Saharan dust particle properties over the central Mediterranean

Abstract

AERONET sun photometer measurements performed at five different sites of the Central Mediterranean during strong Saharan dust outbreaks are used to characterize optical and physical properties of dust particles. Sun photometer retrievals are combined at two of the five sites with lidar observations. It is shown that at low aerosol optical depths (AODs) the dust particle properties are quite dependent on dust load and monitoring site location. Differences on retrieved particle properties reduce with increasing dust load. AERONET retrievals at AOD (440 nm) ≥ 0.6 are then used in this paper to characterize dust particles over the central Mediterranean basin leading to columnar averaged values of the real refractive index < n > = 1.5 ± 0.1, the imaginary refractive index < k > = 0.004 ± 0.002, the single scattering albedo < SSA > = 0.89 ± 0.03, and the Angstrom exponent < Å > = 0.2 ± 0.1. It is shown that Å represents the best marker to trace the temporal evolution of dust events. Volume size distributions show a dominant coarse mode peaking at 1.7–3 μm. In particular, the average coarse mode that is centred at ≅ 2.2 μm at Lampedusa, which is ∼200 km away from the northwest Africa coast, gets peaked at ≅ 1.7 μm at Lecce, which is ∼800 km away. Lidar retrievals are used to characterize the vertical distribution of dust particles by the vertical profiles of the backscatter and extinction coefficients, the lidar ratio, and the depolarization ratio. Lidar retrievals show that over the Mediterranean basin, dust layers generally extend from 1 up to 6 km and that their vertical distribution can significantly change within a few hours. It is also shown that at high values of the AODs dust particles are characterized by lidar ratios spanning the 50–70 sr range and depolarization ratios larger than 30%.