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 < Dp < 100 μm were conducted. The aerosol mixing state was determined below Dp < 2.5 μm. 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 μm, the particles had a non‐volatile core and a volatile coating; larger particles above 0.5 μm consisted of non‐volatile components and contained light absorbing material. In all cases, particles larger than 10 μm were present, and in 80% of the measurements no particles larger than 40 μm were present. The abundance of large particles showed almost no height dependence. The effective diameter Deff in the dust plumes investigated showed two main ranges: the first range of Deff peaked around 5 μm and the second range of Deff around 8 μm. The two ranges of Deff suggest that it may be inadequate to use one average effective diameter or one parametrization for a typical dust size distribution.

During the Saharan Mineral Dust Experiment (SAMUM) conducted in summer 2006 in southeast Morocco, the complex refractive index of desert dust was determined from airborne measurements of particle size distributions and aerosol absorption coefficients at three different wavelengths in the blue (467 nm), green (530 nm) and red (660 nm) spectral regions. The vertical structure of the dust layers was analysed by an airborne high spectral resolution lidar (HSRL). The origin of the investigated dust layers was estimated from trajectory analyses, combined with Meteosat 2nd Generation (MSG) scenes and wind field data analyses. The real part n of the dust refractive index was found almost constant with values between 1.55 and 1.56, independent of the wavelength. The values of the imaginary part k varied between the blue and red spectral regions by a factor of three to ten depending on the dust source region. Absolute values of k ranged from 3.1 × 10−3 to 5.2 × 10−3 at 450 nm and from 0.3 × 10−3 to 2.5 × 10−3 at 700 nm. Groupings of k values could be attributed to different source regions.

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Airborne measurements of pure Saharan dust extinction and backscatter coefficients, the corresponding lidar ratio and the aerosol optical thickness (AOT) have been performed during the Saharan Mineral Dust Experiment 2006, with a high spectral resolution lidar. Dust layers were found to range from ground up to 4-6 km above sea level (asl). Maximum AOT values at 532 nm, encountered within these layers during the DLR Falcon research flights were 0.50-0.55. A significant horizontal variability of the AOT south of the High Atlas mountain range was observed even in cases of a well-mixed dust layer. High vertical variations of the dust lidar ratio of 38-50 sr were observed in cases of stratified dust layers. The variability of the lidar ratio was attributed to dust advection from different source regions. The aerosol depolarization ratio was about 30% at 532 nm during all measurements and showed only marginal vertical variations. ?? 2008 The Author Journal compilation ?? 2008 Blackwell Munksgaard

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