Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) Nadir ozone profile retrieval from SCIAMACHY and its application to the Antarctic ozone hole in the period 2003-2011 SwetaShah1,OlafTuinder1,JacobvanPeet1,AdrianusdeLaat1,andPietStammes1 1RoyalNetherlandsMeteorologicalInstitute(KNMI),DeBilt,theNetherlands Correspondenceto:SwetaShah([email protected])andPietStammes([email protected]) Abstract. The depletion of the Antarctic ozone layer and its changing vertical distribution has been monitored closely by satellitesinthepastdecadeseversincetheAntarctic"ozonehole"wasdiscoveredinthe1980’s.Ozoneprofileretrievalfrom nadir-viewing satellites operating in the ultraviolet-visible range requires accurate calibration of level-1 (L1) radiance data. Herewestudytheeffectsofcalibrationonthederivedlevel-2(L2)ozoneprofilesandapplytheretrievaltotheAntarcticozone 5 holeregion. We retrieve nadir ozone profiles from the SCIAMACHY instrument that flew on-board Envisat using the Ozone ProfilE RetrievalAlgorithm)(OPERA)developedatKNMIwithafocusonthestratosphericozone.Westudyandassessthequality oftheseprofilesandcompareretrieved(L2)productsfromL1SCIAMACHYversions7and8indicatedasrespectively(v7, v8)datafromtheyears2003-2011withoutfurtherradiometriccorrection.Fromvalidationoftheprofilesagainstozonesonde 10 measurements,wefindthatthev8performsbetterduetocorrectionforthescan-angledependencyoftheinstrument’soptical degradation. The instrument spectral response function can still be improved for the L1 v8 data with a shift and squeeze. We find that the contribution from this improvement is a few percent residue reduction compared to reference solar irradiance spectra. Validationfortheyears2003and2009withozonesondesshowsdeviationsofSCIAMACHYozoneprofilesof0.8% 15%in − 15 thestratosphereand2.5% 100%inthetroposphere,dependingonthelatitudeandtheL1versionused.UsingL1v8forthe − years2003-2011leadstodeviationsof 1% 11%instratosphericozoneand 1% 45%introposphericozone.Application ∼ − ∼ − of SCIAMACHY v8 data to the Antarctic ozone hole shows that most ozone is depleted in the latitude range from 70 S to ◦ 90 S.Theminimumintegratedozonecolumnconsistentlyoccursaround15Septemberfortheyears2003-2011.Furthermore ◦ fromtheozoneprofilesforalltheseyearsweobservethatthevalueoftheozonecolumnperlayerreducestoalmostzeroata 20 pressureof100hPainthelatituderangeof70◦Sto90◦S,aswasfoundfromotherobservations. 1 Introduction Ozone (O ) is one of the most important trace gases in our atmosphere. Stratospheric O absorbs the dangerous solar ultra- 3 3 violet(UV)radiationmakingitanimportantprotectoroflife.AsmallamountofO isfoundinthetroposphereoriginating 3 fromairpollutionandphotochemistry-thisozoneisconsideredahealth-risk. 1 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) Dailyozonemonitoringusingsatellitesdatesbacktothelate1970swithinstrumentsliketheTotalOzoneMonitoringSpec- trometer (TOMS, 1979) and Solar Backscatter Ultra Violet (SBUV) instruments, and since the mid-1990s also by the full UV/VISspectrumcoveringsatelliteinstrumentsliketheGlobalOzoneMonitoringExperiment(GOME,GOME-2) (e.g.Bur- rowsetal.,1999;Munroetal.,2016),ScanningImagingAbsorptionspectroMeterforAtmosphericChartograpHY(SCIAMACHY) 5 (Bovensmannetal.,1999),andOzoneMonitoringInstrument(OMI) (Leveltetal.,2006),tonameafew.Thesesuccessions ofinstrumentsallowustocomparelongtermglobalozonelayerbehaviourandcross-checkthequalityofthemeasureddata. Along-termmonitoringoftheozonetrendlayerisprimarilydrivenbyglobalmeasurementsoftotalozonetimeseriesofsuch