Abstract
Abstract. In this study, we discuss the short- and the long-term variability of spectral UV irradiance at Thessaloniki, Greece, using a long, quality-controlled data set from two Brewer spectrophotometers. Long-term changes in spectral UV irradiance at 307.5, 324 and 350 nm for the period 1994–2014 are presented for different solar zenith angles and discussed in association with changes in total ozone column (TOC), aerosol optical depth (AOD) and cloudiness observed in the same period. Positive changes in annual mean anomalies of UV irradiance, ranging from 2 to 6 % per decade, have been detected both for clear- and all-sky conditions. The changes are generally greater for larger solar zenith angles and for shorter wavelengths. For clear-skies, these changes are, in most cases, statistically significant at the 95 % confidence limit. Decreases in the aerosol load and weakening of the attenuation by clouds lead to increases in UV irradiance in the summer, of 7–9 % per decade for 64° solar zenith angle. The increasing TOC in winter counteracts the effect of decreasing AOD for this particular season, leading to small, statistically insignificant, negative long-term changes in irradiance at 307.5 nm. Annual mean UV irradiance levels are increasing from 1994 to 2006 and remain relatively stable thereafter, possibly due to the combined changes in the amount and optical properties of aerosols. However, no statistically significant corresponding turning point has been detected in the long-term changes of AOD. The absence of signatures of changes in AOD in the short-term variability of irradiance in the UV-A may have been caused by changes in the single scattering albedo of aerosols, which may counteract the effects of changes in AOD on irradiance. The anti-correlation between the year-to-year variability of the irradiance at 307.5 nm and TOC is clear and becomes clearer as the AOD decreases.
Highlights
Ultraviolet (UV) radiation is only a small fraction (< 10 %) of the total solar radiation that reaches the Earth’s surface, it is vital for the life on Earth (Asta et al, 2011; Häder et al, 2015; Lucas et al, 2015; Madronich et al, 2015; UNEP, 2010; Williamson et al, 2014)
Solar radiation with wavelengths shorter than 290 nm is entirely blocked by the atmosphere, while for longer wavelengths the fraction that penetrates to the surface depends mainly on the solar zenith angle, the composition of the atmosphere and the characteristics of the surface (Kerr and Fioletov, 2008)
The present study aims at the quantification of the longterm changes in surface UV irradiance using spectral measurements which have been recorded since 1990 at Thessaloniki (Bais et al, 2001; Garane et al, 2006; Gröbner et al, 2006), one of the longest time series globally (Glandorf et al, 2005)
Summary
Ultraviolet (UV) radiation is only a small fraction (< 10 %) of the total solar radiation that reaches the Earth’s surface, it is vital for the life on Earth (Asta et al, 2011; Häder et al, 2015; Lucas et al, 2015; Madronich et al, 2015; UNEP, 2010; Williamson et al, 2014). Solar radiation with wavelengths shorter than 290 nm is entirely blocked by the atmosphere, while for longer wavelengths the fraction that penetrates to the surface depends mainly on the solar zenith angle, the composition of the atmosphere and the characteristics of the surface (Kerr and Fioletov, 2008). The interaction between the UV radiation, the atmospheric constituents and the characteristics of the surface is complicated and not yet fully understood (Bernhard et al, 2007; Kerr and Fioletov, 2008; Meinander et al, 2009). The geophysical parameters that mainly affect the levels of the surface UV irradiance are: ozone, clouds, surface reflectivity and aerosols (Arola et al, 2003; Bais et al, 1993; Bernhard et al, 2007; WMO, 2007).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
