How dust enters the atmosphere
Dust uptake into the atmosphere; Wikimedia Commons
Mineral dust particles are transported into the atmosphere due to dust and sand storms in arid regions. They arise when the wind exceeds a value that allows dust particles to be removed from the surface. This minimum wind speed necessary to mobilize particles is called the “threshold velocity”. Dust events are often triggered by thunderstorm outflows passing topographic depressions fulfilled by dust accumulated over long time (e.g. the Bodélé Depression).
The process of the uptake of sand and dust particles from the soil depends on the near-surface dynamics . It is controlled by the wind intensity, the soil wetness, the soil texture and the land cover. It has the lowest value for disturbed soils, followed by sand dunes, alluvial and aeolian deposits, disturbed playa soils, skirts of playas, playa centers and highest for desert pavements . It also increases with the size of the particles, because they are heavier and therefore more difficult to lift up due to gravity. If the particles are pretty small the threshold wind value is high, since it has to break rather strong cohesion force keeping small particles together.
Within the uptake process there are three modes of aeolian particle motion identified , which are also shown in the diagram below:
Processes of particle motion;
Saltation is the dominant mechanism for the uptake of dust particles by wind from the surface . Further, it leads to the so-called process of saltation bombardment or sandblasting to produce fine dust aerosols [3, 4, 5].
After emission, dust particles are carried up from the source by turbulent diffusion and vertical advection. They are also transported by horizontal advection. They can be transported further away from the source region in the free atmosphere under the influence of strong winds. Through the process of dust dispersion, which is driven by synoptic dynamics, trans-continental scales can be easily achieved.
The dust particles are removed out of the atmosphere by sedimentation (gravitational settling) and dry and wet deposition . The lifetime of the particles/the removal processes is dependent on the particle size . Their original size ranges from sub-microns to scales of about 40 µm . Particles with a diameter smaller than 1 µm have a life time around 2-3 weeks. In contrast, larger particles like sand and large silt have a live time of several hours .
Atmospheric dust process;
Dust sources are associated with arid regions (mostly topographically low) characterized by little rainfall (annual rainfall under 200–250 mm) . Moreover, these areas are often affected by human impacts (e.g. agricultural activity, etc.) . Another important factor controlling dust emission at the source areas is the vegetation cycle. The dust storm frequency is highest in desert/bare ground . It is inversely correlated with the leaf area index, which describes the density of the vegetation. The emissions of dust are correlated with the vegetation types of the areas considered. The diagram below shows six landcover types from which dust potentially could be released to the atmosphere.
Distribution of USGS land covers that are taken as first guess mask for potentially dust productive areas; data of the U.S. Geological Survey
The largest dust source region in the world is North Africa (heat low regions of the Sahara) . The most intense source region in North Africa is the Bodélé Depression (northern Chad). It may be responsible for up to 18% of global dust emissions . Other main source regions are the Middle East, Central and South Asia. The emitted dust from these regions is also most persistent . The dust emission at the Southern Hemisphere is smaller, but still important for countries such as Australia and Southern Africa. The origin (hot spots) of the dust sources is shown in the following diagram:
Hot spots of dust sources; according to 
Physical and chemical features of dust source soils
The emission of mineral particles underlies a large daily variability controlled by the meteorology [14, 15, 11]. The dust mobilization is more active during daytime than nighttime . This can potentially be explained by a peak of dust burden and the dry deposition in the late afternoon. In addition, a clear seasonal cycle exists with its maximum during the dry season , especially in the Sahara desert and central Asia . The emission varies in time and space depending on atmospheric and surface conditions .
However, many sources are point sources which result in wide spread dust weather , therefore it is quite difficult to describe the emission process in atmospheric dust models.
Mineral dust aerosols have complex nonspherical shapes and varying composition . They are generally a highly heterogeneous mixture . The composition of mineral particles is dependent on the source region . Whereas dusts in the Atlantic northeast trades resulting from dust sources in Africa are dominated by kaolinite, are dusts in the northeast monsoons of the northern Indian Ocean resulting from the Rajasthan desert dominated by illite . Furthermore, quartz is occurring in larger amounts in the dusts of the Northern Hemisphere (7%) than in the Southern Hemisphere (3%) over the Atlantic and Indian Oceans . The composition of dust has, among other things, influences on the biosphere, the ocean biogeochemistry and the atmospheric radiation.
Mineralogy of dust particles
Dominant minerals in arid soils are illite, kaolinite, smectite, calcite, quartz, feldspar, hematite and gypsum. Phyllosilicates (illite, kaolinite and smectite) makes up the largest chemical weathering minerals in sedimentary rocks . The crust of the Earth constitutes approximately 90 percent of phyllosilicates and Tectosilicates (quartz, feldspar; three dimensional framework of silicate).
Recently developed mineral global data base  contains distribution of major minerals in dust productive soils. Two exemplary maps are shown below. Such data on mineral fraction could be useful input in dust models in studying impact of mineral dust composition on cloud formation, radiation feedback, ocean bioproductivity and health.
Global distribution of ilite (1 km resolution);
Global distribution of phosphorus (1 km resolution);
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Publication list in alphabetical order (including publications from above):
bibliography-sources [pdf, 95 kB]
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