National Aeronautics and Space Administration

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

GMAO RESEARCH BRIEF

Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System K. Wargan & J.E. Nielsen

BACKGROUND

SUMMARY A faithful representationof polar stratospheric chemistry inmodelsand its connection with dynamical variability is essential for ourunderstandingoftheevolutionoftheozonelayerinachangingclimateand during the projected continuing decline of ozone depletingsubstances in the atmosphere.We use a new configuration of theGoddard Earth Observing System Data Assimilation System with astratosphericchemistrymodeltostudyozonedepletionintheArcticpolarstratosphereduringtheexceptionallycold(inthestratosphere)winters2015/2016and2010/2011.

#2017-2, April 2017

Stratospheric ozone layer shields Earth’s biosphere from harmfulultravioletradiationandgreatly impactsthethermalstructureofthestratosphere, strongly affecting the climate. A growing number ofrecent studies suggest that the full inclusion of realistic ozone inatmosphericmodel simulationsmayhelp to improveboth seasonalweatherpredictionsandclimateprojections.

GMAO RESEARCH BRIEF Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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Thebest-knownandmostseveremanifestationofthe20thcenturystratosphericozoneloss is the seasonal occurrence of ozone holes over Antarctica. Although to a lesserdegree, the same chemical mechanism leads to springtime ozone depletion over theArctic.Themainpathwayofozonedestruction inpolar regions involvesconversionofchlorine reservoir species, chlorine nitrate (ClONO2) and hydrogen chloride (HCl), intoactivechlorinecompounds,ClOx,fromwhichchlorineatomsarereleasedinthepresenceofsunlight.Chlorinethencatalyticallydestroysozone.TheconversiontoClOxtakesplaceonsurfacesofsmallparticlesofpolarstratosphericclouds(PSCs).Thisprocessdependsonthreeconditions:

• Thepresenceofchlorinecompoundsinthestratosphere• LowtemperatureallowingtheformationofPSCs• Isolationoftheairmasswherethereactionstakeplace

Stratospheric chlorine loading is still high, although it is declining following theimplementation of the Montreal Protocol. Low temperatures and air isolation areattainedwithin thestratosphericpolarvortex,a large-scalecyclonicwindpattern thatformsoverthepoleduringwintertimeandlaststhroughthefirsthalfofspring.Whenthevortex isweak, its temperature iselevated inhibitingPSC formationandstrongmixingdispersesozone-depletingcompoundsatthesametimereplenishingthevortexozone.Ontheotherhand,astrongvortexactsinamannersimilartoalaboratorybeakerinwhichchlorine activation and ozone depletion take place with little mixing with themidlatitudinal ozone-rich air masses. The severity of polar stratospheric ozone lossstronglydependson the temperatureandstrengthof thepolarvortex,whichexhibitsstronginterannualvariability.The2015-2016winterwasoneofthecoldestonrecordintheArcticpolarstratosphere.ThisledtohighlevelsofchlorineactivationandrapidozonedepletionuntilthefirstweekofMarch,whenthevortexbrokedownseveralweeksearlierthanusuallypreventingfurtherchemicaldestructionofozone.Thepurposeofthisworkistwofold:

• Toestimatethe2015/2016Arcticozoneloss• TotesttheGoddardEarthObservingSystem(GEOS)DataAssimilationSystem

(DAS)withfullstratosphericchemistryforfutureatmosphericreanalyses.

GMAO RESEARCH BRIEF Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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ExperimentandResults

AnewconfigurationofGEOS-DAS isused to study thisozone loss.Ozoneevolution iscomputedwithSTRATCHEM,astratosphericchemistrymodulethatsimulatesover160chemical reactions among 34 transported and 17 derived species and contains a PSCscheme. STRATCHEM replaces the parameterized chemical module normally used inGEOS-DASexperiments,includingmultidecadalreanalyses.Theobservationsassimilatedby the system consist of radiance data from satellite-borne sensors, conventionalmeasurementsandozoneobservations.Thelatterareobtainedfromtwoinstruments:theMicrowaveLimbSounder(MLS)andtheOzoneMonitoringInstrument(OMI),bothonboardNASA’sEOSAurasatellite.Combiningthereal-worldstratosphericmeteorologyconstrainedbyobservationswithachemistrymodelenablestheproductionofaglobalthree-dimensional representation of the atmospheric composition and its evolutionduringaspecifiedperiodthatcanbereadilyevaluatedagainstsatellitedata.Figure 1 shows comparisons of the vortex-averaged GEOS ozone, HCl and chlorinemonoxide(ClO)withdatafromtheMLSinstrument.Thedatashownarefromthe480-Kpotentialtemperaturesurface,whichcorrespondstoabout20kmabovetheground.TheexcellentagreementbetweenthemodeledandMLSozoneisnotsurprisingbecauseMLSozone data are assimilated in this experiment. But there is also a good qualitativeagreementbetweentheGEOSandMLSHCl,whichisnotassimilated.TheslowerdeclineofthemodelHClconcentrationslikelyresultedfromdeficienciesintheGEOStransport.ThisconclusionissupportedbyadditionalanalysisoftheGEOSnitrousoxideindicatingthat downward transport of air inside the vortex is underestimated in GEOS. Preciseestimationoftransporterrorsandstrategiestoimprovethemodelperformanceareareasof active research in the GMAO. While transport issues affect the atmosphericcompositionandcontributetotheestimatederrorsoftheanalysis,theyalsoprovideaninsightintotheperformanceofthemodelandtheassimilationschemeandhelptomapoutfutureimprovementsofboth.Despitethisshortcoming,theClOconcentrationsfromGEOSfollowMLSverycloselyexceptinMarchwhenthemodelgeneratesahighbiasinClO,indicativeofaslightlydelayedchlorinedeactivation.Thistooislikelyaconsequenceofmodeltransporterrorsbutcanalsoberelatedtoerrorsinthechemistrymodelthatrequire further investigation.Wenote that theuncertainties in theozonebehavior inGEOSthatresultfromthesedeficiencieshavebeenestimatedandincludedaserrorbarsinthefinalanalysisinFigure3.SinceClOisdirectlyresponsibleforozonedepletion,thisclose agreement during most of the winter provides confidence in the modelperformancewithrespecttoozoneloss.

