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A 3-year data set from the AERONET simphotometers is used to remotely sense the aerosol optical depth (AOD) and the Angstrom wavelength exponent (alpha) in four key locations characteristic of different environments., These sites are indicative of the dominance of different aerosol types, representing biomass burning, urban/industrial pollution, marine conditions and desert particles. The multiyear observations show robust differentiation in both values and spectral dependence of the AOD in the different environments. Higher AODs are depicted in tropical areas directly affected by forest fires, in urban areas due to heavy anthropogenic pollution as well as in desert regions under specific dust outbreaks. In contrast, remote oceanic regions can be characterized as areas without local pollution, where their aerosol load approaches background levels. In all locations, significant annual, seasonal and day-to-day variability in AOD and Angstrom alpha is observed. The Angstrom exponent exhibits its lowest values over oceanic and desert areas, similar to 0.2-0.4, while it can reach 2.0, or even more, under intensive fire events. The seasonal variation of the AOD in biomass-burning areas shows a pronounced August-September peak, while in the other months the AOD is low. A clear seasonal pattern with maximum AOD in the period May-June is observed for the desert region, while the AOD’s seasonal variation in urban and oceanic sites does not present a clear annual peak. As regards the Angstrom exponent, a clear seasonal variation is depicted in the biomass-burning region with a peak in the burning season, and in desert areas with a minimum in the months with significant dust loading. (C) 2007 Elsevier Ltd. All rights reserved.