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The paper presents the first retrievals of clean snow properties using spaceborne hyperspectral observations via the Environmental Mapping and Analysis Program (EnMAP). The location close to the Concordia station at the Dome C Plateau (Antarctica) was selected. At this location, the atmospheric effects (except molecular light scattering and absorption) are weak, and the simplified atmospheric correction scheme could be applied. The ice grain size, snow specific surface area, and snow spectral and broadband albedos were retrieved using single-view EnMAP measurements. In addition, we propose a technique to retrieve trace gas concentrations (e.g., water vapor and ozone) from EnMAP observations over the snow surfaces. A close correspondence of satellite and ground-measured parameters was found.

The paper presents the first retrievals of clean snow properties using spaceborne hyperspectral The Environmental Mapping and Analysis Program (EnMAP) observations. The location close to the Dome C in Antarctica has been selected. At this location the atmospheric effects except molecular light scattering and absorption are weak and the simplified atmospheric correction scheme can be applied. The ice grain size, snow specific surface area and snow spectral and broadband albedos have been retrieved using single view EnMAP measurements. In addition, we propose the technique to retrieve trace gas concentrations (e.g., water vapor, ozone) from EnMAP observations over the snow surfaces. Close correspondence of satellite and ground measured parameters has been found.

After the decrease of ozone-depleting substances (ODSs) as a consequence of the Montreal Protocol, it is still challenging to detect a recovery in the total column amount of ozone (total ozone) at northern high latitudes. To assess regional total ozone changes in the “ozone-recovery” period (2000–2020) at northern high latitudes, this study investigates trends from ground-based total ozone measurements at three stations in Norway (Oslo, Andøya, and Ny-Ålesund). For this purpose, we combine measurements from Brewer spectrophotometers, ground-based UV filter radiometers (GUVs), and a SAOZ (Système d'Analyse par Observation Zénithale) instrument. The Brewer measurements have been extended to work under cloudy conditions using the global irradiance (GI) technique, which is also presented in this study. We derive trends from the combined ground-based time series with the multiple linear regression model from the Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS) project. We evaluate various predictors in the regression model and found that tropopause pressure and lower-stratospheric temperature contribute most to ozone variability at the three stations. We report significantly positive annual trends at Andøya (0.9±0.7 % per decade) and Ny-Ålesund (1.5±0.1 % per decade) and no significant annual trend at Oslo (0.1±0.5 % per decade) but significantly positive trends in autumn at all stations. Finally we found positive but insignificant trends of around 3 % per decade in March at all three stations, which may be an indication of Arctic springtime ozone recovery. Our results contribute to a better understanding of regional total ozone trends at northern high latitudes, which is essential to assess how Arctic ozone responds to changes in ODSs and to climate change.

The UV-index (UVI) is a measure of the erythemally effective solar radiation reaching the Earth surface and it was introduced to alert people about the need of Sun protection. The present study applies a model that estimates the UVI over the high Bulgarian mountains for clear sky conditions considering the Total Ozone Content (TOC), which was taken from satellite measurements. The results show that during the periods from May to August at altitudes above 2 000 m a.s.l. very high UVI's (greater than 8) were observed for more than 18 days per month. The UVI values were very high practically for every day of July at altitudes higher than 1 500 m. Extremely high UVI result from episodes with TOC lower than 290 DU during June and July at the highest mountain parts with elevations greater than 2 500 m. High radiation risks were observed during April, especially when the preceding polar vortex was strong and the mountains were snow covered.

The atmospheric humidity in the Polar Regions is an important factor for the global budget of water vapour, which is a significant indicator of Earth’s climate state and evolution. The Global Navigation Satellite System (GNSS) can make a valuable contribution in the calculation of the amount of Precipitable Water Vapour (PW). We focus on Polar Regions, especially Antarctica. 20-year GPS observations, acquired by more than 40 GNSS geodetic stations, were processed with the purpose of ensuring the utmost accuracy of the PW retrieval, adopting homogeneous, consistent, and up-to-date processing strategies. We also estimated PW from radio-sounding stations (RS), which operate Vaisala radiosondes, co-located with GNSS stations. The PW values from global atmospheric reanalysis model were used for validation and comparison, very high correlation coefficients between times series, have been highlighted both in the Arctic and Antarctica. A small dry bias of RS vs. GPS values was found in the Arctic, while no clear behaviour is present in Antarctica. The PWGPS and PWRS seasonal variations are consistent, as also confirmed by scatter plots. After validation, long-term trends, both for Arctic and Antarctic regions, were estimated using Hector scientific software, which allows the estimation of trends from time series with temporal correlated noise. We applied a function to estimate the linear trend plus the annual/semiannual signals, and autoregressive noise model AR(1) which best fits the residuals of all investigated PW time series. We investigated also on the choice of the most suitable noise model, this study was useful in determining the residuals of the time series, once the trend and seasonal signals were subtracted. Positive PWGPS trends dominate at Arctic sites near the borders of the Atlantic Ocean. Sites located at higher latitudes show no significant values. Negative PWGPS trends were observed in the Arctic region of Greenland and North America. A similar behaviour was found in the Arctic for PWRS trends. The stations in the West Antarctic sector show a general positive PWGPS trend, while the sites on the coastal area of East Antarctica exhibit some significant negative PWGPS trends, while in most cases, no significant PWRS trends were found. The present work confirms also that GPS is also able to provide reliable estimates of water vapour content in regions where data are sparse and not easy to collect as the Arctic and Antarctic regions are.

Even though ozone-depleting substances have been substantially reduced due to the Montreal Protocol, it is still not possible to state with confidence that the total column amount of ozone (total ozone) recovers globally. A special focus lies on high latitudes, as they experienced strong stratospheric ozone depletion in the 1980s and 1990s. Especially at northern high latitudes, it is still challenging to detect significant total ozone trends. It is therefore important to use carefully homogenized and stable long-term ozone measurements and advanced trend models to derive ozone trends at northern high latitudes.This study uses ground-based total ozone measurements in Norway and the Arctic to investigate total ozone trends at northern high latitudes. We present combined total ozone time series from Brewer Spectrophotometers at Oslo (60°N) and Andøya (69°N) in Norway, from 2000 to 2020. In addition, measurements from a SAOZ instrument and a Brewer at Ny-Ålesund in Svalbard are used. The combined Brewer time series consist of direct sun (DS) and global irradiance (GI) Brewer measurements and are complemented with measurements from ground-based ultraviolet radiometers (GUV). This makes it possible to obtain measurements during cloudy conditions and in winter and spring, where DS measurements cannot be retrieved due to large solar zenith angles and reduced direct sunlight.We present total ozone trends at the three measurement stations using the LOTUS (Long-term Ozone Trends and Uncertainties in the Stratosphere) multilinear regression model. We test various explanatory variables and select a set of predictors to obtain the best possible regression fit. We found that besides the commonly used predictors QBO, ENSO, and solar cycle, tropopause pressure and stratospheric temperature are also important to improve the fit. We finally present annual total ozone trends and trends for different months at each station. Despite that the annual trends were generally found to be insignificant, we detected significant trends in some months.We believe that our study contributes to a better understanding of long-term ozone changes at northern high latitudes, which is essential to assess how Arctic ozone responds to changes in ozone depleting substances and to climate change.