GMAO RESEARCH BRIEF Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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TheGEOSmodelwritesoutchemicaltendenciesforozone, i.e.ozone’srateofchangedue to chemistry alone, other processes excluded. This allows us to estimate theevolutionoftheozonelossrateandthecumulativechemicalozonechangeduringthewinter-spring.Thetimeseriesofthevortex-averagedchemicalozonetendenciesinpartsperbillionbyvolume(ppbv)perdayisshowninFigure2(bottomleftpanel)asafunctionofpotentialtemperature,aconvenientverticalcoordinateforstratosphericstudies.Here,theverticalrangeis400Kto800K,correspondingtoheightsbetweenabout16kmand30km.Thetoppanelshowsthepoletemperatureat~20kmandthefractionofthepolarvortexilluminatedbysunlight.Asthesunlitfractionreachesabout10%inmid-January,therateofchemicalozonelossacceleratestoabout20ppbv/dayandmaximizesintheendofFebruaryat40ppbv/day.Threedistinctmaximacorrespondto increases inthepoletemperatureinmid-January,thebeginningandtheendofFebruary.Thesejumpsindicateslightdynamicalshiftsofthepositionofthepolarvortexthatslidesoffthepole

Figure1.Timeseriesofvortex-averagedozone(top),HCl(middle),andClO(bottom)atthe480-Kpotentialtemperaturesurface(~20km)fromMLS(black)andGEOS(red).Ozoneisshowninpartspermillionbyvolume(ppmv)andtheotherspeciesinpartsperbillionbyvolume(ppbv).

GMAO RESEARCH BRIEF Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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exposing larger portions of its air mass to sunlight, which accelerates photochemicalreactions that release atomic chlorine and strongly depend on solar illumination. Thechlorine-catalyzedozonedepletionislimitedtoelevationsbelowabout26km(potentialtemperatureofabout640K).Theozone lossabovethat level isduetoreactionswithnitrogenoxidesandisreplacedbyrapidphotochemicalproductioninMarch.

Figure3showsthecumulativeozonelossduetochemistrybetween1December2015and31March2016.Themaximum lossof2.1ppmvoccurredat480K (~20km).Thiscorrespondsto66%ofthevortexozoneon1January,asignificantamount,comparedtomostArcticwinters.Thesamepanelshowsthecumulativelossduringtherecordbreakingwinter 2010/2011.Note thatwhile themaximumdepletion thenwas 3.25 ppmv, thecumulative loss computed only up to 12 March 2011 (the approximate date whendepletionwasterminatedin2016)wasaboutthesameas2016.Thisresultindicatesthatthe2015/2016winterwaslikelyontracktomatchtheextremeozonelossseenin2011,

Figure2.(top)Thepercentageofthe480-Kpolarvortexareailluminatedbysunlightasafunctionofday(1December2015to30March2016)isshowninblack.The480-Ktemperatureatthepoleisplottedinblue.(bottom)Thevortex-averageddailyozonetendenciesduetochemistryinppbvperday.

GMAO RESEARCH BRIEF Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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anoutcomeprecludedonlyby theunusually earlybreakupof the lower-stratosphericpolarvortexaround12March2016.Theerrorbarsaroundthelosscurveareestimatedfrom the differences between six-hourly ozone forecasts and satellite data calculatedwithinthedataassimilationsystem.

