The aerosol optical depth (AOD) at 550 nm, inferred from the solar radiation measurements taken at Belsk, Poland (52 ¿N, 21 ¿E), with a Sonntag pyrheliometer in the period 1980--1996, has been analyzed for detection of its long-term characteristics. The overall mean AOD equals to about 0.27¿0.14 (1&sgr;). The AOD varies in the range 0.1--0.7, with a maximum (0.33) in April--May and a minimum (0.21) in November--December. The long-term decrease of AOD (-7.4% per decade) yields an increase of the surface UV dose (erythemally weighted) of ~1% per decade. The influence of different parameters on the transmission of UV through the atmosphere on a day-to-day timescale has been analyzed on the basis of the multiple regression model that relates the daily UV erythemal dose at the ground level with total ozone, global solar radiation (surrogate for cloud properties variations), and AOD in the period 1993--1996. It is found that a 10% increase (decrease) of AOD at 550 nm relative to its reference value manifests as ~1.5% decrease (increase) of the UV erythemal daily dose at the ground level. The changes of UV radiation due to the AOD variations are, in the mean, comparable with those due to total ozone changes. The mean cloud signal in the UV data has been found to be a few times larger than the mean ozone signal. The statistical model suggests that the extreme values of AOD in the Belsk record (that is, 0.1 and 0.7) would be associated with 20--30% changes of the erythemally weighted UV daily dose. This estimate is corroborated by radiative transfer model calculations. ¿ 1998 American Geophysical Union