The atmospheric humidity in the Polar Regions is an important factor for the global budget of water vapour, which is a significant indicator of Earth’s climate state and evolution. The Global Navigation Satellite System (GNSS) can make a valuable contribution in the calculation of the amount of Precipitable Water Vapour (PW). The PW values retrieved from Global Positioning System (GPS), hereafter PWGPS, refer to 20-year observations acquired by more than 40 GNSS geodetic stations located in the polar regions. For GNSS stations co-located with radio-sounding stations (RS), which operate Vaisala radiosondes, we estimated the PW from RS observations (PWRS). The PW values from the ERA-Interim global atmospheric reanalysis were used for validation and comparison of the results for all the selected GPS and RS stations. The correlation coefficients between times series are very high: 0.96 for RS and GPS, 0.98 for RS and ERA in the Arctic; 0.89 for RS and GPS, 0.97 for RS and ERA in Antarctica. The Root-Mean-Square of the Error (RMSE) is 0.9 mm on average for both RS vs. GPS and RS vs. ERA in the Arctic, and 0.6 mm for RS vs. GPS and 0.4 mm for RS vs. ERA in Antarctica. After validation, long-term trends, both for Arctic and Antarctic regions, were estimated using Hector scientific software. Positive PWGPS trends dominate at Arctic sites near the borders of the Atlantic Ocean. Sites located at higher latitudes show no significant values (at 1σ level). Negative PWGPS trends were observed in the Arctic region of Greenland and North America. A similar behaviour was found in the Arctic for PWRS trends. The stations in the West Antarctic sector show a general positive PWGPS trend, while the sites on the coastal area of East Antarctica exhibit some significant negative PWGPS trends, but in most cases, no significant PWRS trends were found. The present work confirms that GPS is able to provide reliable estimates of water vapour content in Arctic and Antarctic regions too, where data are sparse and not easy to collect. These preliminary results can give a valid contribution to climate change studies.

The present study discusses the effect of the ozone depletion that occurred over the Arctic in 2020 on the ozone column in central and southern Europe by analysing a data set obtained from ground-based measurements at six stations placed from 79 to 42°N. Over the northernmost site (Ny-Alesund), the ozone column decreased by about 45% compared to the climatological average at the beginning of April, and its values returned to the normal levels at the end of the month. Southwards, the anomaly gradually reduced to nearly 15% at 42°N (Rome) and the ozone minimum was detected with a delay from about 6 days at 65°N to 20 days at 42°N. At the same time, the evolution of the ozone column at the considered stations placed below the polar circle corresponded to that observed at Ny-Alesund, but at 42°–46°N, the ozone column turned back to the typical values at the end of May. This similarity in the ozone evolutional patterns at different latitudes and the gradually increasing delay of the minimum occurrences towards the south allows the assumption that the ozone columns at lower latitudes were affected by the phenomenon in the Arctic. The ozone decrease observed at Aosta (46°N) combined with predominantly cloud-free conditions resulted in about an 18% increase in the erythemally weighted solar ultraviolet irradiance reaching the Earth’s surface in May.

Measurements of spectral irradiance between 306 and 1020 nm were performed with a GUVis-3511 multi-channel filter radiometer at Smith Rock State Park, Oregon, during the total solar eclipse of 21 August 2017. The radiometer was equipped with a shadowband, allowing the separation of the global (sun and sky) and direct components of solar radiation. Data were used to study the wavelength-dependent changes in solar irradiance at Earth's surface. Results were compared with theoretical predictions using three different parameterizations of the solar limb darkening (LD) effect, which describes the change in the solar spectrum from the Sun's center to its limb. Results indicate that the LD parameterization that has been most widely used during the last 15 years underestimates the LD effect, in particular at UV wavelengths. The two alternative parameterizations are based on two independent sets of observations from the McMath–Pierce solar telescope. When these parameterizations are used, the observed and theoretical LD effects agree to within 4 % for wavelengths larger than 400 nm and occultation of the solar disk of up to 97.8 %. Maximum deviations for wavelengths between 315 and 340 nm are 7 %. These somewhat larger differences compared to the visible range may be explained with varying aerosol conditions during the period of observations. The aerosol optical depth (AOD) and its wavelength dependence was calculated from measurements of direct irradiance. When corrected for the LD effect, the AOD decreases over the period of the eclipse: from 0.41 to 0.34 at 319 nm and from 0.05 to 0.04 at 1018 nm. These results show that AODs can be accurately calculated during an eclipse if the LD effect is corrected. The total ozone column (TOC) was derived from measurements of global irradiance at 306 and 340 nm. Without correction for the LD effect, the retrieved TOC increases by 20 DU between the first and second contact of the eclipse. With LD correction, the TOC remains constant to within natural variability (±2.6 DU or ±0.9 % between first and second contact and ±1.0 DU or ±0.3 % between third and fourth contact). In contrast to results of observations from earlier solar eclipses, no fluctuations in TOC were observed that could be unambiguously attributed to gravity waves, which can be triggered by the supersonic speed of the Moon's shadow across the atmosphere. Furthermore, systematic changes in the ratio of direct and global irradiance that could be attributed to the solar eclipse were not observed, in agreement with results of three-dimensional (3-D) radiative transfer (RT) models. Our results advance the understanding of the effects of solar LD on the spectral irradiance at Earth's surface, the variations in ozone during an eclipse, and the partitioning of solar radiation in direct and diffuse components.

In a such changing environment like the Arctic, improving the understanding of the thermodynamic state and processes of the atmosphere is critical for the development of accurate prediction and climatic models. This is fundamental for example for studies on sea-ice development as well as on cloud formation. Taking into account the above remarks, it is very important to know the pressure, temperature and moisture conditions of the Arctic atmosphere throughout the year and over the whole tropospheric and stratospheric altitude range. Furthermore, knowing these data is necessary to realistically evaluate the radiative effects involving both the short-wave and long-wave radiation fluxes, which regulate the energy balance of the Arctic surface-atmosphere system.

