Chlorofluorocarbons and Stratospheric Ozone Scott Elliott and F. S. Rowland University of California, I ~ i n eCA . 92717 Strong circumstantial cvidence suggests that the so-called "ozone hole" discovered recently over the continent of Antarctica is caused by photodecomposition products of the synthetic chlorofluorocarbons (CFC's) ( 1 4 ) . This has renewed the high levels of ouhlic concern orieinallv eenerated in the mid-1970's when ihe CFC's were first ide%fied as a potential threat to stratos~heric ozone. In the present article b e review the physical atmospheric chemistri behind CFCozone relationships, first from a historical perspective and then in light of the new developments in Antarctica. Hopefully, our discussion will he useful to nonatmospheric scientists in evaluating the issues involved.
' Ozone Studles before Chlorlne The existence of ozone in the atmosphere is a natural consequence of the interaction of molecular oxygen and light. The first mechanistic explanation of this was given by Chapman in the 1930's (5).
The usual fate of oxygen atoms is termolecular recombination with 0 2 to form ozone. The most likely fate of ozone, in turn, is photolysis.
Occasionally, the two non-O2 species meet and terminate each other, reforming molecular oxygen,
A detailed photochemical equilibrium solution of reactions 1-4 reveals several very useful approximations. Atomic oxygen and ozone are closely coupled through much of the stratosphere in a rapid cycle composed of reactions 2 and 3, the ratio 0310 given by k202MIJ3, where J is the photolysis rate in s-'. Through most of the stratosphere, then, it is convenient to consider the sum 0 3 + 0 as a species in itself, the non-On oxygen compounds, usually called simply "odd oxygen". Ozone concentrations in this region are many orders of magnitude greater than those of atomic oxygen, so that, for most practical purposes, ozone and odd oxygen are identical. The Chapman system illustrates nicely the concept of the ozone "laver" (5). Production of odd oxveen in reaction 1 reaches adistinct maximum in the stratosghere because the photolysis rate J I and molecular oxygen concentration O2 behave very differently as functions of altitude. The earth's. atmosphere is necessarilv a pressure-varving medium. hecause;raviry must he balanced hy a press&e:&adient force. T h r total atmosphrric omrentration n falls exponentially Editor's NOIS This paper and lhe one following were part of the State-01-the-Art Symposium an Issues in Air Quality sponsored by the Division of Chemical Educationand held at the National ACS Mettting at Anaheim, September 1986.
with a scale height H of around 7 km, which means that n = noe-'n, where z is altitude. Over the range with which we are concerned. wind-driven mixine dominates molecular diffusion. The ~ o l e c u l a oxygen r concentration is 20% of n a t all ooints and falls exnonentiallv as well. On the other hand. the flux uf radiation hiving J i'attenuated in passing through theatmosphere and so rises with altitude.The oroduct J1.O7 peaks a t around 30 km. The ozone layer actualiy lies a t lower altitudes because stratospheric circulation moves air downward from the zone of m ~ x i m u mproduction, compressing i t and raising the absolute concentration of its constituents. Ozone is the most intensely studied compound in the atmosphere for many reasons. Perhaps most importantly, it acts as a filter for biologically harmful wavelengths of radiation that would otherwise reach the earth's surface. This can be readilv.aooreciated hv aoolvine .. .. Beer's Law to reactions 1 and 3. Several key hiomilecules, including DNA, absorb and are damaged hv light beeinnine a t around 300 nm and extending t ~ w a r d t h ~ u l t r a ~ i o~l eeta.c t i o u1provides an effective shield at wavelengths less than 250 nm. Beer's Law absorbance is traditionally given by anl, where a is the ahsorption cross section, n is concentration in units of molecules cm-3, and 1 is path length. In a pressure-varying medium, nl is more appropriately replaced by an integrated column density, or, with respect to oxygen, 0.2 Hnoe-z/~,where no is the total surface pressure of 1 atm (2.5 X 1019molecules rm-'I. A convenient reterence altitude for judging thedepth of penetration of light incident from the sun is the reciprocal c heiehr. " . at which the initial tlux has been attenuated bve-I. Absorbance also