The surface aerosol in the NE trade wind region at the island of Sal, Cape Verde Islands, was studied in a comprehensive research program. The knowledge of this aerosol is important in view of the large source represented by the Sahara for mineral particles and the worldwide transport over the Atlantic Ocean and further. Aerosol properties such as total size distribution, mass, sea salt, mineral, organic compound content water adsorption, optical properties, and soil material were studied in a 3-week field campaign in summer 1973. The aerosol observed has a particle concentration comparable to that of 'clean air' and a total mass like that of 'polluted air' (see the definition by Study of Man's Impact on Climate (1971)). Its sea salt content is comparatively small, and its water uptake is somewhat similar to that of pure ammonium sulfate. The content of organic material is similar to that of background aerosols, but the turbidity resembles that of polluted city air. The source strength of the Sahara desert for aerosol particles was estimated from 2 years of optical measurements and found to be greater than was previously estimated.