Optical Properties of Saharan Dust and Asian Dust: Application to Radiative Transfer Simulations
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Because the bulk optical properties of dust are largely dependent on their chemical composition, published reports from numerous dust field studies enabled us to compile observation data sets to derive the effective complex refractive indices of Saharan and Asian dust. We considered the individual mineral components as aggregates and used the Bruggeman approximation to derive the effective refractive indices. Using the results, we calculated the single-scattering properties, including phase matrix, single-scattering albedo and asymmetry factor, with a combination of the T-matrix method and an improved geometric optics method (IGOM). The single-scattering properties were averaged by the measured particle size distribution to provide bulk optical properties for radiative transfer simulations. Using a Rapid Radiative Transfer Model (RRTM), the radiative forcing of mineral dust was computed at both the top of the atmosphere and the surface. By analyzing samples from various in-situ measurements, we assumed the Saharan and Asian dust to have average volume compositions and average aspect ratios. The effective refractive indices for Saharan and Asian dust were derived based on the assumed composition models. Bulk optical properties were integrated using the modified log-normal particle size distributions. The aspect ratio assumed in this study is 1.6 for both Saharan and Asian dust. The longwave radiative (IR) forcings at the top of the atmosphere (TOA) and at the surface were found to be positive and sensitive to wavelength. The shortwave (solar) radiative forcing at TOA, was also positive, but may possibly have been due to the strong absorption components considered in the composition models.