Absorption of Long-Wavelength Infrared

However, this is not the case for airborne particles composed of crustal materials formed by erosion processes. As discussed in Chapter 9.C, mineral dust consists primarily of such crustal materials. Despite the fact that soil dust particles tend to be quite large, of the order of a micron and larger, they can be carried large distances. These particles not only scatter and absorb solar radiation but also absorb long-wavelength infrared emitted by the earth s surface. 

FIGURE 14.35 (a) Real In) and (b) imaginary (k) parts of the index of refraction (rj = 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 el al., 1998). 

Based on their measurements of North African dust transported to the Barbados, Li et al. (1996) estimate that over a 10-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 

FIGURE 14.36 Predicted mean radiative forcing at the top of the atmosphere due to mineral dust from scattering, absorption, and their total (adapted from Tegen et al., 1996). 

In short, the combination of absorption and scattering of light by mineral dusts, combined with an increase in these due to anthropogenic activities, has the potential to contribute to climate change. However, many uncertainties need to be removed before these effects can be confidently quantified. For example, the infrared absorption depends on the composition of the dust and as seen in Fig. 14.35, this can be quite variable from location to location and even as a function of time from one source. This one effect alone can lead to a large variability in the predicted effects on radiative forcing (Sokolik et al., 1998). 

---