Stratosphericozone,troposphericozone,andthecaseforcontinuingozoneresearchNASA’sEarthScienceResearchandAnalysisprogram(R&A)collectsandanalyzesozonedatafromEarth-observingsatellites,airbornecampaignsandsurface-basedinstrumentstoadvanceourunderstandingofstratosphericandtroposphericozone.Thesedataallowscientiststodeterminethedistribution,physicalandchemicalpropertiesofozonethroughouttheatmosphereandhowitaffectsEarth’sclimateandhumanhealth.

Ozone,climateandhealth

Whetherit’sinthestratosphereorthetroposphere,ozoneimpactsourenvironment,healthandsafety.Dependingonitselevationintheatmosphere,however,itcanhaveapositiveornegativeeffectonradiativeforcingandhealth.Forexample,stratosphericozone(~10-50km)istypicallyseenas“good”ozone,whereastroposphericozone(~7-20km),orozoneneartheboundarylayerwherehumansliveandbreathe,istypicallyseenas“bad”ozone.

• Stratosphericozone,climateandhealth:Naturallyoccurringstratosphericozoneactsasaprotectivelayer,absorbingharmfulsolarradiationthatdamagesourskinandeyesandthecellstructureofplants.Human-madechemicalcompoundsthatcontainchlorineandotherhalonsreactwiththesuntodestroystratosphericozone,limitingitsabilitytoabsorbharmfulUVrays.This,inturn,increasesourchancesofbeingexposedtodangerouslevelsofsolarradiationandalsocausesthestratospheretocool,sincefewerozonemoleculesexisttoabsorbthesun’sradiation.

• Troposphericozone,climateandhealth:Althoughozoneoccursnaturallyinsmallquantitiesinthelowertroposphere,unhealthylevelsoftroposphericozonearecreatedwhenhighlevelsofhuman-madepollutants,suchnitrogenoxides(NOx),andvolatileorganiccompounds(VOCs)reactwithsolarradiation.Theseexcessozonemoleculesactaspollutantsandgreenhousegases,creatingharmfulsmogthatdamagesourlungsandcontributestohighertemperaturesnearEarth’ssurface.Highlevelsofozoneinthelowertropospherecauseavarietyofrespiratoryhealthproblems,includingchestpain,coughinganddifficultybreathing.Italsodamagesplantsandagriculture,reducingcropyieldsinsomeareasbyasmuchas50%.However,intheuppertroposphere,ozonecanhaveapositiveeffectonpollution:itreactstoformthehydroxylradical(OH),whichscrubstheatmosphereofpollutantslikemethane.

IntegratedozonemeasurementsBecausestratosphericandtroposphericozonearetransportedworldwide,weneedawaytomonitorthemglobally.Earth-observingsatellitesfromNASA,ourdomesticandinternationalpartnersprovidespace-basedregionalandglobalobservationsonthedistributionandtransportationofozoneatallaltitudes.Complementaryair-andsurface-basedmeasurementstakenaroundtheworldhelpvalidatethesedatasets,ensuringacompleteandaccuratepictureofglobalozone.

• Surface-basedmeasurements:Surface-basedmeasurementsprovidethemostaccuratepictureofozoneanditsprecursorsnearthesurfaceoftheEarth.Theyuseinsituchemicalobservations,ground-basedlidarandozonesondes(meteorologicalballoonsthatcarryozonesensors)toprovidedataandcriticalon-the-groundvalidationmeasurementsforairbornecampaignsandsatellites.Thesedata,however,offeronlyalimitedviewofozoneastheinstrumentsaresparselydistributed,constrainedtomeasurementsprimarilyinthetroposphereandprovidelimitedinformationontheverticaldistributionofozone.

• Airbornemeasurements:Airbornemeasurementsprovidethebestinformationontheverticaldistributionofozoneanditsprecursorsintheatmosphere.Airbornecampaignsuseinsituchemicalobservations,radar,limb-scanninginstrumentsanddropsondestocollectinformationthatprovidecriticalvalidationmeasurementsforsatellites.Althoughthesecampaignsareincrediblydata-rich,theyofferonlyalimitedviewofozone,astheone-offcampaignsareconstrainedintheirregionalcoverageandconfinedmostlytouppertroposphericandlowerstratosphericmeasurements.Theyarealsorelativelyexpensive.

• Satellitemeasurements:Satelliteobservationsprovidethebroadspatialandtemporalcoveragenecessaryforconsistent,long-termdatasets.Scientistsusethesedatasetstoidentifytrendsanddrawconclusionsabouttheseasonalandinter-annualvariationsofozoneanditsprecursorsintheatmosphere.Space-

basedinstrumentsusereflectedandtransmittedlightacrosstheelectromagneticspectrumtomeasureozoneanditsprecursors.Thesedata,however,areatamuchcoarserresolutionthanairborneorsurface-basedmeasurementsandarelimitedprimarilytothestratosphere.Therefore,theyrequiretheadditionaldatafromsurface-andair-basedmeasurementstoprovideacompletepictureofglobalozone.

