823 
5.2. Aerosol retrieval over land 
The class of vegetation called the Dense Dark Vegetation (DDV) has been used to performed aerosols retrieval 
over land (Kaufman and Sendra, 1988). Detection of DDV using channel 3 brightness temperature has been 
assessed by Holben at al, 1991, and later Vermote and al., 1993, proposed to use the surface reflectance defined 
by Stowe et al., 1991. We tried the formula described above to detect the DDV and then to retrieve the aerosol 
optical thickness using the data from SCAR-A experiment. Figure 12 shows the comparisons between optical 
thicknesses derived from AVHRR and sunphotometer measurements. The agreement between the two is good, 
which confirms the retrieval scheme of aerosol optical thickness and thus the efficiency of dark surface detected. 
0.5 
> 
< 
E 0.3 
<§, 
0.2 
H 0 0.1 
0.0 
0 0.1 0.2 0.3 0.4 0.5 
Sunphotometer Measurement (0.55^m) 
Figure 12: Comparison between optical thickness at 0.55 p.m derived from AVHRR 
and sunphotometer measurements performed during SCAR-A. 
5.3. Desertics aerosol studies 
Dust outbreaks generation, transport and deposition are significant biospheric processes [Morales, 1979]. One of 
the important aspect of the desertic aerosol study is the estimate of the total mass of dust. Channel 3 centered at 
a longer wavelength should help in the retrieval of the size distribution which is critical in the mass estimate. 
A preliminary study of an event of dust was performed using images taken over Dakar (Senegal) in 1988 
[Jankowiak and Tanre, 1992]. A plot over a cloud of dust, Figure 13, shows very well that the reflectance in 
channel 3 can be used to identify dusty areas. Quantitative analysis is on going 
0.25 
0.20 
0.15 
0.10 
0.05 
0.00 
channel 2 
y 
channel 3 
Figure 13: Plot of channel 1, 2 and 3 reflectances observed over a dust event in 1988 near Dakar. 
6 - CONCLUSION 
An operational formula to compute the reflectance in channel 3 (3.75mm} has been defined. This formula has 
been validated using sunglint measurements and FIFE-87 data set. Some preliminary applications to surface 
properties studies, atmospheric correction and desertic dust outbreaks have been investigated as further work. 
REFERENCE 
Becker F. and Li Z.L., 1990, Towards a local split window method over land surfaces, Int. J. Remote 
Sensing, 3:369. 
Cox C. and Munk W., 1954, Measurement of the roughness of the sea surface from photographs of the sun's 
glitter, J. Opt. Soc. America, 44:838.