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Introduction
A large portion of airborne particulate matter is derived from arid and barren regions of the
Earth and is distributed all over the globe. The most prominent example of this transport is
the export of desert aerosols from the Saharan area (cf. Figure on front page), which extend
over areas of several 100.000 km2. This dust may be regarded as being distributed over the
Northern Hemisphere. Since transport distances up to 10.000 km are observed, desert dust is
virtually omnipresent within the atmosphere.
There is a marked spatial and temporal variation of the source strength of airborne dust. The same is true for the whole range of transport mechanisms including
i) Those at the surface where differentiations on the microscale is necessary,
ii) The vertical motions lifting the material, and
iii) The large scale flow patterns.
Thus, dust transport has to be considered as a quasi-continuous phenomenon on all atmospheric scales
.
In summer, dust is exported from the African continent mainly towards the Southern North Atlantic. Evidently, this transport often reaches the Americas and high concentrations of dust of African origin are frequently observed there. In winter the dust plumes leave the African continent also mainly in Westerly directions but with a more pronounced Southerly flow. High concentrations of dust often occur in the Northern parts of the South American continent. Episodes of summer transports are observed several times per month, the strength of the outbreaks being variable. Significant episodes of transports in winter are less frequent.
In addition the Saharan desert dust is significantly but less frequently transported in other directions. In winter dust often reaches the Southern part of the Mediterranean: the direction of such plumes being mainly towards the East. Spring is the season when dust outbreaks occasionally are directed towards the North reaching Europe.
The only source of mineral dust in the atmosphere is the Earth’s surface. Prospero et al., (2002) find good agreement of satellite-observed dust emission maxima with the location of topographic depressions, where fine loose sediments can accumulate. Over the large flat surfaces of the North African continent the source strength is proportional to the wind force exerted on this surface. The airborne material takes its kinetic energy from the air motion, thus causing an additional dissipation of kinetic energy. This results in the proportionality of the sediment transport to the third power of the wind speed, more accurately to the friction velocity. Plant coverage, topographic features in general do act as complicating factors. From plausibility, it can be derived that the vertical motion of dust adds a further dimension to the above-mentioned third power. As a result, dust transport is in total proportional to about the fourth power of the wind speed, thus highly non-linear (Gillette and Passi, 1988; Shao et al., 1993).
Soils consist of a wide spectrum of granular material. Only a certain part of this material can become airborne. To enable bigger amounts of material to become airborne soil grains have to be moved by wind action to set free the grains lying underneath the immediate surface layer and which are appropriate to become airborne. Creeping and saltation are known to be responsible for the lifting process. To start the lifting from the surface a threshold force has to be exerted on the granular material at the surface. This threshold, usually coupled with a wind speed of at least 7 m s-1 at 10 meters height above the surface, depends on surface properties like roughness, grains size, and soil moisture. Rough surfaces containing structural elements like rocks, crusts, or vegetation increase the threshold velocity required for dust emission (Marticorena and Bergametti, 1995) since wind energy is partly absorbed by the obstacles.
Climatically, wind speeds exceeding the threshold only occur during a small part of the whole year. To produce high wind speeds, a meteorological process is required, i.e. at least on the convective, more plausible on the synoptic scale. Microscale eddies (dust devils) caused by strongly heated land surfaces play also an important role for the production of dust. This daily phenomenon contributes to the background desert dust level. As a result, dust is lifted from the surface, injected to greater heights and then transported horizontally over long distances. Combining the non-linear supply at the source and the structure of the atmospheric flow systems, the pulse-like appearance of the dust outbreaks becomes plausible.
Detailed observations of the temporal and spatial patterns of the dust load require continuous satellite data. Detailed description of these patterns need continuous model computations considering all relevant processes, including radiation effects of the dust load.



Figure 1
Vertical distribution of Lidar (Light detection and ranging) backscatter signals (532 nm wavelength) for a Latitude/Longitude cross-section over the Sahara during orbit 146 of the LITE (Lidar In-space Technology Experiment) mission at approx. 23 GMT on September 18, 1994. The color assigned to each pixel represents the intensity of the return signal. The color bar shows the color assigned to each range of measured signal returns in digitizer counts. Red and yellow indicate dust; white with black columns underneath indicates clouds.
The Sahara is the most important source of desert dust on a global scale (Washington et al., 2003). Huge dust outbreaks are frequently observed over the Sahara and the surrounding oceans (see cover image) causing considerable atmospheric turbidity. Lidar measurements (cf. Fig. 1) onboard of a spacecraft (Winker et al., 1996) have revealed that even under nondust storm conditions the lower troposphere over deserts can be regarded as a reservoir of atmospheric dust. Due to intensified land use, and changes in human practices in and around deserts a contribution of up to 10% of the total globally emitted annual amount of mineral dust has been estimated as anthropogenic (Tegen et al., 2003).
By its omnipresence, the desert aerosol, consisting of a mixture of mineral matter from soil erosion and significant contributions of sulfur, nitrogen compounds, soot from combustion sources, and particulate matter from the biosphere, is causing a significant impact on the atmospheric radiation field. Dust not only scatters but also absorbs solar radiation, and also absorbs and emits outgoing longwave radiation. The magnitude and even the sign of the direct radiative dust forcing is uncertain. It depends on the optical properties of dust, its vertical distribution, cloud cover, and the albedo of the underlying surface (Liao and Seinfeld, 1998). Strong columnar aerosol mass loadings, which are frequently observed over deserts and during long-range transport of dust outbreaks, cause warming of layers aloft and thus changes of the atmospheric (in)stability. To date no weather model includes any feedback between these strong diabatic heating rates and atmospheric dynamics. A magnitude of direct mineral dust forcing of about –0.5 Wm-2 to 0.5 Wm-2 is actually discussed and thus indicates global significance in comparing with greenhouse gases and other effects.
Dust influences the Earth’s system in several ways. The presence of dust may alter cloudoptical properties by changing the number of cloud condensation or ice nuclei (Sassen, 2002; Sassen et al., 2003). The efficiency of dust particles to form cloud drop nuclei may alter during transport due to mixing with soluble aerosol species (Levin et al., 1996). This can influence both the brightness of clouds and the formation of rain. Dust particles can change chemical reactions in the atmosphere because of their large surface area (Dentener et al., 1996). Micronutrients (e.g., Fe) deposited with dust particles impact on the marine (e.g., Hutchins and Brunland, 1998) and terrestrial (Chadwick et al., 1999; Garstang et al., 1998) ecosystems, thus influencing the carbon cycle and potentially changing atmospheric greenhouse gases. The impacts of these indirect dust effects are very uncertain.
Therefore IPCC (2001) pays special attention to the role of mineral dust with regard to the aerosol radiative forcing issue. Also the recent Strategic Plan for the US Climate Change Science Program emphasizes studies of mineral dust (http://www.climatescience.gov/Library/stratplan2003). Given the important role of dust within the climate system and the relatively limited knowledge about the magnitudes and even the direction of the radiative effects of dust aerosol more research on all aspects of the dust cycle is urgently needed.