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Newsgroups: sci.environment,sci.answers,news.answers Path: bloom-beacon.mit.edu!news.media.mit.edu!uhog.mit.edu!MathWorks.Com!zombie.ncsc.mil!paladin.american.edu!howland.reston.ans.net!agate!boulder!cnsnews!rintintin.Colorado.EDU!rparson From: rparson@rintintin.colorado.edu (Robert Parson) Subject: Ozone Depletion FAQ Part III: The Antarctic Ozone Hole Message-ID: <Cn7D5r.4ux@cnsnews.Colorado.EDU> Followup-To: sci.environment Summary: This is the third of four files dealing with stratospheric ozone depletion. It describes the massive losses measured in the Antarctic spring, and the smaller losses seen in the Arctic. Originator: rparson@rintintin.Colorado.EDU Keywords: ozone layer hole cfc stratosphere antarctic arctic ClO Sender: usenet@cnsnews.Colorado.EDU (Net News Administrator) Nntp-Posting-Host: rintintin.colorado.edu Reply-To: rparson@rintintin.colorado.edu Organization: University of Colorado, Boulder Date: Fri, 25 Mar 1994 04:15:27 GMT Approved: news-answers-request@MIT.Edu Lines: 850 Xref: bloom-beacon.mit.edu sci.environment:18029 sci.answers:1010 news.answers:16822 Archive-name: ozone-depletion/antarctic Last-modified: 25 March 1994 Version: 4.2 These files are posted monthly, usually in the third week of the month. They may be obtained by anonymous ftp from rtfm.mit.edu (18.70.0.209) in the directory: /pub/usenet/news.answers/ozone-depletion which contains the four files intro, stratcl, antarctic, and uv. They may also be obtained by sending the following message to mail-server@rtfm.mit.edu: send usenet/news.answers/ozone-depletion/intro send usenet/news.answers/ozone-depletion/stratcl send usenet/news.answers/ozone-depletion/antarctic send usenet/news.answers/ozone-depletion/uv Leave the subject line blank. If you want to find out more about the mail server, send a message to it containing the word "help". *********************************************************************** * Copyright 1994 Robert Parson * * * * This file may be distributed, copied, and archived. All * * copies must include this notice and the paragraph below entitled * * "Caveat". Reproduction and distribution for personal profit is * * not permitted. If this document is transmitted to other networks or * * stored on an electronic archive, I ask that you inform me. I also * * request that you inform me before including any of this information * * in any publications of your own. Students should note that this * * is _not_ a peer-reviewed publication and may not be acceptable as * * a reference for school projects; it should instead be used as a * * pointer to the published literature. In particular, all scientific * * data, numerical estimates, etc. should be accompanied by a citation * * to the original published source, not to this document. * *********************************************************************** This part deals specifically with springtime antarctic ozone depletion (and with the similar but smaller effects seen in the Arctic spring). More general questions about ozone and ozone depletion, including the definitions of many of the terms used here, are dealt with in parts I and II. Biological effects of the ozone hole are dealt with in part IV. | Caveat: I am not a specialist. In fact, I am not an atmospheric | chemist at all - I am a physical chemist who talks to atmospheric | chemists. These files are an outgrowth of my own efforts to educate | myself over the past two years. I have discussed some of these | issues with specialists but I am solely responsible for everything | written here, including any errors. This document should not be | cited in publications off the net; rather, it should be used as a | pointer to the published literature. *** Corrections and comments are welcomed. - Robert Parson Associate Professor Department of Chemistry and Biochemistry, University of Colorado (for which I do not speak) rparson@rintintin.colorado.edu CONTENTS 1. What is the antarctic ozone hole? 2. How big is the hole, and is it getting bigger? 3. When did the hole first appear? 4. How far back do Antarctic ozone measurements go? 5. But I heard that Dobson saw an ozone hole in 1956-58... 6. Why is the hole in the antarctic? 7. What is the evidence for the present theory? 8. Will the ozone hole keep growing? 9. Why be concerned about an ozone hole over antarctica? Nobody lives down there. 10. Is there an ozone hole in the arctic? if not, why not? 11. Can the hole be "plugged"? 