INTRODUCTIONOzone depletion describes twodistinct, but related observations:a slow, steady decline of about 4%per decade in the total volume ofozone in Earths stratosphere (theozone layer) since the late 1970s,and a much larger, but seasonal,decrease in stratospheric ozoneover Earths polar regions duringthe same period. The latterphenomenon is commonly referred toas the ozone hole. In addition tothis well-known stratospheric ozonedepletion, there are alsotropospheric ozone depletionevents, which occur near thesurface in polar regions duringspring.The detailed mechanism by which thepolar ozone holes form is differentfrom that for the mid-latitudethinning, but the most important
process in both trends is catalyticdestruction of ozone by atomicchlorine and bromin. The mainsource of these halogen atoms inthe stratosphere isphotodissociation ofchlorofluorocarbon (CFC) compounds,commonly called freons, and ofbromofluorocarbon compounds knownas halons. These compounds aretransported into the stratosphereafter being emitted at thesurface.Both ozone depletionmechanisms strengthened asemissions of CFCs and halonsincreased.CFCs and othercontributory substances arecommonly referred to as ozone-depleting substances (ODS). Sincethe ozone layer prevents mostharmful UVB wavelengths (280–315 nm) of ultraviolet light (UVlight) from passing through theEarths atmosphere, observed andprojected decreases in ozone havegenerated worldwide concern leading
to adoption of the MontrealProtocol that bans the productionof CFCs and halons as well asrelated ozone depleting chemicalssuch as carbon tetrachloride andtrichloroethane. It is suspectedthat a variety of biologicalconsequences such as increases inskin cancer, cataracts,damage toplants, and reduction of planktonpopulations in the oceans photiczone may result from the increasedUV exposure due to ozone depletion.The ozone layer is a layer in earthatmosphere which containsrelatively high concentration ofozone(O3) this layer observes 97-99% of the suns high frequencyultra violet light, which isdamaging to life on earth it ismainly located in lower position ofthe stratsophere from approximately13-40kms (8.1-25 mi) above earththough the thickness variesseasonally and geographically theozone layer was discovered in 1913
by the physicist Charles Fabry andHeny buission. Ozone cycle overview Theozone cycleThree forms (or allotropes) ofoxygen are involved in the ozone-oxygen cycle: oxygen atoms (O oratomic oxygen), oxygen gas (O2 ordiatomic oxygen), and ozone gas (O3or triatomic oxygen). Ozone isformed in the stratosphere when
oxygen molecules photodissociateafter absorbing an ultravioletphoton whose wavelength is shorterthan 240 nm. This produces twooxygen atoms. The atomic oxygenthen combines with O2 to create O3.Ozone molecules absorb UV lightbetween 310 and 200 nm, followingwhich ozone splits into a moleculeof O2 and an oxygen atom. Theoxygen atom then joins up with anoxygen molecule to regenerateozone. This is a continuing processwhich terminates when an oxygenatom "recombines" with an ozonemolecule to make two O2 molecules:O + O3 → 2 O2Global monthly average total ozoneamount.
The overall amount of ozone in thestratosphere is determined by abalance between photochemicalproduction and recombination.Ozone can be destroyed by a numberof free radical catalysts, the mostimportant of which are the hydroxylradical (OH·), the nitric oxideradical (NO·), atomic chlorine(Cl·) and bromine (Br·). All ofthese have both natural and manmadesources; at the present time, mostof the OH· and NO· in thestratosphere is of natural origin,but human activity has dramaticallyincreased the levels of chlorineand bromine. These elements arefound in certain stable organiccompounds, especiallychlorofluorocarbons (CFCs), whichmay find their way to thestratosphere without beingdestroyed in the troposphere due totheir low reactivity. Once in the
stratosphere, the Cl and Br atomsare liberated from the parentcompounds by the action ofultraviolet light, e.g. (h isPlancks constant, ν is frequencyof electromagnetic radiation)CFCl3 + hν → CFCl2 + ClThe Cl and Br atoms can thendestroy ozone molecules through avariety of catalytic cycles. In thesimplest example of such a cycle,chlorine atom reacts with an ozonemolecule, taking an oxygen atomwith it (forming ClO) and leaving anormal oxygen molecule. Thechlorine monoxide (i.e., the ClO)can react with a second molecule ofozone (i.e., O3) to yield anotherchlorine atom and two molecules ofoxygen. The chemical shorthand forthese gas-phase reactions is:Cl + O3 → ClO + O2ClO + O3 → Cl + 2 O2
The overall effect is a decrease inthe amount of ozone. Morecomplicated mechanisms have beendiscovered that lead to ozonedestruction in the lowerstratosphere as well.A single chlorine atom would keepon destroying ozone (thus acatalyst) for up to two years (thetime scale for transport back downto the troposphere) were it not forreactions that remove them fromthis cycle by forming reservoirspecies such as hydrogen chloride(HCl) and chlorine nitrate(ClONO2). On a per atom basis,bromine is even more efficient thanchlorine at destroying ozone, butthere is much less bromine in theatmosphere at present. As a result,both chlorine and brominecontribute significantly to theoverall ozone depletion. Laboratorystudies have shown that fluorineand iodine atoms participate in
analogous catalytic cycles.However, in the Earthsstratosphere, fluorine atoms reactrapidly with water and methane toform strongly bound HF, whileorganic molecules which containiodine react so rapidly in thelower atmosphere that they do notreach the stratosphere insignificant quantities.Furthermore, a single chlorine atomis able to react with 100,000 ozonemolecules. This fact plus theamount of chlorine released intothe atmosphere bychlorofluorocarbons (CFCs) yearlydemonstrates how dangerous CFCs areto the environment.
