Airborne measurements of dust layer properties, particle size distribution and mixing state of Saharan dust during SAMUM 2006

Abstract

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 - 100 μm were conducted. The aerosol mixing state was determined below 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 dependency. 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 parameterisation for a typical dust size distribution