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Relationship Between Erythema Effective UV Radiant Exposure, Total Ozone, Cloud Cover and Aerosols in Southern England, UK

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Relationship Between Erythema Effective UV Radiant Exposure, Total Ozone, Cloud Cover and Aerosols in Southern England, UK Atmos. Chem. Phys., 19, 683–699, 2019 https://doi.org/10.5194/acp-19-683-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Relationship between erythema effective UV radiant exposure, total ozone, cloud cover and aerosols in southern England, UK Nezahat Hunter, Rebecca J. Rendell, Michael P. Higlett, John B. O’Hagan, and Richard G. E. Haylock Public Health England, Chilton, Didcot, Oxfordshire, OX11 0RQ, UK Correspondence: Nezahat Hunter ([email protected]) Received: 8 August 2018 – Discussion started: 6 September 2018 Revised: 21 December 2018 – Accepted: 21 December 2018 – Published: 17 January 2019 Abstract. Evidence of an underlying trend in the dependence and CC on Her (i.e. the largest RAF value) was found dur- of erythema effective ultraviolet (UV) radiant exposure (Her) ing spring. Spring was also the season during which TO and on changes in the total ozone, cloud cover and aerosol optical CC variation explained the greatest proportion of variabil- depth (AOD) has been studied using solar ultraviolet radia- ity in Her (82 %). In the later period (2004–2015), the RAF tion measurements collected over a 25-year period (1991– and greatest influence of TO and CC were observed in winter 2015) at Chilton in the south of England in the UK. (67 %) and the AOD effect explained a further 5 % variability The monthly mean datasets of these measures corrected in Her. for underlying seasonal variation were analysed. When a sin- This study provides evidence that both the increasing trend gle linear trend was fitted over the whole study period be- in Her for 1991–2004 and the decreasing trend in Her for tween 1991 and 2015, the analyses revealed that the long- 2004–2015 occur in response to variation in TO, which ex- term variability of Her can be best characterised in two sub- hibits a small increasing tendency over these periods. CC periods (1991–2004 and 2004–2015), where the estimated plays a more important role in the increasing trend in Her for linear trend was upward in the first period (1991–2004) but 1991–2004 than TO, whereas for 2004–2015, the decreasing downward in the second period (2004–2015). trend in Her is less associated with changes in CC and AOD. Both cloud cover (CC) and total ozone (TO) were found to have a highly statistically significant influence on Her, but the influence of the AOD measure was very small. The ra- diation amplification factor (RAF) for the erythema action 1 Introduction 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, Ultraviolet (UV) radiation is only a small portion of the Her decreases by 1.03 %. Over the first period (1991–2004), radiation we receive from the sun, but it has become a the RAF related to CC was slightly higher at 0.97 compared topic of increasing concern because of the harmful health to that for TO at 0.79. The proportion of the change in Her effects it can cause. Stratospheric ozone is a naturally oc- explained by the change in CC (47 %) was much greater than curring gas that filters the sun’s UV radiation. It absorbs the proportion explained by changes in TO (8 %). For the most of the shorter wavelength UV-B radiation, whereas second period (2004–2015), the pattern reversed, with the longer wavelength UV-A radiation mostly passes through the observed RAF related to TO being −1:25, almost double that ozone layer and reaches the ground (WMO, 2014). How- of CC (−0:65). Furthermore, in this period the proportion of ever, in the mid-1970s it was discovered that the release of variation in Her explained by TO variation was 33 %, dou- man-made chlorine-containing chemicals could cause strato- ble that of CC at 16 %, while AOD changes had a negligible spheric ozone depletion. In subsequent years temporary effect (1 %). ozone holes appeared over the Antarctic and to a lesser ex- When the data were examined separately for each season, tent in the Arctic (Farman et al., 1985). Stratospheric ozone for the first period (1991–2004) the greatest effect of TO depletion was also detected over populated areas such as Australia, most notably in spring when the ozone layer over Published by Copernicus Publications on behalf of the European Geosciences Union. 