Individual-particle analysis of airborne dust samples collected over Morocco in 2006 during SAMUM 1

During the course of SAMUM 1 in May and June 2006, airborne samples were collected in southern Morocco at altitudes between 830 and 3340 m above ground. We analysed approximately 22 500 particles of 1-30 mu m by automated individual-particle analysis with a scanning electron microscope (SEM) and an energy-dispersive X-ray (EDX) analyser. The major difference between samples is due to the presence and amount of chlorine-and sulphur-bearing particles, pointing to a varying maritime influence and to different degrees of aging. In contrast, the desert dust component is very homogeneous independent of altitude, activated local source area, and dust storm intensity, implying very fast mixing of the entrained dust with a persistent regional background aerosol. Nevertheless, our study reinforces the usefulness of carbonates and palygorskite as 'compositional fingerprints' for mineral dust from source areas in northwestern Africa. The average median aspect ratio of most particle groups is in the range between 1.5 and 1.6. Higher values are found for S-and Cl-dominated particles (except sodium chloride), and internal mixtures of alumosilicates with carbonates or sulphates. Taken together with other studies, a tendency of increasing aspect ratios with longer transport distances can be observed for Saharan dust.