Complex refractive indices of Saharan dust samples at visible and near UV wavelengths: a laboratory study
1Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, P.O. Box 3640, 76021 Karlsruhe, Germany
2University of Szeged, Department of Optics and Quantum Electronics, Dóm tér 9, 6720 Szeged, Hungary
3Darmstadt University of Technology, Institute of Applied Geosciences, Schnittspahnstr. 9, 64287 Darmstadt, Germany
4Leibniz Institute for Tropospheric Research, Permoserstr. 15, 04318 Leipzig, Germany
Abstract. We have retrieved the wavelength-dependent imaginary parts of the complex refractive index for five different Saharan dust aerosol particles of variable mineralogical composition at wavelengths between 305 and 955 nm. The dust particles were generated by re-dispersing soil samples into a laboratory aerosol chamber, typically yielding particle sizes with mean diameters ranging from 0.3 to 0.4 μm and maximum diameters from 2 to 4 μm. The extinction and absorption coefficients as well as the number size distribution of the dust particles were simultaneously measured by various established techniques. An inversion scheme based on a spheroidal dust model was employed to deduce the refractive indices. The retrieved imaginary parts of the complex refractive index were in the range from 0.003 to 0.005, 0.005 to 0.011, and 0.016 to 0.050 at the wavelengths 955, 505, and 305 nm. The hematite content of the dust particles was determined by electron-microscopical single particle analysis. Hematite volume fractions in the range from 1.1 to 2.7 % were found for the different dusts, a range typical for atmospheric mineral dust. We have performed a sensitivity study to assess how accurately the retrieved imaginary refractive indices could be reproduced by calculations with mixing rule approximations using the experimentally determined hematite contents as input.