TEMPORAL AND SPATIAL VARIABILITY OF AEROSOL OPTICAL DEPTH IN THE SAHEL REGION IN RELATION TO VEGETATION REMOTE-SENSING

A network of Sun photometers was established in the Sahel region of Senegal, Mali and Niger in order to monitor the aerosol characteristics needed for atmospheric correction of remotely sensed data. The aerosol optical thickness tau-a computed from the spectral Sun photometer measurements exhibited very high day-to-day variability ranging from approximately 0.1 to greater than 2.0 at 875 nm for both the wet and dry seasons. A gradient of decreasing tau-a from north-to-south latitudes in the Sahel for the wet season, July-September, was observed, which may be caused jointly by increased washout owing to the gradient of increasing precipitation to the south and the location of source regions for dust in the north. The Angstrom wavelength exponent alpha-was found to vary with the magnitude of the aerosol optical thickness, with values as high as 0.75 for very low tau-a, and values of 0.25 to 0.0 for high tau-a conditions. Analysis of tau-a data from this observation network suggests that there is a high spatial variability of tau-a in the western Sahel region. Statistical analysis performed on the wet season data showed that at a 67 per cent confidence level the instantaneous values of tau-a can be extrapolated approximately 270-400 km with an error tolerance of 50 per cent. Spatial variability in the dry season was of a similar magnitude. The ranges of variations in the NDVI in the Sahel region owing to commonly observed fluctuations in the aerosol optical thickness and aerosol size distribution were shown to be approximately 0.02 and 0.01, respectively.