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<ul><li>1.sis_17_RZ_.qxq:Layout 1 24.11.20109:40 UhrSeite 46 Im ag e courte syof NA AS A hole in the sky Twenty-five years ago,the discovery of the hole in the ozone layer hit the news. How have things developed since?Tim Harrison and Dudley Shallcross investigate.The Antarctic ozone hole at its annual maximum on12 September 2008, stretching over 27 millionsquare kilometres. This is considered a moderatelylarge ozone hole, according to NASADiscovering the hole It was a serendipitous find, as Jonathan Shanklin, one ofthe holes discoverers, remembers: having joined the35British Antarctic Surveyw1 in 1977, he was supposed todigitise their backlog of ozone measurements until then,30handwritten data sheets. As it turned out, this includedthe crucial decade, the 1970s, when ozone levels began toAltitude (kilometres)25 StratosphericPublic domain image; image source: Wikimedia Commonsdrop.Ozone layer ozone There had already been growing concern that industrial20chlorofluorocarbons (CFCs) organic compounds such astrichlorofluoromethane (CFCl3) and dichlorodifluo-romethane (CF2Cl2), then widely used as refrigerants, pro-15pellants (in spray cans) and solvents might destroy theozone layer. For an open day in 1983, Shanklin prepared a 10 Ozone Troposphericgraph ironically to show that the ozone data from that increases ozoneyear were no different from 20 years before. Although this 5 from pollutionwas true for the overall ozone levels, he noticed that thespringtime values did look lower from one year to the0next. Further studies corroborated this, and in 1985Ozone concentrationShanklin and his colleagues Joe Farman and BrianGardiner published their findings: each SouthernOzone is present throughout the lower atmosphere. MostHemisphere spring, a hole gaped in the ozone layer aboveozone is in the stratospheric ozone layer. Near Earths surface,the Antarctic, it was probably caused by CFCs, and it was the ozone levels increase as a result of pollution from humangrowing (Farman et al., 1985).activities46 Science in School Issue 17 : Winter 2010 www.scienceinschool.org</li></ul><p>2. Science topicsWhat is the chemistry behind this, and why is the ozone (h) with a wavelength (l) around 200 nm and dissociateshole dangerous? into two oxygen atoms (O) (reaction 1). Each of these canthen combine with another oxygen molecule to formozone in the stratosphere ozone, if the pressure (M) is high enough (approximatelyOzone (O3) is a much less stable triatomic form of oxy- one thousandth of an atmosphere) to stabilise the newlygen (O2). It is a pale blue gas present at low concentrations formed ozone molecule (reaction 2). The higher the alti-throughout the atmosphere and a double-edged sword: tude, the faster the rate of reaction 1 (below 20 km alti-in the troposphere (see image on page 48), ozone is an airtude, no 200 nm photons occur because they have all beenpollutant which can damage the respiratory systems of absorbed in reaction 1). The rate of reaction 2, however, ishumans and other animals and burn sensitive plants. The faster closer to the ground, where atmospheric pressure isozone layer in the stratosphere, however, is beneficial, pre- higher. As a result, the maximum amount of ozone is cre-venting most of the harmful ultraviolet (UV) light emittedated between about 25 and 30 km altitude (see graph onby the Sun from reaching Earths surface. page 46).The rate of ozone formation maximises in the strato- The stratosphere has two important consequences forsphere, the second highest layer of Earths atmosphere (atlife on Earth. First, ozone itself absorbs high-energyabout 10-50 km altitude; see image), through a photo- UV radiation at around 250 nmchemical mechanism: (reaction 3):O2 + h O + O l ~ 200 nm (1) O3 + h O + O2 l ~ 250 nm DH = - 90 kJ mol-1 (3)O + O2 + M O3 + M (2) Between them, oxygen (reaction 1) and ozone (reactionAn oxygen molecule (O2) absorbs a photon of UV light3) therefore filter out of the atmosphere most of the short-ChemistryEarth sciencecatalytic cycles. Where do those organisms live?Ages 16+ Which chemical substances do they produce?The chemical composition of the atmosphere andThe ozone hole is a topical global issue, and you will its influence on the climate. Which gases is thefind this article really helpful to get into the subject.atmosphere composed of? In which way do theThe chemical processes involved are described in atmospheres properties determine climatic con-full detail. In chemistry lessons, the article can beditions on Earths surface, and how does this dif-used to teach atomic structure and chemical bonds, fer on other planets?free radicals, catalytic cycles, and the influence of The article could also form the basis of a lesson onlight and temperature on chemical reactions.how science is reported in the media. Students couldcompare this articleto those in the general press: doFor the earth science classroom, the article would fitthey provide a balanced view of the