On the characterization of aerosols using the Ångström exponent in the Athens area

Abstract

Ground-based spectroradiometric measurements were taken in the atmosphere of Athens during May 1995 in order to investigate various atmospheric conditions. This study focuses on the Ångström exponent α, which is the slope of the logarithm of the aerosol optical depth (AOD) versus the logarithm of the wavelength, ln λ, and is commonly used to characterize the wavelength dependence of AOD and to provide some basic information on the aerosol size distribution. Using the Volz method, Ångström exponent values, α, were derived in five narrow spectral bands, 340–380, 380–440, 440–500, 500–670 and 670–870 nm. Also using both Volz and least-squares fit methods α values were derived for the whole spectrum 340-870 nm.The results show that α depends strongly on the wavelength interval used due to the curvature of the lnAOD versus ln λ line. Using the spectral dependence of the Ångström exponent and the relationship between α computed in different spectral bands with AOD, an attempt to investigate on the aerosol types is made. The results obtained are rather contradictory and the determination of a dominant aerosol type in the Athens area is not clear. Nevertheless, in the most cases the anthropogenic aerosols seem to be the dominant type with rather significant contributions of coarse-mode particles due to particle growing or due to mixing processes with other aerosol types.