1. What is the Antarctic ozone hole? For the past decade or so, ozone levels over Antarctica have fallen to abnormally low values between late August and late November. At the beginning of this period, ozone levels are already low, about 300 Dobson units (DU), but instead of slowly increasing as the light comes back in the spring, they drop to 150 DU and below. In the lower stratosphere, between 15 and 20 km, about 95% of the ozone is destroyed. Above 25 km the decreases are small and the net result is a thinning of the ozone layer by about 50%. In the late spring ozone levels return to more normal values, as warm, ozone-rich air rushes in from lower latitudes. The precise duration varies considerably from year to year; in 1990 the hole lasted well into December. In some of the popular newsmedia, as well as many books, the term "ozone hole" is being used far too loosely. It seems that any episode of ozone depletion, no matter how minor, now gets called an ozone hole (e.g. 'ozone hole over Hamburg - but only for one day'). This sloppy language trivializes the problem and blurs the important scientific distinction between the massive ozone losses in polar regions and the much smaller, but nonetheless significant, ozone losses in middle latitudes. It is akin to using "gridlock" to describe a routine traffic jam. 2. How big is the hole, and is it getting bigger? During the years 1978-1987 the hole grew, both in depth (total ozone loss in a column) and in area. This growth was not monotonic but seemed to oscillate with a two-year period (perhaps connected with the "quasibiennial oscillation" of the stratospheric winds.) The hole shrank dramatically in 1988 but in 1989-1991 was as large as in 1987, and in 1992-93 was larger still. In 1987 and 1989-93 it covered the entire Antarctic continent and part of the surrounding ocean. The exact size is determined primarily by meteorological conditions, such as the strength of the polar vortex in any given year. The boundary is fairly steep, with decreases of 100-150 DU taking place in 10 degrees of latitude, but fluctuates from day to day. On occasion, the nominal boundary of the hole has passed over the tip of S. America, (55 degrees S. Latitude). Australia and New Zealand are far outside the hole, although they do experience ozone depletion, more than is seen at comparable latitudes in the Northern hemisphere. After the 1987 hole broke up, December ozone levels over Australia and New Zealand were 10% below normal. [WMO 1991] [Atkinson et al.] [Roy et al.]. 3. When did the hole first appear? It was first observed by ground-based measurements from Halley Bay on the Antarctic coast, during the years 1980-84. [Farman, Gardiner and Shanklin.] At about the same time, ozone decreases were seen at the Japanese antarctic station of Syowa; these were less dramatic than those seen at Halley (Syowa is about 1000 km further north) and did not receive as much attention. It has since been confirmed by satellite measurements as well as ground-based measurements elsewhere on the continent, on islands in the Antarctic ocean, and at Ushaia, at the tip of Patagonia. With hindsight, one can see the hole beginning to appear in the data around 1976, but it grew much more rapidly in the 1980's. [Stolarski et al. 1992] 4. How far back do antarctic ozone measurements go? Ground-based measurements began in 1956, at Halley Bay. A few years later these were supplemented by measurements at the South Pole and continent. Satellite measurements began in the early 70's, but the first really comprehensive satellite data came in 1978, with the TOMS (total ozone mapping spectrometer) and SBUV (solar backscatter UV) instruments on Nimbus-7. The TOMS, which finally broke down on May 7 1993, is the source for most of the pretty pictures that one sees in review articles and the popular press. Today there are several satellites monitoring ozone and other atmospheric gases; the Russian Meteor-3 carries a new TOMS, while instrument on NASA's UARS (Upper Atmosphere Research Satellite) simultaneously measure ozone, chlorine monoxide (ClO), and stratospheric pressure and temperature. 5. But I heard that Dobson saw an ozone hole in 1956-58... This is a myth, arising from a misinterpretation of an out-of- context quotation from Dobson's paper. A glance at the original suffices to refute it. In his historical account [Dobson], Dobson mentioned that when springtime ozone levels over Halley Bay were first measured, he was surprised to find that they were about 150 DU below corresponding levels (displaced by six months) in the Arctic. Springtime arctic ozone levels are very high, ~450 DU; in the Antarctic spring, however, Dobson's coworkers found ~320 DU, close to winter levels. This was the first observation of the _normal_, pre-1980 behavior of the Antarctic ozone layer: because of the tight polar vortex (see below) ozone levels remain low until late spring. In the Antarctic ozone hole, on the other hand, ozone levels _decrease_ from these already low values. What Dobson describes is essentially the _baseline_ from which the ozone hole is