ozone layers and its causes

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Ozone Depletion, Properties, Significances and Ozone levels
 What Is Ozone?
Ozone isa molecule composedof three atomsof oxygen -- the oxygenmolecule we breathe thatis
essential tolife hasonlytwo.Manyof us are familiarwithozone asahazardousoutdoorair pollutantin
large cities.While ozone canusuallybe foundatlow levelsinindoorandoutdoorair,harmful levelsof
ozone can be formedinthe atmosphere throughthe interactionof sunlightwithcertainchemicals
emittedtothe environment(suchasautomobileemissions) andbyhighvoltage dischargesinsome
electrical equipment.Because ozoneisreactive,itcanchange the chemical compositionof many
materialsandsubstances,ofteninadestructive way.Manufacturersof ozone generatorsrelyonthis
characteristicto supporttheirclaimsof the benefitsof the devices.
Composition of Ozone layer:
• O3
• a gas composedof three atoms ofoxygen
• bluishgasthat isharmful to breathe
• Nearly90% of the Earth's ozone isinthe stratosphere andisreferredtoasthe ozone layer
• Ozone absorbsa bandof ultravioletradiationcalledUVB
 OZONE DEPLETION:
The ozone layercan be depletedbyfree radical catalysts,including nitricoxide (NO), nitrous
oxide (N2O),hydroxyl (OH),atomicchlorine (Cl),andatomicbromine (Br).While there are natural
sourcesfor all of these species,the concentrationsof chlorine andbromine increasedmarkedlyin
recentdecadesdue tothe release of large quantitiesof man-made organohalogencompounds,
especially chlorofluorocarbons(CFCs) andbromofluorocarbons. Thesehighlystable compoundsare
capable of surviving the rise tothe stratosphere,where Cl and Br radicalsare liberatedbythe actionof
ultravioletlight.Eachradical isthenfree toinitiate andcatalyze achain reactioncapable of breaking

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downover100,000 ozone molecules.By2009, nitrousoxide wasthe largestozone-depletingsubstance
(ODS) emittedthroughhumanactivities.
The breakdownof ozone inthe stratosphere resultsinreducedabsorptionof ultravioletradiation.
Consequently,unabsorbedanddangerousultravioletradiationisable toreachthe Earth’ssurface at a
higherintensity.
Ozone levelshave droppedbyaworldwide average of about 4% since the late 1970s. For approximately
5% of the Earth's surface,aroundthe northand southpoles,muchlargerseasonal declineshave been
seen,andare describedas"ozone holes". The discoveryof the annual depletionof ozone above the
Antarcticwas firstannouncedby Joe Farman, BrianGardinerand JonathanShanklin,inapaperwhich
appearedin NatureonMay 16, 1985.
Ozone Layer :
The ozone layerreferstoa layerinthe atmosphere,whichspreadsinthe stratosphere andisinhigh
concentration.Thislayerhasitsmaximumpartial pressure inthe lowerstratosphereata level of 19-23
Kmsabove the Earth. The stratosphere contain90 % of all ozone inthe atmosphere.If all the ozone
moleculesinthe atmosphere were transferredtothe Earth surface,theywouldassume athicknessof
only2.5-3.5 mm at average surface temperature andpressure (WMO1992).
Ozone,beingamolecule composedof more thantwoatoms has manyabsorptionbandsand
particularlyinthe UV range at wavelengthslessthan0.3 µm.Stratosphericozone layerplaysavery
importantrole forthe protectionof humanlife andthe whole ecosystem.Itfiltersmostof the
ultravioletbetaradiationsfromthe sun,bufferthe ecosystemfromharmful ultravioletraysandalso
absorbsthe solar energyforwarmingthe stratosphere tomaintainthe presentclimate conditions
familiartothe ecosystem.

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Mechanism of Hole Formation:
The mechanismof ozone hole formationisverycomplex.Atsouthpole the polarvortex isformedin
winterandif the polar vortex growssufficientlyitrestrictsthe exchangeof airbetweenthe inside and
outside of the vortex.Inwinterthe polarnightregionnotexposedtosunlightspreadsaroundthe pole
and so the vortex temperature insidedropsevenupto -90oC(Tomoyuki 1992).At -78oC, nitricacidand
waterin the stratosphere are frozentoformcloudscomposedof ice particlesof micronsize i.e.,Polar
stratosphericclouds(PSCs).
Laboratory experimentsconfirmedthatonthe surface of these ice particles,chlorinenitrate reactwith
hydrogenchloride andwater,toformchlorine gasand hypochlorousacid,thusincreasingthe
concentrationof chlorine andhypochlorousacidinthe stratosphere.These twoare photodissociatedto
discharge active chlorine atomtodeplete ozone,thusdecreasingthe ozone inthe polarvortex.
The absorptionof solarenergyinthe periodfromspringto summergraduallyraisesthe temperature,
finallydestroyingthe vortexandallowingthe exchangebetweenthe airinside andoutside of the polar
vortex,thusfritterawaythe ozone hole.The devastatingeffectof the ozone depletion,asobservedby
the satellite (NIMBUS-7).
 Physical Properties of Ozone:
1) Ozone absorbsradiationstronglyinthe ultravioletregionof the atmosphericspectrumbetween
220-290 nm. Thisprotectsthe Earth andits inhabitantsfromthe harmful ultravioletradiation of
the Sun.Withoutthisprotective layer,more ultravioletradiationwouldreachthe surface of the
Earth and cause damage to plant, animal and human life.
2) Pure ozone is a blue gas, with a strong irritating smell. When inhaled, it causes headache and
nausea. In smaller proportions it smells pleasant.
3) It isabout 1.5 timesheavierthanairand has a vapor densityof 24, correspondingtothe formula
O3.
4) It is more soluble than oxygen in water, about 49% by volume at 0°C.
5) It gets liquefied to a deep blue colour liquid, when cooled in liquid air.
6) It boils at 161.2 K and solidifies to violet-black crystals, which melt at 80.6 K.
7) It dissolves readily in turpentine oil and acetic acid.