Observations on ozone layer depletionThe most pronounced decrease inozone has been in the lowerstratosphere. However, the ozonehole is most usually measured notin terms of ozone concentrations atthese levels (which are typicallyof a few parts per million) but byreduction in the total columnozone, above a point on the Earthssurface, which is normallyexpressed in Dobson units,abbreviated as "DU". Markeddecreases in column ozone in theAntarctic spring and early summercompared to the early 1970s andbefore have been observed usinginstruments such as the Total OzoneMapping Spectrometer (TOMS).
Lowest value of ozone measured byTOMS each year in the ozone hole.Reductions of up to 70% in theozone column observed in theaustral (southern hemispheric)spring over Antarctica and firstreported in 1985 (Farman et al.1985) are continuing.Through the1990s, total column ozone inSeptember and October havecontinued to be 40–50% lower thanpre-ozone-hole values. In theArctic the amount lost is morevariable year-to-year than in theAntarctic. The greatest declines,up to 30%, are in the winter andspring, when the stratosphere iscolder.
Reactions that take place on polarstratospheric clouds (PSCs) play animportant role in enhancing ozonedepletion SCs form more readily inthe extreme cold of Antarcticstratosphere. This is why ozoneholes first formed, and are deeper,over Antarctica. Early modelsfailed to take PSCs into accountand predicted a gradual globaldepletion, which is why the suddenAntarctic ozone hole was such asurprise to many scientists.In middle latitudes it ispreferable to speak of ozonedepletion rather than holes.Declines are about 3% below pre-1980 values for 35–60°N and about6% for 35–60°S. In the tropics,there are no significant trends.Ozone depletion also explains muchof the observed reduction instratospheric and uppertropospheric temperatures. The
source of the warmth of thestratosphere is the absorption ofUV radiation by ozone, hencereduced ozone leads to cooling.Some stratospheric cooling is alsopredicted from increases ingreenhouse gases such as CO2;however the ozone-induced coolingappears to be dominant.Predictions of ozone levels remaindifficult. The World MeteorologicalOrganization Global Ozone Researchand Monitoring Project—Report No.44 comes out strongly in favor forthe Montreal Protocol, but notesthat a UNEP 1994 Assessmentoverestimated ozone loss for the1994–1997 period.Chemicals in the atmosphereCFCs in the atmosphereChlorofluorocarbons (CFCs) wereinvented by Thomas Midgley, Jr. inthe 1920s. They were used in air
conditioning/cooling units, asaerosol spray propellants prior tothe 1980s, and in the cleaningprocesses of delicate electronicequipment. They also occur as by-products of some chemicalprocesses. No significant naturalsources have ever been identifiedfor these compounds — theirpresence in the atmosphere is duealmost entirely to humanmanufacture. As mentioned in theozone cycle overview above, whensuch ozone-depleting chemicalsreach the stratosphere, they aredissociated by ultraviolet light torelease chlorine atoms. Thechlorine atoms act as a catalyst,and each can break down tens ofthousands of ozone molecules beforebeing removed from thestratosphere. Given the longevityof CFC molecules, recovery timesare measured in decades. It iscalculated that a CFC moleculetakes an average of 15 years to go
from the ground level up to theupper atmosphere, and it can staythere for about a century,destroying up to one hundredthousand ozone molecules duringthat time.Verification of observationsScientists have been increasinglyable to attribute the observedozone depletion to the increase ofman-made (anthropogenic) halogencompounds from CFCs by the use ofcomplex chemistry transport modelsand their validation againstobservational data (e.g. SLIMCAT,CLaMS). These models work bycombining satellite measurements ofchemical concentrations andmeteorological fields with chemicalreaction rate constants obtained inlab experiments. They are able to
identify not only the key chemicalreactions but also the transportprocesses which bring CFCphotolysis products into contactwith ozone. There has been increasein the depletion also seen due tothe emissions of the cfc gases fromthe electrical appliances like ac,refrigerator that are one of theessentials that we use in dailypurpose.The ozone hole and its causes
Ozone hole in North America during1984 (abnormally warm reducingozone depletion) and 1997(abnormally cold resulting inincreased seasonal depletion).Source: NASAThe Antarctic ozone hole is an areaof the Antarctic stratosphere inwhich the recent ozone levels havedropped to as low as 33% of theirpre-1975 values. The ozone holeoccurs during the Antarctic spring,from September to early December,as strong westerly winds start tocirculate around the continent andcreate an atmospheric container.