In this chapter remote sensing techniques as applied to studies of Arctic aerosol are surveyed. They include the analysis of ground and shipborne observations of atmospheric aerosol using sunphotometers and also airborne/satellite observations using optical instrumentation (lidars, imagers, radiometers).

Arctic surface temperature has been increasing at a rate 2–3 times that of the global average in the last half century. Enhanced warming of the Arctic, or Arctic Amplification, is a climatic response to external forcing. Despite good results obtained by climatic models for the globe, the largest intermodel differences in surface temperature warming are found in the Arctic. The magnitude of this warming drives many different processes and determines the evolution of many climatic parameters such as clouds, sea ice extent, and land ice sheet mass. The Arctic Amplification can be attributed to the peculiar feedback processes that are triggered in the Arctic. Most of these processes include radiation interaction with the atmosphere and with the surface, all of them contributing to the radiation budget. It is then mandatory to correctly evaluate this budget both at the surface and at the top of the atmosphere and in the solar and thermal spectra. This can be done using both direct observations, from ground and from space, and model simulation via radiation transfer codes. This last approach need many observed input parameters anyhow.

Continuous all year-round samplings of atmospheric aerosol and surface snow at high (daily to 4-day) resolution were carried out at Dome C since 2004-05 to 2013 and nitrate records are here presented. Basing on a larger statistical data set than previous studies, results confirm that nitrate seasonal pattern is characterized by maxima during austral summer for both aerosol and surface snow, occurring in-phase with solar UV irradiance. This temporal pattern is likely due to a combination of nitrate sources and post-depositional processes whose intensity usually enhances during the summer. Moreover, it should be noted that a case study of the synoptic conditions, which took place during a major nitrate event, showed the occurrence of a stratosphere-troposphere exchange. The sampling of both matrices at the same time with high resolution allowed the detection of a an about one-month long recurring lag of summer maxima in snow with respect to aerosol. This result can be explained by deposition and post-deposition processes occurring at the atmosphere-snow interface, such as a net uptake of gaseous nitric acid and a replenishment of the uppermost surface layers driven by a larger temperature gradient in summer. This hypothesis was preliminarily tested by a comparison with surface layers temperature data in the 2012-13 period. The analysis of the relationship between the nitrate concentration in the gas phase and total nitrate obtained at Dome C (2012-13) showed the major role of gaseous HNO3 to the total nitrate budget suggesting the need to further investigate the gas-to-particle conversion processes. © 2017 Elsevier Ltd

Measurements of spectral irradiance between 306 and 1020 nm were performed with a GUVis-3511 multi-channel filter radiometer at Smith Rock State Park, Oregon, during the total solar eclipse of 21 August 2017. The radiometer was equipped with a shadowband, allowing to separate the global (sun and sky) and direct components of solar radiation. Data were used to study the wavelength-dependent changes of solar irradiance at Earth's surface. Results were compared with theoretical predictions using three different parameterizations of the solar limb darkening (LD) effect, which describes the change of the solar spectrum from the Sun's center to its limb. Results indicate that the LD parameterization that has been most widely used during the last 15 years underestimates the LD effect, in particular at UV wavelengths. The two alternative parameterizations are based on two independent sets of observations from the McMath-Pierce Solar Telescope. When these parameterizations are used, the observed and theoretical LD effects agree to within 4 % for wavelengths larger than 400 nm and occultation of the solar disk of up to 97.8 %. Maximum deviations for wavelengths between 315 and 340 nm are 7 %. These somewhat larger differences compared to the visible range may be explained with varying aerosol conditions during the period of observations. Aerosol optical depth (AOD) and its wavelength dependence was calculated from measurements of direct irradiance. When corrected for the LD effect, AOD monotonically decreases over the period of the eclipse: from 0.41 to 0.32 at 319 nm and from 0.05 to 0.04 at 1018 nm. These results show that AODs can be accurately calculated during an eclipse if the LD effect is corrected. The total ozone column (TOC) was derived from measurements of global irradiance at 306 and 340 nm. Without correction for the LD effect, the retrieved TOC increases by 20 DU between the 1st and 2nd contact of the eclipse. With LD correction, the TOC remains constant to within natural variability (±2.6 DU or ±0.9 % between 1st and 2nd contact and ±1.0 DU or ±0.3 % between 3rd and 4th contact). In contrast to results of observations from earlier solar eclipses, no fluctuations in TOC were observed that could be attributed to gravity waves, which can be triggered by the supersonic speed of the Moon's shadow across the atmosphere. Furthermore, systematic changes in the ratio of direct and global irradiance that could be attributed to the solar eclipse were not observed. This finding agrees with results of three-dimensional radiative transfer models but contradicts reports from earlier observations, which indicate that the diffuse-to-direct ratio may change by 30 %. Our results advance the understanding of the effects of solar LD on the spectral irradiance at Earth's surface, the variations of ozone during an eclipse, and the partitioning of solar radiation in direct and diffuse components.

The atmospheric water vapor is an important indicator of the Earth's climate state and evolution. We therefore aimed at calculating the content and long-term variation of the precipitable water vapor at five coastal Antarctic stations, i.e., Casey, Davis, Mawson, McMurdo, and Mario Zucchelli. To do that, we processed the 12-year time series of GPS and radiosounding (RS) observations acquired at those stations, with the purpose of ensuring the utmost accuracy of the results adopting homogeneous, consistent, and up-to-date processing strategies for both data sets. Using the two fully independent techniques, rather consistent contents and seasonal variations of precipitable water were detected, mainly ranging from 1 (Austral winter) to 10 mm (Austral summer). At each site, correlation coefficients varying from 0.86 to 0.91 were found between the GPS and RS time series, with mean discrepancies $\leq$0.75 mm. There is no clear indication regarding the possible dry or wet biases of one technique with respect to the other, with only a notable GPS wet bias identified at Mawson and a dry bias at Casey that, nevertheless, correspond to an average difference of $