measured. [Dobson] [WMO 1989] For those interested, here is how springtime antarctic ozone has developed from 1956 to 1991: ------------------------------------------------------------- Halley Bay Antarctic Ozone Data Mean October ozone column thickness, Dobson Units From J. D. Shanklin, personal communication, 1993. See also [Dobson], [Farman et al.], [Hamill and Toon], [Solomon], and [WMO 1991], p. 4.6 1956 321 1975 308 1957 330 1976 283 1958 314 1977 251 1959 311 1978 284 1960 301 1979 261 1961 317 1980 227 1962 332 1981 237 1963 309 1982 234 1964 318 1983 210 1965 281 1984 201 1966 316 1985 196 1967 323 1986 248 1968 301 1987 163 1969 282 1988 232 1970 282 1989 164 1971 299 1990 179 1972 304 1991 155 1973 289 1992 142 1974 274 1993 117 6. Why is the hole in the Antarctic? This was a mystery when the hole was first observed, but it is now well understood. I shall limit myself to a brief survey of the present theory, and refer the reader to two excellent nontechnical articles [Toon and Turco] [Hamill and Toon] for a more comprehensive discussion. Briefly, the unusual physics and chemistry of the Antarctic stratosphere allows the inactive chlorine "reservoir" compounds to be converted into ozone- destroying chlorine radicals. While there is no more chlorine over antarctica than anywhere else, in the antarctic spring most of the chlorine is in a form that can destroy ozone. The story takes place in six acts, some of them occurring simultaneously on parallel stages: i. The Polar Vortex As the air in the antarctic stratosphere cools and descends during the winter, the Coriolis effect sets up a strong westerly circulation around the pole. When the sun returns in the spring the winds weaken, but the vortex remains stable until November. The air over antarctica is largely isolated from the rest of the atmosphere, forming a gigantic reaction vessel. The vortex is not circular, it has an oblong shape with the long axis extending out over Patagonia. (For further information about the dynamics of the polar vortex see [Schoeberl and Hartmann], [Tuck 1989], [AASE], [Randel], [Plumb], and [Waugh]). There is currently some controversy over just how isolated the air in the vortex is. According to Tuck, the vortex is better thought of as a flow reactor than as a containment vessel; ozone-rich air enters the vortex from above while ozone-poor and ClO-rich air is stripped off the sides. Recent tracer measurements lend some support to this view, but the issue is unresolved. See [Randel] and [Plumb].) ii. Polar Stratospheric Clouds ("PSC") The Polar vortex is extremely cold; temperatures in the lower stratosphere drop below -80 C. Under these conditions large numbers of clouds appear in the stratosphere. These clouds are composed largely of nitric acid and water, probably in the form of crystals of nitric acid trihydrate ("NAT"), HNO3.3(H2O). Stratospheric clouds also form from ordinary water ice (so-called "Type II PSC"), but these are much less common; the stratosphere is very dry and water-ice clouds only form at the lowest temperatures. iii. Reactions Catalyzed by Stratospheric Clouds Most of the chlorine in the stratosphere ends up in one of the reservoir compounds, Chlorine Nitrate (ClONO2) or Hydrogen Chloride (HCl). Laboratory experiments have shown, however, that these compounds, ordinarily inert in the stratosphere, do react on the surfaces of polar stratospheric cloud particles. HCl dissolves into the particles as they grow, and when a ClONO2 molecule becomes adsorbed the following reactions take place: ClONO2 + HCl -> Cl2 + HNO3 ClONO2 + H2O -> HOCl + HNO3 The Nitric acid, HNO3, stays in the cloud particle. In addition, stratospheric clouds catalyze the removal of Nitrogen Oxides ("NOx"), through the reactions: N2O5 + H2O -> 2 HNO3 N2O5 + HCl -> ClNO2 + HNO3 Since N2O5 is in (gas-phase) equilibrium with NO2: 2 N2O5 <-> 4 NO2 + O2 this has the effect of removing NO2 from the gas phase and sequestering it in the clouds in the form of nitric acid, a process called "denoxification" (removal of "NOx"). iv. Sedimentation and Denitrification The clouds may eventually grow big enough so that they settle out of the stratosphere, carrying the nitric acid with them ("denitrification"). Denitrification enhances denoxification. If, on the other hand, the cloud decomposes while in the stratosphere, nitrogen oxides are returned to the gas phase. Presumably this should be called "renoxification", but I have not heard anyone use this term :-). v. Photolysis of active chlorine compounds The Cl2 and HOCl produced by the heterogeneous reactions are easily photolyzed, even in the antarctic winter when there is little UV present. The sun is always very low in the polar winter, so the light takes a long path through the atmosphere and the short-wave UV