Wenow turn to the spatial distribution of the chlorine compounds and ozone in thecontext of the stratospheric dynamics. Figure 4 shows maps of the GEOS chlorinereservoirspecies,activechlorineandozoneonthe480-Kpotentialtemperaturesurface(approximately20kmabovetheEarth’ssurface)overtheNorthPoleon11Januaryatthebeginningofozonedepletionandon21March2016afterthebreakupofthepolarvortex.ThemapsaregeneratedfromtheGEOSoutput.Thetoprowshowsthesituationinthemid-winter:lowconcentrationsofchlorinenitrateandHClcombinedwithhighquantitiesofClOxwithinthepolarvortexindicatechlorineactivationbutbeforesignificantozonelosscommenceswhenalargerportionofthevortexbecomesilluminatedbysunlight.Inthebottomrow(overtwomonthslater)thereisonlyaremnantofthepolarvortexleft,chlorineisdeactivated(ClONO2haselevatedconcentrations,ClOxisnearlygone)andverylowozonevalues,theresultofearlierchemicalloss,areseeninsidethevortexremnant.Note the consistency of chemistry and dynamics in GEOS: sharp gradients of species’

Figure3.Timeintegrated(December-March)tendencyfromFigure2inppmv(black),thesamefor6December2010–30April2011(solidblue)andfor6December2010–12March2011(dashedblue).Thebarsindicateanerrorenvelopeestimatedfromassimilationofozonedata.

GMAO RESEARCH BRIEF Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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concentrations are aligned with the edge of the polar vortex and the chemicalcomposition inside thevortex is verydifferent than in the restof thehemisphere.Ananimatedversionofthese,andsomeadditionalmapsatthe500-Kpotentialtemperaturelevelisavailableathttps://gmao.gsfc.nasa.gov/animations/O3loss_arctic_strat.php.

Finally,wecomparethe2015/2016and2010/2011ozonelosswithinadeeplayerwheremost of the polar ozone resides. Figure 5 shows the cumulative change in the vortexozoneintegratedbetween200hPaand40hPa(~12kmto22km).TheresultsareshowninDobsonunits(DU:1DUcorrespondsto2.69×1020ozonemoleculespersquaremeterwithinthespecified layer).Thecumulative losson10Marchwasabout80DU inbothwinters.Whilethe2016lossceasedquicklyafterthebreakupofthepolarvortexanddidnotexceed90DU,in2011theGEOS-estimatedlossreachedabout126DUinApril.

Figure4.MapsofmodeledClONO2(aande),HCl(bandf),ClOx(candg),andozone(dandh)onthe480-K potential temperature surface on 11 January 2016 (a-d) and 21March 2016 (e-h). The blackcontouristheedgeofthepolarvortex.

GMAO RESEARCH BRIEF Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

8

Conclusion

A new configuration of the GEOS Data Assimilation System with the STRATCHEMchemistrymodelallowedustoproduceadetailedrepresentationofpolarstratosphericozone chemistry during the exceptionally cold (in the stratosphere) 2015/2016 and2010/2011winters.Thechemistrymodelsimulates theevolutionofahostofchlorinespecies consistent with the dynamics of the polar vortex. In particular, there is anexcellentagreementbetweenthevortexchlorinemonoxidefromGEOSandMLSdata.Incombinationwithdataassimilation,themodelallowedustoestimatethemagnitudeandtemporalvariationsofchlorine-catalyzedozonelosswithinthepolarvortexandestimateitserrors.Weconcludedthatthemaximumcumulativechemicalozonelossof2.1ppmvtookplaceatabout20kmabovethesurfaceandwascomparabletotherecordArcticozonedepletionof2011uptothetimeoftheearlypolarvortexbreakupinearlyMarch.In2011,thelosscontinueduntilthebeginningofApril.Theseresultsunderscorethevalueof includingafullstratosphericchemistrymodel infuture data assimilation-based atmospheric reanalyses but also help us identifydeficiencies in transportof stratosphericconstituents inGEOS,whichwillguide futureinvestigationsandmodeldevelopments.

Figure5.Cumulativevortex-averagedchemicalozonelossinthe200hPa–40hPacolumn.Black:2010/2011,red:2015/2016.OzonelossvaluesareexpressedinDobsonunits.Thedatesofthe10hPazonalmeanzonalwindreversalat60°N(6March)andthevortexbreakupat480K(12March)in2016areshownasadottedanddashedline,respectively.

GMAO RESEARCH BRIEF Quantifying chemical ozone loss in the Arctic stratosphere with GEOS-STRATCHEM Data Assimilation System

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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Thisworkhasbeendone incollaborationwithourcolleagues in theGMAOandotherinstitutionsandwillbeincludedinapeer-reviewedpublication,Warganetal.(2017)nowinpreparation.

Reference

Wargan,K.,J.E.Nielsen,G.L.Manney,M.L.Santee,N.J.Livesey,Z.D.Lawrence,S.PawsonandL.Coy,2017:ChemicalOzoneLossDuringthe2015/2016ArcticWinterinaGoddardEarthObservingSystemAssimilationExperimentwithStratosphericChemistry.SubmittedtoJ.Geophys.Res.