Temporal and spatial variability of aerosol optical depth in the Sahel region in relation to vegetation remote sensing

Abstract

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 r. 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 r. 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 c< was found to vary with the magnitude of the aerosol optical thickness, with values as high as 0·75 for very low r., and values of 0·25 to 0·0 for high r. conditions. Analysis of r. data from this observation network suggests that there is a high spatial variability of r. 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 r. 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.

Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign

“…The scatteringÅngström exponent (å sp ), estimated for 450 to 700 nm, decreases to near zero in this layer. It has been well documented thatå sp is inversely related to particle size (e.g., Eck et al, 1999;Schuster et al, 2006 and reference therein) and that values less than one are indicative of particles dominated by sea-salt and/or dust (e.g., Schuster et al, 2006;Holben et al, 1991). Tanré et al (2003) showed that theå sp values derived from Cape Verde AERONET observations were generally <0.2 during prevalent dust season, including August and September.…”

Multi-year investigations of aerosols from an island station, Port Blair, in the Bay of Bengal: climatology and source impacts

“…Examining the daily mean spectral AODs in the light of the above, it has been found that α ( aerosol columnar size distributions dominated by fine mode aerosols (r < 1 µm), such as from biomass burning or urban/industrial activities (Holben et al, 1991;Westphal and Toon, 1991;Kaufman et al, 1992;Reid et al, 1999;, and 1.0 < α < 2 suggest the prevalence of mixed type of aerosols with significant contributions from both accumulation mode and coarse mode aerosols to the columnar AOD. The top panel of Fig.…”

Climatic and environmental change in Africa during the last two centuries

“…Five studies (Oluwafemi 1979, ben Mohamed & Frangi 1983, Fouquart et al 1987a, Faizoun et al 1994, Pinker et al 1994) have used sun photometers to make measurements at a single station. Studies by Cerf (1980), Holben et al (1991), d 'Almeida (1986'Almeida ( , 1987, ben Mohamed et al (1992) and Faizoun et al (1994) have some degree of spatial coverage, varying from 2 to 11 stations. In some cases, measurements are limited to a few months; many cover a 1 or 2 yr period at each station.…”