Ultraviolet erythemal radiation (UVER) values have been simulated for different values of column ozone and aerosol optical depth (AOD) using a multiple scattering model: SBDART. The results show that UVER decreased when atmospheric ozone and aerosols increased, with the ozone having greater influence than AOD. To quantify these dependencies the influence of aerosols on the simulated values of UVER has been analysed for different ozone values. This dependence was linear. Similarly, to quantify the influence of ozone on the UVER values this was simulated for fixed values of AOD while varying the column ozone. The expression of this dependence was found to be a second-order polynomial. We also analysed the dependence of the experimental UVER values on the total column ozone and the AOD at 500 nm. Daily values of UVER were used, normalized with respect to a reference value. It was seen that the modified clearness index, k*tUVER, decreased as the total column ozone increased and also as the 500 nm AOD increased. These linear relationships were obtained and were found to be of the same order for the different intervals of AOD and for those total column ozone intervals for which the greater number of experimental data were available. At the same time, the relations between the modified UVER clearness index and the ozone, as between the modified clearness index and the AOD, showed an annual pattern. Finally, an expression was found to represent the combined dependence of the modified UVER clearness index, k*tUVER, on ozone and aerosols. Copyright © 2009 Royal Meteorological Society