FIGURE 14.35 (a) Real (n) and (b) imaginary (k) parts of the index of refraction (t) = n — ki) of some atmospheric dust samples from the Sahara collected in Barbados, Afghanistan, and Whitehill, Texas. Regions of strong absorption of some known common dust components are also shown (adapted from Sokolik et al., 1998).
stretch in quartz near 9 /¿m, and absorptions due to kaolinite [Al2Si205(0H)4] in the 8.5- to 12-/j,m region (Salisbury et al., 1992). Many of these crustal materials also absorb in the 15- to 25-/iim region; the major infrared absorption of hematite (a-Fe203) is also in this region.
This absorption also leads to positive radiative forcing. Figure 14.36, for example, shows a model estimate of the contribution to radiative forcing by dust particles due to scattering of solar radiation and absorption of infrared radiation (Tegen et al., 1996). As expected, the effects are calculated to be the largest in areas having the highest dust, around the Arabian Sea and over the Atlantic Ocean off the coast of Africa. Figure 14.36 shows that the calculated effect of infrared absorption by mineral dust particles can be equal to or greater than that of scattering of solar radiation. Alpert et al. (1998) have proposed that the previously unrecognized contribution of dust particles to heating of the atmosphere was responsible for inaccuracies in weather prediction models over the eastern tropical North Atlantic Ocean.
While dust particles are a natural component of the atmosphere, the amount of airborne dust is believed to have increased due to anthropogenic surface land modifications such as deforestation, cultivation, and shifts in vegetation (e.g., see Tegen and Fung, f994, 1995; Tegen and Lacis, 1996; Li et al., 1996; Tegen et al., 1996; and Tegen et al., 1997). These activities may be responsible for as much as half of the total airborne dust (Tegen and Fung, 1995).
Based on their measurements of North African dust transported to the Barbados, Li et al. (1996) estimate that over a fO-year period, dust contributed about 56% of the total light scattering. Similarly, Tegen et al. (1997) estimate using a global transport model that scattering and absorption of light by submicron soil
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