Comparison between experimental data and modeling estimates of aerosol optical depth over Athens, Greece

Ground-based spectroradiometric measurements taken in the atmosphere of Athens during May 1995 have been used to investigate various atmospheric parameters. From the direct-beam irradiance measurements the spectral aerosol optical depths as well as the Ångström parameters are derived using the least-squares method applied to measured spectral irradiance data. In this way the errors associated with the turbidity coefficient, β, are found to be 0.0029 on average and with the wavelength exponent, α, 0.031. In this paper, a comparative study between the aerosol optical depth derived by the spectral direct-beam irradiance measurements and those estimated through the Ångström's formula is made. The correlation coefficient between the measured and computed optical depth is higher than 0.93 in most cases, while their greatest differences take place at the shorter wavelengths. A further analysis of the above differences at 500 and 1000 nm shows very low values, especially at 500 nm, indicating the accuracy of the fitting. The differences at 1000 nm are much higher due to uncertainties in the water-vapor optical depth determination. The errors in Ångström parameters as well as in the Ångström's formula fitting to the derived spectral aerosol optical depth depend strongly on atmospheric turbidity, reaching to higher values under low atmospheric turbidity conditions.