is selectively absorbed. Molecular chlorine, however, absorbs _visible_ and near-UV light: Cl2 + hv -> 2 Cl Cl + O3 -> ClO + O2 The effect is to produce large amounts of ClO. This ClO would ordinarily be captured by NO2 and returned to the ClONO2 reservoir, but "denoxification" and "denitrification" prevent this by removing NO2. vi. The chlorine peroxide mechanism As discussed in Part I, Cl and ClO can form a catalytic cycle that efficiently destroys ozone. This cycle uses free oxygen atoms, however, which are only abundant in the upper stratosphere, whereas the ozone hole forms in the lower stratosphere. Instead, the principal mechanism involves chlorine peroxide, ClOOCl (often referred to as the "ClO dimer"): ClO + ClO -> ClOOCl ClOOCl + hv -> Cl + ClO2 ClO2 -> Cl + O2 2 Cl + 2 O3 -> 2 ClO + 2 O2 ------------------------------- Net: 2 O3 -> 3 O2 At polar stratospheric temperatures this sequence is extremely fast and in process. The second step, photolysis of chlorine peroxide, requires UV light which only becomes abundant in the lower stratosphere in the spring. Thus one has a long buildup of ClO and ClOOCl during the winter, followed by massive ozone destruction in the spring. This mechanism is believed to be responsible for about 70% of the antarctic ozone loss. Another mechanism that has been identified involves chlorine and bromine: ClO + BrO -> Br + Cl + O2 Br + O3 -> BrO + O2 Cl + O3 -> ClO + O2 ----------------------- Net: 2 O3 -> 3 O2 This is believed to be responsible for ~25% of the antarctic ozone depletion. Additional mechanisms have been suggested, but they seem to be less important. [WMO 1991] (For further information on the "perturbed chemistry" of the antarctic stratosphere, see [Solomon], [McElroy and Salawich], and [WMO 1989, 1991]). 7. What is the evidence for the present theory? The evidence is overwhelming - the results from a single 1987 expedition (albeit a crucial one) fill two entire issues of the Journal of Geophysical Research. What follows is a very sketchy summary; for more information the reader is directed to [Solomon] and to [Anderson et al.]. The theory described above (which is often called the "PSC theory") was developed during the years 1985-87. At the same time, others proposed completely different mechanisms, making no use of chlorine chemistry. The two most prominent alternative explanations were one that postulated large increases in nitrogen oxides arising from enhanced solar activity, and one that postulated an upwelling of ozone-poor air from the troposphere into the cold stratospheric vortex. Each hypothesis made definite predictions, and a program of measurements was carried out to test these. The solar activity hypothesis predicted enhanced NOx, whereas the measurements show unusually _low_ NOx ("denoxification), in accordance with the PSC hypothesis. The "upwelling" hypothesis predicted upward air motion in the lower stratosphere, which is inconsistent with measurements of atmospheric tracers such as N2O which show that the motion is primarily downwards. Positive evidence for the PSC theory comes from ground-based and airborne observations of the various chlorine-containing compounds. These show that the reservoir species HCl and ClONO2 are extensively depleted in the antarctic winter and spring, while the concentration of the active, ozone-depleting species ClO is strongly enhanced. Measurements also show enormously enhanced concentrations of the molecule OClO. This is formed by a side-reaction in the BrO/ClO mechanism described above. Further evidence comes from laboratory studies. The gas-phase reactions have been reproduced in the laboratory, and shown to proceed at the rates required in order for them to be important in the polar stratosphere. [Molina et al. 1990] [Sander et al.] [Trolier et al.] [Anderson et al.]. The production of active chlorine from reservoir chlorine on ice and sulfuric acid surfaces has also been demonstrated in the laboratory [Tolbert et al. 1987,1988] [Molina et al. 1987]. (Recently evidence for these reactions has been found in the arctic stratosphere as well: air parcels that had passed through regions where the temperature was low enough to form PSC's were found to have anomalously low concentrations of HCl and anomalously high concentrations of ClO [AASE].) The "smoking gun", however, is usually considered to be the simultaneous in-situ measurements of a variety of trace gases from an ER-2 stratospheric aircraft (a converted U2 spy plane) in August-October 1987. [Tuck et al.] These measurements demonstrated a striking "anticorrelation" between local ozone concentrations and ClO concentrations. Upon entering the "hole", ClO concentrations suddenly jump by a factor of 20 or more, while ozone concentrations drop by more than 50%. Even the local fluctuations in the concentrations of the two species are anticorrelated. [Anderson et al.] In summary, the PSC theory explains the following observations: 1. The ozone hole occupies the region of the polar vortex where temperatures are below -80 C and where polar stratospheric clouds are abundant. 