Adv. SpaceRes.Vol. 13,No. 1, pp.(l)311—(l)319, 1993 02731177/93$15.00Printed inGreatBritain. 1992 COSPAR

SOMEASPECTSOFTHE INTERACTIONBETWEENCHEMICAL AND DYNAMICPROCESSESRELATING TOTHE ANTARCTICOZONE HOLE

R. S. Eckman,*R. E. Turner,* W. T. Blackshear,*T. D. A. Fairlie**andW. L. Grose*

* AtmosphericSciencesDivision,NASALangleyResearchCenter,Mail Stop

401B, Hampton,VA23665-5225,U S.A.** ScienceandTechnologyCorporation,101 ResearchDrive, Hampton,VA 23666,U S.A.

ABSTRACT

Observationalandmodeling studieshavebeenconductedto examinetheinteractionbetweenthe chemicalanddynamical processesthatoccurduring springtimein the lower stratosphereof the SouthernHemisphere. Thetemporalevolutionof theozonedistributionandthecirculationduring 1987 is contrastedwith thatfor 1988 asanillustrativeexampleof how dynamicalprocessesandthe resultingmeteorologicalconditionsmodulatetheozonedepletion. Concurrentlywith the observationalanalysis,aneffort wasinitiatedto simulatethe ozonedepletionduring australspringusinga three-dimensionalchemical/transportmodel. Themodel includesa parameterizedrepresentationof theheterogeneousprocessesthoughtto beimportantin this region. Thesimulation indicatesthatthe inclusionof this additionalchemistry,whichresultsin thereleaseof freechlorineandtheredistributionof oddnitrogeninto reservoirspecies,reproducesmanyaspectsof theobservations.While significantuncertaintiesanddifficultiesremainin orderto includeheterogeneouschemistryin stratosphericmodels in a self-consistentmanner,thepreliminaiyresultsareencouragingandprovidetheimpetusfor improvingcurrentmodels.

INTRODUCTION

Observationsfrom ground-basedinstrumentsof significantandunexpecteddepletionsof columnozoneaboveAntarcticaduringspring/1/havecausedanunprecedentedeffortto understandthis phenomenon.Substantialdata/2,3,4/ from ground-basedandairborneexpeditionsin the polarregions of both hemisphereshaveled to aconsensusthat theobservedozonereductionsarepredominantlytheresultof chlorine-catalyzedchemistrytakingplace on cloud surfacesin regions of the lower stratosphere.Previously,suchreactionswere consideredofminimal importancein relationto stratosphericchemistry.

Although the primarycauseof the Antarcticozonedepletionis thought to bechemical,it is apparentthat thevariability of dynamicalprocessesact to modulatethephenomenon/5,6,7/. Furthermore,radiative/dynamicalcoupling may be important /8/. Thus, a generalunderstandingexists,but considerableuncertaintyremainsinachievingaquantitativeunderstandingof thephenomenonandthe interactionsbetweenchemical,radiative,anddynamicalprocesses/9/.

This paperhastwo purposes:(1) to illustrate someaspectsof theinteractionbetweenchemistryanddynamicsinmodulatingozonedepletionby contrastingtheevolutionof theozonedistributionandthecirculationfor 1987 withthat of 1988 using observationsfrom satellite-borneinstrumentsand (2) to presentthe resultsof a simulationconductedwith a three-dimensionalchemical/transportmodel which incorporatesparameterizedheterogeneouschemicalprocessesbelievedresponsiblefor theozonedepletion.Resultsfromthesimulation will bediscussedandcomparedwith observations.

Therearestill considerableuncertaintiesanddifficulties in parameterizingheterogeneouschemicalprocessesinstratosphericmodels in a self-consistentmanner. The parameterizationpresentedhere is aninitial attempt tocharacterizethe generalprocessesin a computationallyefficient manner,while retainingmuchof the relevantphysics. Further,it should be emphasizedthat the model simulation reportedon here is not intendedto berepresentativeof ozonedepletionin aspecificyear.

