External Geophysics, Climate (Aeronomy and Meteorology)
Twenty-five years of spectral UV-B measurements over Canada, Europe and Japan: Trends and effects from changes in ozone, aerosols, clouds, and surface reflectivity
Ilias Fountoulakis
a,*, Christos S. Zerefos
b,c,d, Alkiviadis F. Bais
a,
John Kapsomenakis
d, Maria-Elissavet Koukouli
a, Nozomu Ohkawara
e, Vitali Fioletov
f, Hugo De Backer
g, Kaisa Lakkala
h,i, Tomi Karppinen
h, Ann R. Webb
jaDepartmentofPhysics,AristotleUniversityofThessaloniki,Thessaloniki,Greece
bNavarinoEnvironmentalObservatory(N.E.O.),CostaNavarino,Messinia,Greece
cFacultyofGeologyandGeoenvironment,UniversityofAthens,Athens,Greece
dResearchCentreforAtmosphericPhysicsandClimatology,AcademyofAthens,Athens,Greece
eMeteorologicalResearchInstitute,JapanMeteorologicalAgency,Ibaraki,Japan
fAirQualityResearchDivision,EnvironmentandClimateChangeCanada,M3H5T4Toronto,Canada
gRoyalMeteorologicalInstituteofBelgium,Ringlaan3,1180Brussels,Belgium
hFinnishMeteorologicalInstitute–SpaceandEarthObservationCentre,Sodankyla¨,Finland
iFinnishMeteorologicalInstitute,ClimateResearch,Helsinki,Finland
jCentreforAtmosphericSciences,SchoolofEarthandEnvironmentalSciences,UniversityofManchester,Manchester,UK
ARTICLE INFO Articlehistory:
Received9January2018 Acceptedafterrevision2July2018 Availableonline1November2018 HandledbySophieGodin- Beekmann
Keywords:
SolarUVradiation Totalozone Clouds
Surfacereflectivity UV-B
ABSTRACT
SpectralUVrecordsofsolarirradianceatstationsoverEurope,Canada,andJapanwere used to study long-term trends at 307.5nm for a 25-year period, from 1992 to 2016.Ground-basedmeasurementsoftotalozone,aswellassatellitemeasurementsof theAerosolIndex,theTotalCloudCoverandthesurfacereflectivitywerealsousedinorder toattributetheestimatedchangesoftheUVtothecorrespondingchangesofthesefactors.
ThepresentstudyshowsthatovertheNorthernHemisphere,thelong-termchangesin UV-BradiationreachingtheEarth’ssurfacevarysignificantlyoverdifferentlocations,and thatthemaindriversofthesevariationsarechangesinaerosolsandtotalozone.Athigh latitudes,partoftheobservedchangesmayalsobeattributedtochangesinthesurface reflectivity.OverJapan,theUV-Birradianceat307.5nmhasincreasedsignificantlyby 3%/decade during thepast 25 years,possibly due to the corresponding significant decreaseofitsabsorptionbyaerosols.Itwasfoundthatthegreatestpartofthisincrease tookplacebeforethemid-2000s.TheonlyEuropeanstation,overwhichUVradiation increasessignificantly,isthatofThessaloniki,Greece.Analysisoftheclear-skyirradiance fortheparticularstationshowsincreasingirradianceat307.5nmby3.5%/decadeduring theentireperiod ofstudy,with anincreasingrate ofchangeduringthelast decade, possiblyagainduetothedecreasingabsorptionbyaerosols.
CrownCopyrightC 2018PublishedbyElsevierMassonSASonbehalfofAcade´miedes sciences.Allrightsreserved.
* Correspondingauthor.
E-mailaddress:[email protected](I.Fountoulakis).
ContentslistsavailableatScienceDirect
Comptes Rendus Geoscience
w ww . sc i e nce d i re ct . co m
https://doi.org/10.1016/j.crte.2018.07.011
1631-0713/CrownCopyrightC2018PublishedbyElsevierMassonSASonbehalfofAcade´miedessciences.Allrightsreserved.
1. Introduction
Thedeclineofstratosphericozoneinthe1980s(Farman etal.,1985;Hofmannetal.,1994;Solomonetal.,1986)led toincreased levels ofthe UVradiation thatreaches the Earth’s surface (Fioletov et al., 2001;Kerr and McElroy, 1993;Lakkalaetal.,2003;McKenzieetal.,1999)andraised theconcernsovertheimpactthatthesechangesmighthave (WHO, 1994; WMO, 2007). As shown in the studies of Newman and McKenzie (2011) and Chipperfield et al.
(2015),thecontinuousUVincreasemighthavedetrimental effectsonbothhumanpopulationsandecosystems.Inthe efforttoavoidsuchanoutcome,mostcountriessignedthe MontrealProtocolin1987andagreedtoadoptmeasuresto minimize the emissions of ozone-depleting substances (ODS). At the sametime, large global networks for the monitoringof solarUV irradiance at the Earth’s surface wereestablishedinordertoimprovetheunderstandingof the interactions between solar UV radiation and other atmosphericconstituents,andaccuratelyinformthepublic aboutchangesinsolarUVradiation(Fioletovetal.,2004).