2. The ozone hole is confined to the lower stratosphere. 3. The ozone hole appears when sunlight illuminates the vortex, and disappears soon after temperatures rise past -80 C, destroying PSC's. 4. The hole is associated with extremely low concentrations of NOx. 5. The hole is associated with very low concentrations of the chlorine "reservoirs", HCl and ClONO2, and very high concentrations of active chlorine compounds, ClO, and byproducts such as OClO. 6. Inside the hole, the concentrations of ClO and ozone are precisely anticorrelated, high ClO being accompanied by low ozone. 7. Laboratory experiments demonstrate that chlorine reservoir compounds do react to give active chlorine on the surfaces of ice particles. 8. Airborne measurements in the arctic stratosphere show that air which has passed through regions containing PSC's is low in reservoir chlorine and high in active chlorine. The antarctic ozone hole, once a complete mystery, is now one of the best understood aspects of the entire subject; it is much better understood than the small but steadily growing ozone depletion at mid latitudes, for example. 8. Will the ozone hole keep growing? To answer this, we need to consider separately the lateral dimensions (the "area" of the hole), the vertical dimension (its "depth") and the temporal dimension (how long the hole lasts.) a.) Lateral Extent Let us define the "hole" to be the region where the total ozone column is less than 200 DU, i.e. where total ozone has fallen to less than 2/3 of normal springtime antarctic values. Defined thus, the hole is always confined to the south polar vortex, south of ~55 degrees. At present it does not fill the whole vortex, only the central core where stratospheric temperatures are less than ~-80 C. Typically this region is south of ~65 degrees, although there is a great deal of variation - in some years the center of the vortex is displaced well away from the pole, and the nominal boundary of the hole has on a few occasions passed over the tip of Chile. As stratospheric chlorine continues to rise, the hole might "fill out" the vortex; this could as much as double its area. [Schoeberl and Hartmann]. So far this does not seem to be happening. The 1992 hole was 15-25% larger than previous years, and the 1993 hole appears to be almost as large. This increase is probably due to the stratospheric sulfate aerosols from the July 1991 eruption of Mt. Pinatubo, which behave in some respects like polar stratospheric clouds. [Solomon et al. 1993] These aerosols settle out of the stratosphere after 2-3 years, so the increases seen in 1992 are expected to be temporary. In any case, it cannot grow beyond ~55 degrees without a major change in the antarctic wind patterns that would allow the vortex to grow. Such a change could conceivably accompany global warming: the greenhouse effect warms the earth's surface, but _cools_ the stratosphere. There is no reason to expect the hole to expand out over Australia, S. Africa, etc., although these regions could experience further ozone depletion after the hole breaks up and the ozone-poor air drifts north. b. Vertical Depth The hole is confined to the lower stratosphere, where the clouds are abundant. In this region the ozone is essentially gone. The upper stratosphere is much less affected, however, so that overall column depletion comes to ~50%. As stratospheric chlorine concentrations continue to increase over the next 10 years or so, some penetration to higher altitudes may take place, but large increases in depth are not expected. (Once again, aerosols from Mt. Pinatubo have allowed the 1992 and 1993 holes to extend over a larger altitude range than usual, both higher and lower, but this is probably a temporary effect.) c. Duration of the hole Here we might see major effects. The hole is destroyed in late d warm, ozone-rich air rushes in. If the stratosphere cools, the vortex becomes more stable and lasts longer. As mentioned above, the greenhouse effect actually cools the stratosphere. There is a more direct cooling mechanism, however - remember that the absorption of solar UV by ozone is the major source of heat in the stratosphere, and is the reason that the temperature of the stratosphere increases with altitude. Depletion of the ozone layer therefore cools the stratosphere, and in this sense the hole is self-stabilizing. In future years we might see more long-lived holes like that in 1990, which survived into early December. (The relationship between ozone depletion