Satellite data measuring aerosols over the Canary Islands need to be correctly interpreted in accordance with the spatial resolution and spectroscopic channels used. In situ data are still a necessary reference for calibrating and interpreting the aerosol index provided by different spectrometers onboard satellites. Most of the airmass flux component arriving at the Canarian Archipelago comes from the North Atlantic Ocean and consists of sea aerosols, i.e. absorbent chloride in the UV which does not affect the extinction in the visible range. African dust intrusions affect the western and eastern Canary Islands differently. Moreover, the presence of a stable inversion layer and the sharp orography of the western islands (La Palma at Tenerife) produce different mass flux patterns in the low (mixing) layers closer to the sea and in the median-upper (or free) troposphere layer. The aerosol index provided by the TOMS (Total Ozone Mapping Spectrometer) is one of the most widely accepted products to detect the daily aerosol content. On the other hand, several techniques have been developed in situ to characterize the presence of dust locally at the Canarian Observatories. In particular, a parameter related to sky transparency, the atmospheric extinction coefficient, has been measured at the Roque de los Muchachos Observatory (ORM) on La Palma since 1984 by the Carslberg Automatic Meridian Circle Telescope (CAMC). In situ aerosols are also available for the Teide Observatory (OT) on Tenerife, from 1986 onwards. In this work we compare aerosol index data of TOMS/Earth Probe observations and atmospheric optical extinction coefficient from CAMC in the period 1996-2004. A preliminary comparison of both techniques (in situ and remote) is discussed in this paper, showing there is no linear correlation between the aerosol index and the extinction coefficient. Here, we present an interpretation of different situations arising and discuss critically the correct interpretation of the aerosol index and images provided by the TOMS in accordance with the dust presence over the Observatories.