STRATOSPHERICOZONEDEPLETIONGEOG/ENST3331–Lecture17Ahrens:Chapter18;Turco:Chapter13

StratosphericOzoneDeple=on

¨  Stratosphericozone¨  Ozonedistribution¨  Ozonedepletion

OzoneLevelsintheStratosphere

¨  Determinedby:¤ Chemistry¤ Radiation¤ Dynamics

Ahrens: Fig. 18.5

OzoneandRadia=on

¨  Ozonechemistryisdependentontheamountofshortwavelength(ultraviolet)radiationreceivedbythestratosphere.

Crea=onofOzone

O2+UV(<0.25µm)→O+O O+O→O2 orO+O2→O3OxygenisabundantandthesechangesoccurwhereverthereisUV<0.25µm

Destruc=onofozone

O3+UV(<0.30µm)→O+O2O+O2→O3orO+O3→2O2

Ahrens: Fig. 18.6

Ozonereservoir

Turco: Figure 13.2

OzoneandRadia=on

Sunspotactivitycausesavariationininsolation

Greatersunspotactivitycoincideswithgreaterinsolation

Cycleofroughly11years

Ozonelevelsaregreaterduringyearsofhighlevelsofsunspots 1960 1999

BenefitoftheOzoneLayer

¨  UltravioletradiationisdangeroustoEarth’sbiota

¨  OzonelayerinthestratosphereeffectivelyblocksmostUVradiationfromreachingsurface

“Near”UVspectrumregions

¨  UVA¤  0.40–0.32µm¤  Usedintanningsalons

¨  UVB¤  0.32–0.29µm¤  Tanning/sunburns

¨  UVC¤  0.29–0.10µm¤  “Broadlybiocidal”¤  AbsorbedbyO2andO3

Turco: Figure 13.7 1000 nm = 1 µm

IncreasedUVBexposure

¨  Skincancer¨  Long-termvisiondamage¨  Amphibians¨  Leaftissuedamage

¤  Increasedplantmortality¤  Decreasedcropyields

¨  Phytoplankton–baseofmarinefoodchain

Lectureoutline

¨  Stratosphericozone¨  Ozonedistribution

¤  Brewer-Dobsoncirculation¤  QBO¤  ENSO

¨  Ozonedepletion

Ozonedistribu=on

¨  Mostozonecreationoccursinthetropics¤ Greaterintensityofinsolation

¨  Dynamicsredistributesozone:¤ Brewer-DobsonCirculation¤ QBO(Quasi-biennialOscillation)

¨  Highestconcentrationisinthepolarregions¤ Slowerdestruction

MeasuringOzone

¨  GordonDobson(1889-1976)¤  Determinedthattemperatureincreaseswithheightinstratosphere¤  Proposedexistenceoftheozonelayer¤  Inventeddeviceformeasuringozoneinacolumnofair

¨  DobsonUnit(DU)¤  1DU=10µmthickequivalentozonedepth(atSTP)¤  2.69×1020moleculesperm2

Ozonedistribu=on

Greatestathighlatitudes

Globalcoverage

Zonallysymmetric

Turco:Figure13.5

(DobsonUnits)

Brewer-DobsonCircula=on

¨  Prominentinwinterhemisphere

¨  Providesequatortopoletransportofozone

Quasi-biennialoscilla=on(QBO)

¨  2to3yearoscillationintheuppertroposphereandlowerstratosphere

¨  Prevailingwindsshiftfromwesterlytoeasterly

QBOandOzone

¨  QBOappearstobelinkedtotheDobson-Brewercirculation

¨  HighlevelsofozonenearthepolesisassociatedwiththewesterlyphaseoftheQBO

ElNiño/SouthernOscilla=on(ENSO)

