<p><strong>Abstract.</strong> Specific aerosols and cloud properties over large urban regions seem to generate an island, similar to the well-known urban heat island, leading to lower ultraviolet (UV) radiation intensity compared to the surrounding less polluted areas, thus creating a shield against excessive human exposure to UV radiation. The present study focuses on differences between erythemal and UVA (324<span class="thinspace"></span>nm) doses measured by the Brewer spectrophotometers in

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... Warsaw (52.3°<span class="thinspace"></span>N, 21.0°<span class="thinspace"></span>E) and Belsk (51.8°<span class="thinspace"></span>N, 20.8°<span class="thinspace"></span>E). The latter is a rural region located about 60<span class="thinspace"></span>km south-west of the city. Ratios between erythemal and UVA partly daily doses, obtained during all-sky and cloudless-sky conditions for the period May 2013–December 2015, were analysed to infer a specific cloud and aerosol forcing on the surface UV doses over Warsaw. Radiative model simulations were carried out to find sources of the observed differences between the sites. It was found that Warsaw urban agglomeration induced 8 and 6<span class="thinspace"></span>% attenuation of the erythemal and UVA doses respectively. This is mostly due to the lower sun elevation in Warsaw during the near-noon measurements and the larger optical depth of the city aerosols and increased cloudiness. It could be hypothesised that the expected stronger absorption of the solar UV radiation by urban aerosols is compensated for here by a higher surface reflectivity over the city.</p>