Apart from its gaseous components, the atmosphere contains particles, aerosols, which originate from various sources, both natural or associated with human activity.
Active volcanoes eject fine particles of rocks into the atmosphere. In arid and semi-arid regions, the wind erodes soils and every year in this manner the quantity of particles reaching the atmosphere is estimated at a billion tons; about 20% of these particles are transported over long distances. This is how sediments of the North Atlantic Ocean have, to a large extent, been formed from dusts from the Sahara. Moreover, oceans release a very large amount of particles into the atmosphere: when bubbles burst on the surface of the sea, fine droplets are carried by the wind and after the water in them has evaporated, the salts crystallise and form aerosols (Schlesinger, 1991).