In the framework of the WATERMED (WATer use efficiency in natural vegetation and agricultural areas by Remote sensing in the MEDiterranean basin) project, an experimental field campaign was carried out in a semi‐arid region near Marrakech, Morocco, during March 2003. This work focused on the columnar aerosol characterization from spectroradiometric and photometric measurements of direct solar irradiance and sky radiance at ground level. The results show a high dependence of the aerosol optical properties on the air masses present in the area. Two periods with different aerosol loads were observed in the campaign. In the first, Atlantic‐origin aerosols showed a marine particle accumulation mode combined with a dominant mode corresponding to coarse particles of local origin. In the second period the synoptic situation changed towards a Saharan+Atlantic origin, the Atlantic influence persisting at altitude while at lower levels the presence of air masses proceeding from the Saharan desert was observed. This dry air carried a large quantity of mineral particles that increased the area's turbidity, leading to a substantial increase in the volume of the whole distribution, a displacement of the accumulation mode radius towards higher values and an increasingly evident spread of the coarse particle mode, although the radius remained the same as that in the first period of the campaign.