and climate change is complicated, and best dealt with in a separate FAQ, preferably written by someone other than myself :-) ) 9. Why be concerned about an ozone hole over antarctica? Nobody lives down there. First of all, even though the ozone hole is confined to the antarctic, its effects are not. After the hole breaks up in the spring, ozone-poor air drifts north and mixes with the air there, resulting in a transient decrease at middle and high latitudes. This has been seen as far north as Australia [WMO 1991][Roy et al.] [Atkinson et al.] On a time scale of months short-wave UV regenerates the ozone, but it is believed that this "dilution" may be a major cause of the much smaller _global_ ozone depletion, ~3% per decade, that has been observed. Moreover, the air from the ozone hole is also rich in ClO and can destroy more ozone as it mixes with ozone-rich air. Even during the spring, the air in the vortex is not _completely_ isolated, although there is some controversy over the extent to which the ozone hole acts as a "chemical processor" for the earth's atmosphere. ([Tuck 1989] [Schoeberl and Hartmann] [AASE] [Randel] [Waugh].) From a broader standpoint, the ozone hole is a distant early warning message. Because of its unusual meteorological properties the antarctic stratosphere is especially sensitive to chemical perturbations; the natural mechanisms by which chlorine is sequestered in reservoirs fail when total stratospheric chlorine reaches about 2 parts per billion. This suggests that allowing CFC emissions to increase by 3% per year, as was occurring during the 1980's, is unwise, to say the least. The emission reduction schedules negotiated under the Montreal Protocol (as revised in 1990 and 1992) lead to a projected maximum of ~4 ppb total strat. chlorine in the first decade of the 21st century, followed by a gradual decrease. Letting emissions increase at 3%/year would have led to >16 ppb total stratospheric chlorine by 2040, and even a freeze at 1980 rates would have led to >10 ppb. [Prather et al.]. 10. Is there an ozone hole in the arctic? if not, why not? There is no _massive_ ozone loss in the arctic, although there _is_ unusually large springtime ozone depletion, so the word "hole" is not appropriate. I like the expression "arctic ozone dimple" but this is not canonical :-). The arctic polar vortex is much weaker than the antarctic, arctic temperatures are several degrees higher, and polar stratospheric clouds are much less common (and they tend to break up earlier in the spring.) Thus even though wintertime ClO gets very high, as high as antarctic ClO in 1991-2, it does not remain high through the spring, when it counts. [AASE] (Recent UARS measurements, however, indicate that in 1993 arctic stratosphere temperatures stayed low enough to retain PSC's until late February, and ClO remained high into March. Large ozone depletions, ~10-20%, have now (spring 1993) been reported for high latitudes in the Northern Hemisphere; these still do not qualify as an "ozone hole" but they do seem to indicate that the same physics and chemistry are operating, albeit with much less efficiency. [Waters et al.] [Gleason et al.]) If "global warming" does indeed take place during the first few decades of the next century, we may see a dramatic change in arctic ozone. The greenhouse effect warms the surface of the earth, but at the same time _cools_ the stratosphere. Since there is much less air in the stratosphere, 2-3 degrees of surface warming corresponds to a much larger decrease in stratospheric temperatures, as much as 10 degrees. This could lead to a true ozone hole in the arctic, although it would still probably be smaller and weaker than the antarctic hole. [Austin et al.] The 27 August issue of _Science_ magazine contains 8 papers devoted to arctic ozone depletion in the winter of 1991-92. [AASE] 11. Can the hole be "plugged"? The present ozone hole, while serious, is not in itself catastrophic. UV radiation is always low in polar regions since the sun takes a long path through the atmosphere and hence through the ozone layer. There may be serious consequences for marine life in the antarctic ocean, which is adapted to the normally low UV levels. When the hole breaks up in summer, there may be temporary increases in UV-b at high latitudes of the southern hemisphere as air that is poor in ozone and rich in "active", ozone-destroying forms of chlorine mixes with the air outside. Nevertheless it looks like we are stuck with the hole for the next 50 years at least, and we don't know what new surprises the atmosphere has in store for us. Thus, some atmospheric scientists have been exploring the possibility of "fixing" the hole by technological means. All such schemes proposed so far are highly controversial, and there