The direct radiative effect of mineral aerosols, i.e. absorption or diffusion of solar (UV-visible) or telluric (IR) radiation, depends on their optical properties, themselves related to their composition, size and shape.
Mineral aerosols are composed of particles covering a very wide range of size (from 0.1 to 10 µm). Each of these "particles" often consists of an assembly of minerals of nature, and therefore of very different optical properties. Finally, these assemblies have a variety of forms, very rarely spherical. In addition, these various characteristics of the particles are likely to evolve in the transport of aerosols in the atmosphere.
The strategy adopted to ensure the consistency of the measured properties is to conduct experiments of "optical closures." These consist in documenting, from the experimental point of view, the physico-chemical and optical properties, then in trying in reproducing the measured optical from the measured physico-chemical properties by calculation using an appropriated 0-D optical model and this in the most varied situations possible. In practice, these closure studies are conducted during field campaigns in source and short- and long-range transport areas, where desert aerosols are in mixture with other types of aerosols. Modeling of optical properties developed on the basis of these experimental data should allow proposing an operational parameterization to be used in 3D models of the cycle of desert aerosols.
Figure: View from an electronic microscope of mineral particles constituting the desert aerosol. These particles have a non-spherical shape (a, d) and very often consist of fine aggregate (b) or coarse (c) or quartz grains covered in clay platelets (d).