A comparison of the TOMS aerosol index (AI) with both measurements of dust concentrations and synoptic data over the Middle East for the extremely heavy dust storm of March 14--17, 1998, is performed. Time series over Algeria, Libya, and Israel yield the following findings: The peak values in both surface concentrations and TOMS data suggest that AI values of 2.5 and 1.2 correspond to surface mean daily concentrations of 1900 and 1000 μg/m3 respectively. Surface concentrations and TOMS maps show that AI values of 3.0 correspond to about 4000 μg/m3. TOMS AI maximum values were found to increase from Algeria to Israel moving along with the center of the dust plume from 0.9 to 2.1 and 2.5 AI values. This seems to be in contradiction with the fact that the plume moves farther away from the major mineral dust sources over the Sahara and may be caused by either an increased vertical extension of the plume, hence increasing the TOMS AI, or due to convergence of the dust plume over the eastern Mediterranean. The size distribution, morphological, and mineralogical composition of the dust analyzed in this study, as well as other aerosol parameters, are essential for improving the remote sensing methods such as the TOMS AI algorithms. Of course, surface measurements alone do not allow the refinement of the TOMS retrievals; vertical profile dust measurements as well as other physical and optical aerosol parameters are necessary. ¿ 2001 American Geophysical Union