are no plans to carry any of them out until the chemistry and dynamics of the stratosphere are much better understood than they are at present. It should be made clear at the beginning that there is no point in trying to replace the ozone directly. The amounts are far too large to be transported to the stratosphere, and the antarctic mechanisms are so fiendishly efficient that they will easily destroy added ozone (recall that where the catalytic cycles operate, ~95% of the ozone is gone, in spite of the fact that the sun is generating it all the time.) It is far better to try to remove the halogen catalysts. One suggestion made a few years ago was to release sodium metal into the stratosphere, in hopes that it would form sodium chloride crystals which would settle out. The problem is that the microcrystals remain suspended as long as they are small, and can play the same role as clouds and aerosols in converting reservoir chlorine to active chlorine. A second suggestion is to destroy the CFC's while they are still in the troposphere, by photolyzing them with high-powered infrared lasers installed on mountainsides. (CFC's and similar molecules can absorb as many as 30 infrared photons from a single laser pulse, a phenomonon known as infrared multiphoton dissociation). The chlorine atoms released would quickly be converted to HCl and rained out. The power requirements of such a project are daunting, however, and it appears that much of the laser radiation would be shifted out of the desired frequency range by stimulated raman scattering. [Stix] A more serious possibility is being explored by one of the discoverers of chlorine-catalyzed ozone depletion, Ralph Cicerone, together with Scott Elliot and Richard Turco [Cicerone et al. 1991,1992]. They considered the effects of dumping ~50,000 tons of ethane or propane, several hundred planeloads, into the antarctic stratosphere every spring. The hydrocarbons would react rapidly with the Cl-containing radicals to give back the reservoir HCl. The hydrocarbons themselves are fairly reactive and would decompose by the end of a year, so the treatment would have to be repeated annually. The chlorine would not actually be removed from the stratosphere, but it would be bound up in an inert form - in other words, the catalyst would be "poisoned". There are no plans to carry this or any other scheme out in the near future; to quote from Cicerone et al. (1991), "Before any actual injection experiment is undertaken there are many scientific, technical, legal and ethical questions to be faced, not the least of which is the issue of unintended side effects." REFERII A remark on references: they are neither representative nor comprehensive. There are _hundreds_ of people working on these problems. For the most part I have limited myself to papers that are (1) widely available (if possible, _Science_ or _Nature_ rather than archival sources such as _J. Geophys. Res._) and (2) directly related to the "frequently asked questions". This gives very short shrift to much important work; for example, I say very little about stratospheric NOx, even though a detailed accounting of chemistry and transport of the nitrogen oxides is one of the major goals of current research. Readers who want to see "who did what" should consult the review articles listed below, or, if they can get them, the extensively documented WMO reports. Introductory Reading: [Graedel and Crutzen] T. Graedel and P. Crutzen, _Atmospheric Change: an Earth System Perspective_, Freeman, 1993. [Hamill and Toon] P. Hamill and O. Toon, "Polar stratospheric clouds and the ozone hole", _Physics Today_ December 1991. [Stolarski] Richard Stolarski, "The Antarctic Ozone Hole", _Sci. American_ 1 Jan. 1988. (this article is now seriously out of date, but it is still a good place to start). [Toon and Turco] O. Toon and R. Turco, "Polar Stratospheric Clouds and Ozone Depletion", _Sci. Am._ June 1991 [Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises Challenge Atmospheric Scientists", _Chemical and Engineering News_, 24 May 1993, pp. 9-18. ----------------------------------------- Books and Review Articles: [Anderson, Toohey and Brune] J.G. Anderson, D. W. Toohey, and W. H. Brune, "Free Radicals within the Antarctic vortex: the role of CFC's in Antarctic Ozone Loss", _Science_ _251_, 39 (4 Jan. 1991). [McElroy and Salawich] M. McElroy and R. Salawich, "Changing Composition of the Global Stratosphere", _Science_ _243, 763, 1989. [Solomon] S. Solomon, "Progress towards a quantitative understanding of Antarctic ozone depletion", _Nature_ _347_, 347, 1990. [Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd. Ed., Oxford, 1991, Ch. 4. [WMO 1989] World Meteorological Organization Global Ozone Research and Monitoring Project - Report #20, "Scientific