Variability of spectral UV irradiance at Thessaloniki, Greece, from 15 years measurements

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1 Variability of spectral UV irradiance at Thessaloniki, Greece, from 15 years measurements Katerina Garane a, Alkiviadis Bais a, Kleareti Tourpali a, Charickleia Meleti a, Christos Zerefos b, Stylianos Kazadzis a a Laboratory of Atmospheric Physics, Physics Department, Aristotle Univ. of Thessaloniki, Greece b National and Kapodestrian University of Athens, Greece ABSTRACT The temporal variability of global ultraviolet solar spectral irradiance measured regularly at Thessaloniki, Greece during the last 15 years is presented. The measurements were conducted by a single- and a double-monochromator Brewer spectroradiometers which operate at the Laboratory of Atmospheric Physics since 1989 and 1993, respectively. Recently the entire series of measurements was re-evaluated and quality controlled, by revising the calibration history of the two instruments and by comparing these measurements with those obtained by a collocated erythemal radiometer and a pyranometer. In addition, the spectral measurements were corrected for the angular response error of each instrument and for the effect of temperature variations. The longest of the re-evaluated series, which was obtained by the single monochromator, was statistically analyzed to derive estimates of the long-term changes and variability of UV irradiance radiation. Daily integrals were derived with the aid of broadband measurements which were used to simulate the diurnal variation of the spectral irradiance at one minute increments. The effect of clouds and solar zenith angle on the log term variability of UV irradiance are also investigated. Finally, signals of inter-annual natural variations and oscillations on this data set are explored and removed in an attempt to attribute the observed variability to different factors or mechanisms and investigate their effects on the long term changes of UV irradiance at the ground. All long term changes that were calculated have positive signs and vary according to wavelength solar zenith angle and the period of data. Monthly erythemal irradiance increases in the 199 s by about 6%, possibly as a result of reduction of clouds and aerosols. Keywords: UV Radiation, UV variability, solar UV irradiance, long term UV changes 1. INTRODUCTION The observed ozone depletion over middle and high latitudes of both hemispheres in the 198 s and 199 s has stimulated the need for long term measurements of solar UV radiation, in order to assess the consequences of possible increases in the UV radiation received at the ground. Earlier analyses by [Scotto et al., 1988] have shown decreasing UV-B levels in a small number of sites, in contradiction to the expected increase due to ozone depletion. This was attributed to increases in the absorbing tropospheric aerosols, ozone and changes in meteorology, [Bruhl and Crutzen, 1989];[Justus and Murphey, 1994]. Not only ozone but also clouds and haze are very important for controlling the UV-B levels at the ground [[Bais et al., 1993]; [Estupiραn et al., 1996];[Seckmeyer et al., 1996]. Other tropospheric minor constituents, such SO 2 and NO 2, also contribute to the UV variability in the atmosphere. Therefore changes in any or all of these factors may reduce, cancel, or even reverse the expected UV-B amplification due to the ozone decline. Of particular importance for determining the variability of UV irradiance and its long term changes is the quality of the measurements. Many projects were accomplished and numerous studies were conducted with the aim to improve the quality of the measurements [Bais et al., 21; Bernhard and Seckmeyer, 1999; Grφbner et al., 25]. [Blumthaler and Ambach, 199], found the erythemal dose increasing, at Jungfraujoch, a 3.6 km high mountain site in Switzerland, by about 1% per year in the 198 s. Increases in spectral UV irradiances were also reported in the 199 s Ultraviolet Ground- and Space-based Measurements, Models, and Effects V, edited by G. Bernhard, J. R. Slusser, J. R. Herman, W. Gao, Proc. of SPIE Vol (SPIE, Bellingham, WA, 25) X/5/$15 doi: / Proc. of SPIE

2 from Brewer measurements in Toronto [Kerr and McElroy, 1993] and at Thessaloniki under practically cloudless skies and at a constant zenith angle [Zerefos et al., 1995]. More recently, a study for the UV variability over The Netherlands was presented [den Outer et al., 25], reporting increasing of annual UV doses of about 5.5% per decade. The calculation of trends depends strongly on the length of the records, which so far are considered long enough to derive significant results[glandorf et al., 25]. The UV monitoring station of Thessaloniki has one of the longest records of spectral UV irradiance, comprising different instruments. In this paper we present a re-evaluation of the UV spectral irradiance records of Thessaloniki which was done in order to improve the quality of the measurements and the significance of the results. The methodology is discussed and results from the analysis of long term changes are presented. 