ASSESSMENT OF THE AFRICAN AIRBORNE DUST MASS OVER THE WESTERN MEDITERRANEAN-SEA USING METEOSAT DATA

The mass of African dust present over the western Mediterranean during a transport episode from northwestern Africa, which occurred in early July 1985, is estimated using a desert aerosol model, an Earth-atmosphere radiative transfer model and Meteosat visible channel data from 4 days running. Dust pixels are selected from Meteosat images, and their aerosol optical thickness is retrieved. A proportionality factor between aerosol optical thickness and atmospheric columnar aerosol loading is computed and applied to the dust pixels. The total mass of atmospheric particles over the basin is obtained by interpolation and spatial integration. The maximum aerosol optical thickness is 1.8. The maximum aerosol columnar loading is evaluated to be 2.3 g m-2. The integrated mass of particles present at a given time is estimated to raise up to about 0.6 x 10(12) g at the maximum and the total mass of dust exported from Africa to be of the order of 10(12) g. The method is carefully evaluated and uncertainties are discussed, with particular emphasis on the relationship between atmospheric dust mass and aerosol optical depth. The overall uncertainty on the total mass is roughly a factor +/- 3. In the absence of clouds it appears that the major uncertainty results from the lack of knowledge of the actual mass-size distribution of suspended dust particles, pointing out the lack of relevant data on particles larger than 10-mu-m in diameter. A simple calculation based on results from both computations and simultaneous field measurements yields a net transfer velocity of particles from the dust layer of approximately 1 cm s-1.