Assessment of Stratospheric Ozone: 1989". [WMO 1991] World Meteorological Organization Global Ozone Research and Monitoring Project - Report #25, "Scientific Assessment of Ozone Depletion: 1991". ------------------------- More Specialized: [AASE] Papers resulting from the Second Airborne Arctic Stratosphere Expedition, published in _Science_ _261_, 1128-1157, 27 Aug. 1993. [Atkinson et al.] R. J. Atkinson, W. A. Matthews, P. A. Newman, and R. A. Plumb, "Evidence of the mid-latitude impact of Antarctic ozone depletion", _Nature_ _340_, 290, 1989. [Austin et al.] J. Austin, N. Butchart, and K. P. Shine, "Possibility of an Arctic ozone hole in a doubled-CO2 climate", _Nature_ _360_, 221, 1992. [Cicerone et al. 1991] R. Cicerone, S. Elliot, and R. Turco, "Reduced Antarctic Ozone Depletions in a Model with Hydrocarbon Injections", _Science_ _254_, 1191, 1991. [Cicerone et al. 1992] R. Cicerone, S. Elliot, and R. Turco, "Global Environmental Engineering", _Nature_ _356_, 472, 1992. [Dobson] G. M. B. Dobson, "Forty Years' research on atmospheric ozone at Oxford", _Applied Optics_, _7_, 387, 1968. [Farman et al.] J. C. Farman, B. G. Gardiner, and J. D. Shanklin, "Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction", _Nature_ _315_, 207, 1985. [Frederick and Alberts] J. Frederick and A. Alberts, "Prolonged enhancement in surface ultraviolet radiation during the Antarctic spring of 1990", _Geophys. Res. Lett._ _18_, 1869, 1991. [Gleason et al.] J. Gleason, P. Bhatia, J. Herman, R. McPeters, P. Newman, R. Stolarski, L. Flynn, G. Labow, D. Larko, C. Seftor, C. Wellemeyer, W. Komhyr, A. Miller, and W. Planet, "Record Low Global Ozone in 1992", _Science_ _260_, 523, 1993. [Molina et al. 1987] M. J. Molina, T.-L. Tso, L. T. Molina, and F.C.-Y. Yang, "Antarctic stratospheric chemistry of chlorine nitrate, hydrogen chloride, and ice: Release of active chlorine", _Science_ _238_, 1253, 1987. [Molina et al. 1990] M. Molina, A. Colussi, L. Molina, R. Schindler, and T.-L. Tso, "Quantum yield of chlorine atom formation in the photodissociation of chlorine peroxide (ClOOCl) at 308 nm", _Chem. Phys. Lett._ _173_, 310, 1990. [Plumb] A. Plumb, "Mixing and Matching", _Nature_ _365_, 489-90, 1993. (News and Views) [Prather et al.] M.J. Prather, M.B. McElroy, and S.C. Wofsy, "Reductions in ozone at high concentrations of stratospheric halogens", _Nature_ _312_, 227, 1984. [Randel] W. Randel, "Ideas flow on Antarctic vortex", _Nature_ _364_, 105, 1993 (News and Views) [Roy et al.] C. Roy, H. Gies, and G. Elliott, "Ozone Depletion", _Nature_ _347_, 235, 1990. (Scientific Correspondence) [Sander et al.] S.P. Sander, R.J. Friedl, and Y.K. Yung, "Role of the ClO dimer in polar stratospheric chemistry: rate of formation and implications for ozone loss", _Science_ _245_, 1095, 1989. [Schoeberl and Hartmann] M. Schoeberl and D. Hartmann, "The dynamics of the stratospheric polar vortex and its relation to springtime ozone depletions", _Science_ _251_, 46, 1991. [Solomon et al. 1993] S. Solomon, R. Sanders, R. Garcia, and J. Keys, "Increased chlorine dioxide over Antarctica caused by volcanic aerosols from Mt. Pinatubo", _Nature_ _363_, 245, 1993. [Stix] T. H. Stix, "Removal of Chlorofluorocarbons from the earth's atmosphere", _J. Appl. Physics_ _60_, 5622, 1989. [Stolarski et al. 1992] R. Stolarski, R. Bojkov, L. Bishop, C. Zerefos, J. Staehelin, and J. Zawodny, "Measured Trends in Stratospheric Ozone", Science _256_, 342 (17 April 1992) [Tolbert et al. 1987] M.A. Tolbert, M.J. Rossi, R. Malhotra, and D.M. Golden, "Reaction of chlorine nitrate with hydrogen chloride and water at Antarctic stratospheric temperatures", _Science_ _238_, 1258, 1987. [Tolbert et al. 1988] M.A. Tolbert, M.J. Rossi, and D.M. Golden, "Antarctic ozone depletion chemistry: reactions of N2O5 with H2O and HCl on ice surfaces", _Science_ _240_, 1018, 1988. [Trolier et al.] M. Trolier, R.L. Mauldin III, and A. Ravishankara, "Rate coefficient for the termolecular channel of the self-reaction of ClO", _J. Phys. Chem._ _94_, 4896, 1990. [Tuck 1989] A. F. Tuck, "Synoptic and Chemical Evolution of the Antarctic Vortex in late winter and early spring, 1987: An ozone processor", J. Geophys. Res. _94_, 11687, 1989. [Tuck et al.] A. F. Tuck, R. T. Watson, E. P. Condon, and J. J. Margitan, "The planning and execution of ER-2 and DC-8 aircraft flights over Antarctica, August and September, 1987" J. Geophys. Res. _94_, 11182, 1989. [Waters et al.] J. Waters, L. Froidevaux, W. Read, G. Manney, L. Elson, D. Flower, R. Jarnot, and R. Harwood, "Stratospheric ClO and ozone from the Microwave Limb Sounder on the Upper Atmosphere Research Satellite", _Nature_ _362_, 597, 1993. [Waugh] D. W. Waugh, "Subtropical stratospheric mixing linked to disturbances in the polar vortices", _Nature_ _365_, 535, 1993.