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Applications:
 Disinfect laundry in hospitals, food factories, care homes etc.;
 Disinfect water in place of chlorine
 Deodorize air and objects, such as after a fire. This process is extensively used in fabric
restoration
 Kill bacteria on food or on contact surfaces;
 Sanitize swimming pools and spas
 Kill insects in stored grain
 Scrub yeast and mold spores from the air in food processing plants;
 Wash fresh fruits and vegetables to kill yeast, mold and bacteria;
 Chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex
organics lumped together as "color");
 Provide an aid to flocculation (agglomeration of molecules, which aids in filtration, where the iron
and arsenic are removed);
 Manufacture chemical compounds via chemical synthesis
 Clean and bleach fabrics (the former use is utilized in fabric restoration; the latter use is
patented);
 Act as an antichlor in chlorine-based bleaching;
 Assist in processing plastics to allow adhesion of inks;
 Age rubber samples to determine the useful life of a batch of rubber;
 Eradicate water borne parasites such as Giardia lamblia and Cryptosporidium in surface
water treatment plants.
 LEVELS OF OZONE LAYER:
Ozone inthe stratosphere andat groundlevel hasbecome animportantglobal airqualityissue.The
concernabout ozone inthe stratosphere isthatit isdepleting;the concernatgroundlevel isthatit is
increasing.
a) Stratospheric:
In the stratosphere (ataltitudesof 15to 35 km) the ozone layeracts as a natural filterabsorbingmostof
the sun's damagingultravioletrays - those thatburn skinand cause some formsof skincancer. Hence
the concern aboutthe depletingozonelayerinthe earth'supperatmosphere.
Sources:
In the stratosphere the protectivenatural layerof ozone isformedbyoxygen(O2) inthe presence of
ultravioletradiation.Mostof the ozone inthe upperatmosphere isina 20-kilometre-thicklayerlying
between15km to 35 km above the earth'ssurface.Evenat thisheight,industrialpollutantsare reaching

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and depletingthe ozone.The majorozone-depletingsubstancesare chlorofluorocarbons(CFCs),halons,
methane (CH4) andnitrousoxide (N2O).Chlorofluorocarbonsare usedinrefrigeration,foamblowing,
solventsandspecializedaerosol propellants.Halonsare similartochlorofluorocarbonsandare usedin
fire extingusihers.Methane isaproductof agricultural,industrial andminingactivitieswhilenitrous
oxide isfromcombustionandfertilizeruse.These gasesare discussedinotherfactsheets.
b) Ground Level:
At groundlevel,ozoneisamajor componentof photochemical smogwhichhasa noticeable lightbrown
colorand resultsinreducedvisibilityandhealthconcerns.The termsmoghasbeeninuse since 1905
whenitwas usedtodescribe the smoke andfogin manyScottishcities.Inthe 1940s increased
concentrationsof ground-level ozone were observedinLosAngeles.The concernwithtroposphericor
ground-level ozoneisitsverypresence,because asacomponentof smogit isa seriouspollutant.
Sources:
At groundlevel,ozoneisformedwhennitrogenoxides(NOx) andvolatile organiccompounds(VOCs)
react inthe presence of sunlight. The factsheetsNitrogenOxides(NOx) andVolatileOrganic
Compounds(VOCs) givefurtherdetails.Burningof fossilfuelsisamajor man-made cause of nitrogen
oxides,while use of motorvehicles,solvents,andindustrial processesinthe petrochemical industryare
sourcesof volatile organiccompounds.Theseman-madeemissionsare more concentratedinurbanand
industrializedareas.
Some ground-levelozone isactuallytransporteddownfromthe stratosphere;some comesfrom
reactionsof naturallyoccurring volatile organiccompoundsandnitrogenoxidesinsunlight.
 Significance of ozone :
• Absorptionof UV and IR radiation
• Complex troposphericand stratospheric chemistry
• Long term trendsobservedin total ozone
• Stratospheric ozone depletionoverthe South Pole (ozone hole) since the 1970s
• Many studieshave shown evidence ofthe impact ofanthropic perturbations on
atmosphericchemistry (CFCs,NOx,CH4, CO …)
• Stratospheric trendsdue to the inverse greenhouse effect
• Impact ofstratospheric coolingon ozone photochemistryand ozone catalytic
destructionby chlorine compounds