Relationship between erythema effective UV radiant exposure, total ozone, cloud cover and aerosols in southern England, UK

• Main Authors: N. Hunter, R. J. Rendell, M. P. Higlett, J. B. O'Hagan, R. G. E. Haylock
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/683/2019/acp-19-683-2019.pdf
• ISSN: 1680-7316; 1680-7324

<p>Evidence of an underlying trend in the dependence of erythema effective ultraviolet (UV) radiant exposure (<span class="inline-formula"><i>H</i>_er</span>) 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.</p> <p>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 long-term variability of <span class="inline-formula"><i>H</i>_er</span> can be best characterised in two sub-periods (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).</p> <p>Both cloud cover (CC) and total ozone (TO) were found to have a highly statistically significant influence on <span class="inline-formula"><i>H</i>_er</span>, but the influence of the AOD measure was very small. The radiation amplification factor (RAF) for the erythema action spectrum due to TO was <span class="inline-formula">−1.03</span> at constant levels of CC over the whole study period; that is, for a 1.0&thinsp;% increase in TO, <span class="inline-formula"><i>H</i>_er</span> decreases by 1.03&thinsp;%. 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 <span class="inline-formula"><i>H</i>_er</span> explained by the change in CC (47&thinsp;%) was much greater than the proportion explained by changes in TO (8&thinsp;%). For the second period (2004–2015), the pattern reversed, with the observed RAF related to TO being <span class="inline-formula">−1.25</span>, almost double that of CC (<span class="inline-formula">−0.65</span>). Furthermore, in this period the proportion of variation in <span class="inline-formula"><i>H</i>_er</span> explained by TO variation was 33&thinsp;%, double that of CC at 16&thinsp;%, while AOD changes had a negligible effect (1&thinsp;%).</p> <p>When the data were examined separately for each season, for the first period (1991–2004) the greatest effect of TO and CC on <span class="inline-formula"><i>H</i>_er</span> (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 <span class="inline-formula"><i>H</i>_er</span> (82&thinsp;%). In the later period (2004–2015), the RAF and greatest influence of TO and CC were observed in winter (67&thinsp;%) and the AOD effect explained a further 5&thinsp;% variability in <span class="inline-formula"><i>H</i>_er</span>.</p> <p>This study provides evidence that both the increasing trend in <span class="inline-formula"><i>H</i>_er</span> for 1991–2004 and the decreasing trend in <span class="inline-formula"><i>H</i>_er</span> 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 <span class="inline-formula"><i>H</i>_er</span> for 1991–2004 than TO, whereas for 2004–2015, the decreasing trend in <span class="inline-formula"><i>H</i>_er</span> is less associated with changes in CC and AOD.</p>