satellitedata. The vertical profile of ozone has traditionally been measured by in-situ electrochemical instruments attached to balloons 10 (ozone sondes). Although ozone sondes provide the most accurate method for ozone measurement, they are limited in the heights they can reach (< 35km), and their geographical coverage is limited to approximately 300 stations worldwide that provideweeklyozonesondeprofiles,andveryfewstationswithahigherthanweeklymeasurementfrequency.Satellitemea- surements provide an alternative means for obtaining globally vertical ozone profiles. In general limb and occultation mode satellite instruments can well resolve the vertical distribution in stratospheric ozone. However, they are limited in their hor- 15 izontal resolution, and they have no sensitivity to ozone in the middle and lower troposphere. An alternative approach is to use satellite measurements in nadir mode by high-resolution spectrometers in the thermal IR, like IASI and TES, and in the UV/VIS,likeGOME,GOME-2(e.g.Caietal.,2012;vanPeetetal.,2014;Keppensetal.,2015;Milesetal.,2015).OMI(e.g. Liuetal.,2010;Kroonetal.,2011),andSCIAMACHY. The observation principle of nadir ozone profile retrieval in the UV/VIS is based on the strong spectral variation of the 20 ozoneabsorptioncross-sectionintheUV-visible wavelengthrange,combinedwithRayleighscattering.Thekeyhere isthat the short UV wavelengths (265-300nm) are back-scattered from the upper part of the atmosphere whereas the longer UV wavelengths(300-330nm)aremostlyback-scatteredfromthelowerpartoftheatmosphere.Thistransitionintheozonecross- sectionbetween265-330nmisusefulinretrievingitsverticalprofile.NadirUVandvisiblespectraprovidebetterhorizontal resolutioninozonealthoughtheirobservationscanonlybecarriedoutindaytime.Inthethermalinfra-redmeasurementscan 25 bedoneduringbothnightandday. SCIAMACHYhadbothlimbandnadirmodecapability.TherehavebeenseveralstudiesofozoneprofilesusingSCIAMACHY limbdata(e.g.Brinksmaetal.,2006;Mieruchetal.,2012;Hubertetal.,2017).Brinksmaetal.(2006)foundbiasesinozone profileofstratosphere<10%.Alsointheiranalysisoflimbscatterozoneprofilesfrom2002-2008,Mieruchetal.(2012)found thestratosphericozonehaveabiasof 10%againstcorrelativedatasetsandthisbiasincreasedupto100%inthetroposphere ∼ 30 for the tropics. Similarly Hubert et al. (2017) in their more recent study of limb profiles find that the SCIAMACHY ozone biasesareabout 10%ormoreinthestratospherewithshort-termvariabilitiesof 10%.Therehasbeenverylittlepublished ∼ ∼ workonozoneprofileretrievalfromSCIAMACHYnadirmode,probablyduetocalibrationissues. Inthisstudy,wefocusonnadirozoneprofileretrievalfromSCIAMACHYandtheimpactofL1calibrationimprovements. We evaluate the three most recent versions of the SCIAMACHY L1 product dataset (described in Sect. 2) on the basis of 2 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) Table1.SCIAMACHYLevel1(L1)datacharacteristics Wavelengthrange[nm] Clusternumber/Channel Integrationtime[s] spectralresolution[nm] 265-282 3/1 [0.125-1] 0.22 282-304 4/1 [0.125-1] 0.22 304-314 5/1 [0.125-1] 0.22 314-321 10/2 [0.125-1] 0.24 321-330 9/2 [0.125-1] 0.24 Integrationtimerangesfrom0.125sto1sdependingonthewhichpixelitis. retrieved nadir ozone profiles from the SCIAMACHY UV reflectance spectra. The result of this paper shows the improved qualityofthelatestL1datasetversion. TheresultspresentedherehighlighttheneedforfurthercorrectionsoftheL1data.However,adetailedstudyofradiometric