SPECIAL ISSUE

Trends in spectral UV radiation from long-termmeasurements at Hoher Sonnblick, Austria

M. Fitzka & S. Simic & J. Hadzimustafic

Received: 17 October 2011 /Accepted: 1 May 2012 /Published online: 13 June 2012# Springer-Verlag 2012

Abstract High-quality long-term records of spectral UV ir-radiance from the Network for the Detection of AtmosphericComposition Change-affiliated Bentham spectroradiometer atthe high-mountain site Hoher Sonnblick (47.05° N, 12.95° E,3,106 m above sea level) from the period 1997–2011 havebeen investigated for the existence of trends. Throughout theyear, significant upward trends are found at wavelengths of315 nm and longer. The magnitudes at 315 nm range from+9.3±4.5 %/dec at 45° solar zenith angle (SZA) to +14.2±3.7 %/dec at SZA 65° for all-sky conditions. The trend esti-mates at 305 nm are considerably smaller and less significant,yielding between +5.1±6.5 and +7.9±7.3 %/dec, dependingon SZA. Seasonally, the largest trends are found during winterand spring. Total ozone has significantly increased by year-round +1.9±1.3 %/dec since 1997 and therefore cannot ex-plain these significant increases. They are rather attributed todecreases in total cloud cover and aerosol optical depth.

1 Introduction

The intensity of UV radiation reaching the earth’s surface isdecisively influenced by the concentration of stratosphericozone, as well as by the atmospheric parameters clouds,surface albedo, and aerosols. All of these parameters aresubject to changes in the past and present as well as in thecourse of a changing climate. As a consequence of the ban ofproduction on several ozone-depleting substances (ODS) bythe Montreal Protocol signed in 1987, it is expected for

stratospheric ozone to recover from depletion as the concen-tration of ODS has already peaked and is declining (WMO2011; Newman et al. 2006). According to the latest report ofthe World Meteorological Organization (WMO) ScientificAssessment Panel: Ozone depletion (WMO 2011), northernhemisphere mid-latitude (35–60°N) annual mean total col-umn ozone amounts over the period 2006–2009 haveremained at the same level as observed during 1998–2005,approximately 3.5 % below the 1964–1980 average after aminimum of about −5.5 % was reached in the mid-1990s.Newchurch et al. (2003) also found evidence for a globalslowdown in stratospheric ozone losses since 1997. For therecent period 1995–2008, Krizan et al. (2011) report statisti-cally significant increases in minimum and maximum month-ly mean values of total ozone in northern midlatitudes frommerged satellite data.

In WMO (2011), it is stated that decreasing tendencies inUV irradiance are visible since the late 1990s, but that atsome northern hemisphere stations UV irradiance is stillincreasing as a consequence of long-term changes in influ-encing factors other than ozone.

Clouds have a strong impact on surface UV levels andcan seriously hamper the detection of ozone-induced trends(den Outer et al. 2005; Glandorf et al. 2005; Seckmeyer etal. 2008). In the course of a changing climate, change ofcloud cover is a vital prerequisite for estimates of futuresurface UV levels. Yet most recent studies that focus on UVtrends based on ozone and temperature projections stillexplicitly exclude the influence of clouds on future UVlevels (Kazantzidis et al. 2010; Tourpali et al. 2009). Stud-ies, which do include clouds come to differing conclusions.Bais et al. (2011) project slight moderations of erythemallyeffective UV irradiance at midlatitudes until the late twenty-first century, while Watanabe et al. (2011) expect all-sky UVradiation in the northern midlatitudes to increase, as

M. Fitzka (*) : S. Simic : J. HadzimustaficInstitute of Meteorology,University of Natural Resources and Life Sciences,Vienna, Austriae-mail: michael.fitzka@boku.ac.at

Theor Appl Climatol (2012) 110:585–593DOI 10.1007/s00704-012-0684-0

reductions in aerosols and clouds are believed to overcom-pensate for the effect of ozone recovery. As for measure-ments, it is stated in JMA (2009) that, despite slightlyincreasing total ozone, increases in surface UV radiationhave been observed that are attributed to changing weatherconditions and aerosol levels rather than changes in ozone.

It is highlighted in Douglass et al. (2011) that at sites withsmall ozone changes, changes in clouds and albedo, andgaseous and aerosol effects may surpass the influence ofozone on UV radiation. By way of example, reconstructingUV levels at European sites for the past four decades, denOuter et al. (2010) found strongly enhanced positive UVtrends in Thessaloniki through the incorporation of aerosolchanges.

Changes in albedo also play a significant role, especiallyat high-altitude and high-latitude sites, where UV irradiancecan be strongly enhanced due to multiple occurrences ofscattering and reflection between snow-covered ground andthe atmosphere (Bernard 2011; Simic et al. 2011).