POLAR OZONE IN THE SOUTHERN HEMISPHERE DURING 1987 AND 1988

Theozonedatausedin this studyhavebeenobtainedfromtheTotalOzoneMappingSpectrometer(TOMS)aboardthe Nimbus7 satellite. Detailsof theTOMS dataaregivenin /10/. Mapsof temperaturehavebeenderivedfromdaily analysesof geopotentialheightproducedroutinelyatthe U.K. MeteorologicalOffice. Theanalysesarebasedon radiosondedataandradiancemeasurementsmade by stratosphericsoundingunits (SSU’s) aboardNOAAsatellites/ill.

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(1)312 R. S.Eckmanetal.

Theevolutionof polarozonein the stratosphereof theSouthernHemisphereduringspring 1987 wasstrikinglydifferentfrom that in 1988,asshownby timeseriesof TOMS datain Figure 1. In 1987, theozone“hole” wasthedeepestandmostpersistentonrecord/12/. Minimum valuesof columnozonefell below150DU in the middleofSeptemberandremainedso until theendofOctober. Valuesaslow as 109DU wererecordedin earlyOctober.Not until lateNovemberwerevaluesmoretypical of springtime(240DU or so)recovered(datanot shown). InearlySeptember1988,indicationswerethatozonewasbeingdestroyedatarate similar to thatwhichwasobservedat the sametime in 1987. However,minimum valuesstabilizedbetween170 and 180DU in middle to lateSeptemberandrecoveredto 240DU by the endof October,almostamonthearlier thanwas the casein 1987.Figure 2 showssynopticmapsof totalcolumnozonenearthe timesin eachyearwhen ozonewasmostdepleted.They indicatethat the 1987ozoneholewasnotonly deeper,but muchmoreextensivethan thatof 1988.

Therelativeshallownessof the 1988ozoneholedidnot indicatethatspringtimevaluesof polarozonewereon theroad to long-termrecovery. The 1989ozonehole wasalmost asdeepas thatof 1987 /13/. Instead,differentmeteorologicalconditionsin 1987and1988 appearto havehadacrucialinfluenceon theextentof ozonedepletion.

Duringwinterandspring 1987,thepolar lower stratospherewasverycold andrelativelyundisturbeddynamically/14/. A synopticmapof temperaturefor 5 September1987 [Figure3(a)] showsconditionsrepresentativeof latewinter/earlyspring in the lower stratospherein thatyear. A large,cold pool of air with temperaturesaslow as187°Kwascenteredover the polarcap. Thestratosphericpolarvortex whichcoincidedwith the cold air wassurroundedby relatively stronggradientsof potentialvorticity, PV (not shown). Suchconditions favor thepresenceof polar stratosphericclouds(PSC’s), whichexist at temperaturesbelowabout197°Kfor pressurestypical ofthelower polarstratosphereanduponwhichtheanomalouschemicalreactionsthatpreconditiontheair forozonedestructiontakeplace/15/. TheSAM II instrumentrecordedanunusually largenumberof sightingsofPSC’s at 16-km--over200 in September1987andalmost80 in October/16/. StrongPV gradientsthatsurroundedthevortexin 1987 tendto resisttransportinto thepolarregionof air rich in ozoneandodd nitrogenfrom lowerlatitudes/5/; this suggeststhatchemicalreactionswithin thevortexoccurredin relativeisolation. When sunlightreturnedto polarlatitudesin September,total ozonevaluesfell sharplyasdescribedaboveandshownin Figure1.

In latewinterandspring 1988,planetarywavedisturbancesweremoreprevalentin the stratosphereof theSouthernHemisphere,the vortexwascommonlydisplacedfrom thepole, andpolar temperatureswerehigherthanin 1987.At theendof August 1988, aplanetarywavedisturbanceraisedminimumtemperaturesin thelower stratosphere

about8°Kandweakenedgradientsof potentialvorticity /5/. Typically, warmerconditionsexistedoverthepolarcapin early September1988 [Figure 3(b)] than for 1987 [Figure 3(a)]. Thepolarvortex wassmaller,andPVgradientswere weaker(not shown). FewerPSC’s wereobserved--sightingsweredownmore than 50 percentinSeptemberand75 percentin Octobercomparedwith thosemadeduringthesamemonthsin 1987 /16/.