Measurements of the spectral solar UV-B irradiance are thusavailablesincetheendofthe1980sorthebeginningof the 1990sat a numberof historicalstationsaround the world–e.g.,Baisetal.(1993),Kaurolaetal.(2000),Zerefos (2002).TheimportanceofUVradiationforhumansandfor thebalanceofecosystems(Astaetal.,2011;Lucasetal., 2015; UNEP, 2010; Williamson et al., 2014) makes continuedaccuratemonitoringimperative.Sincesatellite instruments perform indirect estimates of UV levels (A Jebaretal.,2017;Cadetetal.,2017;Zempilaetal.,2017;
Zempila et al., 2016), the proper maintenance and uninterruptedoperation ofground-based networksis of exceptionalimportance.
Manyrecentstudiesshowthatthemeasuresadopted aftertheimplementation oftheMontrealprotocolwere successfulandthattheweakeningoftheozonelayerhas decelerated since the mid-1990s (Egorova et al., 2013;
Ma¨deretal.,2010)andthefirstsignsofrecoveryarenow evidentoverboththeNorthern(Kuttippurathetal.,2013;
McLinden and Fioletov, 2011; Newchurch et al., 2003;
Smedley et al., 2012) and the Southern Hemispheres (Chipperfield et al., 2017; Kuttippurathand Nair, 2017;
Solomonetal.,2016)althoughitisstilldebatablewhether therecoverywillcontinueinthefuture(Bernhardetal., 2013a;Manneyetal.,2011).Asaresultofthisreported ozonerecovery,solarUV-Bradiationreachingthesurface hasdecreasedduringthelasttwodecadesoverthehigh latitudes of the Northern Hemisphere (Bernhard, 2011;
Eleftheratoset al., 2015). In the future, climate change drivenshiftsintheatmosphericcirculation patternsare projectedtoinduceadditionalincreaseofozoneleadingto super-recovery, reducing further the levels of UV-B radiation (Hegglin and Shepherd, 2009; Tourpali et al., 2009). Over these high latitudes, other geophysical parametersthatalsostronglyinfluencethelevelsofthe surface UV irradiance, such as clouds and surface reflectivity, are affected enormously by climate change (IPCC,2013).Changesoftheseparametersalready affect thevariabilityoftheUVirradianceoververyhighlatitudes (Bernhard, 2011; Bernhardet al.,2007; Bernhard etal.,
2013b)andareprojectedtoinduceadditionalreductionin itsfuturelevels(FountoulakisandBais,2015;Fountoulakis etal.,2014).
Contrarytowhathappensathigherlatitudes,positive trendsinUV-Birradiancehavebeendetectedovermany locations of the Northern Hemisphere mid-latitudes duringthelasttwodecades(DeBocketal.,2014;Fitzka etal.,2012;Fountoulakisetal.,2016a;Roma´netal.,2014;
Zerefos et al., 2012). The main reason seemsto bethe improvement of air quality levels, since aerosols, the emissionsofwhichhavebeenreducedoverEurope,North AmericaandmanyregionsofAsia(Maoetal.,2014;Wild, 2011; Zerefoset al., 2009), are amongthe mainfactors reducingsurfaceUVlevels(Arolaetal.,2003;Fragkosetal., 2015;Kazadzisetal.,2009b).Zerefosetal.(2012)suggest that there is a decelerationin theincrease oftheUV-B irradianceoverCanada,Europe,andJapansincethemid- 2000s.Fountoulakisetal.(2016a)confirmedthesefindings forthestationofThessaloniki,Greece,andalsosuggested thatthelong-termchangesinUVirradiancecouldbefully explained only by assuming changes in the aerosol absorption efficiency (i.e. in the aerosol physical and chemical properties) in addition to the changes of the aerosolopticaldepth(AOD)andozone.Changesinaerosols andcloudsareprojectedtocontinueplayingamajorrolein future changes of thesurface UV irradiance (Watanabe etal.,2011),generatingtheneedforfurtherstudiesthat willshedlightonthecomplexinteractionsbetweenthese parameters(Baisetal.,2015).
The present study is an extension of the works by Zerefosetal.(2012)andFountoulakisetal.(2016a),this time usingdatafrom10historicalstationsoverCanada, Europe,andJapanfora25-yearperiod(1992–2016).The long-term variability of the UV-B irradiance has been studied with respect to the variability of total ozone, absorption by aerosols, and changes in cloudiness and surfacereflectivity.