Several studies state a minimum length of 12–15 yearsfor a record of UV radiation to reliably exhibit significantlinear changes (Glandorf et al. 2005; Weatherhead et al.1998). It is therefore only just recently that reliable assess-ment of trends becomes viable through the existence ofcontinuous records of sufficient lengths (Bernard 2011;Krzyścin et al. 2011). The long-term measurements of UVradiation that are carried out as part of the Network for theDetection of Atmospheric Composition Change (NDACC)according to its tight quality requirements, as well as thetotal ozone measurements since 1994 at Hoher Sonnblickrepresent a long, uninterrupted series and may thereforeserve as a good basis for trend analysis in measured timeseries since the turnaround in stratospheric ozone.

The long-term evolution of surface UV radiation plays acrucial role for assessing potential beneficiary as well as harm-ful effects on the biosphere. This study aims at an in-depthanalysis of potential trends in surface UV radiation at HoherSonnblick as a follow-up of a previous study (Simic et al.2008), based on since then extended records of global spectralUV irradiance (1997–2011) and total ozone (1994–2011).

2 Data and method

2.1 Data

2.1.1 Measurement site and cloud observations

Sonnblick observatory is a high-mountain station, situatedon top of Hoher Sonnblick (47.05°N, 12.95°E) at 3,106 mabove sea level (Fig. 1). The surroundings exhibit a complextopography: The adjacent valley to the north (Raurisertal) isat least 1,300 m lower, while the nearby summits are about

the same height. Changes in snowline as well as cloudsbelow the summit have a large impact on surface albedoand resulting surface UV radiation through multiple reflec-tions and scattering (Simic et al. 2008, 2011). Being a high-altitude site, the air is relatively clean and under a reducedinfluence of air pollution and tropospheric aerosols: Meas-urements and model calculations from a case study suggestthat the variations in 305 nm irradiance due to changes inaerosol optical depth (AOD) are only in the range of3–4.7 % at 315 and 370 nm (Weihs et al. 1999). Sonnblickobservatory is also an observation site of the AustrianWeather Service (ZAMG). Therefore detailed 3-hourlycloud observations considering cloud cover above and be-low the observatory as well as cloud type (low, mid, andhigh level) are available. These observations are used overthe preferable parameter cloud optical depth, which is notavailable at the site.

2.1.2 Spectral UV measurements

The Bentham DM150 spectroradiometer operated by theUniversity of Natural Resources and Life Sciences, Vienna,Institute of Meteorology (BOKU-Met), installed in late1996, is continually acquiring spectra from 290 to 500 nmin steps of 0.5 nm every 30 min. The instrument is officiallyaffiliated with NDACC, fulfilling the network’s tight qualityrequirements in quality control and assurance (McKenzie etal. 1997). Its entrance optics are regularly freed from snowand ice by permanent onsite staff. It is periodically calibrat-ed with a self-built 1,000 W FEL lamp assembly and severalNational Institute of Standards and Technology, USA andPhysikalisch-Technische Bundesanstalt, Germany calibrated1,000 W FEL standard lamps. Comparisons of the Benthamwith the portable NDACC-affiliated and BOKU-Met oper-ated Bentham (usually stationed in Groß-Enzersdorf nearVienna) are performed on a regular basis. Comparative

Fig. 1 Location of the observatory at Hoher Sonnblick (47.05°N,12.95°E, 3,106 m above sea level) within the complex topography ofthe Austrian Alps

586 M. Fitzka et al.

measurements in the UV-B range lie within ±5 %. Changesin the instrument’s sensitivity between individual calibra-tions typically fall in the range below 2–3 %.

Data from the Bentham have been chosen because of theinstrument’s advantage in temporal resolution and accuracyat the shorter UV wavelengths over the co-situated BrewerMkIV (Section 2.1.3) as a consequence of the Bentham’sdouble monochromator design. The length of the data re-cord covering the period 1996–2011 is considered sufficientfor the analysis of “post-turnaround” trends in UV radiation.

2.1.3 Total ozone measurements

Total ozone measurements for the period 1994–2011 are takenfrom the Brewer MkIV #093 spectrophotometer operated byBOKU-Met. The instrument was installed in late 1993 and issince acquiring measurements of total ozone continuously. Itis periodically compared with the traveling standard Brewer#017 through International Ozone Services Inc. (Canada) foronsite ozone calibration, most recently in May 2011.

2.2 Method

Based on an earlier, less exhaustive assessment with a shorterdata record (Simic et al. 2008), measurements from the period1997–2011 from the Bentham spectroradiometer have beenselected at several fixed solar zenith angles (SZA). For examplefor SZA 55°, single measurements falling in the range of 55±2°have been selected. Selecting measurements within an intervalof SZAs rather than at one value is not supposed to introduceuncertainties into the analysis as no significant changes in theselected measurements’ SZAs is visible over the investigatedperiod. To each measurement, the closest observation of totalcloud cover N (if available within ±1 h) was assigned. Toremove the influence of clouds in a stepwise fashion in orderto facilitate trend detection (Glandorf et al. 2005), subsets withmaximum allowed cloud cover (all-sky, 5/8, 2/8) were created.The case of N≤2/8 is the smallest maximum allowable cloudcover, for smaller values large gaps in the series of availablemeasurements start to appear. Trend analysis has been carriedout on the respective series’ monthly mean anomalies that arecalculated by subtracting the climatological monthly meanfrom each month’s individual monthly mean value, effectivelyremoving the seasonal cycle. Monthly mean values had beencalculated if at least 10 measurements within a month wereavailable, otherwise the given month was excluded from trendanalysis, leading to systematical gaps in year-round series at thesmaller SZAs. As in a previous study (Simic et al. 2008), lineartrend estimates were established using Sen’s Q method (Sen1968; Thiel 1950) while the nonparametric Mann–Kendalltrend test (MK; Mann 1945; Kendall 1975) was used to assessthe linear changes’ significance levels, returning trend signifi-cance at the levels 95, 99, and 99.9 % (referred to as “less