The features of erythemally weighted (EW) and short-wave downwelling (SWD) solar irradiances, observed during the spring-summer months of 2007-2011 at Johann Gregor Mendel (63 degrees 48'S, 57 degrees 53'W, 7 m a.s.I.) and Dome Concordia (75 degrees 06'S, 123 degrees 21'E, 3233 m a.s.l.) stations, placed at the Antarctic coastal region and on the interior plateau respectively, have been analysed and compared to each other. The EW and SWD spectral components have been presented by the corresponding daily integrated values and were examined taking into account the different geographic positions and different environmental conditions at both sites. The results indicate that at Mendel station the surface solar irradiance is strongly affected by the changes in the cloud cover, aerosols and albedo that cause a decrease in EW between 20% and 35%, and from 0% to 50% in SWD component, which contributions are slightly lower than the seasonal SWD variations evaluated to be about 71%. On the contrary, the changes in the cloud cover features at Concordia station produce only a 5% reduction of the solar irradiance, whilst the seasonal oscillations of 94% turn out to be the predominant mode. The present analysis leads to the conclusion that the variations in the ozone column cause an average decrease of about 46% in EW irradiance with respect to the value found in the case of minimum ozone content at each of the stations. In addition, the ratio between EW and SWD spectral components can be used to achieve a realistic assessment of the radiation amplification factor that quantifies the relationship between the atmospheric ozone and the surface UV irradiance. (C) 2016 Elsevier B.V. All rights reserved.

This work focuses on the near-surface O3 variability over the eastern Antarctic Plateau. In particular, eight years (2006–2013) of continuous observations at the WMO/GAW contributing station “Concordia” (Dome C–DMC: 75°06′S, 123°20′E, 3280 m) are presented, in the framework of the Italian Antarctic Research Programme (PNRA). First, the characterization of seasonal and diurnal O3 variability at DMC is provided. Then, for the period of highest data coverage (2008–2013), we investigated the role of specific atmospheric processes in affecting near-surface summer O3 variability, when O3 enhancement events (OEEs) are systematically observed at DMC (average monthly frequency peaking up to 60% in December). As deduced by a statistical selection methodology, these OEEs are affected by a significant interannual variability, both in their average O3 values and in their frequency. To explain part of this variability, we analyzed OEEs as a function of specific atmospheric variables and processes: (i) total column of O3 (TCO) and UV-A irradiance, (ii) long-range transport of air masses over the Antarctic Plateau (by Lagrangian back-trajectory analysis – LAGRANTO), (iii) occurrence of “deep” stratospheric intrusion events (by using the Lagrangian tool STLEFLUX). The overall near-surface O3 variability at DMC is controlled by a day-to-day pattern, which strongly points towards a dominating influence of processes occurring at “synoptic” scales rather than “local” processes. Even if previous studies suggested an inverse relationship between OEEs and TCO, we found a slight tendency for the annual frequency of OEEs to be higher when TCO values are higher over DMC. The annual occurrence of OEEs at DMC seems related to the total time spent by air masses over the Antarctic plateau before their arrival to DMC, suggesting the accumulation of photochemically-produced O3 during the transport, rather than a more efficient local production. Moreover, the identification of recent (i.e., 4-day old) stratospheric intrusion events by STEFLUX suggested only a minor influence (up to 3% of the period, in November) of “deep” events on the variability of near-surface summer O3 at DMC. © 2018 The Authors

The vertical features of the variations in the atmospheric ozone density, temperature and wind velocity observed at Ny-Alesund, Svalbard were studied by applying the principal component analysis to the ozonesounding data collected during the 1992–2016 period. Two data sets corresponding to intra-seasonal (IS) variations, which are composed by harmonics with lower than 1 year periods and inter-annual (IA) variations, characterised by larger periods, were extracted and analysed separately. The IS variations in all the three parameters were found to be composed mainly by harmonics typical for the Madden-Julian Oscillation (from 30- to 60-day periods) and, while the first four principal components (PCs) associated with the temperature and wind contributed about 90% to the IS variations, the ozone IS oscillations appeared to be a higher dimensional object for which the first 15 PCs presented almost the same extent of contribution. The IA variations in the three parameters were consisted of harmonics that correspond to widely registered over the globe Quasi-Biennial, El Nino-Southern, North Atlantic and Arctic Oscillations respectively, and the IA variations turned out to be negligible below the tropopause that characterises the Svalbard troposphere as comparatively closed system with respect to the long-period global variations. The behaviour of the first and second PCs associated with IS ozone variations in the time of particular events, like the strong ozone depletion over Arctic in the spring 2011 and solar eclipses was discussed and the changes in the amplitude-frequency features of these PCs were assumed as signs of the atmosphere response to the considered phenomena.

A two-term algorithm is defined to evaluate the Rayleigh-scattering optical depth (ROD) in the Arctic and Antarctic atmospheres at ultraviolet, visible, and near-infrared wavelengths. The first term accounts for the tropospheric contribution up to 8 km altitude and the second for the upper troposphere and low stratosphere (UTLS) region and the upper atmosphere up to 120 km altitude. In these calculations, the vertical profiles of pressure p(z) and temperature T(z) are used, as derived from the multiyear sets of radiosounding measurements performed from 2000 to 2006 at four Arctic stations (Cambridge Bay, Resolute Bay, Danmarkshavn, and Alert) and four Antarctic stations (Neumayer, McMurdo, Dome C, and South Pole). By analyzing the radiosonde data sets collected at the eight sites, the daily average values of tropospheric ROD are calculated and their seasonal variations versus the mean tropospheric temperature and surface level temperature are determined in order to evaluate the temperature correction functions for tropospheric ROD. Similarly, the vertical profiles of p(z) and T(z) defined in the stratosphere and upper atmosphere are examined in order to (i) calculate the “stratospheric” ROD contribution, (ii) analyze its annual temporal variations, (iii) calculate the average temperature in the UTLS region, and (iv) determine the most suitable temperature anomaly corrections at the eight chosen polar sites. The spectral evaluations of the tropospheric and “stratospheric” ROD terms are given at 21 selected wavelengths from 0.30 to 4.00 µm for each polar site. The present two-term algorithm incorporates the tropospheric and “stratospheric” ROD terms, along with the correction factors derived for the actual pressure and temperature conditions, and can be used to calculate accurate overall ROD values at the given Arctic and Antarctic sites for the average annual thermal conditions of the atmosphere and the average seasonal variations characterizing the temperature conditions of the UTLS region. The ROD estimates obtained using the present method constitutes a nontrivial improvement over estimates made with the traditional method.