Within this polar vortex, over 50%of the lower stratospheric ozone isdestroyed during the Antarcticspring.As explained above, the primarycause of ozone depletion is thepresence of chlorine-containingsource gases (primarily CFCs andrelated halocarbons). In thepresence of UV light, these gasesdissociate, releasing chlorineatoms, which then go on to catalyzeozone destruction. The Cl-catalyzedozone depletion can take place inthe gas phase, but it isdramatically enhanced in thepresence of polar stratosphericclouds (PSCs).These polar stratosphericclouds(PSC) form during winter, inthe extreme cold. Polar winters aredark, consisting of 3 monthswithout solar radiation (sunlight).The lack of sunlight contributes to
a decrease in temperature and thepolar vortex traps and chills air.Temperatures hover around or below-80 °C. These low temperatures formcloud particles. There are threetypes of PSC clouds; nitric acidtrihydrate clouds, slowly coolingwater-ice clouds, and rapid coolingwater-ice(nacerous) clouds; thatprovide surfaces for chemicalreactions that lead to ozonedestruction.The photochemical processesinvolved are complex but wellunderstood. The key observation isthat, ordinarily, most of thechlorine in the stratosphereresides in stable "reservoir"compounds, primarily hydrochloricacid (HCl) and chlorine nitrate(ClONO2). During the Antarcticwinter and spring, however,reactions on the surface of thepolar stratospheric cloud particlesconvert these "reservoir" compounds
into reactive free radicals (Cl andClO). The clouds can also removeNO2 from the atmosphere byconverting it to nitric acid, whichprevents the newly formed ClO frombeing converted back into ClONO2.The role of sunlight in ozonedepletion is the reason why theAntarctic ozone depletion isgreatest during spring. Duringwinter, even though PSCs are attheir most abundant, there is nolight over the pole to drive thechemical reactions. During thespring, however, the sun comes out,providing energy to drivephotochemical reactions, and meltthe polar stratospheric clouds,releasing the trapped compounds.Warming temperatures near the endof spring break up the vortexaround mid-December. As warm,ozone-rich air flows in from lowerlatitudes, the PSCs are destroyed,
the ozone depletion process shutsdown, and the ozone hole closes.Most of the ozone that is destroyedis in the lower stratosphere, incontrast to the much smaller ozonedepletion through homogeneous gasphase reactions, which occursprimarily in the upperstratosphere.Interest in ozone layer depletionWhile the effect of the Antarcticozone hole in decreasing the globalozone is relatively small,estimated at about 4% per decade,the hole has generated a great dealof interest because: The decrease in the ozone layer was predicted in the early 1980s to be roughly 7% over a 60 year period.
The sudden recognition in 1985that there was a substantial"hole" was widely reported inthe press. The especially rapidozone depletion in Antarcticahad previously been dismissed asa measurement error .Many were worried that ozoneholes might start to appear overother areas of the globe but todate the only other large-scaledepletion is a smaller ozone"dimple" observed during theArctic spring over the NorthPole. Ozone at middle latitudeshas declined, but by a muchsmaller extent (about 4–5%decrease).If the conditions became moresevere (cooler stratospherictemperatures, more stratosphericclouds, more active chlorine),then global ozone may decreaseat a much greater pace. Standardglobal warming theory predictsthat the stratosphere will cool[
When the Antarctic ozone hole breaks up, the ozone-depleted air drifts out into nearby areas. Decreases in the ozone level of up to 10% have been reported in New Zealand in the month following the break-up of the Antarctic ozone hole.Consequences of ozone layer depletionSince the ozone layer absorbs UVBultraviolet light from the Sun,ozone layer depletion is expectedto increase surface UVB levels,which could lead to damage,including increases in skin cancer.This was the reason for theMontreal Protocol. Althoughdecreases in stratospheric ozoneare well-tied to CFCs and there aregood theoretical reasons to believethat decreases in ozone will leadto increases in surface UVB, thereis no direct observational evidencelinking ozone depletion to higher
incidence of skin cancer in humanbeings. This is partly because UVA,which has also been implicated insome forms of skin cancer, is notabsorbed by ozone, and it is nearlyimpossible to control statisticsfor lifestyle changes in thepopulace.Increased UVOzone, while a minority constituentin the Earths atmosphere, isresponsible for most of theabsorption of UVB radiation. Theamount of UVB radiation thatpenetrates through the ozone layerdecreases exponentially with theslant-path thickness/density of thelayer. Correspondingly, a decreasein atmospheric ozone is expected togive rise to significantlyincreased levels of UVB near thesurface.
Increases in surface UVB due to theozone hole can be partiallyinferred by radiative transfermodel calculations, but cannot becalculated from direct measurementsbecause of the lack of reliablehistorical (pre-ozone-hole) surfaceUV data, although more recentsurface UV observation measurementprogrammes exist (e.g. at Lauder,New Zealand).Because it is this same UVradiation that creates ozone in theozone layer from O2 (regularoxygen) in the first place, areduction in stratospheric ozonewould actually tend to increasephotochemical production of ozoneat lower levels (in thetroposphere), although the overallobserved trends in total columnozone still show a decrease,largely because ozone producedlower down has a naturally shorterphotochemical lifetime, so it is
destroyed before the concentrationscould reach a level which wouldcompensate for the ozone reductionhigher up.Biological effectsThe main public concern regardingthe ozone hole has been the effectsof increased surface UV andmicrowave radiation on humanhealth. So far, ozone depletion inmost locations has been typically afew percent and, as noted above, nodirect evidence of health damage isavailable in most latitudes. Werethe high levels of depletion seenin the ozone hole ever to be commonacross the globe, the effects couldbe substantially more dramatic. Asthe ozone hole over Antarctica hasin some instances grown so large asto reach southern parts ofAustralia, New Zealand, Chile,Argentina, and South Africa,environmentalists have been