significant” or “*”, “significant” or “**”, “highly significant”or “***”, respectively, in the following). The nonparametricMK test was chosen because of its robustness towards outliersand (nonsystematic) gaps in data series. Additionally, the meth-od proposed byWeatherhead et al. (1998) returning the numberof years required for a given series to exhibit an estimated lineartrend with at least 90 % significance was applied to the series.Due to the very high agreement with and the sufficiency of theMK test alone to assess trend significance, it is refrained fromstating the WH statistics in the following sections. Throughoutthis paper, trends are given in percent per decade (dec)±90 %confidence interval.

3 Results

3.1 Year-round trends

The trend estimates were calculated for wavelengths be-tween 305 nm (strong ozone absorption) and 325 nm (weakozone absorption; Kerr and McElroy 1993) and erythemallyweighted irradiance (ERY) according to McKinlay and Dif-fey (1987) for the period 1997–2011. A summary of the testresults is given in Table 1. Throughout the year, significantupward trends are found at wavelengths of 315 and 325 nm.The trends’ magnitudes at 315 nm range from +9.3±4.5 %/dec** at SZA 45° to +14.2±3.7 %/dec*** at SZA 65° forall-sky conditions. The trends for SZA 45° however are ingeneral deemed less reliable due to systematic data gaps inlate fall and winter: SZAs as small as 45° do not occur at47°N during that period. Consequently, the estimate showsless significance than at larger SZAs. The ERY and 325 nmseries behave similarly: The trend in ERY is significant atSZA 45° and SZA 55°, and highly significant at SZA 65°,while at 325 nm it is highly significant at SZA 65° and SZA55° and shows coinciding magnitudes. The picture is slight-ly different at the shortest wavelength (305 nm): Regardlessof SZA, the trends exhibit smaller magnitudes and less to nosignificance (Table 1). The trend estimates for SZA 65° areshown in Fig. 2.

Additionally, trend analysis was carried out on dailydoses of spectral irradiance and shows coinciding magni-tudes, significance levels and a similar behavior except forthe shortest wavelength (Table 1). Bais et al. (2007) foundupward trends at Bilthoven and Lindenberg (Germany) forthe periods 1999–2004 and 1996–2003, respectively. ForSZA 60°, they calculated smaller +7.7 and +8.5 %/dec thatare attributed to changes in aerosols, ozone, and cloudiness.While the study yields increasing trends, a direct compari-son to Sonnblick is hindered not only by the differentperiods for trend analysis, but also the locations’ settings(e.g., snow, aerosols, and albedo).

Trends in spectral UV radiation from long-term measurements 587

3.2 Seasonal trends

On a seasonal scale, analysis is confined to SZA 65° to avoidbiasing the trend estimates with the systematic gaps during thecolder season at lower solar elevations. In general, the largestand most significant trends are found during spring and winterat all wavelengths (Table 2, Fig. 3). While of notable magni-tude and high significance at longer wavelengths, they aresmaller and insignificant at 305 nm. This is also valid inwinter, where the slopes are even steeper, but show interfer-ingly large confidence intervals. The large variation in UVirradiance during the winter months is caused by the highvariability of total ozone, which also accounts for decreasedtrend significance at 305 nm along with effective surface

albedo: Simic et al. (2008) found that varying surface albedothrough changes in snowline height may introduce meanmaximum variability of about 25 % in global UV irradiance,while ozone may introduce up to 200 % in winter.

Analysis of the summer months reveals smaller and insig-nificant trend estimates at all wavelengths. This is also thecause for smaller trend estimates at smaller SZAs (Table 1) asthose trends are dominated by the summer months due to thenon-occurrence of small SZAs during the winter months. Bycomparison, Krzyścin et al. (2011) found that the mean levelof ERY at Belsk, Poland, during April–October in the 2000swas around 10 % larger compared to the entire period ofobservations from 1976 to 2008. This different increase maybe explained through the inclusion of a longer period that still

Fig. 2 Year-round trendestimates in monthly meananomalies at Sonnblickobservatory (3,106 m above sealevel; MMA) at SZA 65° for all-sky conditions during the period1997–2011 for 305 nm, 315 nm,325 nm, ERY; solid lines indicatetrend lines, significance is givenfor 95 % (*), 99 % (**), and99.9 % (***)