Ozoneandmeteorologicaldatafrom 1987 and1988 suggestthat dynamical variability wasinstrumentalinmodulatingthe depletionof ozoneduringspringtimein thestratosphereof theSouthernHemisphere.Furthermore,it is plausiblethat reducedradiativeheatingdueto reducedlevelsof ozonehelpedsustainlow polar temperaturesin1987anddelaythetransition to summerconditionsuntil theendofNovember,muchlaterthan usual/14/. Thus,aqualitativeunderstandingof theinterplaybetweendynamics,chemistry,andradiationin the polar stratosphereexists. However,adetailedquantitativeanalysisis precludedboth by the uncertaintiesinherentin global satellitedatasets(e.g.uncertaintiesarising fromcalibration,inversion,resolution,andsampling)andby thelackof a fullcomplementof thenecessarydata(i.e., windsandotherconstituents).

Simulationstudiesconductedwith amodelincorporatingtherelevantphysicalandchemicalprocessesarealogicaladjunctto theobservationalstudies. In particular,a three-dimensionalchemistry/transportmodelis ideally suitedfor enhancinginterpretationof theozoneholephenomena.In following sections,suchamodelis brieflydescribed,andpreliminaryresultsfrom a simulationconductedwith themodelarepresented.

MODEL DESCRIPTION

TheLangleyResearchCenterthree-dimensionalchemistry/transportmodelhasbeendescribedpreviously/17,18/.Forthepurposesof this study,severalmodificationsweremadeto accountfor chemistryoccurringduringthepolarnight. The numberof vertical levelsin the model hasbeenincreasedfrom 12 to 24, providing for increasedresolution. Themodel wasconfiguredto transportexplicitly eight families or species: Ox, NOy, HNO3, Clx,N205, H202, HC1, and C1ONO2. The addition of the two chlorine speciesallows for their unambiguousdeterminationat all times andlocations. Prior to their inclusion, the concentrationsof the membersof the oddchlorinefamily remainedindeterminantat night. In addition,thechemistryof thedimer,C1202,believedto beofprimeimportancein the lower stratosphericchlorine-catalyzedozonedestruction,is included/19/.

In thecurrentsimulation,threeheterogeneousreactionsareincluded:

N205+ H20(s)— 2HN03(s)C1ONO2+ H2O(s)—~HOCI + HNO3(s)

C1ONO2+ HCl(s)—~C12 + HNO3(s).

All threereactionsareparameterizedwith aneffective first-orderrate constantof 4.6 x i05 sec~/20,21/. Thiseffective rate is a simplification of the true rate of a gas-surfacereactionwhich is characterizedby a sticking

AntarcticOzoneHole Studies (1)313

coefficientor reactionprobabilitywhich is afunctionof theparticlesurfacearea,itself critically dependenton

temperature.For the purposesof this simulation,thereactionsare includedonly polewardsof 64°Sandbetween128- to 23-mbwhenthetemperature,ascalculatedby theGCM, is lessthan200°K.

As this parameterizationdoesnot considertheexplicit formationof polarstratosphericcloudsandthesubsequentfate of condensedphaseHNO3, eitherin theformof nitric acidtrihydrateor its possibleloss from thestratospherethroughsedimentation/22/, asimplifiedmechanismwasconsideredto simulatetheobserveddenithficationof theAntarcticstratosphere.For thepurposesof this initial simulation, all HNO3in the solidphaseformedby theabovereactionsis consideredlost to the system. Themodel-derivedremovalof oddnitrogenis, therefore,largerthanobserved/23/and, hence,likely overestimatestheoddoxygendestruction,asno activechlorine is convertedtoC1ONO2in this region.