The diurnal variations observed in the ozone column at Ny-Alesund, Svalbard during different periods of 2009, 2010 and 2011 have been examined to test the hypothesis that they could be a result of a chaotic process. It was found that each of the attractors, reconstructed by applying the time delay technique and corresponding to any of the three time series can be embedded by 6-dimensional space. Recurrence plots, depicted to characterise the attractor features revealed structures typical for a chaotic system. In addition, the two positive Lyapunov exponents found for the three attractors, the fractal Hausdorff dimension presented by the Kaplan–Yorke estimator and the feasibility to predict the short-term ozone column variations within 10–20 h, knowing the past behaviour make the assumption about their chaotic character more realistic. The similarities of the estimated parameters in all three cases allow us to hypothesise that the three time series under study likely present one-dimensional projections of the same chaotic system taken at different time intervals.

The sequences, given by a 7D map, are analysed by means of the methods, widely used to detect chaos in the real world in order to test their sensitivity to chaotic features of a non-linear system, determined by comparatively high number of parameters. The same diagnostic approaches are applied to the 3D Lorenz map for comparison. The results show that for some of the sequences yielded from the 7D map, the adopted methods are not able to give a straightforward answer to the question if the system is chaotic as in the 3D case. Since the sequences, subject to the analysis, are not contaminated by noise and are sufficiently long, it could be assumed that such difficulties arise likely due to specific internal features of the more complex system. It is found also that an increase from 0.01 to 0.5 of the sampling step, determining the sequences obtained from the 7D map, masks the chaos in some of them.

New calculations of the relative optical air mass function are made over the 0°–87° range of apparent solar zenith angle θ, for various vertical profiles of background aerosol, diamond dust and thin cirrus cloud particle extinction coefficient in the Arctic and Antarctic atmospheres. The calculations were carried out by following the Tomasi and Petkov (2014) procedure, in which the above-mentioned vertical profiles derived from lidar observations were used as weighting functions. Different sets of lidar measurements were examined, recorded using: (i) the Koldewey-Aerosol-Raman Lidar (KARL) system (AWI, Germany) at Ny-Ålesund (Spitsbergen, Svalbard) in January, April, July and October 2013, (ii) the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite-based sensor over Barrow (Alaska), Eureka (Nunavut, Canada) and Sodankylä (northern Finland), and Neumayer III, Mario Zucchelli and Mirny coastal stations in Antarctica in the local summer months of the last two years, (iii) the National Institute of Optics (INO), National Council of Research (CNR) Antarctic lidar at Dome C on the Antarctic Plateau for a typical “diamond dust” case, and (iv) the KARL lidar at Ny-Ålesund and the University of Rome/National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) lidar at Thule (northwestern Greenland) for some cirrus cloud layers in the middle and upper troposphere. The relative optical air mass calculations are compared with those obtained by Tomasi and Petkov (2014) to define the seasonal changes produced by aerosol particles, diamond dust and cirrus clouds. The results indicate that the corresponding air mass functions generally decrease as angle θ increases with rates that are proportional to the increase in the pure aerosol, diamond dust and cirrus cloud particle optical thickness.

The UV Index was established more than 20 years ago as a tool for sun protection and health care. Shortly after its introduction, UV Index monitoring started in several countries either by newly acquired instruments or by converting measurements from existing instruments into the UV Index. The number of stations and networks has increased over the years. Currently, 160 stations in 25 European countries deliver online values to the public via the Internet. In this paper an overview of these UV Index monitoring sites in Europe is given. The overview includes instruments as well as quality assurance and quality control procedures. Furthermore, some examples are given about how UV Index values are presented to the public. Through these efforts, 57% of the European population is supplied with high quality information, enabling them to adapt behaviour. Although health care, including skin cancer prevention, is cost-effective, a proportion of the European population still doesn't have access to UV Index information.

The dependence functions of relative optical air mass on apparent solar zenith angle θ have been calculated over the θ

For most parts of the year the Antarctic Plateau has a surface temperature inversion with strength c. 20 K. Under such conditions the warmer air at the top of the inversion layer contributes more to the clear sky atmospheric longwave radiation at surface level than does the colder air near the ground. Hence, it is more appropriate to relate longwave irradiance (LWI) to the top of the inversion layer temperature (Tm) than to the ground level temperature (Tg). Analysis of radio soundings carried out at Dome C and South Pole during 2006–08 shows that the temperature at 400 m above the surface (T400) is a good proxy for Tm and is linearly related to Tg with correlation coefficients greater than 0.8. During summer, radiosonde measurements show almost isothermal conditions, hence T400 still remains a good proxy for the lower troposphere maximum temperature. A methodology is presented to parameterize the clear sky effective emissivity in terms of the troposphere maximum temperature, using ground temperature measurements. The predicted LWI values for both sites are comparable with those obtained using radiative transfer models, while for Dome C the bias of 0.8 W m-2 and the root mean square (RMS) of 6.2 W m-2 are lower than those calculated with previously published parametric equations.

Variations in total ozone column and sun exposures able to cause erythema and damage the DNA molecules were observed by the narrow-band filter radiometer UV-RAD in Bologna, Italy from 2005 to 2010. The ozone columns determined from the UV-RAD measurements were found to be close to those provided by the satellite Ozone Monitoring Instrument (OMI) showing an average discrepancy of 1% with standard deviation of ± 6%. Analysis of the data highlights a well-marked annual cycle of the ozone column variations while the oscillations with periods of 8, 18 and 34 months present much smaller amplitudes. The influence of the frequency of solar irradiance measurements on the accuracy of the evaluated daily exposure dose has been studied and it was found that time intervals no longer than 5-10 min between the measurements of erythema and DNA damage effective UV irradiances provide a satisfactory assessment of the corresponding daily exposures. The latter do not present significant year-to-year variations for the period under study, while their annual distributions show slight changes likely due to the specific cloud cover and ozone column variability for different years. The annual erythemal exposure dose for 2007-2010 varied between 603.7 and 638.1 kJ m(-2), while the corresponding sun exposure affecting DNA changed from 6.38 to 7.91 kJ m(-2).

The object of this study was to investigate the different modal behavior of ultrafine aerosol particles collected at the Gruvebadet observatory located in Ny-lesund (Svalbard Islands, 78A degrees 55'N, 11A degrees 56'E). Aerosol particle size distribution was measured in the size range from 10 to 470 nm typically from the beginning of spring to the beginning of fall during four (non-consecutive) years (2010, 2011, 2013 and 2014). The median concentration for the whole period taken into account was 214 particles cm(-3), oscillating between the median maximum in July with a concentration of 257 particles cm(-3) and a median minimum in April with 197 particles cm(-3). The median total number concentration did not present a well-defined seasonal behavior, as shown by contrast looking at the sub/modal number concentration, where distinct trends appeared in the predominant accumulation concentration recorded during April/May and the preponderant concentration of Aitken particles during the summer months. Lastly, the short side-by-side spring 2013 campaign performed at the Zeppelin observatory with a differential mobility particle sizer was characterized by an aerosol concentration mean steady difference between the two instruments of around 14 %, thereby supporting the reliability of the device located at Gruvebadet.

