Saharan dust events observed in the tropics and subtropics by a shipborne Aerosol Raman lidar

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Immler, F. and Schrems, O. (2003): Saharan dust events observed in the tropics and subtropics by a shipborne Aerosol Raman lidar , Lidar measurements in Latin America, Workshop, Camaguey, Cuba .
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Results of Lidar observations made aboard the German research vessel Polarstern will be presented. The measurements have been performed with AWIs mobile Aerosol Raman Lidar (MARL). Extended Saharan dust layers in an altitude range between 2 and 6 km have been observed on two occasions around 10° N and 30° N. The continental, North African origin of the corresponding air masses has been confirmed by backward trajectories. The dust is characterized by an optical depth between 0.1 and 0.3, a depolarization around 10% and high lidar ratios of 45 sr at 532 nm and 75 at 355 nm. Moreover, backscattering in the UV is relatively inefficient, resulting in a negative color index.These results are analyzed based on Mie Theory and non-spherical scattering calculations. The measured optical properties are consistent with an imaginary part of the refractive index of 0.002 and 0.004 at 532 nm and 355 nm, respectively. This increasing absorption with decreasing wavelength may be attributed to the content of hematite. It accounts for the strong wavelength dependence of the lidar ratio and the negative color index. On the other hand, the non-spherical nature of the dust particles is responsible for the high values of the lidar ratios. The effective radius retrieved on this bases, was found to decrease from about 3 µm to 0.6 µm with altitude. Our results are in good agreement with reports based on sun photometer and satellite measurements.

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