieved. The primary particle diameters obtained from the computer simulations (7.3 +/- 0.8 nm for Palas soot, 25 3 nm for Diesel soot) are in excellent agreement with the TEM results. The fractal dimension of Diesel soot received from the COSIMA algorithm of 1.9 +/- 0.2 (overlap of primary particles was taken into consideration) is consistent with the value obtained from TEM image analysis. For Palas soot, the computer simulation yielded a fractal dimension of 2.0 +/- 0.1 (overlap was not corrected, as the overlap coefficient is not known). (C) 2003 Elsevier Ltd. All rights reserved.