Abstract. In this study, we discuss the short-and the longterm 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 anticorrelation between the year-to-year variability of the irradiance at 307.5 nm and TOC is clear and becomes clearer as the AOD decreases.

Abstract. In this study we investigate the climatological behavior of the aerosol optical properties over Thessaloniki during the years [2003][2004][2005][2006][2007][2008][2009][2010][2011][2012][2013][2014][2015][2016][2017]. For this purpose, measurements of two independent instruments, a lidar and a sunphotometer, were used. These two instruments represent two individual networks, the European Lidar Aerosol Network (EAR-LINET) and the Aerosol Robotic Network (AERONET). They include different measurement schedules. Fourteen years of lidar and sunphotometer measurements were analyzed, independently of each other, in order to obtain the annual cycles and trends of various optical and geometrical aerosol properties in the boundary layer, in the free troposphere, and for the whole atmospheric column. The analysis resulted in consistent statistically significant and decreasing trends of aerosol optical depth (AOD) at 355 nm of −23.2 and −22.3 % per decade in the study period over Thessaloniki for the EARLINET and the AERONET datasets, respectively. Therefore, the analysis indicates that the EAR-LINET sampling schedule can be quite effective in producing data that can be applied to long-term climatological studies. It is also shown that the observed decreasing trend is mainly attributed to changes in the aerosol load inside the boundary layer. Seasonal profiles of the most dominant aerosol mixture types observed over Thessaloniki have been generated from the lidar data. The higher values of the vertically resolved extinction coefficient at 355 nm appear in summer, while the lower ones appear in winter. The dust component is more dominant in the free troposphere than in the boundary layer during summer. The biomass burning layers tend to arrive in the free troposphere during spring and summer. This kind of information can be quite useful for applications that require a priori aerosol profiles. For instance, they can be utilized in models that require aerosol climatological data as input, in the development of algorithms for satellite products, and also in passive remote-sensing techniques that require knowledge of the aerosol vertical distribution.

The influence of variations in atmospheric temperature and ozone profiles on the total ozone column (TOC) derived from a Brewer MKII spectrophotometer operating in Thessaloniki, Greece, is investigated using three different sets of ozone absorption cross-sections. The standard Brewer total ozone retrieval algorithm uses the Bass and Paur (1985) cross-sections without accounting for the temperature dependence of the ozone cross-sections which produces a seasonally dependent bias in the measured TOC. The magnitude of this temperature effect depends on the altitude where the bulk of the ozone absorption occurs. Radiosonde measurements for the period 2000 to 2010 combined with climatological ozone profiles were used to calculate the effective temperature of ozone absorption and investigate its effect on the retrieved ozone column. Three different ozone absorption cross-section spectra convolved with the instrument's slit function were used: those of Bass and Paur (hereafter BP), currently used in the standard Brewer retrieval algorithm; those of Brion, Daumont, and Malicet (Malicet et al., 1985; hereafter BDM); and the recently published set by Serdyuchenko et al. (2013 hereafter S13). The temperature dependence of the differential ozone absorption coefficient ranges between 0.09 and 0.13% per degree Celsius for BP, between −0.11 and −0.06% per degree Celsius for BDM, and between 0.018 to 0.022% per degree Celsius for S13, resulting in a seasonal bias in the derived TOC of up to 2%, 1.8%, and 0.4%, respectively. The temperature sensitivity of the differential ozone absorption coefficient for the Brewer spectrophotometer at Thessaloniki for the BP and BDM cross-sections is found to be within the range reported for other Brewer instruments in earlier studies, whereas the seasonal bias in TOC is minimized when using the new S13 cross-sections because of their small temperature dependence.RÉSUMÉ [Traduit par la rédaction] Nous étudions l'influence des variations dans les profils de température atmosphérique et d'ozone sur la colonne d'ozone total dérivée d'un spectrophotomètre Brewer MKII en service à Thessaloniki, en Grèce, au moyen de trois ensembles différents de sections efficaces d'absorption de l'ozone. L'algorithme standard d'extraction de l'ozone total du Brewer utilise les sections efficaces de Bass et Paur (1985) sans tenir compte de la dépendance de la température des sections efficaces de l'ozone, ce qui produit un biais dépendant de la saison dans les colonnes d'ozone total mesurées. La grandeur de cet effet de la température dépend de l'altitude où se produit le gros de l'absorption de l'ozone. Nous nous sommes servis de mesures de radiosonde pour la période 2000 à 2010 de pair avec des profils d'ozone pour calculer la température effective d'absorption de l'ozone et étudier son effet sur la colonne d'ozone extraite. Nous avons utilisé trois spectres différents de sections efficaces d'absorption de l'ozone convolués avec la fonction de fente de l'instrument : celles de Bass et Paur (ci-après BP), présentem...