biascorrectionsinL1dataisbeyondthescopeofthispaper.Thefocusofthisstudyistoanalysestratosphericozoneandthis 5 study is done for almost the entire mission length of 2003-2011 where validation is done globally for the latest L1 version available(v8).Wewillfocusexclusivelyonthestudyofozoneinthestratosphericregion(100–10hPa),butwewillbriefly commentontheaccuracieswegetfortroposphericregion(1000–100hPa).TheOPERAretrievalalgorithmisbrieflyreviewed in Sect. 2.2. In Sect. 3, the analysis of the slit function of the instrument is presented. Results on the ozone profiles, and the comparisonbetweenthelevel-1datasetsareshowninSect.4.ThisisfollowedbycomparisontosondesinSect.5forthemost 10 recentdatasetfromyears2003-2011spanningalmosttheentiremission.WeapplytheSCIAMACHYdatasetinanalysingthe AntarcticozoneholeinSect.6.WediscussthepossibleeffectsofL1radiometricbiascorrectionsandapplyingslitfunction correctionsinSect.7,andfinallyconcludeinSect.8. 2 Instrument,dataandmethods SCanningImagingAbsorptionspectroMeterforAtmosphericChartograpHY(SCIAMACHY)isspace-bornespectrometeron 15 board ESA’s Environmental Satellite Envisat (Burrows et al., 1995; Bovensmann et al., 1999) with both horizontal (limb) andvertical(nadir)modeviewingdesigncoveringthewavelengthrangefrom212nm(UV)to2386nm(infrared,IR)spread over 8 channels. Launched in March 2002, its mission lifetime spanned until 2012 and we have level-1 data (Lichtenberg etal.,2006)fromthespectrometerfromAugust2002untilApril2012.Inthispaperweconcernourselveswiththenadirdata andinretrievingtheverticaldistributionofozoneusing265-330nmUV-VIScontinuousspectraldata.Eachnadirstateisan 20 area on the Earth’s surface defined by the scan speed of the nadir mirror across track direction and the spacecraft speed in the along track direction, the field of view (FoV) and the operation of the instrument. This gives typical ground pixel sizes (or equivalently nadir states) of 240km 30km for an integration time (IT) of 1.0s and 60km 30km for an IT of 0.25s × × (Gottwald and Bovensmann, 2011). Alternatively the nadir viewing corresponds to an instantaneous FoV (IFoV) of 0.045 ◦ 3 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) 2005/06/24, L1- v8 1015 10-1 1014 m] n 2/s/sr/1013 10-2 ance [photons/cm11001112 10-3reflectance [-] di a (ir)r 1010 el 1 el 2 n n n n a a h h 109 C C 10-4 270 280 290 300 310 320 330 λ [nm] Figure 1. An example of the spectra of Earth radiance (solid blue line) and solar irradiance (dotted blue line) with the corresponding radiometricscale[photons/cm2/s/sr/nm]indicatedontheleftsideofthefigure,andreflectance(solidblackline)withthecorresponding scale[unit-less]indicatedontherightsideofthefigure.Theverticaldashedlineingreenseparatesthetwochannelsandverticalgreylines separatestheclustersaslistedinTable1. (acrosstrack) 1.8 (alongtrack).TheITalsovariesforclusters,themeasurementsshouldbecombinedbeforeitisfedtothe ◦ × optimalestimation.ThevariationinITbetweenclusterscangiverisetoaspectraljump(discontinuity)wherethelastvalueof thespectralvalueinthepreceedingclusterdoesnotmatchthefirstvalueofthesameinthefollowingcluster.Thewavelengths atwhichthisoccursforChannels1and2wereidentifiedandblockedfromouranalysis(seeFig.11inSect.7). 5 TheabovespecifiedwavelengthrangestraddlesoverChannels1and2ofSCIAMACHYwithanoverlapbetweenthetwo channels.Weusewavelengths265–314nmfromChannel1and314–330nmfromChannel2.TheextracteddatafromL1are brokenintostates,whicharegroupsofgroundpixels.Thespectrumofeachgroundpixelisdividedintospectralclusterswhich aregroupsofwavelengthshavingtheirownintegrationtime.Thesearethenorganizedintoclustersofdatainsteadofchannels. Themappingofclusterstowavelengths,theresolution,andITarelistedinTable1. 