MODEL RESULTS

Calculatedtotal ozoneasafunctionof latitude andseasonfrom anannualsimulationof themodelwith only gas-phasechemistryis presentedin Figure4. Themodelresultsare in reasonableaccordwith observationsin manyrespects/24/, but differ in somedetails. For example,the SouthernHemispherespring maximumshowsapronouncedpolewardprogressionin time. Themodel resultsdisplayonly a slight tendencyin this respect. Inaddition,themaximumcolumnamountsseenat high latitudesarelargerthantheequivalentvaluesfrom the20-yearclimatology,but not with observationsfor someindividual years.

Thepresentmodelsimulationwasinitialized at thesummersolstice. Theheterogeneouschemistryparameterizationwasenabledin earlyJuly. Figure5(a)showsthecalculatedtotal ozonedistributionabove128-mb(thefirst modellevel abovethe tropopause)in the SouthernHemispherefor late June. Near the pole, valuesnear280 DUpredominate.A collarof higherlevelsofcolumnozoneis presentfrom45°Sto 65°Swith valuesup to 410DU. Atlow latitudes,abroadregionof minimumcolumnozoneis seen,in generalagreementwith climatologicallevels.

During thepolarnight,temperaturesin themodelfall below200Kin aregionof thelower andmiddle stratospherepolewardof 60°S.As aresult, heterogeneousreactionsaretriggeredin the model, liberating chlorine fromreservoirspeciesinto forms which arephotolyzedrapidly with the returnof sunlight to produceactivechlorinespecies.TheHNO3producedby thesereactionsplaysamultiple rolein thechemistryresponsiblefor theenhanceddestructionof oddoxygen. First, its presenceinhibits the productionof activeforms of nitrogen,sinceHNO3photolyzesrelativelyslowly. Active nitrogen,particularlyin theform of NO2,convertsactivechlorinebackto itsreservoirformby three-bodyreactionwith ClO. Second,condensedphaseHNO3participatesin theformationofpolarstratosphericcloudsuponwhichtheseheterogeneousreactionstakeplace. Finally, HNO3may subsequentlybe lost fromthe stratosphereby sedimentationif theparticlesreachsufficientlylargesizesasdiscussedabove.

With thereturnof sunlightin the southernpolarregionin lateAugust,chlorine-catalyzeddestructionof oddoxygenbegins. Figure5(b) showsthe initial impacton polarcolumnozonefor 10 September.Totalozonein theareapolewardof about70°hasbeenreducedby approximately30 DU frommidwintervalues,while ozoneatmid- andlow-latitudesshowlittle changefrom thesolsticevalues. At theendof themodelsimulationin mid-October,Figure5(c) showsthat the total ozonehasbeensignificantly depleted,with minimum total ozonevaluesnear170 DU, adecreaseof 100DUfrom themodelinitialization. During themonthof September,columnozonedecreasedin thepolarregionat arateof 5 DU/day. This is somewhatlargerthanthe3.3 DU/daydecreaseseenduringSeptember1987 /12/.

Zonalmeanprofilesof someof theconstituentsimportantin thechemistryof theozoneholearedisplayedin Figure6for 10Septemberof thesimulation. In Figure6(a),oddoxygenis similar to othermodelingefforts/25/exceptfora small depletionrelative to unperturbedconditionspolewardof 70°S. Below about 1-mb, odd oxygenispredominantlyin theform of ozonesothatoddoxygenmaybeusedasaproxyfor ozone.Maximummixingratiosneartheequatorof 11.5 ppmvarein goodaccordwith ourprevioussimulation/18/, but the vertical structureismuchimprovedandin betteragreementwith observationsas aresultof thedoublingof the numberof verticallevels. Figure6(b) showszonal meanodd nitrogen,definedhereas all nitrogen-containingspecies. Maximumlevels of 22 ppbv are in accordwith othermodeling studies/25/, but somewhatbelow the24 ppbv maximumestimatesderivedby summingLIMS nighttimeN02 andHNO3measurements/26/. Theimpactof heterogeneous

reactionsis seenclearly polewardof 60°Sbetween100-mband10-mbwherethe deniirificationprocessis almostcomplete. Chlorine nitrate(C1ONO2),anotherkeyconstituentinvolvedin thepolarozonedepletion,is showninFigure6(c), Its distributionis in agreementwith othertwo-dimensionalmodelingstudies/25/, but theredistributionof chlorinefrom its reservoirto activeformsis evidentin southernpolarregionswhereCIONO2is nearlydepleted.