The present report shortly summarises the conclusions about the ozone column and solar UV irradiance variability that were achieved through the analysis of the data obtained from the narrow-band filter radiometer UV-RAD, operating at Ny-Alesund from 2008 to 2014. The polar summer gives the opportunity to register the solar irradiance 24 h per day during several months that provides comparatively long continuous time series as regards for the short- (diurnal) and medium-term (monthly) variations. To exclude the hypothesis about the artificial nature of the large amplitudes registered in the short-term ozone column variations, which can be due to the measurement or methodological errors, they were related to the corresponding variations in the solar UV radiation. In addition, these oscillations were studied using the methods developed for the analysis of non-linear dynamical systems that revealed a complex chaotic interaction between the ozone column and five other atmospheric factors. This approach leads to the conclusion that the short-term variations can be predicted for 10–20 h if a long history is available. The effect of a sporadic phenomenon, such as the ozone depletion event, occurred over Arctic in the spring of 2011, which can be considered an impulse perturbation of the medium-term ozone variations, on the mid-latitude ozone column and surface solar UV irradiance has been studied by analysing the data collected from six surface stations.

The reliability of astronomical observations at millimeter and sub-millimeter wavelengths closely depends on a low vertical content of water vapor as well as on high atmospheric emission stability. Although Concordia station at Dome C (Antarctica) enjoys good observing conditions in this atmospheric spectral windows, as shown by preliminary site-testing campaigns at different bands and in, not always, time overlapped periods, a dedicated instrument able to continuously determine atmospheric performance for a wide spectral range is not yet planned. In the absence of such measurements, in this paper we suggest a semi-empirical approach to perform an analysis of atmospheric transmission and emission at Dome C to compare the performance for 7 photometric bands ranging from 100 GHz to 2 THz. Radiosoundings data provided by the Routine Meteorological Observations (RMO) Research Project at Concordia station are corrected by temperature and humidity errors and dry biases and then employed to feed ATM (Atmospheric Transmission at Microwaves) code to generate synthetic spectra in the wide spectral range from 100 GHz to 2 THz. To quantify the atmospheric contribution in millimeter and sub-millimeter observations we are considering several photometric bands in which atmospheric quantities are integrated. The observational capabilities of this site at all the selected spectral bands are analyzed considering monthly averaged transmissions joined to the corresponding fluctuations. Transmission and pwv statistics at Dome C derived by our semi-empirical approach are consistent with previous works. It is evident the decreasing of the performance at high frequencies. We propose to introduce a new parameter to compare the quality of a site at different spectral bands, in terms of high transmission and emission stability, the Site Photometric Quality Factor.

A four-year set of vertical profiles of pressure, temperature and relative humidity derived from 1113 radiosoundings performed at Dome C (Antarctica) at 12h00 UT of each day, from late March 2005 to the end of March 2009, was examined by following a complex procedure for removing the most important lag errors and dry biases from the temperature and moisture data. The analysis provides evidence of annual cycles over the four years, characterizing the pressure and temperature conditions at the surface and at the various troposphere and low stratosphere levels, with maxima in summer and wide minima in winter for both parameters. Specific studies of the thermal parameters characterizing the ground layer and the tropopause region are also presented to describe their annual average variations. The analysis of moisture parameters indicates that absolute humidity varies regularly with season within the low troposphere, presenting well marked peaks in the summer months. Consequently, precipitable water was found to vary regularly during the year, from values of 0.2–0.4 mm in the winter to more than 0.6 mm in summer. The main year-to-year variations characterizing the monthly mean vertical profiles of pressure, temperature and moisture parameters are also described.

An updated set of time series of derived aerosol optical depth (AOD) and Angstrom’s exponent a from a number of Arctic and Antarctic stations was analyzed to determine the long-termvariations of these two parameters. The Arctic measurements were performed at Ny-Alesund (1991e2010), Barrow (1977e2010) and some Siberian sites (1981e1991). The data were integrated with Level 2.0 AERONET sun-photometer measurements recorded at Hornsund, Svalbard, and Barrow for recent years, and at Tiksi for the summer 2010. The Antarctic data-set comprises sun-photometer measurements performed at Mirny (1982e2009), Neumayer (1991e2004), and Terra Nova Bay (1987e2005), and at South Pole (1977e2010). Analyses of

Measurements performed at polar latitudes by the MIPAS limb-scanning Fourier Transform spectrometer aboard ENVISAT, from July 2002 to March 2004 (FR mission) and January 2005 to April 2010 (OR mission) were analysed with the 2-D tomographic Geofit Multi-Target Retrieval (GMTR) procedure (Dinelli et al., 2010. The MIPAS2D database of MIPAS/ENVISAT measurements retrieved with a multitarget 2-dimensional tomographic approachAtmos. Meas. Techn. Discuss. (AMTD) 2, 2639–2688.) to obtain the MIPAS2D database, and extract more than 386,000 vertical profiles of pressure p, temperature T and water vapour volume mixing ratio Q. They were subdivided into 12 latitudinal classes selected in steps of 51 from 651 Nt o 901N and 651 St o 901S, each vertical profile consisting of values measured at 14 altitude levels from 12 to 60 km. Each latitudinal set was subdivided into 12 monthly sets to determine the multi-year monthly mean vertical profiles of the three parameters: those of p (monthly average standard deviations (SD) 5–15%) provide evidence of marked seasonal variations above 30 km; those of T (SD values of a few percent) show large seasonal variations, with summer maxima at all stratospheric levels; and those of Q (SD lower than 20% from 20 to 50 km) present values ranging in general between 2 and 6 ppmv at the 12–25 km levels and 4 and 7 ppmv at higher altitudes. To verify the reliability of the MIPAS results, the pressure profiles are compared with those obtained from radiosounding data-sets taken at Arctic and Antarctic sites from 2000 to 2003; those of T with both radiosounding measurements and MLS/Aura satellite data from 2005 to 2010; and those of Q with the MLS/Aura satellite data, finding a substantial agreement in all cases. Comparison of MIPAS pressure and temperature profiles with pre ozone-hole CIRA models at 701 and 801 latitudes highlights the variations occurring in the polar atmosphere over the last 3 decades, with relative pressure decreases of 5–10% on average, and overall average decreases in temperature of 0.4 and 2.0 K in the Arctic and Antarctica, respectively. Using the MIPAS profiles of p, T and Q, the monthly mean vertical profiles of absolute humidity were also calculated, from which the monthly values of stratospheric water vapour content from 12 to 50 km were determined, varying between 0.0047 and 0.0070 mm at Arctic latitudes and between 0.0026 and 0.0055 mm at Antarctic latitudes.