Table 1 Summary of trends inpercent per decade±90 % confi-dence interval, year-round trendsat SZA 45°, SZA 55°, SZA 65°,and daily sums (DS) for all-skyconditions, N≤5 and N≤2, re-spectively; trend significance isindicated for 95 % (*), 99 % (**)and 99.9 % (***); “samples”states the number of data pointsin each subseries

SZA/DS N 305 nm 315 nm 325 nm ERY No of samples

45 All +7.9±7.3 +9.3±4.5** +5.9±4.9* +8.4±5.2** 95

45 5 +3.4±6.1 +8.7±3.3*** +4.3±4.4 +6.2±4.1* 76

45 2 −1.1±8.0 +5.1±4.2 +1.1±5.7 +2.8±5.3 39

55 All +5.1±6.5 +11.8±4.3*** +10.0±3.9*** +9.7±4.6** 122

55 5 +6.5±7.6 +11.4±4.1*** +7.7±3.6*** +9.3±4.5*** 103

55 2 −3.9±7.9 +7.3±4.2** +3.8±3.0 +2.8±4.7 59

65 All +6.6±6.0* +14.2±3.7*** +11.5±3.5*** +12.6±3.6*** 148

65 5 +5.4±5.8 +11.9±3.2*** +10.5±2.8*** +11.1±3.1*** 134

65 2 +2.5±7.7 +11.2±3.6*** +9.4±2.9*** +9.3±3.8*** 93

DS All +3.4±5.7 +10.6±3.3*** +9.5±3.1*** +8.3±3.4*** 129

DS 5 −4.7±11.0 + 6.2±4.6* + 5.8±3.7** + 3.1±5.7 66

DS 2 −7.6±11.3 +8.1±6.5* +9.7±5.0** +5.2±7.4 58

588 M. Fitzka et al.

falls into the time span of significant stratospheric ozonedepletion, while the shift of the largest trends to the coldseason at Sonnblick may stem from an increased influenceof surface albedo at the high-mountain site.

3.3 Explanation for increasing surface UV radiation

Overall increasing UV intensities lead to expect a decline instratospheric ozone over the investigated period. Trend analysisof total ozone measurements from the Brewer MkIV at Sonn-blick observatory, however, proves the opposite: After a declineuntil around 1997, total ozone exhibits a small, yet significantyear-round increase of +1.9±1.3 %/dec* (Fig. 4). Similar be-havior is reported by Vigouroux et al. (2008) at the midlatitudestations Zugspitze and Jungfraujoch, despite of slightly highermagnitude but less significance, most likely due to the shorterpost-turnaround subseries (1995–2005). While the exact turningpoint cannot be unambiguously determined due to the short

series before its occurrence (Fig. 4), the increase since 1997 isconsistent with studies discussing the evolution of ozone in thenorthernmidlatitudes (Steinbrecht et al. 2006; Zanis et al. 2006).

The increase in total ozone may explain the smaller trendmagnitudes at the shorter wavelengths and to a reduced extentfor ERY (Table 1), but cannot account for the overall positivetrends. It rather seems that the changes in total ozone aremasked or even surpassed by a changing influence of cloudi-ness: Synoptic cloud observations by the ZAMG at Sonnblickobservatory reveal a seasonally differential decline in totalcloud cover over the period 1997–2011 (Table 3). During thesummer months, the reduction amounts to about −5.8 % perdecade, which is also supported by the homogenized sunshineduration record from Hoher Sonnblick from the HISTALPdataset (Auer et al. 2007), indicating a marked increase inobserved sunshine duration since the mid 1960s and largerincreases in winter (+8.6±6.8 %/dec*) and spring (+7.2±7.3 %/dec) since the 1980s. Although the dataset only covers

Table 2 Summary of seasonaltrends in percent per decade ±90 % confidence interval at SZA65° and for daily sums (DS) forall-sky conditions

SZA/DS N Season 305 nm 315 nm 325 nm ERY

65 All Spring +3.0±10.2 +12.6±5.7** +9.4±5.4** +9.3±5.6**

65 All Summer +0.2±12.2 +7.6±9.4 +6.7±9.3 +5.8±10.1

65 All Fall +3.8±12.7 +13.7±8.6 +11.0±8.8 +11.8±9.5

65 All Winter +21.7±31.1 +23.1±13.7** +16.3±10.2** +19.7±13.8**

DS All Spring +5.1±8.8 +12.3±4.2*** +11.4±4.5*** +9.7±5.5**

DS All Summer +6.0±9.8 +7.1±8.3 +5.6±7.8 +6.1±8.4

DS All Fall +12.5±13.5 +15.0±8.7* +11.4±8.6 +11.6±10.1

DS All Winter −7.0±19.8 +9.0±8.9* +11.1±7.3** +8.4±9.3

Fig. 3 Seasonal trends inerythemally weightedirradiance at Sonnblickobservatory (3,106 m above sealevel) for all-sky conditions

Trends in spectral UV radiation from long-term measurements 589

the period until spring 2009, the magnitude of the increase stillleads to expect visible effects on surface UV irradiance in theperiod 1997–2011.

