Vertical profiles, optical and microphysical properties of Saharan dust layers determined by a ship-borne lidar F. Immler and O. Schrems Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Abstract. A unique data set of ship-borne lidar measurements of Saharan dust layers
above the Atlantic ocean has been collected aboard the research vessel Polarstern with a mobile Aerosol Raman Lidar (MARL) during the
LIMPIDO-campaign in June 2000. Extended Saharan dust layers have been observed in the region between
8.5º N and 34º N in an altitude range between 2 and 6 km. The continental, North African
origin of the probed air masses is confirmed by 8-day backward trajectories.
The Saharan dust is characterized by an optical depth in the range of 0.1 and
0.3, a depolarization around 10% and high lidar ratios of 45 sr at 532 nm and
75 sr at 355 nm. The backscattering by the dust particles at the UV-wavelength is relatively weak, resulting in a
negative color index. From the measured optical properties the effective radius and the refractive index of the dust particles are derived using a new
approach based on Mie Theory and non-spherical scattering calculations. The
low backscatter coefficient observed at 355 nm is due to significant absorption which increases with decreasing wavelength. This finding agrees
very well with results from satellite and sun photometer measurements. The
effective radii decrease from about 3 mm at the base to
at the top of the dust plumes. The non-spherical shapes of the dust particles
are responsible for the high values of the lidar ratios.
Citation: Immler, F. and Schrems, O.: Vertical profiles, optical and microphysical properties of Saharan dust layers determined by a ship-borne lidar, Atmos. Chem. Phys., 3, 1353-1364, doi:10.5194/acp-3-1353-2003, 2003.