A blind intercomparison of ground-based ultraviolet (UV) instruments has been organized for the first time in Italy. The campaign was coordinated by the Environmental Protection Agency of Aosta Valley (ARPA Valle d'Aosta) and took place in Saint-Christophe (45.8° N, 7.4° E, 570 m a.s.l.), in the Alpine region, from 8 to 23 June 2010. It involved 8 institutions, 10 broadband radiometers, 2 filter radiometers and 2 spectroradiometers. Synchronized measurements of downward global solar UV irradiance at the ground were collected and the raw series were then individually processed by the respective operators on the base of their own procedures and calibration data. A radiative transfer model was successfully applied as an interpretative tool. The input parameters and output results are described in detail. The comparison was performed in terms of global solar UV Index and integrated UV-A irradiance against a well-calibrated double monochromator spectroradiometer as reference. An improved algorithm for comparing broadband data and spectra has been developed and is discussed in detail. For some instruments, we found average deviations ranging from −16 % up to 20 % relative to the reference and diurnal variations as large as 15 % even in clear days. Remarkable deviations were found for the instruments calibrated in the manufacturers' facilities and never involved in field intercomparison. Finally, some recommendations to the UV operators based on the campaign results are proposed.

Areas with dimensions of 1000–3000 km in which the total ozone content (TOC) decreases fast are called ozone mini-holes. They are generated mainly dynamically in two ways, either by poor-ozone air mass transport from the tropics to higher latitudes by planetary wave activity or, they are connected with strong adiabatic uplifting of the tropopause height. An ozone mini-hole, generated by the second mechanism, was observed over the Balkan Peninsula on 19/21 March 2005. In the middle of March, the polar vortex was strongly disturbed by Rossby waves, reaching up to the lower stratosphere. Warming episodes over a geographical area, covering the Barents Sea and the Polar Sea north from Central Siberia, displaced a polar vortex fragment extremely southwards. However, the vorticity was weak and the stratospheric temperatures did not reach low values, providing conditions for ozone chemical destruction via heterogenic reactions. At the same time, a Rossby wave ridge was located below the European polar fragment. In the period from 13 to 19 March, the thermal tropopause over Sofia was uplifted almost by 3 km. Ozone distributions observed by the SCIAMACHY instrument on 18–21 March show a fast TOC decrease westwards from Ireland, which was moving eastwards during the next days, increasing the area in which the ozone content decreased. On 20/21 March low ozone content was observed above the Stara Zagora (42°N, 25°E) ground-based station by means of the GASCOD instrument, using DOAS technique. The TOMS Earth probe instrument detected 237 DU over Sofia. This is a record low March value from the beginning of the TOMS instrument measurements in 1978. In March/April the ozone distribution was characterized by its mean annual maximum of 360 DU at 42°N. Using the TUV model, the UV-index was calculated for regions of 2000 m height, typical for the mountains southwards of Sofia. The UV-index was very high (almost 10). This is a result not only of the low TOC but also of the relatively high zenith angle at middle latitudes in March.

This presentation deals with the recent investigations of the dynamics of the total ozone content (TOC) and of the lower troposphere ozone over Bulgaria, South-Eastern Europe. For analysis of the TOC we have used data obtained through measurements carried out by using a ground-based spectrophotometer Photon and data from satellite experiments TOMS-Nimbus 7 and TOMS-EP. The spectrophotometer Photon measures the direct solar ultraviolet (UV) radiation in the range 290–360 nm with 1 nm resolution, using which TOC is calculated. The ozone values received from the ground-based and satellite observations in the period 1999–2005 have been compared. In this period the ozone column did not show a statistically significant trend. The TOC data demonstrated typical for the middle latitudes seasonal variations. The monthly mean ozone values observed during the last 9 years have been examined also. It was found that the largest TOC fluctuations were detected in February and the smallest – in July. Ozone behaviour in the planetary boundary layer (PBL) has been analyzed by using data obtained with remote-sensing technique – ground-based aerosol LIDAR (LIght Detection And Ranging), which give information about the evolution of the PBL. The surface ozone concentrations have been measured with a fast and sensitive ozone monitor, based on chemiluminescent method for ozone detection. The investigation shows that the simultaneous observations of the surface ozone and of the boundary layer dynamics provide the opportunity accurately to establish relation between the BL structure and the features of ozone variations. In particular, the influence of the residual layer on the ozone values near the ground is demonstrated.

Ozone concentration measurements were made during December from 2001–2005 to quantify the contributions of different processes to near-surface ozone concentrations (O3) in Terra Nova Bay, Antarctica. The average O3 concentration was 20.3 ppbv. On days characterized by high solar radiation fluxes (HSR), significantly higher concentrations of O3 (21.3 ppbv) were recorded compared to days with low solar radiation fluxes (LSR days, 16.8 ppbv). High O3 concentrations could be related to strong winds from SW–NW. Three-dimensional back-trajectories show that air from the interior of the continent could affect O3 at Terra Nova Bay. Moreover, during HSR days, high O3 concentrations were also recorded in connection with weak circulation, suggesting that emissions from the Italian base (located 2 km north) could also represent a significant source of O3. To clarify the role of local pollution in Terra Nova Bay, O3 values were also calculated using the photochemical steady state (PSS) approximation under clear sky and cloudy conditions.

Two-channel sun photometers can be easily employed at Antarctic sites, where harsh environmental conditions prevail, to carry out measurements of precipitable water W. In the very dry air conditions observed in the Antarctic atmosphere, water vapor does not produce strong absorption features along the sun path. Therefore, these instruments need to be calibrated using analytical forms different from the square root regime, which can be determined by simulating the output voltages measured at Antarctic sites, for the spectral near-IR curves of extraterrestrial solar irradiance, instrumental responsivity parameters, and atmospheric transmittance, relative to various measurement periods. For this purpose, average models of the Antarctic atmosphere from the ground level up to the 30-km altitude were considered for different solar zenith angles and relative humidity conditions. The ratios between the output voltages simulated in the band and window channels were plotted as a function of total water vapor content Cw, for each site and each period, to define the best-fit calibration curves, which were subsequently normalized to the field measurements to take into account the aging effects on the filter transmission characteristics. Each of the five calibration curves was found to present a slope coefficient decreasing gradually with Cw from values higher than 0.8 to about 0.6. Using these curves, measurements of W were obtained, which differ appreciably at both sea level and high-altitude sites from those given by the square root calibration curves, avoiding large overestimation errors of 10%–40% at the high-altitude sites and underestimation errors of 5%–15% at the sea level site.