question, men-in the context of the following topics:tioning both chemical and natural components lead- Atmospheric influence on climateing to the formation of the ozone hole? Do they min- Biological processes occurring on Earths surface imise or overemphasise the phenomenon as aand affecting marine organismswhole? Why due to journalists ignorance, political Earths morphology and the distribution of moun-strategy or both? For further ideas on using news intain ranges on Earths surfacethe science classroom, see Veneu-Lumb &amp; Costa The changing of the seasons, Earths axial tilt and (2010).rotation. Finally, the text is suitable as the basis of a compre- REViEWThere is the opportunity for interdisciplinary work hension exercise, too. Possible questions are:linking chemistry and earth sciences. Possible topicsinclude: Why is this topic a much debated questionnowadays? The geographical distribution of the organisms What is the role of natural factors in the growththat produce chemicals that are active in natural of the ozone hole? What about human factors?Teresa Celestino, Italywww.scienceinschool.org Science in School Issue 17 : Winter 2010 47 3. sis_17_RZ_.qxq:Layout 124.11.20109:40 Uhr Seite 48Image courtesy of Marlene RauLayers of atmosphere leading to space 10 000 km Exosphere690 kmPublic domain image; image source: Wikimedia CommonsThe four main reactions of oxygen in the ozone layer. Blue arrows indicate reac-tions, green dotted arrows indicate that a molecule from one reaction goes on totake part in another reaction. M denotes the pressure required for reaction 2wave UV radiation between 200 and 300 nm, which would otherwise be verydamaging to life on Earth.Second, reaction 3 produces a lot of heat, so the stratosphere is a warmerlayer than the top of the troposphere (see image left), making the weather inthe troposphere less extreme than it would otherwise be. ThermosphereReactions 2 and 3 rapidly interconvert oxygen atoms and ozone. There isanother slow reaction, though, which is known to destroy both oxygen atomsand ozone, namely the reaction between these two species:O + O3 O2 + O2(4)Reactions 1-4 are summarised in the diagram above. 85 kmNatural catalytic cycles reduce the levels of ozone Mesosphere In 1995, Paul Crutzen, Mario Molina and F Sherwood Rowland were awardedthe Nobel Prize in Chemistry for their work on the formation and decomposi-tion of ozone in the stratosphere. What had they learned? In the 1970s, Crutzenand others discovered the existence of natural catalytic cycles that speed up 50 kmreaction 4 and reduce the amount of ozone in the stratosphere (Crutzen, 1970,1971): water (H2O), methane (CH4), nitrous oxide (N2O) and chloromethane(CH3Cl) are released into the atmosphere from biological processes occurring on StratosphereEarths surface, and lead to the formation of radicals such as hydroxyl (OH),nitric oxide (NO) and chlorine (Cl), which catalyse the decomposition of ozone. Reaction 5 shows how chloromethane releases chlorine radicals into the strat-osphere through photolysis, and reactions 6 and 7 are an example of a catalytic 6-20 kmcycle (see diagram on page 49). The reactions of the other catalysts are analo-gous with reactions 6 and 7. Chloromethane is released in part by both marine Troposphereand terrestrial organisms, such as red macroalgae, white rot fungi and higherplants, to regulate chloride ion levels in the cells and after 30 to 40 years canreach the upper stratosphere (around 40 km altitude) where it is broken down 0 kmby sunlight (photolysis):48Science in School Issue 17 : Winter 2010 www.scienceinschool.org 4. sis_17_RZ_.qxq:Layout 1 24.11.2010 9:40 Uhr Seite 49 Science topicsImage courtesy of Marlene RauChlorine radicals(for example fromreaction 5) enter acatalytic cycle(reactions 6 and 7)of net ozonedecomposition,which can be ter-minated by reac-tions 8 and 9. Bluearrows indicatereactions, greendotted arrows indi-cate that a mole-cule from one reac-tion goes on to takepart in anotherreaction. M denotesthe pressurerequired for reac-tion 9CH3Cl + h CH3 + Cl ~ 200 nm (5) predicted that CFCs would cause a significant additionalThe resulting chlorine free radical (Cl) can then partici- loss of ozone at around 40 km altitude (see Molina &amp;pate in a catalytic cycle:Rowland, 1974). However, when the ozone hole was final-ly found in 1985, it was in fact at around 20 km altitude,Cl + O3 ClO + O2 (6)ClO + OCl + O2(7) over the South Pole in the Southern Hemisphere spring-time (see Farman et al., 1985).Reactions 6 and 7 taken together are in fact equivalent toIt soon emerged that chlorine free radicals from thereaction 4, but happen much faster in the case of theCFCs were responsible, but many questions remainedchlorine / chlorine monoxide (ClO) radical cycle, aboutunanswered. Why did the hole occur over the Pole? If it30 000 times faster. So why do these catalytic cycles notoccurred over the South Pole, why not also over the Northdestroy all the ozone? The answer lies in the terminationPole? Why