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 D_eff in the dust plumes investigated showed two main ranges: the first range of D_eff peaked around 5 μm and the second range of D_eff around 8 µm. The two ranges of D_eff suggest that it may be inadequate to use one average effective diameter or one parameterisation for a typical dust size distribution.