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The main aim of this work is to assess the performance of the Total Ozone Mapping Spectrometer (TOMS) on board the Earth Probe satellite aerosol index (AI) version 8.0 retrieved from the daily measurements of the TOMS instrument as an indicator of the presence of absorbing aerosols in the atmosphere. The analysis is carried out over the region of Northern Greece, an area exposed to various aerosol types from different sources. The aerosol optical depth (AOD) measured by a Brewer spectroradiometer situated in the centre of the city of Thessaloniki, at 40.6 degrees N, 22.9 degrees E, was employed as a measure of the aerosol loading in the atmosphere during the Earth Probe overpass. Discrete aerosol episodes that affect the area, such as biomass burning and Saharan dust events, have been isolated and studied in order to assess the effectiveness of using the TOMS AI in this highly complex atmospheric environment. The yearly values for the AOD measured at 355 nm range from 0.03 to 1.97 with a mean of 0.58 +/- 0.32 and for the AI from -1.67 to 3.06, with a mean of 0.22 +/- 0.73. This wide range of values results from the co-existence of urban, continental, marine and dust aerosols over Thessaloniki, a fact that makes it complicated to segregate optical effects on the observed radiation depending on different aerosol types. A total of 223 biomass burning events that affected Thessaloniki showed a correlation of 0.67 between the AOD and AI values whereas a correlation of 0.57 was deduced for the 37 cases of Saharan dust loading. A cluster analysis on the back-trajectories was performed in order to study the origins of the air masses over Thessaloniki, which permitted that the optical depth and AI were analysed depending on the possible source of the aerosols. The highest AOD values coupled with the highest AI values were seen for trajectories that transport air from the Saharan desert over Thessaloniki and those that bring polluted air from Eastern Europe and the Balkan regions. Even though some deductions were made vis-A-vis the origin and importance of different aerosol types over the city corroborating the usefulness of the AI in climatological or global climate studies, auxiliary information is essential for studying in detail the effects of the atmospheric aerosol loading over a local point of interest. (c) 2006 Elsevier Ltd. All rights reserved.