10 ThemostimportantinputforretrievingozoneprofilearetheEarth’sreflectancespectra(fromtheL1product).Anexample ofthesespectraareshowninFig.1.Theobservedreflectancespectrumisdefinedas: πI(λ) R (λ)= , (1) meas µ E(λ) 0 whereI,µ ,andEaretheradiancescatteredandreflectedbytheEarth’satmosphere,thecosineofthesolarzenithangle(θ ), 0 0 andtheincidentsolarirradianceatthetopoftheatmosphereperpendiculartothesolarbeam,respectively. 4 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) 2.1 Versionsoflevel-1(L1)data We make use of three different versions of L1 products (described below) from the nadir spectral data and present their differences using L2 products (ozone retrieval). Specifically the calibrated L1c data are reproduced using ESA’s SciaL1C programofv3.2.61.Theseversionsweuseinthispaperaredescribedbelow: 5 1. v7: L1 data products from SCIAMACHY are collocated spectra, radiometrically and spectrally calibrated radiances, includingsun/moonoccultationgeometries.Theversion7(v7)specificallyrefersto7.04 W releasedinearly2012, − where‘W’meansprocessingstageflagand7.04isthesoftwareversionnumberusedforconvertingrawlevel-1toL1 product(SCIAMACHYL1processor,IPPv7.04 W)(vanSoestetal.,2005). − 2. v7 : This data set is identical to the one described above, except here we use the degradation corrections that were mfac 10 provided independently as auxiliary data files2. Thus, the data structure allows us to turn on and turn off to check the effectsofthedegradationcorrectionsindependentofothercalibrationcorrections.Degradationcorrectionisobtainedby theso-called‘m-factors’.Them-factorsaredeterminedbythemonitoringofthelightpathwhichisgivenbytheratio of the measured spectrum of a constant source (Sun) to that obtained for the same optical path at a given time. This givesthereforeanindicationofthepartofthedegradationoftheopticalpathastheinstrumentages.Thesem-factorsare 15 simplemultiplicationfactorstothesolarspectraaftertheabsoluteradiometriccalibration (GottwaldandBovensmann, 2011). 3. v8:TheIPPv8.02isthe2016versionofSCIAMACHYL1product.Themaindifferencebetweenthisversionwiththe onesaboveistheimplicitimplementationofastandarddegradationcorrection.Thedegradationinthisversiontakesinto account the scan angle dependence of the nadir viewing geometry of the instrument with the optical path. We use the 20 slit-functionkeydataprovidedinv8fortheinstrumentalslitfunctionretrieval(seeSect.3).Specificallytheradiometric calibrationusesascanmirrormodelwhichtakesintoaccountthephysicaleffectfromthecontaminationlayersinthe mirror.Thedegradationusingthismodelgivesascanangledependence(Bramstedt,2014). 2.2 OPERAretrievalalgorithm TheOzoneProfileRetrievalAlgorithm(OPERA)hasbeendevelopedinKNMI (vanOssandSpurr,2002;vanderAetal., 25 2002). It retrieves the vertical ozone profile using nadir satellite observations of back scattered UV sunlight from the atmo- sphereusingUVandvisiblewavelengths.Thealgorithmmakesuseofthelawsofradiativetransferincomputingthetop-of- atmosphereradiancesgivenanumberofatmosphericscatteringandabsorptionparameters.Theozoneabsorptioncross-section decreasesfrom265nmto330nmwhichallowsustoretrievetheamountofozoneasafunctionofatmosphericheight.The retrievalmethodisbasedonaforwardmodelwithamaximumposterioriapproachfollowingRodgers(2000).Thisamounts 