Ultraviolet-B (UV-B), UV-A, and erythemal solar irradiance over ocean-covered areas north of 55°N are simulated for the past (1950-1960), present (2005-2015), and future (2090-2100) using a radiative transfer model. The simulations focus mainly on the effects of changes in ocean surface reflectivity, cloudiness, and stratospheric ozone. Based on projected changes in sea ice cover and thickness, changes in irradiance transmitted into the ocean are also derived. The input parameters of the radiative transfer model were obtained from four Coupled Model Intercomparison Project phase 5 Earth System Models driven by the emission scenarios Representative Concentration Pathway (RCP) 4.5 and RCP 8.5. Over a large fraction of the area under study, the overall effect from the projected changes in the factors considered is a reduction in the ultraviolet solar irradiance by the end of the 21st century relative to the levels in the 1950s. Increases were projected only for all skies during August for locations below 65°N due to the projected decrease in cloudiness. The reduction in clear-sky UV-A irradiance (on average 4-7% depending on scenario and season) is entirely driven by the reduction in surface reflectivity, while the projected ozone recovery is responsible for a great portion of the reduction in clear-sky UV-B irradiance (10-18% on average). Under all skies, the changes in the monthly mean noontime erythemal irradiance range from +15% to À38%, depending on the location and season. Compared to the 1950s, up to 10 times higher levels of UV-B irradiance are projected to enter large parts of the Arctic Ocean by 2100, mainly because of the partial disappearance of sea ice.

Abstract. The main aim of the paper is to demonstrate an approach for post-processing of the Dobson spectrophotometers' total ozone columns (TOCs) in order to compensate for their known stratospheric effective temperature (Teff) dependency and its resulting effect on the usage of the Dobson TOCs for satellite TOCs' validation. The Dobson observations employed are those routinely submitted to the World Ozone and Ultraviolet Data Centre (WOUDC) of the World Meteorological Organization, whereas the effective temperatures have been extracted from two sources: the European Space Agency, ESA, Ozone Climate Change Initiative, Ozone-CCI, GODFIT version 3 (GOME-type Direct FITting) algorithm applied to the GOME2/MetopA, GOME2A, observations as well as the one derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) outputs. Both temperature sources are evaluated utilizing co-located ozonesonde measurements also retrieved from the WOUDC database. Both GODFIT_v3 and ECMWF Teffs are found to be unbiased against the ozonesonde observations and to agree with high correlation coefficients, especially for latitudes characterized by high seasonal variability in Teff. The validation analysis shows that, when applying the GODFIT_v3 effective temperatures in order to post-process the Dobson TOC, the mean difference between Dobson and GOME2A GODFIT_v3 TOCs moves from 0.63 ± 0.66 to 0.26 ± 0.46 % in the Northern Hemisphere and from 1.25 ± 1.20 to 0.80 ± 0.71 % in the Southern Hemisphere. The existing solar zenith angle dependency of the differences has been smoothed out, with near-zero dependency up to the 60–65° bin and the highest deviation decreasing from 2.38 ± 6.6 to 1.37 ± 6.4 % for the 80–85° bin. We conclude that the global-scale validation of satellite TOCs against collocated Dobson measurements benefits from a post-correction using suitably estimated Teffs.