A sensitivity study on the effects of particle chemistry, asphericity and size on the mass extinction efficiency of mineral dust in the terrestrial atmosphere: from the near to thermal IR

Abstract

Abstract. To determine a plausible range of mass extinction efficiencies (MEE) of terrestrial atmospheric dust from the near to thermal IR, sensitivity analyses are performed over an extended range of dust microphysical and chemistry perturbations. The IR values are subsequently compared to those in the near-IR, to evaluate spectral relationships in their optical properties. Synthesized size distributions consistent with measurements, model particle size, while composition is defined by the refractive indices of minerals routinely observed in dust, including the widely used OPAC/Hess parameterization. Single-scattering properties of representative dust particle shapes are calculated using the T-matrix, discrete dipole approximation and Lorenz-Mie light-scattering codes. For the parameterizations examined, MEE ranges from 0–1.2 m2 g−1, with large contributions from non-spheres composed of quartz and gypsum. At near-IR wavelengths, MEE for non-spheres generally exceeds those for spheres, while in the IR, shape-induced changes in MEE strongly depend on volume median diameter (VMD) and wavelength, particularly for MEE evaluated at the mineral resonant frequencies. MEE distributions appear to follow particle geometry and are evidence for shape dependency in the optical properties. It is also shown that non-spheres best reproduce the positions of prominent absorption peaks found in silicates. Generally, angular particles exhibit wider and more symmetric MEE distribution patterns from 8–10 μm than those with smooth surfaces, likely due to their edge-effects. Lastly, MEE ratios allow for inferring dust optical properties across the visible-IR spectrum. We conclude MEE are significant for the parameter space investigated, and are a key component for applications including sea surface temperature retrievals and the study of direct aerosol radiative effects.