2. INSTRUMENTATION AND DATA Spectral measurements of solar ultraviolet irradiance on a horizontal surface were conducted by a single- (MKII) and a double-monochromator (MKIII) Brewer spectroradiometers which operate on a regular basis at the Laboratory of Atmospheric Physics, University of Thessaloniki, Greece, since 1989 and 1993, respectively. The Brewer MKII spectroradiometer (numbered #5) operates in the wavelength range of nm and records spectral irradainces with a step of.5nm, while the Brewer MKIII (numbered #86) operates in the range nm (step.5nm). The two instruments are positioned side by side on the roof of the Physics Department (latitude N, longtitude E, altitude 6m a.s.l.), in the center of the city of Thessaloniki. The horizon of the instruments is free to the South and West. Buildings and local obstructions block the East side up to an angle of 1 and the north side up to an angle of 3. Besides the two spectroradiometers, two erythemal radiometers (YES UVB-1 and Solar Light 51), a UVA radiometer and a pyranometer operate also on a regular basis, providing ancillary data that have been also used during the quality control process that is described below Calibration of the two spectroradiometers To achieve and maintain a reliable absolute calibration of a spectrometer is a complicated task, but this is the most important requirement in UV spectroradiometry. Some of the most significant sources of errors and uncertainties during the calibration procedure are the accuracy of lamp current, the precision in setting the distance from the lamp, the stability of the conditions in the calibration room, reflections on surroundings and the positioning of the lamp. The absolute irradiance calibration of the two Brewer spectroradiometers is performed once a month using working standard 1 W DXW tungsten-halogen lamps. The primary standards were provided by Optronics Laboratories and thus are traceable to NIST standards. Once every six months the calibration is performed using a primary standard 1 W DXW lamp along with the working standard so that the latest is re-calibrated every six months. Following the schedule that is described above the use of the primary lamp is reduced to the minimum. The time series of the calibration factors of the two instruments at 32 ± 5 nm is shown in Fig. 1. The filled circles show the time series of the calibration factor for Brewer MKIII #86, while the open circles correspond to the calibration factor of Brewer MKII #5. The jump in the calibration factor of Brewer #86 on 27/6/22 is due to the replacement of one of the gratings of the spectrometer. Smaller fluctuations of the shape of the curves are because of smaller instrument changes, either due to temperature variations or due to changes in the sensitivity of the photomultiplier. The systematic decrease of the calibration factor of Brewer #86 at this wavelength through the years is about 25% for the period and about 15% for the period 22 25, resulting to an overall degradation of the instrument s sensitivity by about 4% in 11 years. The degradation of the sensitivity of Brewer #5 is about 6% in 15 years. The stability of the instruments is checked once a week using a set of five 5 W lamps. Usually two or three different lamps are measured each time for consistency checking. Since 22, one pair of lamps from the set are measured every week, together with a third one which is different each week. As a result, the stability of the instrument from week to week is checked by the first two lamps and the third one is used to have a link to the past measurements. Proc. of SPIE

3 8 λ = 32 ± 5 nm Brewer MKIII #86 Brewer MKII #5 Change in responsivity (%) Years Fig. 1: Time series of the calibration factor of the two Brewer spectroradiometers at 32 ±5 nm Quality control and re-evaluation methodology The entire time series of measurements was re-evaluated and quality controlled, first, by revising the calibration history of the two instruments, second, by intercomparing the two instruments and third, by comparing the measurements with those obtained by a collocated erythemal radiometer and a pyranometer. Both spectroradiometers are automated, resulting occasionally to false measurements that are caused mainly by mispositioning of the iris, the filters or the director prism. Such problems are usually evident when comparing two different instruments. The first step of the quality control methodology was the correction of the raw data for extreme values (spikes) that might appear randomly in the spectrum. The correction is achieved by comparing the erroneous measurement with that of a reference spectrum, and is calculated with respect to the relation between two consecutive wavelengths. The next step was the day-by-day check of the record of the internal standard