Variability of aerosol optical thickness and atmospheric turbidity in Tunisia

The aerosol optical thickness (AOT) tau(a) computed from the spectral sun photometer in Thala (Tunisia) exhibited variability ranging from approximately 0.03 to greater than 2.0 at 870 nm for March-October 2001. These measurements are compared to the aerosol optical thickness computed in Ouagadougou (Burkina-Faso), Banizoumbou (Niger), IMC Oristano (Sardinia) and Rome Tor Vergata (Italy). Analysis of tau(a) data from this observation network suggests that there is a high temporal and spatial variability of tau(a) in the different sites. The Angstrom wavelength exponent alpha was found to vary with the magnitude of the aerosol optical thickness, with values as high as 1.5 for very low tau(a), and values of -0.1 for high tau(a) situations. The relationship between the two parameters tau(a) and alpha is investigated. Values of the turbidity coefficient beta have been determined in Thala (Tunisia) for 8 months in 2001 based on a direct fitting method of the Angstrom power law expression using sun photometer data. The monthly averaged values of the turbidity coefficient vary between 0.15 and 0.33. The months of July and October experienced the highest turbidity, while April experienced the lowest aerosol loading on average. The turbidity shows a maximum and minimum values for the Southwest and the Northwest wind directions, respectively. The single scattering albedo omega(o) for the 870 nm wavelength obtained from solar aureole data in Thala is analysed according to the particles' origin. (C) 2002 Elsevier Science B.V. All rights reserved.