concerned that the increase insurface UV could be significant.Effects on humansUVB (the higher energy UV radiationabsorbed by ozone) is generallyaccepted to be a contributoryfactor to skin cancer. In addition,increased surface UV leads toincreased tropospheric ozone, whichis a health risk to humans.1. Basal and Squamous CellCarcinomas — The most common formsof skin cancer in humans, basal andsquamous cell carcinomas, have beenstrongly linked to UVB exposure.The mechanism by which UVB inducesthese cancers is well understood—absorption of UVB radiation causesthe pyrimidine bases in the DNAmolecule to form dimers, resultingin transcription errors when theDNA replicates. These cancers arerelatively mild and rarely fatal,although the treatment of squamous
cell carcinoma sometimes requiresextensive reconstructive surgery.By combining epidemiological datawith results of animal studies,scientists have estimated that aone percent decrease instratospheric ozone would increasethe incidence of these cancers by2%.2. Malignant Melanoma — Anotherform of skin cancer, malignantmelanoma, is much less common butfar more dangerous, being lethal inabout 15–20% of the casesdiagnosed. The relationship betweenmalignant melanoma and ultravioletexposure is not yet wellunderstood, but it appears thatboth UVB and UVA are involved.Experiments on fish suggest that 90to 95% of malignant melanomas maybe due to UVA and visible radiationwhereas experiments on opossums
suggest a larger role for UVB.Because of this uncertainty, it isdifficult to estimate the impact ofozone depletion on melanomaincidence. One study showed that a10% increase in UVB radiation wasassociated with a 19% increase inmelanomas for men and 16% forwomen. A study of people in PuntaArenas, at the southern tip ofChile, showed a 56% increase inmelanoma and a 46% increase innonmelanoma skin cancer over aperiod of seven years, along withdecreased ozone and increased UVBlevels.3. Cortical Cataracts — Studies aresuggestive of an associationbetween ocular cortical cataractsand UV-B exposure, using crudeapproximations of exposure andvarious cataract assessmenttechniques. A detailed assessment
of ocular exposure to UV-B wascarried out in a study onChesapeake Bay Watermen, whereincreases in average annual ocularexposure were associated withincreasing risk of corticalopacity. In this highly exposedgroup of predominantly white males,the evidence linking corticalopacities to sunlight exposure wasthe strongest to date. However,subsequent data from a population-based study in Beaver Dam, WIsuggested the risk may be confinedto men. In the Beaver Dam study,the exposures among women werelower than exposures among men, andno association was seen. Moreover,there were no data linking sunlightexposure to risk of cataract inAfrican Americans, although othereye diseases have differentprevalences among the differentracial groups, and cortical opacityappears to be higher in AfricanAmericans compared with whites.
4. Increased Tropospheric Ozone —Increased surface UV leads toincreased tropospheric ozone.Ground-level ozone is generallyrecognized to be a health risk, asozone is toxic due to its strongoxidant properties. At this time,ozone at ground level is producedmainly by the action of UVradiation on combustion gases fromvehicle exhausts.Effects on non-human animalsA November 2010 report byscientists at the Institute ofZoology in London found that whalesoff the coast of California haveshown a sharp rise in sun damage,and these scientists "fear that thethinning ozone layer is to blame"The study photographed and tookskin biopsies from over 150 whalesin the Gulf of California and found"widespread evidence of epidermaldamage commonly associated with
acute and severe sunburn," havingcells which form when the DNA isdamaged by UV radiation. Thefindings suggest "rising UV levelsas a result of ozone depletion areto blame for the observed skindamage, in the same way that humanskin cancer rates have been on theincrease in recent decades."Effects on cropsAn increase of UV radiation wouldbe expected to affect crops. Anumber of economically importantspecies of plants, such as rice,depend on cyanobacteria residing ontheir roots for the retention ofnitrogen. Cyanobacteria aresensitive to UV light and theywould be affected by its increase.Public policy
NASA projections of stratosphericozone concentrations ifchlorofluorocarbons had not beenbanned.The full extent of the damage thatCFCs have caused to the ozone layeris not known and will not be knownfor decades; however, markeddecreases in column ozone havealready been observed (as explainedbefore).After a 1976 report by the UnitedStates National Academy of Sciencesconcluded that credible scientificevidence supported the ozone
depletion hypothesis a fewcountries, including the UnitedStates, Canada, Sweden, Denmark,and Norway, moved to eliminate theuse of CFCs in aerosol spray cans.At the time this was widelyregarded as a first step towards amore comprehensive regulationpolicy, but progress in thisdirection slowed in subsequentyears, due to a combination ofpolitical factors (continuedresistance from the halocarbonindustry and a general change inattitude towards environmentalregulation during the first twoyears of the Reagan administration)and scientific developments(subsequent National Academyassessments which indicated thatthe first estimates of themagnitude of ozone depletion hadbeen overly large). A criticalDuPont manufacturing patent forFreon was set to expire in 1979Chlorofluorocarbon
Regulation_and_DuPont. The UnitedStates banned the use of CFCs inaerosol cans in 1978. The EuropeanCommunity rejected proposals to banCFCs in aerosol sprays, and in theU.S., CFCs continued to be used asrefrigerants and for cleaningcircuit boards. Worldwide CFCproduction fell sharply after theU.S. aerosol ban, but by 1986 hadreturned nearly to its 1976 level.In 1993, DuPont shut down its CFCfacility.The U.S. Governments attitudebegan to change again in 1983, whenWilliam Ruckelshaus replaced AnneM. Burford as Administrator of theUnited States EnvironmentalProtection Agency. UnderRuckelshaus and his successor, LeeThomas, the EPA pushed for aninternational approach tohalocarbon regulations. In 1985 20nations, including most of themajor CFC producers, signed the