lamp measurements. The standard lamp of the instrument is measured at least two times per day. Any evident changes in the measurement history can help in understanding the instruments behavior and in the correction of the measured irradiances. It is mentioned that the standard lamp measurements depend strongly on temperature, which however is recorded and can be taken into account when comparing measurements taken at different times. The spectral calibration factors of the instruments are derived both from primary and working 1 W standard lamps. The less accurate 5W lamp scans were used to track any prominent changes in the instrument s sensitivity between subsequent absolute calibrations. As a result the index that contains information about the time period for which each calibration factor is valid becomes more precise. The quality control of the spectral irradiances recorded by the two spectroradiometers, as far as their calibration is concerned, was achieved by comparing all spectra that were recorded almost synchronously, within a certain time difference. The irradiance average in the wavelength range nm was chosen for the comparison. For selecting a pair of scans (one for each instrument) to be used for that purpose, the following conditions had to be filled: (a) the two scans are taken at the same time and (b) the weather conditions during an observation remain stable. These restrictions arise from the fact that the irradiance changes significantly with solar zenith angle and with changes in cloudiness. The duration of a spectral scan is a few minutes and is different for the two Brewers. Therefore the time stamp for each averaged irradiance value was calculated as the average of the measurement times of all wavelengths between 35 and 325 nm. Scans with a difference in the mean time of less than 2 minutes were considered as synchronous [Meleti, 1996]. Continuous pyranometer measurements with a time resolution of one minute were used to flag all the scans recorded by the two instruments for the stability of the radiation field using the methodology described by [Vasaras et al., 21]. In addition to the comparison of two spectrophotometers to each other, the comparison of the broadband measurements with the two Brewers was also used for the re-evaluation of the calibration history of the Brewers. Specifically, the ratio of the erythemal irradiance that was calculated from each spectrum was compared to the irradiance that was measured by Proc. of SPIE

4 the erythemal detector averaged over the duration of the scan. In case the irradiance ratio of the two Brewers was out of limits for a number of days, especially if some of these days were cloudless, the ratio of each Brewer to the erythemal detector was used to detect which of the two spectroradiometers was behaving wrongly. Then its calibration factor was revised and the irradiance measurements for this period were corrected accordingly. Finally, the ratio of the Brewer derived erythemal irradiance, to the simultaneous measurements of the erythemal detector was checked day by day to detect any remaining erroneous measurements. Contrary to checking the ratio of the two Brewers, this method allows to check all the available spectra from both instruments and not only those that happened to be simultaneous, because the erythemal detector is sampled every one minute Corrections of the data Before converted to spectral irradiances, all raw measurements of the two spectroradiometers were previously corrected for the dark current and the dead time of their photomultipliers. In order to correct the single-monochromator Brewer measurements for the stray light effect, the mean irradiance measured between 29 and nm is subtracted from the whole spectrum. This procedure is not necessary for the double monochromator Brewer. The whole data set was also corrected for the effect of temperature variations. The temperature of the PMT of the Brewer is recorded during all measurements. The temperature coefficients of each instrument were calculated using the record of the internal standard lamp measurements. For the days that 4 or more measurements were performed, a linear fit of the measured counts as a function of temperature was used to determine the temperature coefficient for each day. In Fig. 2, the time series of the temperature coefficients is shown. The temperature coefficient for Brewer #5 is mainly positive and it ranges from to +.5 %/ C, while for Brewer #86 it is mostly negative and it ranges between +.1 and.4 %/ C..8 Brewer MKII #5 Brewer MKIII #86 Temperature coefficient (%/ C) Years Fig. 2: Time series of temperature coefficients (%/ C) calculated for each day from the internal standard lamp measurements for Brewer #86 (full circles) and Brewer #5 (open circles). As the day to day variation of the coefficients has significant noise, the use of average values was considered as more appropriate. In addition, the coefficients for both instruments