Saharan dust particle properties over the central Mediterranean

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. AERONETretrievals at AOD (440 nm)z0.6 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 bnN=1.5F0.1, the imaginary refractive index bkN=0.004F0.002, the single scattering albedo bSSAN=0.89F0.03, and the Angstrom exponent bA° N=0.2F0.1. It is shown that A° 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 Am. In particular, the average coarse mode that is centred ati2.2 Am at Lampedusa, which is ~200 km away from the northwest Africa coast, gets peaked at i1.7 Am 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%.