Variability of aerosol optical thickness and atmospheric turbidity in Tunisia

Abstract The aerosol optical thickness (AOT) τ 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 τ a data from this observation network suggests that there is a high temporal and spatial variability of τ a in the different sites. The Angstrom wavelength exponent α was found to vary with the magnitude of the aerosol optical thickness, with values as high as 1.5 for very low τ a , and values of −0.1 for high τ a situations. The relationship between the two parameters τ a and α is investigated. Values of the turbidity coefficient β 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 β 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 ω o for the 870 nm wavelength obtained from solar aureole data in Thala is analysed according to the particles' origin.