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The Saharan Mineral Dust Experiment (SAMUM) was conducted in May/June 2006 in southern Morocco. As part of SAMUM, airborne in situ measurements of the particle size distribution in the diameter range 4 nm < D-p < 100 mu m were conducted. The aerosol mixing state was determined below D-p < 2.5 mu m. Furthermore, the vertical structure of the dust layers was investigated with a nadir-looking high spectral resolution lidar (HSRL). The desert dust aerosol exhibited two size regimes of different mixing states: below 0.5 mu m, the particles had a non-volatile core and a volatile coating; larger particles above 0.5 mu m consisted of non-volatile components and contained light absorbing material. In all cases, particles larger than 10 mu m were present, and in 80% of the measurements no particles larger than 40 mu m were present. The abundance of large particles showed almost no height dependence. The effective diameter D-eff in the dust plumes investigated showed two main ranges: the first range of D-eff peaked around 5 mu m and the second range of D-eff around 8 mu m. The two ranges of D-eff suggest that it may be inadequate to use one average effective diameter or one parametrization for a typical dust size distribution.