Vienna Convention for theProtection of the Ozone Layer whichestablished a framework fornegotiating internationalregulations on ozone-depletingsubstances. That same year, thediscovery of the Antarctic ozonehole was announced, causing arevival in public attention to theissue. In 1987, representativesfrom 43 nations signed the MontrealProtocol. Meanwhile, the halocarbonindustry shifted its position andstarted supporting a protocol tolimit CFC production. The reasonsfor this were in part explained by"Dr. Mostafa Tolba, former head ofthe UN Environment Programme, whowas quoted in the 30 June 1990edition of The New Scientist,...the chemical industry supportedthe Montreal Protocol in 1987because it set up a worldwideschedule for phasing out CFCs,which [were] no longer protected bypatents. This provided companies
with an equal opportunity to marketnew, more profitable compounds.’At Montreal, the participantsagreed to freeze production of CFCsat 1986 levels and to reduceproduction by 50% by 1999. After aseries of scientific expeditions tothe Antarctic produced convincingevidence that the ozone hole wasindeed caused by chlorine andbromine from manmadeorganohalogens, the MontrealProtocol was strengthened at a 1990meeting in London. The participantsagreed to phase out CFCs and halonsentirely (aside from a very smallamount marked for certain"essential" uses, such as asthmainhalers) by 2000 in non-Article 5countries and by 2010 in Article 5(less developed) signatories. At a1992 meeting in Copenhagen, thephase out date was moved up to1996. At the same meeting, methylbromide (MeBr), a fumigant used
primarily in agriculturalproduction, was added to the listof controlled substances. It shouldbe noted that for all substancescontrolled under the Protocol,phaseout schedules were delayed forless developed (Article 5(1))countries, and phaseout in thesecountries was supported bytransfers of expertise, technology,and money from non-Article 5(1)Parties to the Protocol.Additionally, exemptions from theagreed schedules could be appliedfor under the Essential UseExemption (EUE) process forsubstances other than methylbromide and under the Critical UseExemption (CUE) process for methylbromide. See Gareau and DeCanioand Norman for more detail on theexemption processes.To some extent, CFCs have beenreplaced by the less damaginghydro-chloro-fluoro-carbons
(HCFCs), although concerns remainregarding HCFCs also. In someapplications, hydro-fluoro-carbons(HFCs) have been used to replaceCFCs. HFCs, which contain nochlorine or bromine, do notcontribute at all to ozonedepletion although they are potentgreenhouse gases. The best known ofthese compounds is probably HFC-134a (R-134a), which in the UnitedStates has largely replaced CFC-12(R-12) in automobile airconditioners. In laboratoryanalytics (a former "essential"use) the ozone depleting substancescan be replaced with various othersolvents.Ozone Diplomacy, by RichardBenedick (Harvard University Press,1991) gives a detailed account ofthe negotiation process that led tothe Montreal Protocol. Pielke andBetsill provide an extensive reviewof early U.S. government responses
to the emerging science of ozonedepletion by CFCs.More recently, policy experts haveadvocated for efforts to link ozoneprotection efforts to climateprotection efforts. Many ODS arealso greenhouse gasses, somesignificantly more powerful agentsof radiative forcing than carbondioxide over the short and mediumterm. Policy decisions in one arenaaffect the costs and effectivenessof environmental improvements inthe other. Thus many methods havebeen adopted to prevent the ozonelayer depeletion, because of theimplementation of these plans thereare seen results that the amount ofdepletion have decreased to someextent only, more can be seen ifthe the reductions of theproduction of the fuel burningvehicles that causes more of thedepeletion of the ozone layer.
Prospects of ozone depletionOzone-depleting gas trends.Since the adoption and strengtheningof the Montreal Protocol has led toreductions in the emissions of CFCs,atmospheric concentrations of the mostsignificant compounds have beendeclining. These substances are beinggradually removed from the atmosphere—since peaking in 1994, the EffectiveEquivalent Chlorine (EECl) level inthe atmosphere had dropped about 10%by 2008. It is estimated that by 2015,
the Antarctic ozone hole will havereduced by 1 million km² out of 25(Newman et al., 2004); completerecovery of the Antarctic ozone layeris not expected to occur until theyear 2050 or later. Work has suggestedthat a detectable (and statisticallysignificant) recovery will not occuruntil around 2024, with ozone levelsrecovering to 1980 levels by around2068. The decrease in ozone-depletingchemicals has also been significantlyaffected by a decrease in bromine-containing chemicals. The data suggestthat substantial natural sources existfor atmospheric methyl bromide (CH3Br).The phase-out of CFCs means thatnitrous oxide (N2O), which is notcovered by the Montreal Protocol, hasbecome the most highly emitted ozonedepleting substance and is expected toremain so throughout the 21st century.When the 2004 ozone hole ended inNovember 2004, daily minimumstratospheric temperatures in theAntarctic lower stratosphereincreased to levels that are too
warm for the formation of polarstratospheric clouds (PSCs) about 2to 3 weeks earlier than in mostrecent yearsThe Arctic winter of 2005 wasextremely cold in the stratosphere;PSCs were abundant over many high-latitude areas until dissipated bya big warming event, which startedin the upper stratosphere duringFebruary and spread throughout theArctic stratosphere in March. Thesize of the Arctic area ofanomalously low total ozone in2004-2005 was larger than in anyyear since 1997. The predominanceof anomalously low total ozonevalues in the Arctic region in thewinter of 2004-2005 is attributedto the very low stratospherictemperatures and meteorologicalconditions favorable for ozonedestruction along with thecontinued presence of ozone