The changes in all observations of cloud cover are, how-ever, nonsignificant, similar to recent studies covering UVtrends in Europe (e.g., Zerefos et al. 2012). It is only in cloudobservations that are attributed to individual UV measure-ments that significant changes in cloudiness become apparent.For example, year-round cloud observations (total cloud coverN) for UV measurements at SZA 55° (N≤5) indicate a de-crease in total cloud cover of −10.4±9.2 %/dec*.

One might argue though that trend analysis in these “fil-tered” cloud observations may introduce artificial trendsmerely by selection of the filter criteria. However, looking atall observations likely misjudges the sky-conditions duringthe time of acquiring the measurement as it takes into accountmore cloud observations during any single day than the seriesat a fixed SZA actually perceives. Therefore, the filtered seriesare still a physically valid representation of what trend incloudiness the UV measurements themselves actually “see”and are consequently influenced by these changes. In the caseof decreasing total cloud cover, this effectively means anincreased partition of measurements taken under lower frac-tions of cloud coverage and therefore reduced attenuationthrough clouds over the course of the investigated period,contributing to the overall positive trends.

The effect of reduced cloud cover is also visible in the trendresults: Gradually lowering the maximum allowed cloud cov-er in the filtered subsets (and thus reducing cloud influence onchanges in UV from the analysis) yields decreasing trendestimates at all wavelengths (Table 1). The estimates at305 nm exhibit the same behavior, although the change-in-trend is reduced and significance levels are generally below95%, which is most likely caused by enhanced ozone absorp-tion at the shorter wavelengths. At low cloudiness (N≤2)however, still significant positive trends are found at thelonger wavelengths. First results of AOD measurements fromdirect sun observations with the Brewer under clear-sky con-ditions suggest that a significant decrease in AOD at UVwavelengths (313.5, 316.8, 320.1 nm) of about −5 to −6 %/dec occurred over the investigated period. This view is sharedby Zerefos et al. (2012), who found an even stronger declinein AOD at five stations in Europe over the period 1995–2011.Further investigations are needed to clarify whether thedecreases in AOD can fully explain the remaining trends atlow to no cloudiness, but the results do suggest that the role ofaerosols is not entirely negligible at Sonnblick. It is thereforebelieved that the discontinued reduction of stratospheric ozoneis overcompensated for by changes in cloudiness and AOD.This view is shared with JMA (2009) at the Japanese sitesSapporo, Tsukuba, and Naha, where increasing surface UVradiation was observed since the early 1990s in spite of slightlyincreasing stratospheric ozone since then. Likewise, Chubarova(2008) and Bais et al. (2011) report increases in surface UVradiation through cloud effects in past periods, and in futuremodel projections respectively, as do Smedley et al. (2012),while Zerefos et al. (2012) attribute increases in UV radiationmainly to reductions in AOD over Europe during 1995–2006.

Moreover, if one only considers cases for N≤5, a (nonsig-nificant) increase in cloud cover below the observatory of about+11 %/dec for SZA 55° becomes apparent. It is known fromearlier studies at Hoher Sonnblick that clouds below the

Fig. 4 Left total ozone record from the Brewer MkIV #093 spectro-photometer at Sonnblick observatory from 1994 to 2011, a turnaroundis suggested in 1997; right: monthly mean anomalies in total ozone

column 1997–2011 at Sonnblick observatory; the year-round trendestimate is given in percent per decade±90 % confidence interval,trend significance is 95 % (*)

Table 3 Summary of seasonal trends inmonthlymean anomalies of totalcloud cover (N) and cloud cover below the observatory (Nh1) at HoherSonnblick over the period 1997–2011 in percent per decade ±90 %confidence interval

Cloudcover

Year Spring Summer Fall Winter

N −2.6±3.9 −3.1±5.9 −5.8±7.3 −1.9±10.9 −1.5±12.7

Nh1 +4.3±10.9 +2.2±28.0 +2.0±19.4 +11.0±20.6 +16.4±25.9

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observatory contribute about +0.28±0.15 to effective albedo atthe site and can consequently enhance UV irradiance by about7 % on average for 4/8 cloud cover and larger (Simic et al.2011). In the time series of effective surface albedo at HoherSonnblick, reconstructed from detailed snow observations(Simic et al. 2008), no significant long-term change was found.Consequently, it is believed that the increases in UV irradianceare mainly caused by changes in cloudiness and AOD alongwith potential contributions to effective surface albedo throughclouds below the observatory.

