Short- and long-term variability of spectral solar UV irradiance at Thessaloniki, Greece: effects of changes in aerosols, total ozone and clouds

摘要

In this study, we discuss the short- and the long-term variability ofspectral UV irradiance at Thessaloniki, Greece, using a long,quality-controlled data set from two Brewer spectrophotometers. Long-termchanges in spectral UV irradiance at 307.5, 324 and 350 nm for the period1994–2014 are presented for different solar zenith angles and discussed inassociation with changes in total ozone column (TOC), aerosol optical depth(AOD) and cloudiness observed in the same period. Positive changes in annualmean anomalies of UV irradiance, ranging from 2 to 6 % per decade, havebeen detected both for clear- and all-sky conditions. The changes aregenerally greater for larger solar zenith angles and for shorter wavelengths.For clear-skies, these changes are, in most cases, statistically significantat the 95 % confidence limit. Decreases in the aerosol load and weakeningof the attenuation by clouds lead to increases in UV irradiance in thesummer, of 7–9 % per decade for 64° solar zenith angle. Theincreasing TOC in winter counteracts the effect of decreasing AOD for thisparticular season, leading to small, statistically insignificant, negativelong-term changes in irradiance at 307.5 nm. Annual mean UV irradiancelevels are increasing from 1994 to 2006 and remain relatively stablethereafter, possibly due to the combined changes in the amount and opticalproperties of aerosols. However, no statistically significant correspondingturning point has been detected in the long-term changes of AOD. The absenceof signatures of changes in AOD in the short-term variability of irradiancein the UV-A may have been caused by changes in the single scattering albedoof aerosols, which may counteract the effects of changes in AOD onirradiance. The anti-correlation between the year-to-year variability of theirradiance at 307.5 nm and TOC is clear and becomes clearer as the AODdecreases.