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Evidence of an underlying trend in the dependence of erythema effective ultraviolet (UV) radiant exposure (H-er) on changes in the total ozone, cloud cover and aerosol optical depth (AOD) has been studied using solar ultraviolet radiation measurements collected over a 25-year period (1991-2015) at Chilton in the south of England in the UK.

The monthly mean datasets of these measures corrected for underlying seasonal variation were analysed. When a single linear trend was fitted over the whole study period between 1991 and 2015, the analyses revealed that the longterm variability of H-er can be best characterised in two subperiods (1991-2004 and 2004-2015), where the estimated linear trend was upward in the first period (1991-2004) but downward in the second period (2004-2015).

Both cloud cover (CC) and total ozone (TO) were found to have a highly statistically significant influence on H-er, but the influence of the AOD measure was very small. The radiation amplification factor (RAF) for the erythema action spectrum due to TO was -1.03 at constant levels of CC over the whole study period; that is, for a 1.0% increase in TO, H-er decreases by 1.03 %. Over the first period (1991-2004), the RAF related to CC was slightly higher at 0.97 compared to that for TO at 0.79. The proportion of the change in H-er explained by the change in CC (47 %) was much greater than the proportion explained by changes in TO (8 %). For the second period (2004-2015), the pattern reversed, with the observed RAF related to TO being -1.25, almost double that of CC (-0.65). Furthermore, in this period the proportion of variation in H-er explained by TO variation was 33 %, double that of CC at 16 %, while AOD changes had a negligible effect (1 %).

When the data were examined separately for each season, for the first period (1991-2004) the greatest effect of TO and CC on H-er (i.e. the largest RAF value) was found during spring. Spring was also the season during which TO and CC variation explained the greatest proportion of variability in H-er (82 %). In the later period (2004-2015), the RAF and greatest influence of TO and CC were observed in winter (67 %) and the AOD effect explained a further 5% variability in H-er.

This study provides evidence that both the increasing trend in H-er for 1991-2004 and the decreasing trend in H-er for 2004-2015 occur in response to variation in TO, which exhibits a small increasing tendency over these periods. CC plays a more important role in the increasing trend in H-er for 1991-2004 than TO, whereas for 2004-2015, the decreasing trend in H-er is less associated with changes in CC and AOD.