1https://earth.esa.int/documents/700255/708683/RMF_0140_SCI _NL__1P_v1.1_Dec2016.pdf/ba0b5e7b-f253-4e65-94b3-51fc23d1143d 2http://www.iup.uni-bremen.de/sciamachy/mfactors/ 5 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) Channel 1 Channel 2 ] M S)/ 0.2 0.2 - M [( 0.0 0.0 s al u 0.2 0.2 d si re 0.4 0.4 e v ati 0.6 G = O(15), O = O(2) 0.6 G = O(4), O = O(1) el G = O(15), O = O(2), D = O(2) G = O(4), O = O(1), D = O(1), S = O(2) R 0.8 0.8 270 280 290 300 310 315 320 325 330 λ [nm] λ [nm] Figure2.RelativeresidualsofdailySCIAMACHYL1v8solarspectraforChannels1and2.G,O,D,SareGain,Offset,Displacement/Shift andSqueezerespectively,whereMisSCIAMACHYMeasurementandSisSimulation(ormodifiedmodelreference,seetext).Forvisibility theresidualsfor 300spectraareshownspreadthroughoutthemissionlengthfromAugust2002toApril2012. ∼ toobtainingthestateoftheatmospherebyusingtheradiativetransfermodelandinversiontechniqueiterativelytillthemodel atmospherematchesthemeasurement.Foracomprehensivealgorithmoverviewandretrievalconfiguration,alongwithade- scription of the evaluation of the algorithm and the application to GOME-2 data, we refer to (Mijling et al., 2010; van Peet etal.,2014).TheconfigurationchosenforalltheretrievalsaretabulatedinTable2.Theretrievalgridortheverticalresolution 5 ofthenadirprofileinOPERAcanbechosenaccordingtotheNyquistcriterion.ForSCIAMACHYdatawefindthatsetting theverticalgridto32layersormoregivesthesamevalueforthedegrees-of-freedom(DFS).DFS(usedintheResultssection below)isanumberrelatedtotheaveragingkernelsoftheinstrumentorthesensitivityoftheinstrumentwithverticalheight. Inpractice,themeasurement(reflectancespectrum)R (λ)inEq.1ispreparedinthebeginningoftheOPERAalgorithm, meas which is then passed to the Forward model. This model contains vertical atmospheric profiles, temperature, a priori ozone 10 profile, geolocation, cloud data and surface characteristics. The forward model is used to compute simulated radiance at the top-of-atmosphere at wavelengths determined from the measured instrumental spectral data. This is further used to generate reflectancesbyusingconvolvedsimulatedsolarirradiancespectrum.TheinversionstepthatfollowsisbasedontheOptimal Estimation method requiring measurement, simulation and measurement uncertainties in vector/matrix forms. An inversion usingderivativesofreflectanceswithrespecttothedesiredparametertobesolvediscarriedoutuntilconvergenceisreached 15 oruntilthemaximumnumberofiterationsisreached.Foracomprehensivedescriptionoftheflowofthealgorithmwerefer totheOPERAmanual (Tuinderetal.,2014). 6 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) Gain Offset Shift 1.08 0.08 0.11 300 0.91 300 0.06 300 0.08 m] 290 0.74 290 0.04 290 0.05 n [λ280 0.58 280 0.02 280 0.03 270 0.41 270 -0.00 270 -0.00 0.24 -0.02 -0.03 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 Gain Offset Shift Squeeze 1.24 0.08 -0.01 0.26 325 1.08 325 0.01 325 -0.04 325 0.16 m] 320 0.93 320 -0.07 320 -0.07 320 0.07 n [λ315 0.78 315 -0.15 315 -0.10 315 -0.03 0.63 -0.22 -0.13 -0.12 310 310 310 310 0.48 -0.30 -0.16 -0.22 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 Year Year Year Year Figure3.TemporalevolutionoftheslitfunctionparametersshownforChannels1(toprow)and2(bottomrow).Thisisshownfortheentire missionfor3466dayswhereYear0means2002andYear8means2010. 