destroying chemicals in thestratosphere.A 2005 IPCC summary of ozone issuesconcluded that observations andmodel calculations suggest that theglobal average amount of ozonedepletion has now approximatelystabilized. Although considerablevariability in ozone is expectedfrom year to year, including inpolar regions where depletion islargest, the ozone layer isexpected to begin to recover incoming decades due to decliningozone-depleting substanceconcentrations, assuming fullcompliance with the MontrealProtocol.Temperatures during the Arcticwinter of 2006 stayed fairly closeto the long-term average until lateJanuary, with minimum readingsfrequently cold enough to producePSCs. During the last week of
January, however, a major warmingevent sent temperatures well abovenormal — much too warm to supportPSCs. By the time temperaturesdropped back to near normal inMarch, the seasonal norm was wellabove the PSC threshold.Preliminary satellite instrument-generated ozone maps show seasonalozone buildup slightly below thelong-term means for the NorthernHemisphere as a whole, althoughsome high ozone events haveoccurred. During March 2006, theArctic stratosphere poleward of 60°North Latitude was free ofanomalously low ozone areas exceptduring the three-day period from 17March to 19 when the total ozonecover fell below 300 DU over partof the North Atlantic region fromGreenland to Scandinavia.The area where total column ozoneis less than 220 DU (the accepteddefinition of the boundary of the
ozone hole) was relatively smalluntil around 20 August 2006. Sincethen the ozone hole area increasedrapidly, peaking at 29 million km²24 September. In October 2006, NASAreported that the years ozone holeset a new area record with a dailyaverage of 26 million km² between 7September and 13 October 2006;total ozone thicknesses fell as lowas 85 DU on 8 October. The twofactors combined, 2006 sees theworst level of depletion inrecorded ozone history. Thedepletion is attributed to thetemperatures above the Antarcticreaching the lowest recording sincecomprehensive records began in1979.On October 2008 the EcuadorianSpace Agency published a reportcalled HIPERION, a study of thelast 28 years data from 10satellites and dozens of groundinstruments around the world among
them their own, and found that theUV radiation reaching equatoriallatitudes was far greater thanexpected, climbing in some verypopulated cities up to 24 UVI, theWHO UV Index standard considers 11as an extreme index and a greatrisk to health. The reportconcluded that the ozone depletionaround mid latitudes on the planetis already endangering largepopulations in this areas. Later,the CONIDA, the Peruvian SpaceAgency, made its own study, whichfound almost the same facts as theEcuadorian study.The Antarctic ozone hole isexpected to continue for decades.Ozone concentrations in the lowerstratosphere over Antarctica willincrease by 5%–10% by 2020 andreturn to pre-1980 levels by about2060–2075, 10–25 years later thanpredicted in earlier assessments.This is because of revised
estimates of atmosphericconcentrations of Ozone DepletingSubstances — and a larger predictedfuture usage in developingcountries. Another factor which mayaggravate ozone depletion is thedraw-down of nitrogen oxides fromabove the stratosphere due tochanging wind patterns.History of the researchThe basic physical and chemicalprocesses that lead to theformation of an ozone layer in theEarths stratosphere werediscovered by Sydney Chapman in1930. These are discussed in thearticle Ozone-oxygen cycle —briefly, short-wavelength UVradiation splits an oxygen (O2)molecule into two oxygen (O) atoms,which then combine with otheroxygen molecules to form ozone.Ozone is removed when an oxygenatom and an ozone molecule
"recombine" to form two oxygenmolecules, i.e. O + O3 →2O2. In the 1950s, David Bates andMarcel Nicolet presented evidencethat various free radicals, inparticular hydroxyl (OH) and nitricoxide (NO), could catalyze thisrecombination reaction, reducingthe overall amount of ozone. Thesefree radicals were known to bepresent in the stratosphere, and sowere regarded as part of thenatural balance – it was estimatedthat in their absence, the ozonelayer would be about twice as thickas it currently is.In 1970 Prof. Paul Crutzen pointedout that emissions of nitrous oxide(N2O), a stable, long-lived gasproduced by soil bacteria, from theEarths surface could affect theamount of nitric oxide (NO) in thestratosphere. Crutzen showed thatnitrous oxide lives long enough toreach the stratosphere, where it is
converted into NO. Crutzen thennoted that increasing use offertilizers might have led to anincrease in nitrous oxide emissionsover the natural background, whichwould in turn result in an increasein the amount of NO in thestratosphere. Thus human activitycould have an impact on thestratospheric ozone layer. In thefollowing year, Crutzen and(independently) Harold Johnstonsuggested that NO emissions fromsupersonic aircraft, which fly inthe lower stratosphere, could alsodeplete the ozone layer.The Rowland-Molina hypothesisIn 1974 Frank Sherwood Rowland,Chemistry Professor at theUniversity of California at Irvine,and his postdoctoral associate
Mario J. Molina suggested thatlong-lived organic halogencompounds, such as CFCs, mightbehave in a similar fashion asCrutzen had proposed for nitrousoxide. James Lovelock (mostpopularly known as the creator ofthe Gaia hypothesis) had recentlydiscovered, during a cruise in theSouth Atlantic in 1971, that almostall of the CFC compoundsmanufactured since their inventionin 1930 were still present in theatmosphere. Molina and Rowlandconcluded that, like N2O, the CFCswould reach the stratosphere wherethey would be dissociated by UVlight, releasing Cl atoms. (A yearearlier, Richard Stolarski andRalph Cicerone at the University ofMichigan had shown that Cl is evenmore efficient than NO atcatalyzing the destruction ofozone. Similar conclusions werereached by Michael McElroy andSteven Wofsy at Harvard University.
