Broadband solar radiation has been measured during several decades, but the history of spectral measurements is relatively short. Several models exist for the calculation of the spectral atmospheric aerosol thickness from broadband solar radiation data. Using the method elaborated at Moscow University, aerosol optical thickness at 550 nm was calculated for Tartu–Tõravere Meteorological Station (Estonia) for the period 1951–2004. A significant increase in the spectral as well as broadband optical thickness is characteristic until the late 1980s, while during the last 15–20 years a sharp decrease was observed. Besides changes in the aerosol burden, changes in the features of dominating particles, as well as in the seasonal cycle, were found during the period under study.
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