2. Dataandmethodology
Datafromground-basedstationsandsatellitesensors havebeenusedforthisstudy.UV-Birradianceandtotal ozonedatawerederivedfromground-basedinstruments;
aerosols,cloudiness,andsurfacereflectivitywerederived fromsatelliteinstruments.Assuggestedbyseveralstudies, thespatialvariabilityoftheUVradiationwithinasatellite pixel maybelarge, evenoftheorderof100%(Kazadzis et al., 2009a), mainly due to the corresponding large variabilityofaerosolsandcloudswithinthepixel(Cadet etal.,2017;Zempilaetal.,2018).Overcomplexterrains with variable surface reflectivity, differences in the UV irradiance of theorder of 15% within20km have been reported,evenunderlow-aerosolconditionsandcloudless skies(Kreuteretal.,2014).Sincesatelliteretrievalsprovide averages within pixels of 5050km2 or wider, quantitativeestimatesofthevariabilityof theseparam- eters may not be fully representative for all stations.
However, reliableground-based measurementsof aero- sols,cloudiness,andsurfacereflectivityarenotavailable forallstationsandfortheentireperiodofthestudy.These uncertainties are more important for the short-term
variability of UV-Bradiation, and less important for its long-term variability. Nevertheless, the measurements usedinthisstudyareaveragesfrom3or4stationsforeach continent,whichisexpectedtosuppresstheseuncertain- ties andleadtoreliableestimatesregarding atleastthe directionofthechanges.
2.1. SpectralUV-Birradiance
For thepresentstudy,Northern Hemisphere stations providingcontinuousspectralUV-Bmeasurementsduring a 25-year period (1992–2016) werechosen. The list of stationsandthecorrespondingdataavailabilityforeach stationarelistedinTable1.Althoughavailableforsome sites,measurementsbefore1992arenotusedforthesake ofconsistencyandcomparabilitybetweentheresultsfrom differentstations.SpectralUVmeasurementswereobtai- nedfromtheWorldOzoneandUltravioletRadiationData Centre (WOUDC) (WMO/GAWUV RadiationMonitoring Community),theEuropeanUVDatabase(EUVDB)(Heik- kila¨ etal.,2016)andfromtheLaboratoryofAtmospheric PhysicsoftheAristotleUniversityofThessaloniki,Greece (LAP-AUTH).
Atnineoftheoveralltenstationsusedforthisstudy, spectral measurementsin theUV-B region arecontinu- ouslyperformedsinceatleast1992byasinglemonochro- matoroftypeBrewerMkIII(Kerr,2010).Measurementsare fromadoublemonochromatorBrewerMkIIIspectropho- tometerinSapporoandTatenoafter1January2001,andin Nahasince9September2006.AtthestationofReading, UK, spectral measurements in the UV-B for the period 1992–2003 have been performed by an Optronics 742spectrophotometerwhile,since2003,measurements areperformedbya BenthamDM150spectrophotometer (BartlettandWebb,2000;Smedleyetal.,2012).
Thesolarirradianceat307.5nmishighlyaffectedby changesintotalozoneandatthesametimeisabsorbed lesseffectivelybysulfurdioxidecomparedtowavelengths intherange306–309nm.Thus,theparticularwavelength was chosen to study the changes of the solar UV-B radiationrelativetothecorrespondingchangesofthetotal ozonecolumn.Althoughwavelengthsshorterthan306nm are affected more effectively by changes in ozone, the measurementsatthesewavelengthsaremoreuncertain
(e.g., Bais et al. (2001)) due to the stray light effect (Karppinen et al., 2014) in single monochromator ins- trumentsandthelowsignal-to-noiseratio(Fountoulakis etal.,2016b),especiallyatlargesolarzenithanglesand/or undercloudyconditions.Inthefollowing,theirradianceat 307.5nmisreferredtoasUV-Birradiance.
Wellmaintainedandcalibratedscanningspectropho- tometersprovidereliablemeasurements,withlowuncer- taintyoftheorderof5%(BernhardandSeckmeyer,1999;
Garaneetal.,2006;Gro¨bneretal.,2006),thoughusually with low temporal resolution – e.g., Zempila et al.
(2016). For that reason, calculating the daily integrals maybemisleading.Hence,takingintoaccountthatmost instruments are regulated to perform at least one measurementnearlocalnoon,weanalyzedtheirradiance reported for that time of day. Furthermore, changes of irradiance at local noon are highly representative of changes in the corresponding daily doses (integrals of theUVirradianceovertheday)sincethegreatestfraction ofthesolarirradianceatshortUVwavelengthsreachesthe surfaceinatimeperiodof1–2haroundlocalnoon(i.e.the timeofthedaywhenthesolarelevationismaximum).
