Nadir ozone profile retrieval from SCIAMACHY and its application to the Antarctic ozone hole in the

31 Pages·2017·2.73 MB·English

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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.
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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
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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
◦
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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.
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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/
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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
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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
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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
−
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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
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ManuscriptunderreviewforjournalAtmos.Meas.Tech.
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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