exhibit an annual variation which is not constant for the entire record. Therefore, functions describing the variation of the temperature coefficient as a function of the day of year were calculated using statistical fits, for different periods in the record. An example is shown in Fig. 3 for both Brewers and for only one period for each instrument. The temperature correction was applied to the Brewer data based on the difference of the instrument s temperature during each UV scan to the temperature that corresponds to the measurement of the irradiance standard that was used to determine the calibration factor for this period. As a result of the application of temperature corrections, the irradiances measured by the two instruments during the entire period were changed to within ±6% for Brewer MKIII and ±8% for MKII. Proc. of SPIE

5 .3 MKIII #86.5 MKII #5.2.4 Temperature coefficient (%/ C) Temperature coefficient (%/ C) Day of the Year Day of the Year Fig. 3: Calculation of the temperature coefficient as a function of day of the year for Brewers #86 (left) and #5 (right). Finally, the spectral irradiances of the two spectrophotometers were corrected for the non-ideal angular response of their entrance optics. The method that was applied for Brewer MKIII after 1996 is based on measured direct to global irradiance ratio [Bais et al., 1998], while for the entire period of operation of Brewer MKII and between 1993 and 1996 for MKIII the angular response corrections were calculated from a model that provided the direct to global irradiance ratio [Bais et al., 25]. In both methods isotropic distribution of the diffuse irradiance is assumed. 3. STATISTICAL ANALYSIS OF UV IRRADIANCE TIME SERIES The statistical analysis of the spectral ultraviolet irradiance time series, from the two Brewer spectroradiometers, covers a time period of 15 years. Daily erythemal irradiance integrals were derived with the aid of broadband measurements which were used to simulate the diurnal variation of the spectral irradiance at one minute increments. In particular, the ratio of the erythemal irradiance calculated for each scan of the Brewer, to the irradiance measured simultaneously by the broadband detector, was used to reproduce the diurnal variation. The characterization of the daily integral of the erythemal irradiance with respect to weather conditions was achieved using cloud-cover observations. If for the entire day the cloudiness was less than 2 octas, the day was flagged as cloudless. Missing ancillary information that were used for the calculation of the daily integrals prevented the use of the entire data set for the estimation of long-term changes and the variability of UV irradiances. For example, the erythemal detector was installed in 1991 and the pyranometer used for flagging the radiation field of the spectra is operating since It should be noted also that the time series of the two instruments have different lengths, as well as number of available measurements. In fact the time series for the daily erythemal irradiances calculated from Brewer MKII starts in 1991, when the broadband detector started operating, although there are spectral measurements since Furthermore, there is a period of missing data between day 322 of 1998 and day 13 of 2, because an instrument problem did not allow measurements below 3 nm, a range which is critical for the calculation of the erythemal irradiance. Finally, the time series of the double monochromator MKIII has many gaps due to participation of the instrument in field and intercomparison campaigns at other locations. The daily erythemal irradiances from both instruments were used for the calculation of monthly means. A monthly mean was calculated only when: a) there were at least 2 days of daily data available, and b) no more than5 consecutive days were missing for this month. To remove the dominant annual cycle form the monthly series, the monthly mean erythemal irradiances were deseasonalized by subtracting the mean annual course. The series of these departures is shown in Fig. 4 and correspond to data recorded under all-sky conditions. The calculated by a linear fit long term change for MKIII for the period is +6.7 % per decade, while for MKII it is +7.9 % for the same period and +4.2 % for the period Such differences are expected because of the relatively short length of the data series. Proc. of SPIE

