Deriving an effective aerosol single scattering albedo from spectral surface UV irradiance measurements

The modification of clear-sky global and diffuse irradiances and the direct-to-diffuse irradiance ratio by the aerosol single scattering albedo, omega, as a function of aerosol optical depth, tau(a) and solar zenith angle (SZA) is investigated using radiative-transfer model calculations. The model-derived relations are combined with UV irradiances at the surface and the aerosol optical depth measured with a Brewer MkIII spectroradiometer, with the aim to develop and test an indirect method of estimating an effective single scattering albedo. The uncertainties introduced from different sources are quantitatively discussed in order to determine the overall uncertainty of the method. The overall accuracy in determining omega indirectly depends strongly on the radiation quantity used and the amount of aerosols, increasing the uncertainty of the method in situations with tau(a) at 340 nm below 0.2. From the three radiation quantities, the direct-to-diffuse irradiance ratio provides the highest accuracy in the estimation of the effective omega. As an example, the effective to is determined for 2 days with different aerosol loadings and composition. Finally, 5 years of measurements of global irradiance and tau(a) under cloud-free conditions are analysed in order to estimate the range of variation of the effective omega at Thessaloniki, Greece. For tau(a) at 340nm larger than 0.8, omega ranges between 0.85 and 0.99, while for lower-aerosol optical depths the derived effective omega covers the entire range from 0.64 to 0.99. (C) 2004 Elsevier Ltd. All rights reserved.