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- ItemNumerical simulations of optical properties of Saharan dust aerosols with emphasis on lidar applications(Milton Park : Taylor & Francis, 2017) Wiegner, M.; Gasteiger, J.; Kandler, K.; Weinzierl, B.; Rasp, K.; Esselborn, M.; Freudenthaler, V.; Heese, B.; Toledano, C.; Tesche, M.; Althausen, D.In the framework of the Saharan Mineral Dust Experiment (SAMUM) for the first time the spectral dependence of particle linear depolarization ratios was measured by combining four lidar systems. In this paper these measurements are compared with results from scattering theory based on the T-matrix method. For this purpose, in situ measurements—size distribution, shape distribution and refractive index—were used as input parameters; particle shape was approximated by spheroids. A sensitivity study showed that lidar-related parameters—lidar ratio Sp and linear depolarization ratio δp—are very sensitive to changes of all parameters. The simulated values of the δp are in the range of 20% and 31% and thus in the range of the measurements. The spectral dependence is weak, so that it could not be resolved by the measurements. Calculated lidar ratios based on the measured microphysics and considering equivalent radii up to 7.5μm show a range of possible values between 29 and 50 sr at λ = 532 nm. Larger Sp might be possible if the real part of the refractive index is small and the imaginary part is large. A strict validation was however not possible as too many microphysical parameters influence Sp and δp that could not be measured with the required accuracy.
- ItemComplex refractive indices of Saharan dust samples at visible and near UV wavelengths: A laboratory study(München : European Geopyhsical Union, 2012) Wagner, R.; Ajtai, T.; Kandler, K.; Lieke, K.; Linke, C.; Müller, T.; Schnaiter, M.; Vragel, M.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 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.