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Volume 16, issue 4
Atmos. Chem. Phys., 16, 2493-2505, 2016
https://doi.org/10.5194/acp-16-2493-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 2493-2505, 2016
https://doi.org/10.5194/acp-16-2493-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 01 Mar 2016

Research article | 01 Mar 2016

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

Ilias Fountoulakis1, Alkiviadis F. Bais1, Konstantinos Fragkos1, Charickleia Meleti1, Kleareti Tourpali1, and Melina Maria Zempila1,a Ilias Fountoulakis et al.
  • 1Aristotle University of Thessaloniki, Laboratory of Atmospheric Physics, Thessaloniki, Greece
  • anow at: Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, USA

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.

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Short- and long-term variability of spectral UV irradiance at Thessaloniki, Greece, is discussed in association with changes in total ozone column, aerosols and cloudiness. The UV data set from two Brewer spectrophotometers is used for the analysis. For the entire period 1994–2014, positive, statistically significant increases of UV irradiance were found, mainly attributable to changes in aerosols. UV irradiance is mainly increased from 1994 to 2006 and remains relatively stable thereafter.
Short- and long-term variability of spectral UV irradiance at Thessaloniki, Greece, is discussed...
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