3.4 Influence of chosen calculation period

One must, however, not forget the importance of the chosencalculation period: Despite the availability of spectral UVmeasurements from the Brewer MkIV #093 starting alreadyin 1994, it was decided to settle for the 1997–2011 period(during which trend estimates from Brewer and Bentham datashow high agreement for significant trends), mainly due to theevolution of total ozone (Fig. 4): The record shows a perceiv-able change-in-trend around 1997, coinciding with the findingsof earlier studies (Newchurch et al. 2003; Reinsel et al. 2005).Therefore, it had been concluded that this nonlinear coursewould mask trends in surface UV and that the results wouldbe more meaningful and unambiguous for the period after theturnaround only. This is supported by calculations based ondata from the Brewer, which indicate much larger trend varia-tions between wavelengths, N and SZAs as well as greatlyreduced trend significance if the calculation period is movedfrom 1997–2011 to 1996–2011, and even negative trend esti-mates for the periods 1995–2011 and 1994–2011. Downwardtrends were also found in a previous study at the same station(Simic et al. 2008) for the earlier period 1994–2006, under-lining that changes in the factors responsible for the trends (totalozone, cloudiness, and aerosols) are occurring in a nonlinearcourse over different time periods or seasons (e.g., stronglyelevated AOD after eruption of Mt. Pinatubo 1991; Trickl et al.2009) with their respective influences possibly adding up orcanceling each other out over certain periods of time. Depend-ing onwhat period is chosen to derive linear trend estimates, theresults may vary accordingly. The behavior of changing trendswith the selected period is also covered in Douglass et al.(2011). Therefore, caution has to be exercised when directlycomparing trend estimates across different studies.

4 Conclusions

Long-term records of spectral UVirradiance from theNDACC-affiliated Bentham spectroradiometer measured at the high-altitude mountain site Hoher Sonnblick from the period1997–2011 have been investigated for potential trends usingnonparametric statistics.

Throughout the year, (highly) significant upward trendsare found at wavelengths of 315 nm and longer. The trends’magnitudes at 315 nm range from +9.3±4.5 %/dec** atSZA 45° to +14.2±3.7 %/dec*** at SZA 65° for all-skyconditions. At 305 nm, the trend estimates are considerablysmaller and less significant, yielding between +5.1±6.5 and+7.9±7.3 %/dec, depending on SZA.

Seasonally, the trends show similar behavior: In general,the largest and most significant trends are found during springand winter at all wavelengths, being larger and more signifi-cant at longer wavelengths (e.g. +9.4±5.4 %/dec** at 325 nmin spring, all-sky conditions, SZA 65°) and markedly smallerand less significant at 305 nm (+3.0±10.2 %/dec, spring).

Analysis of total ozone measurements shows an overallincrease of +1.9±1.3 %/dec* over the period 1997–2011around the year, which may explain the smaller increases at305 nm where ozone absorption plays an important role.The overall positive trends in surface UV however cannotbe attributed to losses in stratospheric ozone.

The increases are rather attributed to changing cloudcover and decreasing AOD, as a decrease in total cloudcover of −10.4±9.2 %/dec* (N≤5) is visible for UV meas-urements at SZA 55°. Lowering the maximum allowedcloud cover in the filtered subsets consequently yields de-creasing trend estimates: At SZA 65° and 325 nm, the sloperesults in +11.5±3.5 %/dec*** for all-sky conditions,whereas it is reduced to +9.4±2.9 %/dec*** for N≤2.305 nm trend estimates show weaker changes in trend withcloud cover and reduced significance because of the en-hanced influence of ozone absorption at the shorter wave-lengths, as smaller changes are less likely to turn outsignificant in the MK test. Also, it is known from previousstudies that clouds below the observatory have a consider-able influence on observed UV irradiance.

The increasing tendency of total ozone at Sonnblick leadsto expect decreasing tendencies in the time–series of spec-tral UV irradiance, while the opposite is observed. It isconcluded that the increases in stratospheric ozone are over-compensated for by changing cloudiness and aerosol opticaldepth while contributions through enhanced effective sur-face albedo through (nonsignificant) changes in cloud coverbelow the summit at Hoher Sonnblick may exist.

It has also been found that the selection of the investiga-tion period has a substantial influence on trend results andassessment. It can therefore be stated that prolonged, con-tinuous high-quality measurement series still play a crucialrole for the unambiguous assessment of ozone recovery andtrends of surface UV radiation.

Acknowledgments This work has been funded by the Austrian Fed-eral Ministry for Agriculture and Forestry, Environment and WaterManagement within the project “Long-term measurements of totalozone and high-resolution spectral UV radiation at Hoher Sonnblickand Groß-Enzersdorf”.

Trends in spectral UV radiation from long-term measurements 591

References

Auer I, Böhm R, Jurkovic A, Lipa W, Orlik A, Potzmann R, Schöner W,Ungersböck M, Matulla C, Briffa K, Jones PD, Efthymiadis D,Brunetti M, Nanni T, Maugeri M, Mercalli L, Mestre O, MoisselinJ-M, Begert M, Müller-Westermeier G, Kveton V, Bochnicek O,Stastny P, Lapin M, Szalai S, Szentimrey T, Cegnar T, Dolinar M,Gajic-Capka M, Zaninovic K, Majstorovic Z, Nieplova E (2007)HISTALP—Historical Instrumental Climatological Surface Time Se-ries of the Greater Alpine Region 1760–2003. Int J Climatol 27:17–46

