Spatial and temporal variability of aerosol: size distribution and optical properties

Abstract

Spectral optical thickness measurements together with sky radiance measurements at Ouagadougou (Burkina-Faso), Banizoumbou (Niger), Thala (Tunisia), IMC Oristano (Sardinia) and Rome Tor Vergata (Italy) during April, May and June 2001 have been used to derive aerosol characteristics as spatial and temporal variations of aerosol optical thickness τ a, aerosol size distribution and single-scattering albedo. The analysed τ a data at the different sites show a high spatial variability of the aerosol optical thickness. The τ a values at 870 nm obtained during the study period show that 97% of the data vary between 0.1 and 1.5 for central African sites (Ouagadougou and Banizoumbou) and between 0.03 and 1 for Mediterranean sites (IMC Oristano, Rome Tor Vergata and Thala). The Angström wavelength exponent α was found to vary with the magnitude of the aerosol optical thickness. The higher and lower values of α correspond to the lower and higher values of τ a, respectively. In European sites, α varies between 0.2 and greater than 2.0. In Thala, it ranges from 0.0 to 1.8 and for central African stations, α exhibited variability from (−0.1) to approximately 0.6. The size distribution inversion obtained by combining the spectral optical thickness from direct sun measurement and aureole data presents three radius modes in both Mediterranean sites and central African sites: the first mode with radius near 0.1 μm and two second modes with radius at 1–4 μm. The effective and volume weighted radii are computed for the five studied sites and compared. The single-scattering albedo ω o, obtained from solar aureole data at African sites, shows an increase of ω o with wavelength for different aerosol optical thickness values.