6 4 ERYTHEMAL IRRADIANCE (daily integrals) THESSALONIKI (all skies) Montly mean departures (%) MKIII #86: +6.7%/dec MKII #5: +4.2%/dec +7.9%/dec Years Fig. 4: Percentage departures of monthly erythemal irradiances from their mean calculated from Brewer MKIII series (crosses) and from Brewer MKII (full circles). Fig. 5, shows the departures of Fig. 4 but for different months: March, June, September and December. The calculated linear changes are also shown for each month. These changes are all positive and range between about 1 and 6%, but they are statistically insignificant, and the large year to year variability complicates even more the uncertain picture. June is the least and December the most variable month, mainly due to difference in the variation ofcloudiness. Similar positive changes are calculated also when only local noon erythemal irradiances are considered. After removing the annual cycle by deseasonalization, the changes in erythemal irradiance are +2.5 %/dec for all-sky and +3.9 %/dec for clear-sky conditions. 4 ERYTHEMAL IRRADIANCE (daily integrals) THESSALONIKI (all skies) MKII #5 Montly mean departures (%) March: +2.6 (±1.) %/dec June: +1.9 (±4.7) %/dec September: +.9 (±11.3) %/dec December: +5.7 (±15.2) %/dec Years Fig. 5: Same as Fig. 4, but only for months March, June, September and December. Besides the changes in daily erythemal irradiances, the variability of spectral UV irradiances at selected wavelengths and solar zenith angles for both Brewers was also investigated. For this data set the characterization of the measurements as clear or cloudy was made by the flags that are derived from the data of a collocated pyranometer (see section 2.2). In Fig. 6, the wavelength dependence of the calculated percentage changes per decade for five solar zenith angles and for Proc. of SPIE

7 both the instruments is displayed. The results in Fig. 6 refer only to clear-sky measurements. The changes for data of Brewer #86 data vary from -.7 to +12.7%/dec, while for Brewer #5 from +.7 to +17.3%/dec (for up to 324 nm only), depending on wavelength and solar zenith angle. For all-sky conditions the corresponding changes vary from -.9 to +19.8%/dec for #86 and from -5.5 to 18.9%/dec for #5. The changes that were calculated for the single monochromator Brewer #5 and for 6 are much higher than at other angles, both for clear and all sky conditions. The reason for this deviation is not clear. In general, and taking into account the uncertainties of the measurements the calculated changes for both instruments are similar in magnitude and wavelength behavior. Change per decade (%) Clear skies #86 #5 SZA: 3 deg 4 deg 5 deg 6 deg 75 deg Wavelength (nm) Fig. 6: Decadal changes in spectral erythemal irradiance for five solar zenith angles and for both Brewer spectroradiometers as a function of wavelength. The wavelength range of Brewer #5 stops at 325 nm. Monthly mean AOD at 32 nm MKIII #86 MKII # : -8 %/dec : -7 %/dec : -1 %/dec (#86) -9 %/dec (#5) YEAR Fig. 7: Time series of the monthly mean aerosol optical depths as calculated from Brewer MKIII #86 and MKII #5. The straight lines correspond to linear fits on the data for the period starting in 1991 for Brewer #5 and 1997 for Brewer #86. Fig. 6 suggests also that at the shorter UV-B wavelengths, which are most affected by ozone, the changes in spectral irradiances are smaller, which can be explained by the observed small increase (1.6% per decade) in total ozone over Thessaloniki for the period The increases at longer wavelengths could be explained only if the optical depth Proc. of SPIE

8 of clouds and aerosols decrease at the same period. The available cloud information is not sufficient to support any conclusion about the long term changes on the cloud optical depth over Thessaloniki in the last decades. There is only a slight indication that the fraction of cloud free days is increasing at a rate of about 5% per decade since 1991, which however cannot be supported statistically. In addition, there is indication that also aerosols decrease during this period. The time series of the aerosol optical depth at Thessaloniki derived from direct irradiance measurements of the two Brewer spectroradiometers is displayed in Fig. 7. Although these measurements are derived from the same instruments, they are completely independent of the measurements of global spectral irradiance, because different measurement principles and calibration procedures are used. The methodologies used for the calculation of the aerosol optical depth are described in [Grφbner and Meleti, 24] and [Kazadzis et al., 25]. Both series show a decreasing tendency in the aerosol optical depth during the last two decades. The data derived from the direct sun measurements of Brewer MKII show a decrease of about 8%/dec for the period The aerosol optical depth from Brewer #86 is derived from the direct spectral irradiances, starting in During their common period of operation, the changes that are calculated for the two datasets are similar: -1 %/dec for #86 and -9 %/dec for #5. Similar decreasing tendency is observed in the aerosol concentrations from the data of the network of the Municipality of Thessaloniki [Petrakakis and al., 25] which support the Brewer optical depth observations and suggest that the air quality in Thessaloniki is improving during the last decade, leading to cleaning in the atmosphere. The influence of the Quasi Biennial Oscillation (QBO) on total ozone has been documented already in many studies, for example [Oltmans and London, 