Optical properties and mineralogical composition of different Saharan mineral dust samples:: a laboratory study

In aerosol chamber experiments optical properties of resuspended mineral dust samples of defined size distributions were measured. Extinction coefficients (b(ext)) and mass specific extinction cross sections (sigma(ext)) were determined for Saharan dust samples from different locations. The results for sigma(ext) were not very sensitive to the type of dust and varied at lambda = 550 nm between 3.3 +/- 0.4 m(2) g(-1) and 3.7 +/- 0.4 m(2) g(-1). The absorption coefficients (b(abs)) and mass specific absorption cross sections (sigma(abs)) were determined with a novel multiwavelength photo-acoustic absorption spectrometer ( PAS). The single scattering albedo was close to 1 (0.98 to 0.99) at 532 nm and 1064 nm, but significantly lower (0.63 to 0.76) at 266 nm. Additionally the chemical and mineralogical composition of the dust samples were analysed with special regard to the iron oxide phases hematite and goethite. At lambda = 266 nm the mineral dust sample without any detectable iron oxides showed a significantly higher SSA compared to the sample with a hematite content of 0.6 wt-%.