3 InstrumentalSlitFunctioncalibrationoftheSolarspectralmeasurement Theaccuracyoftheretrievedgeophysicalproductisprimarilydrivenbythequalityofthemeasuredspectra,R (λ)(Eq.1), meas and its spectral and radiometric calibration. The most important SCIAMACHY specific calibration applied to the level-0 (raw data) is described in Slijkhuis et al. (2001). One of the spectral calibrations done often to assess the degradation of the 5 instrument in-flight is a fit of the instrument slit function (SF). It describes the behaviour of the projection of the incoming lightontothedetectorpixels.TheseweremeasuredforSCIAMACHYongroundbeforelaunch(2002)andtheyareprovided asL1keydatafortheinstrumentthatmeasuresthesolarspectra,E(λ)(Eq.1).Theslitfunctionoftheinstrumenthasdifferent functional forms depending on which channel they belong to. For Channels 1-2 (relevant for our ozone retrieval) they are describedbyasinglehyperbolicfunction: 1 10 (λ)= . (2) S a2+λ2 TheL1keydataprovidesthefullwidthhalfmaximum(FWHM)measuredongroundwhichisrelatedtotheaparameter in the equation above. This parameter can be solved in terms of the FWHM by using the fact that at the central point (λ ): 0 S(λ )=1/a2. Thus, a= FWHM2. At each given solar spectrum wavelength, λ , this functional shape is numerically com- 0 2 i putedbetween[-1,1]nmcentredatλ .ThisshapecanbemanipulatedwiththeSFparameters:additiveconstant(Offset,O), i 15 multiplicationfactor(Gain,G),Displacementofthepeakalongwavelength(Shift,D)andexpansionandcontractionofthe spectralpeak(Squeeze,S).Thehighresolutionsolarspectrum(Simulation)fromDobberetal.(2008)ismodifiedwiththese four parameters to best match the SCIAMACHY measured solar intensity (Measurement, E(λ)). First spectral parameters, 7 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) shift,Dandsqueeze,S areappliedtoEq.2: SS0D(λ)=S(λ[1−S(λ)]+D(λ)), (3) followedbyradiometricparameters,gainGandoffsetO: SG0 OSD(λ)=[SS0D(λ)G(λ)]+O(λ), (4) 5 TheunitofShift,DisnmandofOffset,O,Gain,GandSqueeze,S arenumericfactorsandallfourparametersdependon wavelength.TheparametersFWHMandλ ateachwavelengthtakenfromthev8L1keydatawereidenticalforallthesolar 0 spectrathroughoutthemission.ThesevaluesaregivenatcertainwavelengthsspreadthroughoutChannels1and2andwere interpolatedforthewavelengthsinbetween.AnOptimalEstimation(OE)algorithmisusedtosolveforthebestfitparameter values using the solar measurements after the launch. The retrieved values at the solar spectrum wavelengths are then also 10 appliedtotheEarthradiancespectraI(λ)(Eq.1)interpolatedatthesolarspectrumwavelengths. Thusforeachsolarspectrumwavelengththeparametervaluesareusedintheretrievalalgorithm.Thesebestfitvaluesare computedusingtheslitfunctionmodelwiththeabovementionedfourwaysofmanipulation.Themanipulationsarethateach spectral peak of the model spectrum (reference) can be transformed with: Offset, Gain, Shift, and Squeeze. Each of these spectral manipulations can be modelled as a polynomial of order n for each slit function of the instrument at the desired 15 channels. The OE was run using this model from which the best values of the Offset, Gain, Shift, Squeeze as a function of wavelengthofChannels1and2areretrieved.Thebestfitvalueischeckedbyevaluatingtherelativedifferencebetweenthe SolarIrradianceMeasurementandSimulationwhichistherelativeresidual.Therelativeresidualsforthebestfit{O,G,D,S} are shown in Fig. 2. Each curve is a residual for one solar spectrum where the blue line is achieved by using only gain and offsetinthemodelandtheredlineisachievedbyincludingtheshiftand/orsqueeze. 