Theirradianceatlocalnooniscalculatedastheaverage of the irradiance measured within 30min around local noon.Forclear-skyconditions,theaveragingisextended to60minaroundlocalnoontoincreasetheamountofdata enteringthe mean.Therebythevalue assigned tonoon irradianceisslightlyunderestimated,buttheuncertainty ofthetrendisnotstronglyaffectedbecausealargenumber of measurements are considered in calculating UV-B trends and climatology. Climatological values for each dayof the yearand each stationare calculated for the entireperiodofstudybyaveragingthenoonirradiancefor eachdayoftheyearacrossall25years.Thesevaluesare subtractedfromtheentiredatasetandthedailyanomalies (% change relative to the climatological values) are calculated. The daily anomalies for each month are averaged to derive the monthly anomalies. Monthly anomaliesareusedintheanalysisonlyifdataareavailable foratleast15daysofeachmonth.The11-yearsolarcycle andtheQuasi-BiennialOscillation(QBO)ofthewindsin theequatorialstratosphereaffectstratosphericozoneand clouds, and subsequently UV-B radiation. Thus, their effects have been removed using the methodology Table1
ListoftheNorthernHemispherestationsprovidingspectralUVmeasurementsatleastsince1992,usedforthepresentstudy.
Station(latitude,longitude) Instrumenttype Datasource
Canada
Churchill(58.758N, 94.078E) BrewerMKII WOUDC
Edmonton(53.558N, 114.108E) BrewerMKII WOUDC
SaturnaIsland(48.788N, 123.138E) BrewerMKII WOUDC
Japan
Naha(26.208N,127.678E) BrewerMKII,BrewerMKIII WOUDC
Sapporo(43.058N,141.338E) BrewerMKII,BrewerMKIII WOUDC
Tateno(36.058N,140.138E) BrewerMKII,BrewerMKIII WOUDC
Europe
Uccle,Belgium(50.808N,4.368E) BrewerMKII WOUDC
Reading,UK(51.448N, 0.948E) Optronics,Bentham EUVDB
Sodankyla¨,Finland(67.368N,26.638E) BrewerMKII EUVDB
Thessaloniki,Greece(40.638N,22.968E) BrewerMKII LAP-AUTH
Theinstrumentsthatperformmeasurementsarealsolisted.
describedinZerefosetal.(2012).Monthlymeansforthe solarflux at 10.7cm were downloadedfrom theNOAA NationalGeophysicalDataCentre(http://www.ngdc.noaa.
gov/stp/space-weather/solar-data/solar-features/
solar-radio/noontime-flux/penticton/),whilefortheQBO wind datawere downloadedfrom theFreie Universita¨t Berlin (http://www.geo.fu-berlin.de/en/met/ag/strat/
produkte/qbo/index.html).Finally,themonthlyanomalies were averaged to derive annual mean anomalies. For Sodankyla¨,the annual meananomalies werecalculated using months from April to September to avoid high uncertaintiesduetotheverylargenoonsolarzenithangles (SZAs)inwinter.Forthenineremainingstations,annual anomaliesarenotusedintheanalysisforeachindividual station,ifdataareavailableforlessthansevenmonths.The annualmeananomalieswereaveragedforthestationsin Canada,Europe, andJapanusingequal weightsforeach yearandeachstation,andthetrendsforeachofthesethree regionswerecalculated.
ForthestationofThessaloniki,analysisofthespectra measuredunderclearskieswasperformedinadditionto theanalysisfortheall-skycases.Forthisstation,detection ofthecloudycasesisachievedusingdatafromacollocated pyranometerasdescribedinVasarasetal.(2001).Although detection of the clear sky cases is also possible for Sodankyla¨ andReadingusingthealgorithmimplemented inEUVDB(Heikkila¨ etal.,2016),thesmallnumberofclear- skycases makestheanalysishighly uncertainfor these stations.
2.2. Totalozonefromground-basedmeasurements
Archivedtotalozonecolumnmeasurementsperformed by Brewer and Dobson spectrophotometers under the auspicesoftheWorldMeteorologicalOrganization,Global Atmosphere Watch network, WMO/GAW, are routinely depositedatWOUDCandwereusedinthisstudy.WOUDC totalozonecolumndatafromalargenumberofstations havealreadybeenusedextensivelybothforozonetrend andozonelayermonitoringstudies(WMO,2014)aswell asforsatellitetotalozonedatavalidationpurposes(e.g., Baket al., 2015;Balis et al., 2007;Garane et al.,2017;
Koukouli et al.,2015; Labowet al., 2013; Loyolaet al., 2011). According to Kerr et al. (1985), the estimated uncertaintyintheretrievaloftotalozonefromtheBrewer spectrophotometers is about 1%. The corresponding uncertainty for the Dobson spectrophotometeris about 1% for cloud-free direct-sun observations and 2–3%for zenith-skyorcloudy observations(VanRosendaeletal., 1998). In this work, daily mean total ozone columns reportedbetween1992and2016byBrewerspectropho- tometers (for stations over Europe and Canada) and Dobson spectrophotometers (for the three Japanese stations)havebeenused. For Reading,ozonedata from threesatellitesthatwereusedasground-basedmeasure- ments are not available before 2003. A more detailed descriptionofthethreedifferentsatelliteinstrumentsand thecorresponding datasets is providedin thefollowing section,since datasetsfromthe sameinstrumentshave beenusedtostudytheaerosolindex.Themethodologyfor thecalculationof theannual meananomalies fromthe
dailymeansandthecorrespondingtrendsisthesameas thatforthespectralUVirradiance.