only in spring? And why was the ozone hole atof these cycles via the formation of stable molecules:20 km altitude instead of at 40 km, as predicted? After all,Cl + CH4 CH3 + HCl (8) CFCs could not be broken down by sunlight at an altitudeClO + NO2 + M ClONO2 + M(9) as low as 20 km, since the photon density was insufficient.Eventually, a chlorine free radical will encounter aFor the same reason, not enough oxygen atoms are pro-methane molecule and react to form hydrochloric acidduced at this altitude for reaction 7 to occur. Many years(HCl, reaction 8). Similarly, a chlorine monoxide radical of further research revealed the complete story.will bind to a nitrogen dioxide radical, forming chlorine First, chlorine free radicals released from the CFCs, e.g.nitrate (ClONO2, reaction 9) another pressure-dependentCFCl3 + hCFCl2 + Cl~ 200 nm (10)reaction that therefore works better at lower altitudes.Both hydrochloric acid and chlorine nitrate are very stable,could react with methane (reaction 8) forming hydrochlo-and the removal of chlorine and chlorine monoxide radi- ric acid, or with ozone (reaction 6) forming chlorinecals eventually stops the catalytic cycle.monoxide radicals, and through reaction 9 could subse-quently form chlorine nitrate. This sequence of reactionsThe Antarctic ozone hole puzzle would increase the concentrations of hydrocholoric acid It was not long before scientists realised that CFCs could and chlorine nitrate at around 40 km altitude globally.trigger a similar catalytic cycle of ozone degradation: inEach Southern Hemisphere winter, the South Pole is1974, Molina and Rowland not only warned that levels of plunged into darkness for approximately three months.CFCs continued to increase without regulation, but also The air in the stratosphere above the South Pole coolswww.scienceinschool.org Science in School Issue 17 : Winter 201049 5. sis_17_RZ_.qxq:Layout 1 24.11.2010 9:40 UhrSeite 50All year round over the Equator, in summer and autumn over the South Pole Early winter The formation and dissolution of the ozone hole over the year.40 km(10) CFCl3 + hv ( ~ 200 nm) CFCl2 + Cl Reactions 5 to 9 happen over(6) Cl + O3 ClO + O2 (8) Cl + CH4 CH3 + HCl the Equator all year round, and (7) ClO + O Cl + O2(9) ClO + NO2 ClONO2 over the South Pole in summer and autumn. In early winter, the 30 km HCl and vortex forms over the SouthClONO2 Pole, followed by the formationVortex forms of polar stratospheric clouds in winter. In early spring, the sun-20 km Cold and dark shine returns, but the vortexair sinks remains, and the reactions lead- ing to ozone removal over the South Pole take their course. InEquator late spring, the vortex breaks down, and ozone from mid-lati- tudes can mix indown; without UV radiation, reaction 3 does not occur, so HCl + ClONO2 HNO3 + Cl2 polar stratospheric clouds (11)no heat is released. The air sinks and Earths rotation causesThis reaction can take place all winter, if it is coldit to spin and form a vortex as it does so, like water goingenough to form polar stratospheric clouds. When the sun-down a plughole. This vortex is so strong that no air fromshine returns in spring, there are plenty of chlorine mole-outside can get in, and no air from inside can get out. Aircules at around 15-25 km altitude, which are photolysed tothat is rich in hydrochloric acid and chlorine nitrate from 40produce chlorine radicals:km altitude is drawn down into this cold and dark vortex.Cl2 + hCl + Cl~ 350 nm(12) In the extreme cold of the polar winter, the air in thisvortex becomes so cold that below -78C (195 K) and at an and subsequently chlorine monoxide radicals via reaction 6.altitude of 15-25 km, polar stratospheric clouds form fromHowever, in the polar spring, reaction 7 (the formationwater and / or acid ice crystals. of chlorine radicals and oxygen molecules from chlorine The first peculiar bit of chemistry is that hydrochloric monoxide radicals and oxygen radicals) is very slow, sinceacid and chlorine nitrate can adsorb onto polar stratos-there are so few oxygen atoms present due to the lack ofpheric clouds and undergo a fast heterogeneous reaction 200 nm photons at this altitude, and here is where a sec-from gaseous to solid phase, producing nitric acid (HNO3) ond peculiar piece of chemistry occurs. At low tempera-that becomes incorporated into the ice crystals, whilst the tures, such as in the polar vortex which is still very coldchlorine (Cl2) is released back into the gas phase. even in spring chlorine monoxide radicals can form a3%15%HCI Image courtesy of Andrew Ryzhkov; image source: Wikimedia CommonsCH3CI Natural Public domain image; image source: Wikimedia Commons28%3% HCFC-22CFC-12 CFC-1136% Anthropogenic CH3CCI310% CFC-11 CCI1 23%12%Sources of stratospheric chlorine accord-ing to the WMO / UNEP ScientificAssessment of Ozone Depletion: 1998 Polar stratospheric clouds in Asker, Norway50 Science in School Issue 17 : Winter 2010www.scienceinschool.org 6. sis_17_RZ_.qxq:Layout 124.11.2010 9:40 Uhr S...</p>