Verticaldistributionsof calculatedozonefor 2 Septemberand16 Octoberat 8l°Sarepresentedin Figure7. Themajority of the ozoneloss occursbetween120-mb and15-mb. This rangeis somewhathigherin the polarstratospherethan observedby balloon-borneinstrumentsin 1986,whichshowedamaximumdepletionbetween200-mband30-mb/27/. This is not entirely surprising,as thetemperaturescalculatedby the GCM reachtheirminimaabove100-mb(not shown)and,hence,the heterogeneouschemistrywhichis drivenby the temperature-dependentparameterizationwouldbemosteffectiveat theselevels.

z

z0(I,000

1987

Fig. 1. Minimum columnozonesouthof 60°Smeasuredby the TOMS instrumentfor SeptemberandOctober1987(full line) and1988 (dashedline). (Units: DU).

z0()

Fig. 2. Synopticmapsof columnozonemeasuredby TOMS for (a)2 October1987and(b) 23 September1988.Themapsextendfromthe SouthPole(center)to 20°S. Blank (white)areasindicatewherenodatawasavailable.In (a)ozonevaluesbelow175DU (darkblue)coveroverathird of thetotal areasouthof 60S. In (b), hardlyanyvaluesbelow175DU appear.

(a)

90 -: 90 E

(b)

90 W. -~90 E

Fig. 3. (a) Synopticmapof temperatureat90-mb(about17-km)for 5 September1987,derivedfromU.K.MeteorologicalOffice analysesof geopotentialheight.(Contourinterval:5°K).(b) As for (a), butfor 5 September1988.

1 50

11 21

~EPTCMBER OCTOBEP

i ~

5 SEP 1987 ~ S.HEM.

90 mb--~180

(1)314

0$ COLUMN ABOVE 120 ~ (a)JUNE 26

410380

320

~ 2900

~ 2600~ 230

200170

0~COLUMN ABOVE 128 ~rSEPTEMBER 1.0

410380

;Zl 350320

~ 2900

,~ 2600~ 230

200170

0$ COLUMN ABOVE 120 mb

OCTOBER 16 (c)410380350

320

~ 2900

•~ 2600

~ 230

200170

Fig. 5. (a)Calculatedtotalozone(DobsonUnits) above128-mbfor 26 June.(b) As in (a)exceptfor 10

September.(c) As in (a), exceptfor 16 October.

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MODEL 0 SEPTEMBER 10 (a)

E‘2)

U)U)

—90 —80 —30 0 30 60 90Latitude (deg)

MODEL NO~ SEPTEMBER 10(b)

—90 —80 —30 0 30 60 90Latitude (deg)

MODEL C1ONO2 SEPTEMBER10 (C)

‘2)

U)U)

—30Latitude (deg)

0.0 0,2 0.4 0.6 0.8 1.0 1.2 1.4 1.6Mixing Ratio (ppbv)

Fig. 6. (a) Calculatedzonalmaanmixing ratio ofozone(ppmv)asafunctionof latitudeandpressurefor 10September.(b)Sameas(a), exceptfor totaloddnitrogen(,ppbv). (c) Sameas (a), exceptfor C1ONO2 (ppbv).

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0

-J

90

60

30

0

—30

—60

—90

Antarctic OzoneHoleStudies

Fig. 4. Variationof zonalmeantotal columnozone(DobsonUnits) from an annualcontrol simulation.

.0

E

100

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Fig.7. Verticalprofileof thezonalmeanozonemixingratiocalculatedby the3-D modelat 8l°Sfor 2 September(solid line)andfor 16 October(dottedline).

.0E

0U)a,0~

0.1

1.0

10.0

100.0

September 10 Latitude 75US

1000.00.0 0.5 1.0 1.5 2.0 2.5

010 Mixing Ratio (ppbv)3.0

Month

2 3 4 5Ozone Mixing Ratio (ppmv)

Fig. 8. Verticalprofileof thecalculatedClO mixingratio for 10Septemberat75°S.

