Investigation about the dependence of spectral diffuse-to-direct-beam irradiance ratio on atmospheric turbidity and solar zenith angle

The modifications of the solar spectral diffuse and direct-beam irradiances as well as the diffuse-to-direct-beam ratio, E-d lambda/E-b lambda, as a function of the aerosol optical depth, AOD, and solar zenith angle, SZA, is investigated. The E-d lambda/E-b lambda ratios decrease rapidly with wavelength and exponential curves in the form E-d lambda/E-b lambda = a lambda(-b) can be fitted with a great accuracy. These curves are strongly modified by the solar spectrum distribution, which is affected by the aerosol loading, aerosol optical properties and SZA. The spectral dependence of the above E-d lambda/E-b lambda ratios in logarithmic coordinates does not yield a straight line, while a significant departure from the linearity is revealed. The reasons for this departure are investigated in detail and it is established that the aerosol physical properties such as single scattering albedo and size distribution along with the effect of SZA are responsible. These parameters strongly affect the scattering processes in the atmosphere and as a consequence the diffuse spectral distribution. The E-d lambda/E-b lambda ratio, which is an indicator of the atmospheric transmittance (King, 1979), exhibits a strong wavelength and aerosol-loading dependence. The observed differences between turbid and clear atmospheres constitute a manifestation of contrasting air properties and influence solar irradiance spectra. The present work aims at investigating the effect of atmospheric turbidity and SZA on the E-d lambda/E-b lambda ratio. For this reason, two distinct cases are examined: one having different atmospheric turbidity conditions but same SZA and a second having different SZAs and same atmospheric turbidity levels.