Optical response of pure materials composing mineral dust in Los Andes

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The optical response of pure materials (quartz, halite, pyrite, and pyrrhotite) typically composing mineral dust in central Los Andes was studied through their controlled deposition on a reflective surface and the measurement of the solar irradiance and the reflected radiance with a hyperspectral radiometric system. Results show that the refractive index, derived from the crystalline structure of the material, has a primary effect on its optical properties, while the size distribution of the particles has a secondary effect, which in some cases is counteracted by the type of crystallinity. These results are useful for understanding the effect of mineral dust on radiative forcing in sensitive areas, particularly on highly reflective surfaces such as snow or ice. Iron-rich minerals, like pyrite and pyrrhotite, contribute significantly to local warming due to their significant ability to reduce albedo, whereas quartz and halite show much lower radiation absorption. The experimental results were compared to results from a radiative transfer model based on the Mie theory, i.e., assuming spherical size- and composition-homogeneous particles. Although these assumptions are far from realistic, especially for anisotropic materials, modeled results show similar decreasing trends for spectral albedo with increasing concentrations of particles on the surface and were useful to suspect inaccurate refractive index spectral data (particularly the imaginary part) for some materials, such as the type of quartz used here. In the case of halite, whose imaginary part of the refractive index is nearly zero, the albedo was not substantially affected by the particle concentration, neither in experimental nor modelled results. On the contrary, the model was unable to predict the non-linear albedo reduction for pyrrhotite dust, which can be explained by the heterogeneities in the content of sulfur and impurities, therefore contributing to an increase in the light scattering, and thus in the albedo.