684 N. Hunter et al.: Erythema effective UV radiant exposure, total ozone, cloud cover and aerosols Antarctica is dramatically thinned (Gies et al., 2013). Since increase during their study periods. In contrast, the Belgian the late 1970s, the effects of ozone depletion on UV radi- study reported the strong impact of TO, while the individual ation have been the subject of a large number of studies contribution of AOD to the non-significant negative trend of published in the literature. These studies have demonstrated the erythema UV dose was very low (De Bock et al., 2014). that the ozone level decreased up to the mid-1990s, result- Nevertheless, the influence of the aerosols on UV irradiance ing in an increase in the amount of UV radiation reaching is still not been fully understood due to high spatial and tem- the Earth’s surface (WHO, 2006). Concern was raised that in poral variability (WMO, 2007). the long term, ozone depletion would result in significantly In 1990, due to the concern that the depletion of the ozone increased ultraviolet radiation (UVR) which in turn may re- layer would cause an increase in population UV exposure sult in an increased incidence of skin cancers, particularly and possible impacts on human health, the former National melanoma. An increase in UVR may also cause other nega- Radiological Protection Board (NRPB – now part of Pub- tive health impacts, such as sunburn, ocular pathologies, pre- lic Health England, PHE) set up monitoring stations at three mature skin aging and a weakened immune system (AGNIR, locations in the UK to measure terrestrial solar UV (HPA, 2002; WHO, 2006; Lucas et al., 2010; UNEP, 2010; Nor- 2012). The Chilton monitoring site is located in a rural area in val et al., 2011). However, UV radiation exposure also has the south-east of England at approximately 51.6◦ N, 1.3◦ W. known benefits to health such as the production of vitamin D, An analysis of annual erythema effective UV radiant expo- which promotes healthy bones and may help in the preven- sure (Her) at Chilton over 25 years (1991–2015) has previ- tion of certain illnesses including heart diseases and cancers ously been carried out; the results revealed a statistically sig- (Holick, 2007; McKenzie et al., 2009; Young, 2009; Epplin nificant increasing linear trend between 1991 and 1995 and and Thomas, 2010). a small decreasing linear trend from 1995 to 2015 (Hooke et The Montreal Protocol came into effect in 1989, banning al., 2017). The analyses described in this paper are comple- multiple substances responsible for ozone depletion. From mentary to those undertaken by Hooke et al. (2017), which 1997 to the mid-2000s it became apparent that a decline in use the same data but with methodological differences, as total column ozone (TO) had stopped at almost all non-polar discussed later in the paper. In particular, this work focuses latitudes (WMO, 2007). However, the pace of the recov- on whether the long-term trend of monthly Her can be linked ery is affected by changes in temperatures, circulation and to changes in ozone, cloud cover and aerosols, the most sig- the nitrogen and hydrogen ozone-loss cycles (Waugh et al., nificant atmospheric factors that affect terrestrial UV. 2009). The ozone level has remained relatively unchanged since 2000, with most studies reporting a plateau or a limited 2 Materials and methods increase in total ozone (WMO, 2014). The most important factor affecting UV radiation at the 2.1 Erythema effective UV radiant exposure (Her) Earth’s surface is the elevation of the sun in the sky – this causes terrestrial ultraviolet radiation (UVR) to vary with Details of the methodology for UV monitoring at Chilton are time of day, day of the year and geographical location (Dif- presented elsewhere (Hooke et al., 2017). A short descrip- fey, 2002). Aside from solar elevation, the most significant tion of materials and methods is given here, and additional factors affecting solar UV radiation are stratospheric ozone analyses using the same data are pointed out. Erythema ef- and cloud cover (CC). A number of other factors also af- fective UV irradiance in the wavelength range 280–400 nm fect UV,including aerosol optical depth (AOD), other aerosol is measured by Robertson–Berger meters (RB-500 and RB- optical parameters, albedo and changes in other trace gases; 501 since 2004, manufactured by Solar Light Co. Philadel- climate change influences many of these factors (Bais et al., phia, USA). Data from these sensors are sampled to calculate 2011). These factors often interact with each other in a com- 5 min mean values. To convert these values to Her per day, the plex way, making their effect on terrestrial UV hard to quan- erythema effective UV irradiance data were summed up daily tify. from half an hour before sunrise to half an hour after sunset The effects of TO, clouds and aerosols on surface UV ir- under all weather conditions (Hooke and Higlett, 2016). The radiance have been studied widely in Europe (den Outer et units of Her are defined as the amount of energy (joules) de- al., 2010; Fitzka et al., 2012; Zerefos et al., 2012; De Bock et posited per square metre (J m−2).
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