Neither group, however, hadrealized that CFCs were apotentially large source ofstratospheric chlorine — instead,they had been investigating thepossible effects of HCl emissionsfrom the Space Shuttle, which arevery much smaller.)The Rowland-Molina hypothesis wasstrongly disputed byrepresentatives of the aerosol andhalocarbon industries. The Chair ofthe Board of DuPont was quoted assaying that ozone depletion theoryis "a science fiction tale...a loadof rubbish...utter nonsense".Robert Abplanalp, the President ofPrecision Valve Corporation (andinventor of the first practicalaerosol spray can valve), wrote tothe Chancellor of UC Irvine tocomplain about Rowlands publicstatements (Roan, p 56.)Nevertheless, within three yearsmost of the basic assumptions made
by Rowland and Molina wereconfirmed by laboratorymeasurements and by directobservation in the stratosphere.The concentrations of the sourcegases (CFCs and related compounds)and the chlorine reservoir species(HCl and ClONO2) were measuredthroughout the stratosphere, anddemonstrated that CFCs were indeedthe major source of stratosphericchlorine, and that nearly all ofthe CFCs emitted would eventuallyreach the stratosphere. Even moreconvincing was the measurement, byJames G. Anderson andcollaborators, of chlorine monoxide(ClO) in the stratosphere. ClO isproduced by the reaction of Cl withozone — its observation thusdemonstrated that Cl radicals notonly were present in thestratosphere but also were actuallyinvolved in destroying ozone.McElroy and Wofsy extended the workof Rowland and Molina by showing
that bromine atoms were even moreeffective catalysts for ozone lossthan chlorine atoms and argued thatthe brominated organic compoundsknown as halons, widely used infire extinguishers, were apotentially large source ofstratospheric bromine. In 1976 theUnited States National Academy ofSciences released a report whichconcluded that the ozone depletionhypothesis was strongly supportedby the scientific evidence.Scientists calculated that if CFCproduction continued to increase atthe going rate of 10% per yearuntil 1990 and then remain steady,CFCs would cause a global ozoneloss of 5 to 7% by 1995, and a 30to 50% loss by 2050. In responsethe United States, Canada andNorway banned the use of CFCs inaerosol spray cans in 1978.However, subsequent research,summarized by the National Academyin reports issued between 1979 and
1984, appeared to show that theearlier estimates of global ozoneloss had been too large.Crutzen, Molina, and Rowland wereawarded the 1995 Nobel Prize inChemistry for their work onstratospheric ozone.The ozone holeThe discovery of the Antarctic"ozone hole" by British AntarcticSurvey scientists Farman, Gardinerand Shanklin (announced in a paperin Nature in May 1985) came as ashock to the scientific community,because the observed decline inpolar ozone was far larger thananyone had anticipated. Satellitemeasurements showing massivedepletion of ozone around the southpole were becoming available at thesame time. However, these wereinitially rejected as unreasonableby data quality control algorithms(they were filtered out as errors
since the values were unexpectedlylow); the ozone hole was detectedonly in satellite data when the rawdata was reprocessed followingevidence of ozone depletion in insitu observations. When thesoftware was rerun without theflags, the ozone hole was seen asfar back as 1976.Susan Solomon, an atmosphericchemist at the National Oceanic andAtmospheric Administration (NOAA),proposed that chemical reactions onpolar stratospheric clouds (PSCs)in the cold Antarctic stratospherecaused a massive, though localizedand seasonal, increase in theamount of chlorine present inactive, ozone-destroying forms. Thepolar stratospheric clouds inAntarctica are only formed whenthere are very low temperatures, aslow as -80 degrees C, and earlyspring conditions. In suchconditions the ice crystals of the
cloud provide a suitable surfacefor conversion of unreactive.chlorine compounds into reactivechlorine compounds which candeplete ozone easily.Moreover the polar vortex formedover Antarctica is very tight andthe reaction which occurs on thesurface of the cloud crystals isfar different from when it occursin atmosphere. These conditionshave led to ozone hole formation inAntarctica. This hypothesis wasdecisively confirmed, first bylaboratory measurements andsubsequently by directmeasurements, from the ground andfrom high-altitude airplanes, ofvery high concentrations ofchlorine monoxide (ClO) in theAntarctic stratosphere.Alternative hypotheses, which hadattributed the ozone hole tovariations in solar UV radiation or
to changes in atmosphericcirculation patterns, were alsotested and shown to be untenable.Meanwhile, analysis of ozonemeasurements from the worldwidenetwork of ground-based Dobsonspectrophotometers led aninternational panel to concludethat the ozone layer was in factbeing depleted, at all latitudesoutside of the tropics.These trendswere confirmed by satellitemeasurements. As a consequence, themajor halocarbon producing nationsagreed to phase out production ofCFCs, halons, and relatedcompounds, a process that wascompleted in 1996.Since 1981 the United NationsEnvironment Programme has sponsoreda series of reports on scientificassessment of ozone depletion,based on satellite measurements.The 2007 report showed that the
hole in the ozone layer wasrecovering and the smallest it hadbeen for about a decade.The 2010report found that "Over the pastdecade, global ozone and ozone inthe Arctic and Antarctic regions isno longer decreasing but is not yetincreasing... the ozone layeroutside the Polar regions isprojected to recover to its pre-1980 levels some time before themiddle of this century... Incontrast, the springtime ozone holeover the Antarctic is expected torecover much later.Ozone depletion and global warmingThere are five areas of linkagebetween ozone depletion and globalwarming:
Radiative forcing from variousgreenhouse gases and other sources. The same CO2 radiative forcing that produces global warming is expected to cool the stratosphere. This cooling, in turn, is expected to produce a relative increase in ozone (O3) depletion in polar area and the frequency of ozone holes. Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposing effects: Reduced ozone causes the stratosphere to
