Aerosol optical properties in Mediterranean basin underSaharan dust outbreaks

Four measurement campaigns held in three different sites of the Mediterranean basin and in different years revealed the intrusion of mineral dust coming from North Africa, as confirmed even by the HYSPLIT back-trajectories analysis. Two of the sites are located in South Italy, the third one is in South Spain.. One data-set was obtained by a Monolight spectrometer (400 nm – 800 nm, resolution 3 nm), the other by means of an Avantes USB2000 spectrometer (400 nm – 900 nm, resolution 1.5 nm). A fitting procedure and a non-parametric inversion t echnique were applied to the measured AODs to retrieve, respectively, the Ǻngström parameters α and β and the Aerosol Size Distributions. Each site is characterised by different size distributions (bi-modal and Junge functions) but, independently from thes e functions, all the measurements affected by Saharan dust showed higher particles density in the radii range 0.43 μm ≤ r ≤ 3.0 μm. The second result is the strong correlation between α and ln( β ) for dust data points obtained in all sites, suggesting that mineral particles properties predominate over the background ones. Finally, Aerosol Size Distributions have been simulated, modifying their log-normal parameters and fixing, time by time, the refractive index values, to reproduce the experimental α vs. ln( β ) behaviour. In this way, an “equivalent refractive index” common to the dust data has been retrieved.