Bais AF, Kazadzis S, Meleti C, Kouremeti N, Kaurola J, Lakkala K,Slaper H, den Outer PN, Josefsson W, Feister U, Janouch M (2007)Variability in spectral UV irradiance at seven European stations. In:Gröbner J (ed) One century of UVradiation research. Proceedings ofthe UV conference, Davos, Switzerland, July 2007, 1:27

Bais AF, Tourpali K, Kazantzidis A, Akiyoshi H, Bekki S, Braesicke P,Chipperfield MP, Dameris M, Eyring V, Garny H, Iachetti D, JöckelP, Kubin A, Langematz U, Mancini E, Michou M, Morgenstern O,Nakamura T, Newman PA, Pitari G, Plummer DA, Rozanov E,Shepherd TG, Shibata K, Tian W, Yamashita Y (2011) Projectionsof UV radiation changes in the 21st century: impact of ozonerecovery and cloud effects. Atmos Chem Phys 11:7533–7545

Bernard G (2011) Trends of solar ultraviolet irradiance at Barrow,Alaska, and the effect of measurement uncertainties on trenddetection. Atmos Chem Phys Discuss 11:26617–26655

Chubarova NY (2008) UV variability in Moscow according to long-term UV measurements and reconstruction model. Atmos ChemPhys 8:3025–3031

den Outer P, Slaper H, Tax R (2005) UV radiation in the Netherlands:assessing long-term variability and trends in relation to ozone andclouds. J Geophys Res 110:D02203

den Outer PN, Slaper H, Kaurola J, Lindfors A, Kazantzidis A, BaisAF, Feister U, Junk J, Janouch M, Josefsson W (2010) Recon-structing erythemal ultraviolet radiation levels in Europe for thepast 4 decades. J Geophys Res 115:D10102

Douglass A, Fioletov V, Godin-Beekmann S, Müller R, Stolarski RS,Webb A, Arola A, Burkholder JB, Burrows JP, Chipperfield MP,Cordero R, David C, den Outer PN, Diaz SB, Flynn LE, HegglinM, Herman JR, Huck P, Janjai S, Jánosi IM, Krzyścin JW, Liu Y,Logan J, Matthes K, McKenzie RL, Muthama NJ, PetropavlovskikhI, Pitts M, Ramachandran S, Rex M, Salawitch RJ, Sinnhuber B-M,Staehelin J, Strahan S, Tourpali K, Valverde-Canossa J et al (2011)Scientific assessment of ozone depletion: 2010,WorldMeteorologicalOrganisation, Geneva, Switzerland Global ozone Research and Mon-itoring Project. Report No. 52:438

Glandorf M, Arola A, Bais A, Seckmeyer G (2005) Possibilities todetect trends in spectral UV irradiance. Theor Appl Climatol81:33–44

JMA (Japan Meteorological Agency) (2009) Climate Change Moni-toring Report 2008. Japan Meteorological Agency, Tokyo,2008:87

Kazantzidis A, Tourpali K, Bais A (2010) Variability of cloud-freeultraviolet dose rates on global scale due to modeled scenarios offuture ozone recovery. Photochem Photobiol 86:117–122

Kendall MG (1975) Rank correlation methods. Griffin, LondonKerr JB, McElroy CT (1993) Evidence for large upward trends of

ultraviolet-B radiation linked to ozone depletion. Science262:1032–1034

Krizan P, Miksovsky J, Kozubek M, Gengchen W, Jianhui B(2011) Long term variability of total ozone yearly minimaand maxima in the latitudinal belt from 20 N to 60 Nderived from the merged satellite data in the period 1979–2008. J Adv Space Res 48:2016–2022

Krzyścin JW, Sobolewski PS, Jarosławski J, Podgórski J, Rajewska-Więch B (2011) Erythemal UV observations at Belsk, Poland, in

the period 1976–2008: data homogenization, climatology, andtrends. Acta Geophys 59:55–182

Mann HB (1945) Nonparametric tests against trend. Econometrica13:245–259

McKenzie RL, Johnston PV, Seckmeyer G (1997) UV Spectroradiom-etry in the Network for the Detection of Stratospheric Change(NDSC). In: Zerefos CS, Bais AF (eds) Proceedings of NATOAdvanced Study Institute (ASI) “Solar Ultraviolet RadiationModelling, Measurements & Effects”, NATO ASI Series, I(52).Springer, Berlin, pp 279–287

McKinlay AF, Diffey BL (1987) A reference action spectrum forultraviolet induced erythema in human skin. CIE J 6:17–22

Newchurch MJ, Yang E-S, Cunnold DM, Reinsel GC, Zawodny JM,Russel JM (2003) Evidence for slowdown in stratospheric ozoneloss: first stage of ozone recovery. J Geophys Res 108:D16

Newman PA, Nash ER, Kawa SR, Montzka SA, Schauffler SM (2006)When will the Antarctic ozone hole recover? Geophys Res Lett33:L12814

Reinsel GC, Miller AJ, Weatherhead EC, Flynn LE, NagataniRM, Tiao GC, Wuebbles DJ (2005) Trend analysis of totalozone data for turnaround and dynamical contributions. JGeophys Res 110:D16