absorb less solar radiation,thus cooling the stratospherewhile warming the troposphere;the resulting colderstratosphere emits less long-wave radiation downward, thuscooling the troposphere.Overall, the cooling dominates;the IPCC concludes that"observed stratospheric O3 lossesover the past two decades havecaused a negative forcing of thesurface-troposphere system" ofabout −0.15 ± 0.10 watts persquare meter (W/m²).One of the strongest predictionsof the greenhouse effect is thatthe stratosphere will cool.Although this cooling has beenobserved, it is not trivial toseparate the effects of changesin the concentration ofgreenhouse gases and ozonedepletion since both will leadto cooling. However, this can be
done by numerical stratosphericmodeling. Results from theNational Oceanic and AtmosphericAdministrations GeophysicalFluid Dynamics Laboratory showthat above 20 km (12.4 miles),the greenhouse gases dominatethe cooling.As noted under Public Policy,ozone depleting chemicals arealso often greenhouse gases. Theincreases in concentrations ofthese chemicals have produced0.34 ± 0.03 W/m² of radiativeforcing, corresponding to about14% of the total radiativeforcing from increases in theconcentrations of well-mixedgreenhouse gases.The long term modeling of theprocess, its measurement, study,design of theories and testingtake decades to document, gainwide acceptance, and ultimatelybecome the dominant paradigm.Several theories about the
destruction of ozone were hypothesized in the 1980s, published in the late 1990s, and are currently being proven. Dr Drew Schindell, and Dr Paul Newman, NASA Goddard, proposed a theory in the late 1990s, using a SGI Origin 2000 supercomputer, that modeled ozone destruction, accounted for 78% of the ozone destroyed. Further refinement of that model accounted for 89% of the ozone destroyed, but pushed back the estimated recovery of the ozone hole from 75 years to 150 years. (An important part of that model is the lack of stratospheric flight due to depletion of fossil fuels.)Misconceptions about ozone depletionA few of the more commonmisunderstandings about ozonedepletion are addressed brieflyhere; more detailed discussions can
be found in the ozone-depletionFAQ.CFCs are "too heavy" to reach thestratosphereIt is commonly believed that CFCmolecules are heavier than air(nitrogen or oxygen), so that theCFC molecules cannot reach thestratosphere in significant amount.But atmospheric gases are notsorted by weight; the forces ofwind can fully mix the gases in theatmosphere. Despite the fact thatCFCs are heavier than air and witha long lifetime, they are evenlydistributed throughout theturbosphere and reach the upperatmosphere.Man-made chlorine is insignificantcompared to natural sources
Another misconception is that "itis generally accepted that naturalsources of tropospheric chlorineare four to five times larger thanman-made one". While strictly true,tropospheric chlorine isirrelevant; it is stratosphericchlorine that affects ozonedepletion. Chlorine from oceanspray is soluble and thus is washedby rainfall before it reaches thestratosphere. CFCs, in contrast,are insoluble and long-lived,allowing them to reach thestratosphere. In the loweratmosphere, there is much more
chlorine from CFCs and relatedhaloalkanes than there is in HClfrom salt spray, and in thestratosphere halocarbons aredominant . Only methyl chloridewhich is one of these halocarbonshas a mainly natural source ,and itis responsible for about 20 percentof the chlorine in thestratosphere; the remaining 80%comes from man made sources.Very violent volcanic eruptions caninject HCl into the stratosphere,but researchers have shown that thecontribution is not significantcompared to that from CFCs. Asimilar erroneous assertion is thatsoluble halogen compounds from thevolcanic plume of Mount Erebus onRoss Island, Antarctica are a majorcontributor to the Antarctic ozonehole.An ozone hole was first observed in1956
G.M.B. Dobson (Exploring theAtmosphere, 2nd Edition, Oxford,1968) mentioned that whenspringtime ozone levels over HalleyBay were first measured in 1956, hewas surprised to find that theywere ~320 DU, about 150 DU belowspring levels, ~450 DU, in theArctic. These, however, were atthis time the known normalclimatological values because noother Antarctic ozone data wereavailable. What Dobson describes isessentially the baseline from whichthe ozone hole is measured: actualozone hole values are in the 150–100 DU range.The discrepancy between the Arcticand Antarctic noted by Dobson wasprimarily a matter of timing:during the Arctic spring ozonelevels rose smoothly, peaking inApril, whereas in the Antarcticthey stayed approximately constantduring early spring, rising
abruptly in November when the polarvortex broke down.The behavior seen in the Antarcticozone hole is completely different.Instead of staying constant, earlyspringtime ozone levels suddenlydrop from their already low wintervalues, by as much as 50%, andnormal values are not reached againuntil December.The ozone hole should be above thesources of CFCsSome people thought that the ozonehole should be above the sources ofCFCs. However, CFCs are well mixedin the troposphere and thestratosphere. The reason foroccurrence of the ozone hole aboveAntarctica is not because there aremore CFCs concentrated but becausethe low temperatures help formpolar stratospheric clouds. Infact, there are findings ofsignificant and localized "ozone
holes" above other parts of theearth.The "ozone hole" is a hole in theozone layerThere is a common misconceptionthat ―ozone hole‖ is really a holein the ozone layer.When the "ozonehole" occurs, the ozone in thelower stratosphere is destroyed.The upper stratosphere is lessaffected, so that the amount ofozone over the continent decreasesby 50 percent or even more. Theozone hole does not disappearthrough the layer; on the otherhand, it is not a uniformthinning of the ozone layer. Itis a "hole" which is a depression,not in the sense of "a hole in thewindshield."