Desert dust

Desert dust is a source of mineral aerosols and it is one of the largest source of aerosols in general. Mineral aerosol sources are mainly in deserts and dry lake beds, and sometimes in the literature the terms mineral aerosols and desert dust aerosols are equated. The other sources of mineral aerosols are semi-arid soils, regions with reduced vegetation or soils disturbed by human activities, pastures, traffic, construction.

Composition Desert dust aerosols are eroded soils and therefore, their com-position resembles to the comcom-position of the crust and crustal rock. They are composed of various oxides and carbonates, where silicon and aluminium oxides dominate. Studies show that in general about 60 % is silicon dioxide (SiO2), and about 10 %–15 % aluminium oxide (Al2O3). Other abundant oxides are iron(III) oxide (Fe2O3), magnesium oxide (MgO) and calcium oxide (CaO), but their percentages can vary depending on the source location (Goudie and Middleton,2001). The elemental composition of desert dust is quite constant around the globe (Gomes and Gillette, 1993), but still, in the particles we can find a variety of different types of minerals (Usher et al., 2003). Dust from desert is mainly from sand, and dust from other arid and semi-arid areas contain more silt and clay (Alfaro et al.,1998).

Formation Desert dust aerosols are, as already mentioned, mechanically disintegrated parts of soils. Its formation and emission highly depend on the wind, but also on the physical properties and conditions of soils. Particles are rarely injected into the air directly by the wind, but instead mainly by an intermediary process — saltation. When the wind achieves a high enough velocity, it starts to move particles. This wind velocity is called erosion threshold velocity, and it depends on the size of the particle which starts to move. It depends also on the soil properties: how rough it is, or how moist, or how strong are cohesive forces between particles in the soil, or is there any vegetation present. It the vegetation is present it is important to which degree is present and which type. When the erosion wind velocity is reached, particles start to move horizontally in the layer close to the surface – saltate. The biggest particles do not bounce because of their weight and gravitational drag, but just roll on the surface. Saltating particles when impacting the surface can break down, or break down the soil aggregates which they hit. The impact could provide enough of energy to break the cohesive forces which link the soil particles and release small particles of dust into the air (Marticorena et al., 1997; Laurent et al., 2006). Typically, particles 100 µm–500 µm can saltate and their size distribution resembles closely to the size distribution of the soil particles (Kok et al.,2012); particles that are sandblasted are generally 0.1 µm–10 µm in diameter (Alfaro et al.,1998).

Transport The particularity of desert dust aerosols is that they are often transported over very long distances, commonly over several thousands of kilometres (Prospero,1999), or even more (Uno et al.,2009). They often travel in high, horizontally layered plumes. The transatlantic path of the African dust from Sahara is a well studied topic (Prospero et al., 1981; Prospero, 1999; Perry et al.,1997), and it has also a biochemical significance, because

1.4. Aerosol types 27

Figure 1.6:Illustration of the process emission of desert dust particles. The biggest particles roll (creep) over the surface under the influence of the wind. Smaller particles rise and then fall (saltate) hitting the soil aggregates and eject fine particles in the air. Figure fromHatfield and Sauer(2011).

desert dust has a fertilizing role for oceanic and continental ecosystems by providing micro nutrients, like phosphorus (P) or iron (Fe). This impact seams to be especially important for the Amazon forest and the equatorial Atlantic Ocean (Bristow et al., 2010). Besides westward transatlantic path to North and South America, Saharan dust is also transported northward to western Mediterranean sea (Guieu et al.,2002) and Europe up to Scandinavia (Franzen et al.,1995), or eastward towards eastern Mediterranean (Levin et al.,1996) and Middle East (Alpert and Ganor,2001). Asian dust, whose the most importante source is the desert Gobi, usually takes eastern path and there are common examples of its detection on Hawai (Parrington et al., 1983) or continental United States (Duncan Fairlie et al.,2007). Also, the dust transport depends on the meteorological and climatic conditions. The transatlantic transport is increased during El Niño period (Prospero and Nees,1986), and the transport to Mediterranean is correlated with the phase of the North Atlantic Oscillation (Moulin et al.,1997).

Ageing The chemical reactivity of desert dust aerosols is very low, but they can provide a surface for chemical reactions. Interactions with reactive inorganic or organic chemical species or aerosols can change the chemical nature of the desert dust particles. They can change and alter their surface or make a coating on it which can enhance or prevent other reactions. For example, desert dusts often get covered with sulphates or other soluble materials. Levin et al. (1996) found that this process is frequent and is probably surface dependant, because the amount of coating is bigger on bigger particles.

Desert dust aerosols are insoluble and hydrophobic, but they are efficient ice nuclei (IN) particles (DeMott et al., 2003;Field et al.,2006). The ageing process, where they may interact with other aerosols and compounds and become coated with hydrophilic material, could turn them also in good cloud condensation nuclei (CCN) without limiting their efficiency as IN particles (Levin et al.,2005). There are also other interactions that desert dust aerosols have on clouds (for example Karydis et al.,2011).

Optical properties Desert dust aerosols are efficient in the scattering of the radiation. Also, although less efficient than black carbon, they absorb the radiation. But because of its high mass abundance (especially compared to BC), its global total absorption is significant (Sokolik and Toon,1996). Also, because of its abundance, it can largely influence the visibility.