2.3. Aerosolindexandtotalozonefromsatelliterecords
The aerosol index (AI) is a measure of theradiative effectof aerosolsandhasbeenderivedusingtheTOMS aerosol algorithm that has been described in detail by Herman et al. (1997) and Torres et al. (1998). Non- absorbingaerosols,suchassulfatesandsea-saltparticles, resultinnegativeAIvalues,whereasUV-absorbingaero- sols,suchasdustandsmoke,resultinpositiveAIvalues.
The aerosolindexhasbeenvalidatedand analyzedin a number ofstudiessuchasthoseof Gkikasetal.(2016), Koukoulietal.(2006),deGraafetal.(2005).AIwaschosen forthepresentstudyinsteadoftheAerosolOpticalDepth (AOD),mainlybecauseAIisameasureoftheabsorptionof UVradiationbyaerosolswhileAODisameasureofthe attenuationofthedirectsolarbeam.Furthermore,AODin theUVis availablesince 1992fromonlya verylimited numberofgroundstations(e.g.,CheymolandDeBacker (2003)). However, changes of AI arestrongly correlated withchangesinAOD,i.e.higher(morepositive)valuesof AI denote higher AOD while lower (more negative) AI denoteslowerAOD,asshowninKoukoulietal.(2006).The AIvaluesusedinthepresentstudyhavebeenderivedfrom measurements performed by three different satellite instruments, namely the TOMS/Nimbus7 (October 1978 to May 1993), TOMS/EarthProbe (July 1996 and December 2006) and OMI/Aura[October2004–to date].
The Total Ozone Mapping Spectrometer (TOMS) instru- ment,designed anddevelopedbyNASA,is adownward nadirviewingspectrometerthatmeasuresbackscattered Earth radiances at six discrete ultraviolet wavelengths (312–380nm for TOMS/Nimbus-7 and 308–360nm for TOMS/EP(Kruegeretal.,1998)).Thetotalozonedataare derived from the backscattered UV reflectance by a dedicated TOMS retrieval algorithm (e.g., Ziemke et al.
(2005)). AI estimates from these sensors have been downloaded fromthe publicly availableNASA GSFC ftp location, ftp://toms.gsfc.nasa.gov/pub/. TheOzoneMoni- toringInstrument,OMI,hasbeenflyingonboardtheAura spacecraftsince2004(Leveltetal.,2006).OMIderivesits heritagefromNASA’sTotalOzoneMappingSpectrometer (TOMS)instrumentandtheEuropeanSpaceAgency(ESA) GlobalOzoneMonitoringExperiment(GOME)instrument on the ERS-2 satellite. The wealth of atmospheric compositioninformationprovidedbyOMIisunparalleled (Levelt et al., 2018), with the instrument prominently featuredinallozonetrendandozonerecoverystudies(e.g., Weberetal.(2017)andreferencestherein).Inthisstudy, AIhasbeenextractedforselectedlocationsfromtheAura DataValidationCenter,https://avdc.gsfc.nasa.gov/.
Inthecaseoftheaerosolindex,theabsolute(notthe relative ones as for spectral UV radiation and ozone) annualmeananomaliesareusedinthestudy.Themean values oftheoverpassesforeachdaywereaveragedfor eachmonth,ifatleast15dayswereavailable.Themonthly climatologicalvaluesweresubtractedfromthemonthly averagestoderivethemonthlyabsoluteanomalies,which in turn were used to derive the annual anomalies. As
discussedinLietal.(2009),thedatasetsfromthethree differentsatelliteinstrumentsarereliableandconsistent witheach otherfor theperiods1980–1993(N7-TOMS), 1996–1999(EP-TOMS),and2004–presentday(OMI).
2.4. Clouds
Monthlytotalcloudcover datais retrievedfromthe MERRA-2 reanalysis (Gelaro et al., 2017) for the time period1991–2016.TheModern-EraRetrospectiveanalysis for Researchand Applications version2 (MERRA-2)is a NASAatmosphericreanalysisforthesatelliteerausingthe GoddardEarthObservingSystemModel,Version5(GEOS- 5)withitsAtmosphericDataAssimilationSystem(ADAS), version 5.12.4.The MERRAproject focuses on historical climateanalysesforabroadrangeofweatherandclimate timescalesandplacestheNASAEOSsuiteofobservations in a climate context. MERRA-2 was initiated as an intermediateprojectbetweentheagingMERRAdataand thenextgenerationofEarthsystemanalysisenvisionedfor the future coupled reanalysis. The spatial resolution of MERRA-2reanalysisis0.580.6258(5050km2atmid- latitudes).Inthisstudy,themonthlycloudcoverdatahas been extracted for selected locations from the NASA Giovanni data visualization base (https://giovanni.gsfc.
nasa.gov/Giovanni). The monthly climatological values weresubtractedfromthemonthlyaveragestoderivethe monthlyanomalies,whichinturnwereusedtoderivethe annual anomalies and finally the annual percentage anomaliesbydividingwiththeannualmean.
