quantifying chemical ozone loss in the arctic stratosphere ... · quantifying chemical ozone loss...

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.

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.

https://ntrs.nasa.gov/search.jsp?R=20170004587 2020-04-18T21:15:03+00:00Z

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-knownandmostseveremanifestationofthe20thcenturystratosphericozonelossistheseasonaloccurrenceofozoneholesoverAntarctica.Althoughtoalesserdegree,thesamechemicalmechanismleadstospringtimeozonedepletionovertheArctic.Themainpathwayofozonedestructioninpolarregionsinvolvesconversionofchlorinereservoirspecies,chlorinenitrate(ClONO2)andhydrogenchloride(HCl),intoactivechlorinecompounds,ClOx,fromwhichchlorineatomsarereleasedinthepresenceofsunlight.Chlorinethencatalyticallydestroysozone.TheconversiontoClOxtakesplaceonsurfacesofsmallparticlesofpolarstratosphericclouds(PSCs).Thisprocessdependsonthreeconditions:

• Thepresenceofchlorinecompoundsinthestratosphere• LowtemperatureallowingtheformationofPSCs• Isolationoftheairmasswherethereactionstakeplace

Stratosphericchlorineloadingisstillhigh,althoughitisdecliningfollowingtheimplementationoftheMontrealProtocol.Lowtemperaturesandairisolationareattainedwithinthestratosphericpolarvortex,alarge-scalecyclonicwindpatternthatformsoverthepoleduringwintertimeandlaststhroughthefirsthalfofspring.Whenthevortexisweak,itstemperatureiselevatedinhibitingPSCformationandstrongmixingdispersesozone-depletingcompoundsatthesametimereplenishingthevortexozone.Ontheotherhand,astrongvortexactsinamannersimilartoalaboratorybeakerinwhichchlorineactivationandozonedepletiontakeplacewithlittlemixingwiththemidlatitudinalozone-richairmasses.Theseverityofpolarstratosphericozonelossstronglydependsonthetemperatureandstrengthofthepolarvortex,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

3

ExperimentandResults

AnewconfigurationofGEOS-DASisusedtostudythisozoneloss.OzoneevolutioniscomputedwithSTRATCHEM,astratosphericchemistrymodulethatsimulatesover160chemicalreactionsamong34transportedand17derivedspeciesandcontainsaPSCscheme.STRATCHEMreplacestheparameterizedchemicalmodulenormallyusedinGEOS-DASexperiments,includingmultidecadalreanalyses.Theobservationsassimilatedbythesystemconsistofradiancedatafromsatellite-bornesensors,conventionalmeasurementsandozoneobservations.Thelatterareobtainedfromtwoinstruments:theMicrowaveLimbSounder(MLS)andtheOzoneMonitoringInstrument(OMI),bothonboardNASA’sEOSAurasatellite.Combiningthereal-worldstratosphericmeteorologyconstrainedbyobservationswithachemistrymodelenablestheproductionofaglobalthree-dimensionalrepresentationoftheatmosphericcompositionanditsevolutionduringaspecifiedperiodthatcanbereadilyevaluatedagainstsatellitedata.Figure1showscomparisonsofthevortex-averagedGEOSozone,HClandchlorinemonoxide(ClO)withdatafromtheMLSinstrument.Thedatashownarefromthe480-Kpotentialtemperaturesurface,whichcorrespondstoabout20kmabovetheground.TheexcellentagreementbetweenthemodeledandMLSozoneisnotsurprisingbecauseMLSozonedataareassimilatedinthisexperiment.ButthereisalsoagoodqualitativeagreementbetweentheGEOSandMLSHCl,whichisnotassimilated.TheslowerdeclineofthemodelHClconcentrationslikelyresultedfromdeficienciesintheGEOStransport.ThisconclusionissupportedbyadditionalanalysisoftheGEOSnitrousoxideindicatingthatdownwardtransportofairinsidethevortexisunderestimatedinGEOS.PreciseestimationoftransporterrorsandstrategiestoimprovethemodelperformanceareareasofactiveresearchintheGMAO.Whiletransportissuesaffecttheatmosphericcompositionandcontributetotheestimatederrorsoftheanalysis,theyalsoprovideaninsightintotheperformanceofthemodelandtheassimilationschemeandhelptomapoutfutureimprovementsofboth.Despitethisshortcoming,theClOconcentrationsfromGEOSfollowMLSverycloselyexceptinMarchwhenthemodelgeneratesahighbiasinClO,indicativeofaslightlydelayedchlorinedeactivation.Thistooislikelyaconsequenceofmodeltransporterrorsbutcanalsoberelatedtoerrorsinthechemistrymodelthatrequirefurtherinvestigation.WenotethattheuncertaintiesintheozonebehaviorinGEOSthatresultfromthesedeficiencieshavebeenestimatedandincludedaserrorbarsinthefinalanalysisinFigure3.SinceClOisdirectlyresponsibleforozonedepletion,thiscloseagreementduringmostofthewinterprovidesconfidenceinthemodelperformancewithrespecttoozoneloss.

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.Thesejumpsindicateslightdynamicalshiftsofthepositionofthepolarvortexthatslidesoffthepoleexposing larger portions of its air mass to sunlight, which accelerates photochemical

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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reactions that release atomic chlorine and strongly depend on solar illumination. Thechlorine-catalyzedozonedepletionislimitedtoelevationsbelowabout26km(potentialtemperatureofabout640K).Theozone lossabovethat level isduetoreactionswithnitrogenoxidesandisreplacedbyrapidphotochemicalproductioninMarch.

Figure3showsthecumulativeozonelossduetochemistrybetween1December2015and31March2016.Themaximumlossof2.1ppmvoccurredat480K(~20km).Thiscorrespondsto66%ofthevortexozoneon1January,asignificantamount,comparedtomostArcticwinters.Thesamepanelshowsthecumulativelossduringtherecordbreakingwinter2010/2011.Notethatwhilethemaximumdepletionthenwas3.25ppmv,thecumulativelosscomputedonlyupto12March2011(theapproximatedatewhendepletionwasterminatedin2016)wasaboutthesameas2016.Thisresultindicatesthatthe2015/2016winterwaslikelyontracktomatchtheextremeozoneloss

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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seenin2011,anoutcomeprecludedonlybytheunusuallyearlybreakupofthelower-stratosphericpolarvortexaround12March2016.Theerrorbarsaroundthelosscurveareestimatedfromthedifferencesbetweensix-hourlyozoneforecastsandsatellitedatacalculatedwithinthedataassimilationsystem.

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

9

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.