Dust and Haze

Mobilization: Strong sand storms often occur in the Sahara. If they occur in certain, so-called source regions, aerosol particles can be suspended by "soil creep" and "saltation" into the atmospheric boundary later located directly above the ground. In the case of "soil creep" large grains of sand are blown by the wind over the desert ground. When the wind is strong enough single grains can be raised in the air and move a bit further. These large and heavy grains fall back to the ground as if they just have jumped. Thereby they destroy agglomerates of other grains and fine particles on the ground. During a strong sand storm there is often a huge layer of several meters high consisting of turbulent swirling and saltating grains of sand, so that the process of soil creep and saltation finally acts like a sandblast with immense erosion power. The swirling grains of sand are too large to be transported far away or up to high levels. But because agglomerates are destroyed by the "sandblast" aerosol particles are formed (i.e. "mobilized") which have a size smaller than few micrometers down to the submicron range. Their sedimentation velocities are small enough that the particles are suitable for the so-called "long range transport".

Long range transport: The small and light particles of the mobilized desert dust aerosol are carried within the atmospheric boundary layer up to altitudes of more than one kilometre by the aid of turbulences. Then they may be transported many thousands kilometres by the steadily blowing winds. Often the small particles are blown over the Atlantic Ocean to the Caribbean Islands where they can be detected the aerosol.

Europe.jpg shows a long range transport event that occurred in 2001. Here the aerosol was finally detected in Northern Europe. The haziness of the atmosphere above France as well as in its north-eastern direction and the English Channel can be clearly noticed. The aerosol particles often pass also the Alps. There the airborne particles can be deposited as yellow, fine, dry precipitation. Or the particles can be washed out from the atmosphere by rain. In 2003 aerosol particles from the Chinese Taklamantan desert was finally found in the snow of the Alps. This event includes a transport range of 20 000 km passing the Pacific Ocean, North America and the Atlantic Ocean within approximately fourteen days.

SaharaDust1+2.jpg:
These images show a significant layer of Saharan Dust during sunset above the coast of Lanzarote.

DustStorm3+4.jpg were taken from the American Seawifs satellite. The aerosol particles derived from a sand storm in the Sahara and were lifted by the warm thermal updraft up to altitudes of 4500 meters. Within a few days the particle cloud extended over the Canaries to the Azores.

Mars.jpg: In the martian polar regions dust storms frequently occur during the spring season when frozen carbon dioxide and water ice evaporate from the polar caps. On this picture, taken by the NASA Mars Global Surveyor satellite the polar cap of the northern hemisphere of Mars is shown. The evaporation generates a temperature gradient in the interface between polar cap and ice-free ground as a consequence of sublimation energy being consumed in this process. Such temperature gradients cause often strong winds which mobilize dust in the same way ("soil creep" and "saltation") as on earth. The upper image showing a dust storm in the neighbourhood of the North Pole on Mars was taken at the 29th of August from the Mars Global Surveyor. The dynamics of the extension is comparable to the ones desert dust has on earth. The dust particles in the Mars atmosphere form also the background aerosol which leads to Cirrus clouds via heterogeneous nucleation. The pictures are presented in Special Cirrus Clouds.

Effects: All in all deserts globally produce around 2 billion tons of dust. Approximately 80 % derive from the deserts Gobi, Sahara and Taklamatan. The desert dust plumes absorb shortwave radiation of the sun and - depending on their optical thickness - can lead to heating inside the dust layer. On the other hand they also reflect sun-light into space. Due to the loss by absorption and reflection the earth surface beneath the dust layer gets less sun-light. Hence there might be minor, locally and temporally limited cooling.

Recently it is assumed that so-called heterogeneous chemical reactions take place at the surface of the dust particles which destroy ozone. Furthermore it is known that the deposited desert dust has a fertilizing effect on the environment. For example minerals - mostly phosphates and iron - are imported that way. Sahara dust events over the Eastern Atlantic Ocean often coincide with plankton blooming in this area. It is assumed that the constituents of the dust particles might have a positive influence on the African rainforests, too. This might be also the case for the plants living in the Amazon basin which can extract nutrients from the deposited dust. However, strong dust precipitation can pollute corals of the Caribbean and inhibit their growth. Plankton blooms deriving from Sahara dust fertilization can also lead to algae growth which again inhibits the growth of the corals. The iron-containing soils on some of the West Indies have formed as a consequence of long-time accumulation of Saharan Dust. All in all the airborne desert dust is related to a lot of physical, chemical and biological mechanisms.




Sahara1+2.jpg: S. Borrmann, flight from Lanzarote to Frankfurt, 7 October 2003, around 7:00 Uhr p.m.

Duststorm3+4.jpg: Saharan Dust Eruption over Northwest Africa, 28 and 26 February 2000; Seawifs-Project at the Seastar Satellite of NASA, Copyright: NASA Goddard Space Flight Centre and ORBIMAGE

Europe.jpg: Saharan Dust long distance transport to Europe, 30 October 2001, Seawifs-Project at the Seastar Satellite of the NASA, Copyright: NASA Goddard Space Flight Centre and ORBIMAGE.

Mars.jpg: Northern polar region of Mars at 29 October 2002, Global-Surveyor Satellite, Copyright: NASA