2.5. Surfacereflectivity
InthepresentstudydatafromtheMERRA-2reanalysis areused tocalculatechangesin thesurfacereflectivity.
Monthly surfacereflectivitydata hasbeenextractedfor selected locations from the Giovanni data visualization basehttps://giovanni.gsfc.nasa.gov/Giovanni,forthetime period1991–2016.Asforthecaseofcloudcover,firstthe monthlyclimatologicalvaluesweresubtractedfromthe monthlyaveragestoderivethemonthlyanomalies,which in turn wereused toderive the annual anomalies and, finally,theannualpercentageanomalieswerecalculated bydividingbytheannualmean.
3. Resultsanddiscussion
Theannualmeananomaliesofthe307.5nmdailynoon irradiance,thedailymeantotalozone,theAI,andthetotal cloudcoverarepresentedinFig.1forCanada,Europe,and Japan. Although analysis of the surface reflectivity was performed,theseresultsareonlydiscussedandnotshown inFig.1.ThedetectedtrendsarelistedinTable2.Thetrends foreachindividualstationarelistedinTable3.Althoughthe detected trends for total ozone, AI and clouds are very similarforthethreeregions,thisisnotthecasefortheUV-B irradiance.
Overallthreeregions,verylowozonewasrecordedin 1992 and 1993, possibly as a result of the eruption of MountPinatubo(Aquilaetal.,2013;Randeletal.,1995).In thecaseofCanada,theverylowozoneinconjunctionwith
thelowTCCintheseyearsledtoveryhighUV-Birradiance (10% above the climatological mean). Thus, different conclusionsmaybeextracteddependingonwhetherthese years are included or not in the analysis of the UV-B irradiance and ozone (i.e. the mean UV-B decrease is 0.29%/yearvs. 0.43%/year,whiletheozoneincreaseis 0.04% vs. 0.11%). The corresponding differences in the trendsofUV-BirradiancearelesssignificantforEurope andJapan,asshowninTable2.
AlthoughozoneincreasessignificantlyoverEuropeand Japan,thereductionoftheUV-BirradianceoverEuropeis not statistically significant (unless 1992 and 1993 are excludedfromtheanalysis),whileoverJapanastatistically significantpositivetrendinUV-Bhasbeendetected.Over theseregions,bothAIandTCCdecrease,thoughonlythe changes of the former were found to be statistically significant.Thus,thelong-termchangesinUV-Birradiance canbeattributedmainlytothechangesofAI.Asdiscussed inthefollowing,theroleofchangesinsurfacereflectivityis alsonotnegligibleoverhigherlatitudes.
Over Japan, positive trends in UV-B irradiance were observedforallthethreestations,indicatingthattheeffect of theAI reduction fullyovercomes theeffect of ozone recovery.ThetrendsofTCCwerefoundtobeverysmall and statistically insignificant for all stations, while a negative significant trend of surface reflectivity was estimatedforSapporo.ItisalsoevidentfromFig.1that overJapantheUVirradianceincreasesfaster beforethe mid-2000s as compared to the period after this point, confirmingthefindingsofZerefosetal.(2012).
ResultsoftheanalysesaremorecomplexoverEurope.
OverUcclethemeanUV-Bchangeisnearlyzero,whileover ThessalonikitheUV-BlevelsincreaseandoverReadingand Sodankyla¨ theUV-Blevelsdecrease.Analysisofclear-sky noon data for Thessaloniki confirms that the long-term changesofUV-Birradiancearemainlydrivenbychangesin aerosolload.AsshowninFig.2,clear-skyUV-Birradiance increases significantly during the period of study (by 0.350.13%/year),followingthedecreaseofAI.Analysisof satellite data confirms that surface reflectivity is nearly invariablethroughouttheyearsoverthisparticularstation.
Over the higher-latitude stations, changes in ozone are estimatedtobethemaindriverofchangesinUV-B,though the role of changes in surface reflectivity and cloudiness cannotbeconsiderednegligible,especiallyforthestationof Sodankyla¨.Forthisstation,astatisticallysignificantreduction of the surface reflectivity by 0.460.13%/year has been detected,probablyduetoacorrespondingreductioninsnow cover(Derksen etal.,2016). Despitethelargerincreasein ozone and the negative trend in surface reflectivity over Sodankyla¨,thenegativetrendsinUV-Birradianceoverthis particularstationareweakerthanthecorrespondingnegative trendsatthelower-latitudestationofReading.Thisispossibly duetothesignificantnegativetrendinTCCoverSodankyla¨.