20 WefindthatinChannel1,thewavelengthrange265-308nmistheoptimalrangeforobtainingsmallestresiduals.However becausetheozoneprofileretrievalalgorithmrequiresdatafrom260nm,wedotheoptimalestimationfrom260-308nm.The higherwavelengthvaluesaround314nminChannel1areknowntohavehadcalibrationproblemsandthereforeweusethem onlyuntil308nmwheretheslitfunctionretrievalsarestillwellbehaved.Theirradiancesat308-314nminChannel1failed tomatchthemodelspectrum.WerantheoptimalestimationonallsolarmeasurementsoftheSCIAMACHYmissionforeach 25 daywhichamountedto3463solarspectra.Weconvolvedallspectrawith{Offset,Gain,Shift,Squeeze}ofpolynomialorders of= 2,15,2, andfoundthatthecostfunctionforthemajorityofcasesreacheslessthan1(asexpected)within10loopsof { −} retrievalintheOptimalEstimationroutine.WefindthatusingShiftinrange265-308nminChannel1reducestheresiduals significantlylookingattheleft-panelinFig.2. The residuals in Channel 2 are larger throughout. Dividing the relevant range of 308-330nm into smaller ranges to get 30 smallerresidualsdidnotreducetheresidualsanyfurther.Fortheoptimalwavelengthdivisions,wefoundtheleastresiduals givenbythepolynomialordersofthesetof{Offset,Gain,Shift,Squeeze}= 1,4,1,2 .InChannel2,shownintherightpanel, { } wefindthatusingShift,Squeezeintherange308-330nmreducestherelativeresidualssignificantly.Therelativeerrorsofthe observationinthewavelengthrangewhichwewilluseforozoneretrieval,265-330nm,areintheorderof10 5.Therelative − 8 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) residualsareintheorderofafewpercent.Soweexpectanerrorofafewpercenttopropagateintotheozoneretrievaldespite moreaccuratesolarirradiances.Thereareanomaliesintheresidualsataround279nm,280nmand285nm.Thesearedueto thestrongMgIandMgIIlinesfromthesolarspectraandwillnotbeusedforozoneretrievalasindicatedinTable2. Figure3showsthetemporaldependenceofalltheslitfunctionparametersforChannel1(toprow)andChannel2(bottom 5 row)asdensityplotswherethevalueforeachwavelengthandeachdayoftheyearisshown.Therangesofthevaluesareto therighttoeachfigure.TheseasonaldependenceofsolarradiationisobservedasexpectedintheparameterGain(firstcolumn ofthefigure)forbothchannels.Theotherparametersdonotshowanyseasonaldependenceoverthemissiontime. 2003 2003 2 10-1 3 4 100 measR 65 hPa] 101 og 7 p [ l 8 102 9 10 103 270 280 290 300 310 320 330 5 0 5 10 15 20 25 30 2009 2009 2 10-1 v7 3 v7_mfac 4 100 v8 measR 65 hPa] 101 og 7 p [ l 8 102 9 10 103 270 280 290 300 310 320 330 5 0 5 10 15 20 25 30 λ [nm] O3 [DU] Figure 4. Top-left: Measured reflectance spectra for the year 2003. The spectra are an average over all the pixels in the range of the geolocationsaroundthesondestationswithinanarrowtropiclatitudebandsfrom10◦Nto10◦S.Top-right:Correspondingretrievedozone profiles(inDobsonUnitsperlayer)withdifferentcoloursrepresentingvariousversionsofL1SCIAMACHYdataused(describedinSection 2). The versions 7 with and without the degradation corrections almost overlap (blue and green lines) whereas version 8 with improved degradation correction has a visible difference in the ozone profile. Bottom rows are for the year 2009. Note the remarkable difference between v7 and later versions with degradation corrections. Here the later two versions almost overlap compared to the case where no degradationistakenintoaccountshowingthesignificantdifferenceinL1databetweenthethreeversionswhendegradationoftheinstrument isnottakenintoaccount.ThenumberofpixelsineachdatasetwithcorrespondinguncertaintiesarelistedinTable3. 