Investigation into the wavelength dependence of the aerosol optical depth in the Athens area

Ground-based solar spectral irradiance measurements obtained by a spectroradiometer have been used to retrieve the aerosol optical depth, tau(alpha), and the Angstrom wavelength exponent, a. These measurements were obtained in the Athens centre in May 1995. Exponent alpha is frequently computed from spectral measurements both from ground-based photometers and satellite remote sensing retrievals. Although the retrieval of alpha has been the subject of many studies worldwide, its spectral variation is typically not considered. In this study we analyse the spectral measurements of ln tau(alpha) versus ln lambda, where lambda is the wavelength, at six selected wavelengths which cover the spectrum from 340 to 870 nm. It is obvious that there is significant curvature in the ln tau(alpha) versus In X relationship, and the reasons for this are examined. It was found that this curvature depends on both atmospheric turbidity and aerosol optical properties, and a second order polynomial fit provides excellent agreement. The curvature of the polynomial equation becomes greater as the aerosol loading alters from high to low. The curvature is characterized by the second derivative of alpha, which can be utilized in conjunction with alpha to characterize the spectral dependence of aerosol optical depth, and can also be used as an indicator of the relative influence of fine-versus coarse-mode particles.