Saharan desert

Figure 7.2 Solid-state speciation in a crustal aerosol collected close to the Saharan Desert and an urban aerosol collected in the UK (Spokes et al., 1994). Results are expressed as a percentage of the total elemental concentration and obtained using a three stage sequential leach using the method of Chester et al. (1989). Stage one uses 1.0 M ammonium acetate to release loosely bound metals. Stage two involves addition of 1.0M hydroxylamine hydrochloride in 25% acetic acid to the residue to release metals held within the oxide and carbonate phases. Stage three uses nitric acid and hydrofluoric acid to break down the aluminosilicate lattice and release metals associated with the crustal fraction of the aerosol.

Wliile most researchers in the field agree on the relative importance of biospheric feedbacks operating at high northern latitudes, the discussion becomes more interesting and diverse as the subtropics are concerned. Climate reconstructions and data on fossil pollen compiled by Jolly et al. (1998), Hoelzmann et al. (1998), Pe-tit-Maire (1996), and Anhuf et al. (1999) indicate that North Africa was much greener in the mid-Holocene than today. The Saharan desert was, presumably to a large extent, covered by annual grasses and low shrubs. The Sahel reached at least as far north as 23 °N, more so in the western than in the eastern part. 

Schaeffer OA, Nagel K, Fechtig H, Neukum G (1981) Space erosion of meteorites and the secular variation of cosmic rays (over 10 years). Planet Space Sci 29 1109-1118 Scherer P, Schultz L (2000) Noble gas record, collisional history, and pairing of CV, CO, CK, and other carbonaceous chondrites. Meteoritics Planet Sci 35 145-153 Scherer P, Zipfel J, Schultz L (1998) Noble gases in two new ureihtes from the Saharan desert. Lunar Planet Sci XXIX, Lunar Planet Institute, Houston, TX Abstr 1383 (CD ROM)... 

As shown in Fig. 2, the reported averages range between 93 and 9.7 pg/m for PM2.5 and 202.6 and 36.4 pg/m for PMio with the highest values of PMio and PM2.5 being measured in a residential site in Egypt. In fact, PMio levels in Egypt exceeded in residential sites by 63% the maximum level that was measured in other Mediterranean countries. This could be attributed to the high population (7.5 million in Cairo city), its proximity to the Arabian and Saharan deserts and... 

Dust storms originating from the Saharan desert occur in autumn, winter and spring lead to the increase in coarse particles. In the summer, levels of fine particles are enhanced as a result of increased humidity, photochemically and induced secondary PM, and long-range transport of sulfur. 

The continental margin off the coast of Northwest Africa is an area of upweUing and marine productivity with special characteristics (a) a wide shelf area, (b) a wide zone of upweUing, and (c) proximity to a large dust source— the Saharan desert. The Canary Islands divide a northern area of primary productivity and accumulation of organic matter (OM) and a southern area. Accumulation of marine OM is supplemented by input of Saharan dust containing adsorbed terrigenous OM. 

Slurry is essentially a mixture of solids and liquids. Its physical characteristics are dependent on many factors such as size and distribution of particles, concentration of solids in the liquid phase, size of the conduit, level of turbulence, temperature, and absolute (or dynamic) viscosity of the carrier. Nature offers examples of slurry flows such as seasonal floods that carry sUt and gravel. Every year during the flood season, the Nile transports massive amounts of silt over thousands of miles to the Saharan desert. To rephrase Herodotus, who once said Egypt is the gift of the NUe, one may consider that one of the most ancient civilizations was dependent on natural slurry flows for its survival. 

For thousands of years the Pyramids and the Sphinx have stared at this immense natural slurry clarifier that is the Valley of the Nile in the middle of the Saharan Desert (Figure 1-... 

Cold periods in the north meant drought to Saharan Africa and India. About 5,000 years ago a sudden drying spell changed the Sahara from a green region spotted with lakes to a hot sandy desert. 

As shown in Figure 14.8, kaolinite concentrations are highest in tropical and equatorial latitudes, particiflarly off the western coasts of North Africa and Australia (>40%) and the northeastern coasts of Australia and South America (30%). The first two are the result of aeolian transport by the Trade Winds from the Saharan and Australian deserts, respectively. The other two are the result of river input from the eastern Australian continent and the Amazon River. 

Prospero JM, Saharan dust transport over the North Atlantic Ocean and Mediterranean An overview, in Guerzoni S, Chester R. (eds.). The Impact of Desert Dust Across the Mediterranean, Kluwer Academic Publishers, Amsterdam, pp. 133— 151, 1996. 

In the case of Algeria, the distribution of the solar resource can be subdivided into three zones coast al (humid climate), atlas (cold in winter and hot in summer), and Saharan (dry desert climate) with typical average values of annual daily horizontal irradiation 4 kWh/m2, 5 kWh/m2, and 6 kWh/m2, respectively. Unfortunately the solar potential is still not accurately known because of the lack of data recordings.1 Thus, to overcome such problem one needs to search for other data sources. Among the existing ones,2 3 4 5 we have chosen the NASA website which proposes a ten-year solar radiation database that covers globally the whole Algerian territory. 

Guieu, C. and Thomas, A.J. (1996) Saharan aerosols from the soil to the ocean. In The Impact of Desert Dust Across the Mediterranean (eds Guerzoni, S. and Chester, R.). Kluwer, Dordrecht. 

The SEA number density field (Cn SEA) is also rather variable. From data of complex observational programmes GAAREX and CAENEX [6, 9] Cn sea in the air surface layer of the Karakum desert (for particles with r > 0.2 Pm) constitutes 8-270 cm" . In the air surface layer (0.5-2 m) of north-eastern Kazakhstan (for particles with r > 0.2 pm) Cn averages 50-80 cm" [11]. During the Saharan aerosol outbreak across the Atlantic the values of Cn near the sea surface reached 6-40 cm" (for particles with 0.3 pm < r < 10 pm). 

The atmosphere, in turn, affects soil development by providing oxygen and by wind erosion and deposition. Sand dunes are only the most obvious example. Loess soils are deposits of silt-sized particles carried by winds from riverbeds and glacial outwash. A large fraction of the clay content of the soils along the eastern shores of the Mediterranean Sea has been carried by winds several thousand kilometers from the Sahara Desert and Atlas Mountains of North Africa. Trade winds carry Saharan clay particles several thousand kilometers out into the Atlantic Ocean. 

Merchel S, Altmaier M, Faestermann T, Herpers U, Knie K, Korschinek G, Kubik PW, Neumann S, Michel R, Suter M (1999) Saharan meteorites with short or complex exposure histories. In Workshop on extraterrestrial materials from cold and hot deserts. Lunar Planet. Institute, Houston, Contrib No. 

Rahn KA, Borys RD, Shaw GE, Schiitz L and Jaenicke R (1979) Long range impact of desert aerosol and atmospheric chemistry two examples. In Saharan Dust. SCOPE 14, Wdey, New York. 

Individual dust storms from the Sahara desert have been shown to transfer material from the northwest coast of Africa, across the Atlantic, to the east coast of the United States (Ott et al. 1991). For example, Prospero et al. (1987) suggested that enough Saharan dust is carried into the Miami area to significantly reduce visibility during the summer months. Similar dust transport occurs from the deserts of Asia across the Pacific Ocean... 

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