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In this paper, the spectral aerosol optical depth (AOD) and Angstrom exponents (αs) and their relations with relative humidities (RHs) were determined and analyzed to obtain information about the adequacy of the simple use of α for characterizing aerosols. The data were taken from four selected OPAC models and compared with AERONET stations for four sites, which are representative of four aerosol types as biomass burning, urban pollution, desert dust and maritime. Using regression analysis, the αs were determined in the spectral interval 350–900 nm, along with the coefficients α1 andα2 of the second order polynomial. The results show that the spectral curvature can provide important additional information about the different aerosol types but depending on the RH. We also determined good correlations between the αs and the coefficients α1 and α2 of the second-order polynomial fit but it is higher for larger particles and decreases with the decrease in particle size. The relations of αs with RHs show increase and decrease of αs and changing in the sign of α2 from negative to positive and vice versa which seem to be closely related to the hygroscopic growth factor of the mixture of the aerosols with the increase in RHs. Finally the relationship between α and wavelength is determined.

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