AdvancingozonescienceSincethe1970s,NASA’sEarthScienceResearchandAnalysisProgramhasbeenfundingresearchacrossdifferentcontinents,countriesandsciencecenterstoadvanceourunderstandingofozoneintheatmosphere.Sincethattime,NASAanditspartnershavediscoveredimportantphysicalandchemicalpropertiesofstratosphericandtroposphericozone,aswellasareasforfutureresearch.

Ozoneuphigh:Stratosphericozone

CurrentNASAassetsthatmeasureozoneinthestratosphereinclude:Aurasatellite-OzoneMonitoringInstrument(OMI),UV/vis;Aurasatellite-MicrowaveLimbSounder(MLS),microwave;Suomi-NPPsatellite-OzoneMappingProfilerSuite(OMPS)UV/vis;Aquasatellite-AtmosphericInfraredSounder(AIRS),IR;InternationalSpaceStation-StratosphericAerosolandGasExperimentIII(SAGEIII),UV/vis

• Whatweknew:ResearcherswiththeBritishAntarcticSurveyfirstdiscoveredthethinningofthestratosphericozonelayerusingaground-basedDobsonSpectrophotometerattheHalleyBayObservatoryinthe1950s(figure1).Afterthisinitialdiscovery,scientistsatNASAandotherinternationalspaceagencieslaunchedsatellitestodirectlymeasurethethinningstratosphericozonelayerandwhatlatercametobeknownastheAntarcticOzoneHole.Theseearlyozonemonitoringinstrumentswerelimitedtonadir-scanningviews,whichprovidedtotalozonemeasurements,butgavelittleinsightintotheverticaldistributionofozoneintheatmosphere—especiallythelowertroposphere.NASA’sTotalOzoneMappingSpectrometer(TOMS)ontheNimbus-7andlatertheMeteor-3satellitefurtherconfirmedthethinningstratosphericozonelayer,aswellasthepresenceofhuman-madechemicalprecursorsthatledtoit(figures2and3).Scientiststhenbegandevisingwaystohaltthetroublingtrend,resultingintheestablishmentoftheMontrealProtocolin1987.

• Whatweknow:Today,NASA’sAurasatellitehasgreatlyenhancedourmodernunderstandingofglobalstratosphericozone.Launchedin2004,Aura’sseriesofozonemonitoringinstrumentshaverefinedthewaysinwhichweareabletodetecttotalozoneconcentrations(OMI),aswellastheverticaldistributionofozone(MLS).Thesetools,alongwithcomplementaryairbornecampaignsandground-basedmeasurements,havenowbeguntoshowtheOzoneHole’srecovery(figure4),whichislargelyattributedtothesuccessfulMontrealProtocolandthelimitingofCFCsandotherozone-destroyinghalons.Today,wehaveamuchbettergraspofwhichchemicalcompoundswillreacttodestroystratosphericozone,aswellasitsseasonaltrends,thankstothelong-termdatasetsaffordedbyNASA’sozonemonitoringsatellites.

• Whatwestillneedtofigureout:Toensurethestratosphericozonelayer’scontinuedrecovery,itisnecessarytomaintainNASA’sstratosphericozonemeasurements.NASA’sOMPSinstrumentontheUS/FinnishSuomi-NPPsatelliteisdesignedtotracktheHole’sprogress,offeringdailytotalatmosphericozonecolumnmeasurementsand15kmto60kmverticalozonemeasurements.Itisstilltooearlytotellwhetheroureffortstocurbthethinningofthestratosphericozonelayerareaseffectiveoraslong-lastingaswehopeandwhetheritsrecoveryisduetohuman-madeornaturalchanges.Forexample,recentshiftsintheQuasi-BiennialOscillation(QBO)haveledsomeresearcherstobelievethatnaturalcausesmaybehelpingtofurtherexpeditethehealingoftheOzoneHole(figure5).Furthermore,newchemicals,suchasdichloromethane,continuetobeinventedandintroducedthatcouldpotentiallyreversethesmallamountofprogressmadeinrecentdecades.

Ozonedownlow:Troposphericozone

CurrentNASAassetsthatmeasureozoneinthetroposphereinclude:Aurasatellite-TroposphericEmissionsSpectrometer(TES),Aurasatellite-MicrowaveLimbSounder(MLS),IR;Ozonesondes,whichcantakeverticalprofilesandmeasureozonechemically;andground-basedsystems,whichmeasureozonechemicallyandwithlidartechnology.

