720 
5 pg/cm 2 to one of 35 pg/cm 2 causes a large variation in NDVI across its whole range particularly 
between 5 and 15 pg/cm 2 (Figure 4a]. AVI, however, is relatively insensitive to leaf chlorophyll 
concentration fra- low LAI values. Beyond an LAI of approximately 2 the effect of chlorophyll starts to 
produce a similar dynamic response in AVI to that due to LAI itself (Figure 4b). 
0.0 1.0 2.0 3.0 4.0 5.0 6.0 
LAI 
Figure 2: Variation of NDVI and AVI with leaf area index (LAI) 
Figure 3: Variation of a) NDVI and b) AVI as a function of LAI and soil brightness 
42 Effect of Ground Visibility and Aerosol Size Distribution on AVI 
The comparison of the sensitivity of AVI and NDVI with decreasing ground visibility for a mid- 
latitude summer atmosphere and the rural aerosol model is shown in Figure 5. NDVI clearly exhibits 
large variation with optical thickness. At the maximum LAI this equates to a difference in NDVI of 0.2 
and for such an error in ground visibility the estimated LAI occupies a range from 1.7-6.0 (Figure 5a). 
By contrast the AVI shows little variation with ground visibility particularly over the range 20-50 km 
(Figure 5b). The effect of error in aerosol size distribution was small for both NDVI and AVI. For 10 
km visibility the NDVI exhibited a decrease in value at low LAI as the aerosol size distribution was 
altered from Maritime to Rural. At 50 km, however, the principal effect was at high LAI with an