Over Canada, no significant trend in the changes of ozoneorcloudinesshas been detected,whilesignificant reductionofthesurfacereflectivityhasbeenfoundonlyfor thestationofChurchill.However,UV-Birradiancedecrea- sessignificantlyoverEdmontonandSaturnaIsland,despite thesignificantdecreaseoftheAI,whichis strongerthan the corresponding decrease over Europe and Japan. The
stronger reduction of the UV-B irradiance over Saturna Island,comparedtotheothertwoCanadianstations,may beagainrelatedtothe statisticallyinsignificant positive trendincloudinessoverthisstation.Itshouldbenotedat thispointthatthemeanlatitudeoftheCanadianstationsis muchhigherthanthatofthestationsofJapanusedinthe present study, leading to a relatively stronger effect of ozonecomparedtotheeffectofaerosols(e.g.,Meletietal.
(2009)).However,itisuncertainwhetherthisisenoughto explainthedifferencesinthebehaviorofUV-Birradiance betweenthesetworegions.Additionalexplanationsmight bethatthehighvariabilityofcloudiness,surfacereflecti- vity,andozoneoverCanadamakestrenddetectiondifficult andthefactthattheinteractionbetweenUV-Birradiance andallthefactorsaffectingitisverycomplex.Thus,further investigationisnecessarytoclarifythisresult.
Fig.1.Annualmeananomaliesoftheall-skiesirradianceat307.5nm(first),thetotalozone(second),theTotalCloudCover(TCC)(third),andtheAI(fourth row)overCanada(left),Japan(middle),andEurope(rightcolumn).
Table2
Long-termtrendsoftheirradianceat307.5nm,thetotalozoneandtheTCCin%peryear,andtheAIinabsoluteunits,forCanada,Japan,andEurope.
307.5nmirradiance (Change%)
Totalozone (Change%)
TCC (Change%)
AI(AbsoluteChange)
IP NP IP NP IP IP
Canada 0.430.13 0.290.13 0.110.06* 0.040.06* 0.030.06* 0.050.00
Japan 0.330.08 0.320.10 0.150.03 0.130.03 0.040.05* 0.040.00
Europe 0.170.12* 0.190.14* 0.140.05 0.080.06* 0.120.08* 0.040.00
*Theasteriskdenotesanon-statisticallysignificanttrendatthe95%confidencelevel.Forthe307.5nmirradianceandtotalozone,thetrendshavebeen derivedincluding(IP)andnotincluding(NP)theyears1992and1993intheanalysis,whileforTCCandAIthedifferencesbetweenthetwocasesarevery small,thusonlytheIPtrendsarepresented.
4. Summaryandconclusions
Data from stations over Europe, Canada, and Japan, where spectral UV-B irradiance is measured since the beginningofthe1990s,wereusedtostudythelong-term trendsoftheirradianceat307.5nmfora25-yearperiod, from1992to2016.Ground-basedmeasurementsoftotal
ozone,as wellassatellitemeasurementsof theAerosol Index,theTotalCloud Coverand thesurface reflectivity werealsousedinordertoattributetheestimatedchanges inUV-Birradiancetothecorrespondingchangesinthese factors, which are generally considered as the main regulatoryfactorsforthelevelsofthesolarUV-Birradiance thatreachestheEarth’ssurface(Baisetal.,1993;Bernhard etal.,2007).
OverJapan,UV-Birradiancehasincreasedsignificantly duringthepast25years,possiblyduetothecorresponding significant decrease in the absorption by aerosols, i.e.
changesintheAOD–e.g.,Wild(2011)–andtheabsorption efficiencyofaerosols(Kudoetal.,2012).Itwasfoundthat thegreatestpartofthisincreasetookplacebeforethemid- 2000s,confirmingthefindingsofZerefosetal.(2012).All threeJapanesestationsshowconsistentresults.
InEurope,althoughchangesinaerosolsaresimilarover all stations, their effects on UV-B irradiance are more pronouncedovercentralandsouthernEurope,whileatthe stationsof northernEurope,long-termchangesin UV-B irradiance were found to be driven mainly by ozone recovery. This is probably due to the more significant positivetrendsof ozone,whichin conjunctionwiththe lowersunelevationoverhigherlatitudeshaveastronger impactonthelong-termchangesinsolarUV-Bradiation.