Seckmeyer G, Pissulla D, Glandorf M, Henriques D, Johnsen B, WebbA, Siani A, Kjeldstad ABB, Brogniez C, Lenoble J, Gardiner B,Kirsch P, Koskela T, Kaurola J, Uhlmann B, Slaper H, den OuterP, Janouch M, Werle P, Gröbner J, Mayer B, de la Casiniere A,Simic S, Carvalho F (2008) Variability of UV irradiance inEurope. Photochem Photobiol 84:172–179

Sen PK (1968) Estimates of the regression coefficient based on Ken-dall’s tau. J Am Stat Assoc 63:1379–1389

Simic S, Weihs P, Vacek A, Kromp-Kolb H, Fitzka M (2008) SpectralUV measurements in Austria from 1994 to 2006: investigations ofshort- and long-term changes. Atmos Chem Phys 8:7033–7043

Simic S, Fitzka M, Schmalwieser S, Weihs P, Hadzimustafic J (2011)Factors affecting UV irradiance at selected wavelengths at HoherSonnblick. Atmos Res 101:869–878

Smedley ARD, Rimmer JS, Moore D, Toumi R, Webb AR (2012)Total ozone and surface UV trends in the United Kingdom: 1979–2008. Int J Climatol 32:338–346

Steinbrecht W, Claude H, Schönenborn F, McDermid IS, Leblanc T,GodinS ST, Swart DPJ, Meijer YJ, Bodeker GE, Connor BJ,Kämpfer N, Hocke K, Calisesi Y, Schneider N, de la Noë J,Parrish AD, Boyd IS, Brühl C, Steil B, Giorgetta MA, ManziniE, Thomason LW, Zawodny JM, McCormick MP, Russell JM,Bhartia PK, Stolarski RS, Hollandsworth-Frith SM (2006) Long-term evolution of upper stratospheric ozone at selected stations ofthe Network for the Detection of Stratospheric Change (NDSC). JGeophys Res 111:D10308

Thiel H (1950) A rank-invariant method of linear and polynomialregression analysis, Part 3. In: Proc. Koninalijke NederlandseAkademie van Weinenschatpen A53:1397–1412

Tourpali K, Bais A, Kazantzidis A, Zerefos C, Akiyoshi H, Austin J,Brühl C, Butchart N, ChipperfieldM, DamerisM, DeushiM, EyringV, Giorgetta M, Kinnison D, Mancini E, Marsh D, Nagashima T,Pitari G, Plummer D, Rozanov E, Shibata K, Tian W (2009) Clearsky UV simulations for the 21st century based on ozone and tem-perature projections from Chemistry-Climate Models. Atmos ChemPhys 9:1168–1172

Trickl T, Giehl H, Jäger H, Scheel HE (2009) 32 years of stratosphericaerosol measurements at Garmisch-Partenkirchen (1976–2008).In: EGU General Assembly Conference Abstracts, Vienna, Aus-tria, April 2009, 9520

Vigouroux C, De Mazière M, Demoulin P, Servais C, Hase F,Blumenstock F, Kramer I, Schneider M, Mellqvist J, StrandbergA, Velazco V, Notholt J, Sussmann R, Stremme W, Rockmann A,Gardiner T, Coleman M, Woods P (2008) Evaluation of

592 M. Fitzka et al.

tropospheric and stratospheric ozone trends over Western Europefrom ground-based FTIR network observations. Atmos ChemPhys 8:6865–6886

Watanabe S, Sudo K, Nagashima T, Takemura T, Kawase H, Nozawa T(2011) Future projections of surface UV-B in a changing climate.J Geophys Res 116:D16118

Weatherhead EC, Reinsel GC, Tiao GC, Meng X, Choi D, Cheang W,Keller T, DeLuisi J, Wuebbles DJ, Kerr JB, Miller AJ, OltmansSJ, Frederick JE (1998) Factors affecting the detection of trends:Statistical considerations and applications to environmental data.J Geophys Res 103:D17

Weihs P, Simic S, Laube W, Mikielewicz W, Rengarajan G, Mandl M(1999) Albedo influences on surface UV irradiance at the

Sonnblick High-Mountain Observatory (3106-m altitude). J ApplMeteorol 38:1599–1610

World Meteorological Organization (WMO) (2011) Scientific assess-ment of ozone depletion: 2010. Global Ozone Research andMonitoring Project–Report No. 52:516

Zanis P, Maillard E, Staehelin J, Zerefos C, Kosmidis E, Tourpali K,Wohltmann I (2006) On the turnaround of stratospheric ozonetrends deduced from the reevaluated Umkehr record of Arosa,Switzerland. J Geophys Res 111:D22

Zerefos CS, Tourpali K, Eleftheratos K, Kazadzis S, Meleti C, FeisterU, Koskela T, Heikkilä A (2012) Evidence of a possible turningpoint in solar UV-B over Canada, Europe and Japan. Atmos ChemPhys 12:2469–2477

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