Finding the aerosol optical thickness over the Baltic Sea - comparison of two methods

The results of two methods used to estimate the aerosol optical thickness over the Baltic Sea are compared. The standard method is based on measurements of the direct component of the downward irradiance at the sea surface in 8 spectral bands (412, 443, 490, 510, 555, 670, 765, 865 nm - the same as SeaWiFS). In the pyranometric method, Baltic aerosols are assumed to be a mixture of model aerosol types with strictly defined optical properties, i.e. maritime, continental and stratospheric types. Their proportion in the Baltic aerosol is found from broadband spectral downward irradiance measurements (VIS, IR) using the radiative transfer model. Simultaneous measurements of the spectral downward irradiance and its direct component on cloudless days in the southern Baltic were used in the comparison. The pyranometric method of estimating the aerosol optical thickness proved to be a satisfactory tool. Depending on the wavelength, the statistical errors in it are not in excess of plus_min.0.06 - 0.08.