Airborne dust collections down the Atlantic

The airborne dust concentration in the Atlantic varies from 102μg/m3 of air near the Sahara coast to as little as 10−5μg/m3 over the Antarctic ‐ for particles > 0.5μm diameter. Dust collector meshes have been exposed on the bows of a ship (February‐March 1971) during the outward and return voyage between the Canary Isles and the River Plate. There is an order of magnitude change in concentration on crossing the Equator around 30°W. The dusts south of the Equator are borne on the south‐east trades and probably come from the Namib desert; their particle sizes are smaller than those coming from the nearer Sahara desert. The colour of the dusts on either side of the Equator are grey due to an admixture of black humus; off the Sahara coast they are brown and clean‐looking. The quartz to clay ratio increases with nearness to land but the kaolinite to illite ratio is symmetrical about the Equator, the savannah lands being rich in kaolin. Daily satellite photographs clearly show veils of dust crossing the coast at Dakar and some attempt has been made to relate these to the ship‐borne collections.