• Whatweknew:Despitethesmog-inducingozonethatusedtoblanketLosAngeles,incompletemeasurementsoftroposphericozonebeforethe1970sledscientiststobelievethatthelowertropospherewasrelativelyozonefree.Withoutspace-basedmeasurements,scientistsreliedonsparselydistributedground-basedsystemsthatonlyprovidedsnapshotsoftroposphericozoneandwerefurthercomplicatedbyitsrelativelyshortlifespanlowerintheatmosphere.Thesemeasurementsledscientiststobelievethattroposphericozonewasrelativelyisolated.However,withthelaunchofNASA’sTOMSinstrumentin1978,scientistswereabletoindirectlymeasuretheamountofozoneinthetropospherebysubtractingstratosphericozonefromtotalozonemeasurements(figure6).Inaddition,theintroductionofadvancedcomputermodelingfurtherrevealedthatozonewascollectinginthetroposphereinlargerquantitiesthanpreviouslybelieved.

• Whatweknow:WenowhaveamuchclearerpictureofthedistributionoftroposphericozonethankstotheTESinstrumentaboardNASA’sAurasatellite.TESemploysnadir-andlimb-scanningviewstodirectlymeasureozoneanditsprecursorsusinginfraredradiation.Today,scientistsunderstandthattroposphericozone,likestratosphericozone,cantravelthousandsofmilesandisnotisolatedtoanemissionsource.Infact,someofthemosthazardouslevelsoftroposphericozonecanbefoundmilesfromthesourceofpollutiondownwindfrommoreindustrialemissioncenters.Forexample,despitethewesternUnitedStates’effortstodecreaseitstroposphericozonelevelsthroughreductionsinNOxemissions,troposphericozonelevelsthereremainedhighduetotransportedpollutionfromAsia(figure7).Inaddition,wenowalsounderstandthatsometroposphericozonecanbegoodwhenit’slocatedintheuppertroposphere.Upthere,ithelpstoformOH,a“detergentmolecule”thatscrubstheatmosphereofotherpollutants.

• Whatwestillneedtofigureout:Wearejustbeginningtoscratchthesurfaceofthecomplexchemicalanddynamicpropertiesoftroposphericozone.Becauseunhealthylevelscanformfromhuman-inducedandnaturalcauses,scientistsneedawaytoidentifychemicalsignaturessothattheycandeterminehowbesttomitigatehazardouslevelswhentheyarise.Also,becausethedistributionoftroposphericozoneissodependentonatmosphericcirculationandweatherpatterns,scientistsneedabetterwaytomeasurehowmuchtroposphericozonewasproducedinsituandhowmuchwastransportedfrom(orinto)thestratospherethroughinjection.TheTroposphericEmissions:MonitoringofPollution(TEMPO)satellite,ageostationarysatellitesettolaunchin2018thatwillhoverovertheUnitedStates,willprovidemoredetailedinformationonthedynamicsanddistributionoftroposphericozoneinthefuture.

ThefutureofozoneresearchatNASAInthefuture,NASAwillcontinueitseffortstoresearchstratosphericandtroposphericozoneinaholisticmanner,integratingdatasetsfromacrossNASAfacilities,domesticandinternationalpartners,andacrossinstrumentplatforms.Similartohowwaterisnowviewedasanintegratedsystem,nolongerseparatedintosurface-andgroundwater,ozonetoowillberesearchedasawholesystem.WiththeonsetofgeostationarysatellitesovertheUnitedStatesandaroundtheworld,wewillbegintodiscoverwithmoreprecisionhowozoneistransportedfromthestratosphereintothetroposphere,andviceversa.However,assatellitesprogress,itiscriticalthatwemaintainourair-andsurface-basedmeasurementstoensuretheseglobalmeasurementsremainaccurate.

Figure1.MinimumOctoberozonemeasurements(DobsonUnits)from1956to2017.Imagecredit:https://legacy.bas.ac.uk/met/jds/ozone/

Figure2.TotalozonemeasurementsfromNASA'sNimbus7/TOMSinstrumentfrom1985-1988.Imagecredit:TheCaseofOzoneDepletion,NASAandtheEnvironment

Figure3.NHinorganicchlorinelevelsfrom1983-2017.Imagecredit:KenJucksetal.

Figure4.TotalozonecolumnmeasurementsfromtheTOMS/OMIinstrumentsshowingthetentativerecoveryoftheOzoneHole.Imagecredit:https://www.nature.com/articles/s41598-017-00722-7

Figure5.Changinthequasibiennialoscillationin2015-2016.Imagecredit:http://onlinelibrary.wiley.com/doi/10.1002/2016GL070373/full

Figure6.SeasonalclimatologyoftroposphericozonederivedfromtheTOMSandSAGEdatasetsusingthetroposphericozoneresidualmethod.Imagecredit:https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20010021137.pdf

Figure7.SummertimetroposphericO3andNO2distributionsfor2005-2006and2009-2010fromTES/AuraandOMI/Aura.Imagecredit:http://www.readcube.com/articles/10.1038/ngeo2493