Temperature variations registered above the southeast part of the Po Valley, Italy, have been examined by applying the principal component analysis of radiosounding profiles recorded during the period from 1987 to 2010. Two datasets, considered to describe intra- and interannual oscillations, respectively, were extracted from the measurements data and the results show that both types of fluctuations can be projected onto four empirical orthogonal functions (EOFs), interpreted as vertical distributions of oscillation amplitudes and four uncorrelated time series that represent the evolution of corresponding EOFs. It was found that intra-annual oscillations composed of periods between 30 and 120 days, together with interannual variations of 1- to 7-year period contribute to the highest extent (about 70%) of the temperature oscillations up to 20 km, changing in both cases the phase in the tropopause region. The other three EOFs indicate prevailing weight of the oscillations in the upper troposphere-low stratosphere region and are characterised by longer periods in both types of fluctuations. The intra-annual variations can be accounted for an interaction between Madden-Julian and Arctic oscillations, while the spectral features of interannual fluctuations could be associated with those of Quasi Biennial, El Niño, and North Atlantic global oscillations.

International audience; The strong ozone depletion event that occurred in Arctic during spring 2011 was found to cause appreciable reduction in the ozone column (OC) in Europe, even at lower latitudes. The features of this episode have been analysed using the data recorded at 34 ground-based stations located in the European area and compared with the similar events in 2000 and 2005. The results provided evidence that OC as far south as 40°N latitude was considerably influenced by the Arctic ozone loss in spring 2011. The reduction of OC at the northernmost sites was about 40% with respect to the mean value calculated over the previous six-year period, while a similar decrease at the southern extreme ranged between 15 and 18%, and were delayed by nearly two weeks compared to the Arctic region. The ozone distributions reconstructed for the West Europe sector show that the decline of OC lasted from late March to late April 2011. The echo of the Arctic ozone depletion on mid-latitude UV irradiance has been analysed trough model computations that show an increase of the midday erythemal dose by 3-4 SED (1 SED = 100 J m-2) that was slightly higher than at polar regions. On the other hand it was assessed that the biosystems in the northernmost regions were a subject of about 4 times higher UV stress than those at mid-latitudes. Despite indications of an OC recovery, the event examined here shows that the issue of ozone depletion episodes cannot be belittled.

© 2014 R. Traversi et al. Here we present the first multi-year record of nitrate in the atmospheric aerosol (2005-2008) and surface snow (2006-08) from central Antarctica. PM10 and size-segregated aerosol, together with superficial snow, have been collected all year-round at high resolution (daily for all the snow samples and for most of aerosol samples) at Dome C since the 2004/05 field season and analysed for main and trace ionic markers. The suitability of the sampling location in terms of possible contamination from the base is shown in detail. In spite of the relevance of nitrate in Antarctic atmosphere, both for better understanding the chemistry of N cycle in the plateau boundary layer and for improving the interpretation of long-term nitrate records from deep ice core records, nitrate sources in Antarctica are not well constrained yet, neither in extent nor in timing. A recurring seasonal pattern was pointed out in both aerosol and snow records, showing summer maxima and winter minima, although aerosol maxima lead the snow ones of 1-2 months, possibly due to a higher acidity in the atmosphere in mid-summer, favouring the repartition of nitrate as nitric acid and thus its uptake by the surface snow layers. On the basis of a meteorological analysis of one major nitrate event, of data related to PSC I extent and of irradiance values, we propose that the high nitrate summer levels in aerosol and snow are likely due to a synergy of enhanced source of nitrate and/or its precursors (such as the stratospheric inputs), higher solar irradiance and higher oxidation rates in this season. Moreover, we show here a further evidence of the substantial contribution of HNO3/NOx re-emission from the snowpack, already shown in previous works, and which can explain a significant fraction of atmospheric nitrate, maintaining the same seasonal pattern in the snow. As concerning snow specifically, the presented data suggest that nitrate is likely to be controlled mainly by atmospheric processes, not on the daily timescale but rather on the seasonal one., This research was performed in the framework of the Air-Glacs1181 'Station Concordia' project, which was funded by the MIUR through the Italian 'Programma Nazionale di Ricerche in Antartide' (PNRA) and in the framework of 'PRIDE' (2009/A2.21) and 'DECA-POL' (2010/A3.05) PNRA projects. The field operations benefited from the support of the French-Italian Concordia Station. This paper is a contribution to the HOLOCLIP project, which is funded by the European Science Foundation. This is HOLOCLIP publication no. 25. Meteorological data and information were obtained from IPEV/PNRA Project >Routine Meteorological Observation at Station Concordia - www.climantartide.it. Moreover, the authors appreciate the support of the AMRC, SSEC, UW-Madison for the data set, NSF grant numbers ANT-0944018 and ANT-1245663. B. Funke was supported by the Spanish MCINN under grant AYA2011-23552 and EC FEDER funds. We are grateful to the anonymous reviewers whose comments markedly improved the final manuscript.

The results from direct ground-based solar UV irradiance measurements and the total ozone content (TOC) over Stara Zagora (42° 25′N, 25° 37′E), Bulgaria are presented. During the period 1999–2003 the TOC data show seasonal variations, typical for the middle latitudes – maximum in the spring and minimum in the autumn. The comparison between TOC ground-based data and Global Ozone Monitoring Experiment (GOME) satellite-borne ones shows a seasonal dependence of the differences between them. A strong negative relationship between the total ozone and the 305 nm wavelength irradiance was found. The dependence between the two variables is significant ( r = −0.62 ± 0.18) at 98% confidence level. The direct sun UV doses for some specific biological effects (erythema and eyes) are obtained. The estimation of the radiation amplification factor RAF shows that the ozone reduction by 1% increases the erythemal dose by 2.3%. The eye-damaging doses are more influenced by the TOC changes and in this case RAF = −2.7%. The amount of these biological doses depended on the solar altitude over the horizon. This dependence was not so strong when the total ozone content in the atmosphere was lower.