9 Atmos.Meas.Tech.Discuss.,https://doi.org/10.5194/amt-2017-136 ManuscriptunderreviewforjournalAtmos.Meas.Tech. Discussionstarted:9June2017 c Author(s)2017.CCBY3.0License. (cid:13) 4 Results:ozoneprofilesfordifferentL1versions WeperformOPERAretrievalsonSCIAMACHYnadirdatafortheentiremissionlength(2003-2011) ongeolocationsclose inspaceandtimetoozonesondes.Hereweuseprofilesinthevicinityofthesondesasaguidanceforageneralcomparison between the three datasets used. Thus the results presented in this section are for pixels (states) with global coverage for all 5 the months of the years 2003 to 2011. In Sect. 4.1 we show the comparison of the validation results for the three different dataset versions for selected years. This is followed by validation results using v8 dataset for all the years, 2003-2011. The mainfocusofthisstudyistoanalyseSCIAMACHYnadirozoneprofilesinthestratosphere,usingv7,v7 ,andv8L1data mfac thusverifyingtheirusefulnessforozoneprofilestudies.Withanadir-viewinginstrumentitisveryhardtoretrieveanaccurate ozoneprofileinthetropospherewhichrangesinpressureheightfrom 1000 100hPa.Thelower-middlestratosphereranges ∼ − 10 from 100 10hPawhichisthemainfocusofthispaperasmotivatedinSect.1above.Thusinthesubsectionsbelowwe ∼ − willpresentadiscussionofthequalityoftheSCIAMACHYnadirO profilesinthelower-middlestratosphereandcompareit 3 totheresultsfromotherexistingstudies. 4.1 Comparisonoftheozoneprofilesusingdatasetsv7,v7mfac,andv8 We make a direct comparison of nadir ozone profiles and their corresponding reflectance spectra (with converged retrievals) 15 betweenthedifferentdatasetversions.Weshowthecomparisonsfortheyears2003and2009toshowhowthedifferencesin themeasurements(andthereforetheprofilesderivedfromthem)varyfromearlytolateintheSCIAMACHYmission.InFig.4 theresultsfor2003areshowninthetoppanelsandtheresultsfor2009areshowninthebottompanels.Theleftpanelsshowthe measuredreflectancespectrausedbytheOPERAretrievalalgorithminestimatingtheozoneprofileshownintherightpanels. Eachcurveisamedianofmanypixels,347foreachdatasetversionofyear2003and 400fortheyear2009.Theresultinthe ∼ 20 figureisanaverageforallthepixelsoveranarrowlatitudebandinthetropicsfrom10◦Nto10◦S.Thedifferentcoloursoflines representdifferentdatasetsaslabelledinthebottomrightpanelofthefigure.Thecurvesofozoneprofilesinthetop-rightpanel for2003representingv7,v7 (blueandgreenrespectively)almostoverlapeachothershowingminimaldifferencesbetween mfac thetwoversionsandthereforethedegradationcorrections(m-factors)arealsominimal.However,thecurverepresentingv8 in red deviates from the other two visibly, which is hard to see in the reflectance spectra in the left panel. These differences 25 areexacerbatedfortheyear2009(latertimeofthemission)wheretheprofileofv7issignificantlydifferentfromv7mfac and v8initsshapeandamountofozone.Thecorrespondingmeasuredspectrainthebottom-left-panelconfirmthesedifferences. We also observe visible differences between v7 and v8 indicating the intrinsic differences in the implementation of the mfac degradation corrections between the two dataset versions. In the right panels of the figure are horizontal black-dashed lines demarcatingthelower-middlestratosphere(100-10hPa)withanotherlineat50hPa.Indatasetv7therearelargevariations 30 in the troposphere (1000-100hPa) and a significant reduction of the peak of the ozone value in the stratosphere, suggesting the unreliability of this dataset for later years of the SCIAMACHY mission. The median errors and standard deviations (st. dev.) along with number of pixels and convergence statistics for the retrievals in Fig. 4 are listed in Table 3. The maximum 10