(1)318 R. S.Eckmanetal.

Correspondingto theozoneloss,Figure8 showsthecalculatedverticaldistributionof chlorinemonoxideon 10Septemberat 75°S. The maximumat about2-mb is dueto ‘standard gas-phasechemistry. At 20-mb,asecondarymaximumis evidentwith peakmixingratiosof 2ppbv. Themaximumis dueto theenhancedlevelsofactivechlorineformedby theheterogeneouschemistry.Themaximumlevelis in reasonableaccordwith ClO levelsmeasuredin the Antarctic during1987, wherepeaklevelsof 1.6 ppbv at 19-km were noted/28/. Again, thecalculatedpeakoccursatsomewhatlower pressuresthan observeddueto theGCM-derivedthermalstructure.

Theresultsof this initial simulationshowthatseveralaspectsof the springtimedepletionof Antarctic ozonearereproduced.Theredistributionof chlorineto its activeform andnitrogento itsreservoirform, vital for oddoxygendestructionto proceedthroughthe ClO dimer mechanism,occursin the simulation,but with somequantitativedifferenceswith observedClO anddenitrificationlevels. Thelevel ofmaximumcalculatedozonedestructionoccurshigher in the stratospherethanobserved. This is amanifestationof the vertical structureof the temperatureminimum in the 0CM. The secondarymaximumin C1O producedasaresultof theheterogeneouschemistryisapproximately20percentgreaterthanobservations,whichis dueto thehighly denitrifiedstateof this regionof thestratosphere.Nonetheless,theseinitial three-dimensionalchemisty/transportmodel results aresufficientlyencouragingto validatetheutility of this typeof modelin theexaminationof theozonehole.

SUMMARY

We haveexaminedthemorphologyof the springtimedepletionin polarozonein the SouthernHemisphereusingobservationsof ozoneandtemperaturederivedfrom satellite-borneinstrumentsfor 1987 and1988. Thedramaticdifferencesin themorphologyof theozoneholebetweenthe2yearsillustratedhow dynamicalvariability playedanimportantrole in modulatingthedepletion.

A three-dimensionalchemical/transportmodel simulation of the formation of theAntarctic ozonehole wasconducted.Themodelincludedaparameterizedrepresentationof theheterogeneouschemicalreactionsimportantinredistributingoddchlorineinto activeforms,while sequesteringoddnitrogenin reservoirspecies.

The model simulatedmanyaspectsof the observeddepletion. The calculatedozonelossduring the month ofSeptemberin thepolarvortexregionis 5 DU/daycomparedto anobservedrateof 3.3 DU/day in 1987. Calculatedlevelsof theenhancedClO resultingfrom chemistryoccurringon cloudsurfacesarein qualitativeagreementwithrecentobservations.Thedenitrificationof thelower stratosphereis reproduced,but in amountshigherthanthoseobserved.

Improvementsto theparameterizationof the heterogeneouschemistryarenow underway to remedysomeof theshortcomingsof the simulationreportedhere. A more physicallyrealistic model of polarstratosphericcloudformationis underdevelopment.Treatmentof thecondensedphaseHNO3is beingenhancedto considerexplicitlytheremovalof oddnitrogenfrom the stratosphereby sedimentation.Thesemodificationsshould allow us toexaminethe subsequentfateof ozoneandoddnitrogenpoorair following thebreakupof the polar vortexin lateautumn. This “dilution” effecthadbeenstudiedpreviouslywith this model /17/, but with an imposedozonedepletionandno pertubationto the nitrogenor chlorinechemistry. Themore realistictreatmentdescribedherewould allow for amorecarefulassessmentof thepossibleimpactof thedilution onthe globalozonebudget.

ACKNOWLEDGMENTS

We would like to thankArlin Kruegerfor providing the TOMS ozonedata. We arealso gratefulto GretchenLingenfelserfor producingtheTOMS color03 mapsandto Mary Kellermanfor theproductionof themodelcolormaps. Thanksarealsodueto SheilaJohnsonfor preparationofthe manuscript.

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