¨  LinkedtochangesintheDobson-BrewerCirculation

¨  EnhancedduringElNiñoyears

Lectureoutline

¨  Stratosphericozone¨  Ozonedistribution¨  Ozonedepletion

¤  Catalyticdestructionn  Chlorinen  Nitricoxiden  Bromine

Moreozonechemistry

X+O3→XO+O2 XO+O→X+O2

___________________Net: O+O3→2O2¨  ThecatalystXcanbeNO,OH,ClorBr

¨  Catalystsdrivereactionsbutarenotconsumed¤  Xremainsinatmospheretoreactagain

Chlorine(Cl)intheStratosphere

¨  Biologicalprocesses:methylchloride(CH3Cl)¤ Accountsfor25%ofchlorineinstratosphere

¨  Volcanoes:hydrogenchloride(HCl)¤ Negligible

¨  Anthropogenic:chloromluorocarbons(CFCs)¤ Massiveinmluxofchlorine

CFCs¨  ‘Halogen’familyofchemicals

¤  Aerosolpropellants(spray)¤  Refrigerants,degreasers¤  Foamblowingagents

¨  Lowreactivity,lowtoxicity¨  Thermalabsorption,solventproperties

¨  SplitupbyUVradiationintheupperstratosphere

Turco: Figure 13.11

CFCSuspicions¨  1974–MarioMolinaandF.SherwoodRowlandnotedthattherearenotroposphericsinksforCFCs¤  HypothesizedthatCFCsreleasedintotheatmospherecouldhaveadverseeffectsontheozonelayer

¨  Suchanimpactwasdetectedinthe1980sandafterward,particularlyoverAntarctica

¨  MolinaandRowlandreceivedtheNobelPrizeinChemistryin1995

¨  WhyAntarctica?

Molina

Rowland

Colouredcontoursrepresentozoneconcentration(DobsonUnits).Thebottomofthemigureistheheightofthetropicaltropopause.Gases–includingCFCs–enterthestratosphereatthetropicsImageSource:Stevenson(2009)

Brewer-DobsonCircula=on

AtmosphericlifecycleofCFCs

1.  Manufacture2.  Useandeventualescape3.  Build-upintroposphere4.  Slowexchangewithstratosphere

n  0.1Mt-Cl/year

5.  DissociationbyUVradiation6.  Chlorinecatalyzesozonedestruction

n  EventuallyturnsintoHCl

7.  Returnstotroposphereandrainsout

CFCsinthestratosphere

¨  Reservoir:roughly1Mt-Cl¤ EachCFCmoleculecouldcatalyzedestructionofupto100000O3molecules

¨  Sink:0.06Mt-Cl/year¤  Intheabsenceofanysourceitwouldtake20yearsforCFCstoberemoved

¤ However,tropospherehasanadditional10Mt-Cl

Nitrogenmonoxide

¨  NOalsocatalyzesdestructionofozone¨  NOintroposphereislargelyremovedbyrain

¨  SourceofstratosphericNOisN2O¤  Fossilfuels:4Mt-N/yearofN2O

¨  Othersource:supersonicaircraft

¨  NotnearlyassevereasCFCs

Bromine

¨  Similargastochlorine,alsocatalyzesozonedestruction

¨  ‘Halon’familyofchemicals¤ Fumigation(termitesandcroppests)¤ Usefulasmireretardants/extinguishers

¨  Chlorineis100timesasabundantintheatmosphere;butbromineis10timesaseffectiveperatom

TheparadoxofOzone

¨  StratosphericozoneessentialforvariouslifeattheEarth’ssurface

¨  Ozoneinthetroposphereharmfultolifeonlandandoceans

Ozone

Earth Observing System Science Plan, Fig. 7.1

Ozoneinthetroposphere

¨  Maincomponentofphotochemical(LosAngeles)smog

¨  Noxioussubstancethatirritateseyesandlungsn Asthma,bronchitis

¨  Alsoharmfultotrees,cropsn US:severalbillionUSDperyearincropdamage

Nextlecture

¨  Ozoneholes¨  Remediation¨  TheMontréalProtocol¨  Otherenvironmentalglobalagreements