Analysisoftheclear-skyirradianceforthestationof ThessalonikishowsacontinuousincreaseinUV-Birradi- anceduringtheentireperiodofstudy,whichwasstronger duringthelastdecade.Thesefindingsarenotinagreement withFountoulakisetal.(2016a),whosuggestthattherate oftheUVincreaseweakensafter2006.However,inthat study,thetrendsinUV-Birradiancewererestrictedtoa specificSZAof648whereattenuationbyozonepossibly playsamoresignificantrole.Formostoftheyear,thisSZA islargerthantheSZAatlocalnoon.Amorein-depthstudy ofthelong-termchangesinaerosolsoverthestationof Thessaloniki should be performed to reveal whether aerosolschangedifferentlyatdifferenttimesoftheday.
Thismightalsoexplainpartofthedifferencebetweenthe twostudies.
The UV-B irradiance has been found to increase significantlyoverCanada,withoutbeingabletodirectly attribute it to the changes of any of the major factors affecting it. This result points tothe complexity of the interaction of UV-B radiation with its main regulatory Table3
Long-termtrendsoftheirradianceat307.5nm,thetotalozone,thesurfacereflectivity,andtheTCCin%peryear,andtheAIinabsoluteunits,forindividual stationsinCanada,Japan,andEurope.
Region Station 307.5nmirradiance
(Change%)
Totalozone (Change%)
TCC (Change%)
SurfaceReflectivity (Change%))
AI(Absolute Change)
Canada Churchill 0.340.18* 0.060.08* 0.000.10* 0.460.15 0.070.01
Edmonton 0.290.15 0.090.07* 0.000.12* 0.010.25* 0.060.00
SaturnaIsland 0.650.17 0.110.07* 0.090.13* 0.050.06* 0.040.00
Japan Naha 0.300.13 0.170.03 0.040.20* 0.030.02* 0.050.00
Sapporo 0.110.12* 0.170.03 0.150.12* 0.240.12 0.030.00
Tateno 0.580.12 0.100.04 0.050.18* 0.000.02* 0.050.00
Europe Uccle,Belgium 0.060.17* 0.260.06 0.220.10 0.030.02* 0.040.00
Reading,UK 1.200.27 0.070.08* 0.150.13* 0.100.05* 0.040.00
Sodankyla¨,Finland 0.320.20* 0.080.07* 0.210.11 0.460.11 0.040.00
Thessaloniki,Greece 0.340.12 0.130.06 0.110.25* 0.080.06* 0.050.00
* Theasteriskdenotesanon-statisticallysignificanttrendatthe95%confidencelevel.
Fig.2. Annualmeananomaliesoftheclear-skyirradianceat307.5nm (upperpanel),thetotalozone(middlepanel),andtheAI(lowerpanel) overThessaloniki,Greece.
factors.Changesin surfacereflectivitywerefoundtobe non-negligibleregardingtheirimpactonthelevelsofUV-B irradiance over the high-latitude stations of Churchill, Sapporo, and Sodankyla¨, confirming the findings of Bernhard (2011) and providing a sign that they might play an important role for thefuture changes of UV-B irradianceovertheArctic.
Concluding, the present study shows that over the NorthernHemisphere,thelong-termchangesintheUV-B radiationthatreachestheEarth’ssurfacevarygreatlyover different locations, and that the main drivers of these changes are changes in aerosols and total ozone. Over higher latitudes, part of the observed changes may be attributedtochangesofthesurfacereflectivityandclouds.
Since the connection between the changes in UV-B irradianceandchangesinthedifferentfactorsisnotclear, effortstoreducetheuncertainties inthemeasurements and to improve the understanding of the interactions between solarUVradiation andtherelated geophysical variablesarenecessary.
Acknowledgments
Theground-baseddatausedinthis publicationwere obtainedaspartoftheWMOGlobalAtmosphereWatch publiclyavailableviatheWorldOzoneandUVDataCentre (http://woudc.org),andfromtheEUVDB(http://uv.fmi.fi/
uvdb/) database. We would like to acknowledge and warmlythank all theinvestigatorsthat providedata to theserepositoriesonatimelybasis,aswellasthehandlers ofthesedatabasesfortheirupkeepandqualityguaranteed efforts.TheQBOindexesarefromtheCDASReanalysisdata andarethezonallyaveragedwindsat30and50hPaand takenfromovertheequator(http://www.cpc.ncep.noaa.
gov/data/indices/).TheF10.7cm solarradiofluxdensity, usedasproxyofthe11-yearsolarcycle,wasacquiredfrom theSolarandTerrestrialPhysicsDivision(STP)ofNOAA’s NationalGeophysicalDataCenter(NGDC)Website(http://
www.ngdc.noaa.gov/stp/stp.html). We further acknowl- edgetheTOMSNASAGSFCteamformaintainingtheftp://
toms.gsfc.nasa.gov/siteaswellastheAuraDataValidation Centerteamformaintainingthehttp://avdc.gsfc.nasa.gov/
site.ThemonitoringsiteatReading,UK,isfundedbythe UKDepartment of Environment,Food and Rural Affairs (DEFRA). The Academy of Finland has funded the UV measurementsatSodankyla¨ bytheFARPOCCandSAARA projects.
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