1982; Zerefos et al., 1992]. Similar signals can be expected to be detected in surface UV irradiance series due to the relation between UV and ozone. Anomalies in % of the respective monthly mean Super Posed Epoch key-dates: QBO west phase max MKIII #86 MKII #5 95% significance Lag in months Fig. 8: Composite erythemal irradiance anomalies (in % of monthly mean) at different lags. Zero lag corresponds to the month of QBO west phase maximum. Brewer #5 measurements correspond to solid line filled circles and Brewer #86 to dashed line open circles. To assess the possible effects of QBO on the erythemal irradiance series measured by the two Brewers, a super posed epoch analysis over the last 6 QBO cycles was performed, using as key-date (zero lag) the.month of the maximum west phase of the monthly mean 3 hpa zonal wind at Singapore. The resulting composite is seen in Fig. 8, for both instruments as change in percent relative to the respective monthly mean. Evidently, there is a clear signal of the QBO cycle on both records sixth months after the event, which is statistically significant at the 95% level for the series of Brewer 5, which has the longest record. Significant signals at negative lags arise from the analysis as a result of previous QBO cycles. Proc. of SPIE

9 4. CONCLUSIONS In this study, the re-evaluation methodologies of the spectral UV irradiances recorded at Thessaloniki with two Brewer spectroradiometers and the first results on the new data set are presented. Daily integrals of the erythemal irradiance at Thessaloniki have increased during the last 15 years by between 4 and 7% per decade depending on the period of the measurements. All calculated changes are positive, but statistically insignificant. UV changes at single wavelengths also increase between 5% and 2%, depending on wavelength and solar zenith angle. Larger positive changes are observed at the long UV wavelengths which are unaffected by ozone absorption. Changes in clear-sky irradiances are generally smaller compared to those under all-sky conditions. These results suggest that the observed UV irradiance increases could be attributed to decreasing cloud cover and aerosol optical depth, since the total ozone over Thessaloniki at the same period has remained almost constant, showing a small insignificant increase of about 1.6 % per decade.. Statistically significant QBO signal is detected 6 months following the 3 hpa zonal wind at Singapore. ACKNOWLEDGEMENTS This work was conducted in the frame of the HRAKLEITOS program (Contract: Environment ) and the European funded project SCOUT-O3 (Contract: 5539-GOCE-CT-24). REFERENCES 1. Bais, A., A. Kazantzidis, S. Kazadzis, and N. Kouremeti, Application of a model derived cosine correction method on Brewer spectral measurements, in European Geosciences Union, General Assembly 25, Vienna, Austria, Bais, A.F., M. Blumthaler, S. Kazadzis, H. Slaper, C. Brogniez, P. Eriksen, B.G. Gardiner, D. Gillotay, J. Ehramjian, W. Josefsson, B. Kjeldstad, T. Koskela, F. Kuik, K. Leszczynski, R.L. McKenzie, A. Redondas, H. Reinen, G. Seckmeyer, T. Svenψe, D.I. Wardle, A.R. Webb, P. Weihs, W. Allabar, G. Bernhard, M. Gay, D. Goebel, J. Groebner, M. Huber, P.V. Johnston, J.-E. Karlsson, J.B. Kerr, P.J. Kirsch, M. Kotkamp, J. Manzano, D. Masserot, C. Meleti, T. Mestechkina, E. Pachart, T. Persen, G. Rengarajan, J. Robertson, E. Saarinen, R. Schmitt, J. Schreder, R. Tax, T.M. Thorseth, R. Visuri, B. Walravens, and W. Wauben, The effect of the cosine response on global solar UV spectra measured during the SUSPEN intercomparison campaign, in ECUV Meeting, EC, Bais, A.F., B.G. Gardiner, H. Slaper, M. Blumthaler, G. Bernhard, R. McKenzie, A.R. Webb, G. Seckmeyer, B. Kjeldstad, T. Koskela, P.J. Kirsch, J. Grφbner, J.B. Kerr, S. Kazadzis, K. Leszczynski, D. Wardle, W. Josefsson, C. Brogniez, D. Gillotay, H. Reinen, P. Weihs, T. Svenoe, P. Eriksen, F. Kuik, and A. Redondas, SUSPEN intercomparison of ultraviolet spectroradiometers, Journal of Geophysical Research-Atmospheres, 16 (D12), , Bais, A.F., C.S. Zerefos, C. Meleti, I.C. Ziomas, and K. Tourpali, Spectral measurements of solar UVB radiation and its relations to total ozone, SO 2, and clouds, Journal of Geophysical Research, 98 (D3), , Bernhard, G., and G. Seckmeyer, Uncertainty of measurements of spectral solar UV irradiance, Journal of Geophysical Research-Atmospheres, 14 (D12), , Blumthaler, M., and W. Ambach, Indication of increasing solar ultraviolet-b radiation flux in alpine regions, Science, 248, 26-28, Bruhl, C., and P.J. Crutzen, On the disproportionate role of tropospheric ozone as a filter against solar UV-B radiation, Geophysical Research Letters, 16 (7), 73-76, den Outer, P.N., H. Slaper, and R.B. Tax, UV radiation in the Netherlands: Assessing long-term variability and trends in relation to ozone and clouds, Journal of Geophysical Research-Atmospheres, 11 (D2), -, 25. Proc. of SPIE

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