stratospheric ozone depletion · 2016-03-07 · measuring ozone ¨ gordon dobson (1889-1976) ¤...
TRANSCRIPT
OzoneandRadia=on
¨ Ozonechemistryisdependentontheamountofshortwavelength(ultraviolet)radiationreceivedbythestratosphere.
Crea=onofOzone
O2+UV(<0.25µm)→O+O O+O→O2 orO+O2→O3OxygenisabundantandthesechangesoccurwhereverthereisUV<0.25µm
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)
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
Ozoneinthetroposphere
¨ Maincomponentofphotochemical(LosAngeles)smog
¨ Noxioussubstancethatirritateseyesandlungsn Asthma,bronchitis
¨ Alsoharmfultotrees,cropsn US:severalbillionUSDperyearincropdamage