Optical Properties of Mineral Dust Aerosol in the Thermal Infrared

Abstract

The optical properties of mineral dust in the thermal infrared (TIR) will be discussed based on FTIR measurements collected during the Saharan Mineral Dust Experiment 2 (SAMUM-2). The discussion will focus on the influence of particle shape, refractive index and mixture state on the upwelling and downwelling radiance in the spectral region 800-1200 cm-1 (8 – 12 µm). The downwelling radiance at the surface was measured by the author using a D&P Model 102 FTIR within the scope of SAMUM-2. Additional coincident IASI measurements have been evaluated to analyze the radiative effect at top of the atmosphere. It will be demonstrated in a couple of case studies, that dust has a distinct effect at both top and bottom of the atmosphere. Although this influence is far more pronounced at the surface, it is clearly visible in IASI spectra, too, and it may e.g. lead to a bias of approximately 1 K in retrieved sea surface temperature retrieval if not properly accounted for. Additionally our measurements confirm recent laboratory studies and field experiments which indicate that the assumption of spherical model particles is a poor choice for mineral dust. Ellipsoids with rather large aspect ratios result in a much better agreement of our measurements with radiative transfer simulations. It will be shown though, that the ellipsoid model may not be an optimal choice for dust particles either, as there still remain significant discrepancies between the microphysical properties retrieved from the ground based FTIR measurements and those determined by other SAMUM-2 groups in elaborate laboratory analysis. It is planned to present first results of ongoing radiative transfer simulations with other particle shapes, which potentially lead to a more satisfying result.