equals In 1011,where I's are intensities or radiation fluxes. For our purposes e-' attenuation is achieved where absorbance equals 1, or a-I = 0.2Hnoe-z'7. At less than 250 nm the molecular oxvgen a is consistentlv greater than around cm2molec;kl (6), and e-I filtriis tion is achieved ahove 25 km. However, beyond 250 nm, 0% transparent, and it is left to ozone to remove photons that would otherwise damage DNA in biological tissues. Typical column densities for ozone fall around 5 X 10lS molecules lying mostly in the ozone layer and ahove, or, at greater than around 20 km (7).Between 250 and 300 nm, the 0 3 r hovers near 10-l7 cm2 molecule-' (61, so that e-' filtration occurs well ahove the ozone layer. From 300 nm into the visible, the atmosphere is transparent, hut key hiomolecules are not sensitive to these waveleneths. Accordine to current theories of evolution, life couldnot have leftuthe sea to colonize land without some sort of shielding in the 250-300nm portion of the spectrum. We owe ou; very existence, then, to the ozone layer. The consequences of losing it now that we have arrived are not clear cut, hut there is general agreement that they could he catastrophic (5). In the 1950's and 1960's it was realized that the Chapman system overestimated stratospheric odd oxygen levels, and chemical removal mechanisms additional to reaction 4 are required (5).Stable atmospheric species react with ozone too slowly to solve this puzzle, leaving radicals as the likely suspects. Atmospheric radicals, however, are not abundant enough to influence ozone on a one-for-one basis, and it became clear that they must be involved as catalysts, or, in other words, that they must he regenerated as ozone is removed. The odd hydrogen species (HO,) were the first to he
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the human environment. in case. for examnle. of leaks from equipment into the home. when productscontaining them are discarded. CFC's 11 and 12 are eventnallv released as vapor-phase waste. The first workers to measuie them in air found that this stability carried over into the atmosphere. By best estimates of integrated production to that time, it appeared that every molecule of CFC's 11and 12 ever made had simply accumulated within the troposphere (6). Initially this seemed to he just an interesting footnote, hut were soon asking themselves aqnestion that several apparently had never been adequately answered: What does eventually happen to the CFC's (5)? Certainly they could not build up unchecked forever. But their inertness did render them insensitive to traditional tropospheric sinks such as rainout and oxidation, and it became clear that ultimately they would simply rise through the troposphere to the level of the ozone layer and above, where they would finally encounter wavelengths of UV light capable of photodissociating them. Photolysis releases one chlorine atom immediately,
I = CH3C I
0.0 1950
1960
1970
1980
1990
YEAR Figure 1. Contribution of various molecules to rising atmospheric chlorine content.
investigated in this context. Two important members of the family are OH and HOz, and two important interactions with the odd oxygen family are as follows:
The sum of these reactions is the equivalent of reaction 4, with no net consumption of odd hydrogen. Through most of the stratosphere, however, and in particular a t ozone-layer altitudes. this additional odd oxveen termination is still insufficient to bring ozone budgeginto balance. In the late 1960's it was found that NO and NO?. -. members of the odd nitrogen family (NO,), perform the majority of stratospheric ozone removal through the catalytic cycle,
which again sums to reaction 4 and is precisely analogous with the reaction 5 and 6 HO, cycle. The NO, family introduced humankind to its own potential to influence stratospheric ozone. There were plans on various drawing hoards to build entire fleets of supersonic transports, aircraft with cruising altitudes in the lower stratosphere. As is the case for combustion engines a t the earth's surface, their jets would convert molecular nitrogen and oxygen into the nitrogen oxides of reactions 7 and 8 and so pose a threat to the ozone shield. Due in part to this NO, dilemma, SST fleets have never materialized. The Flrsl Chlorine-Ozone Controversy
By the mid-1970's, quite apart from NO, chemistry and the SST, profound changes in the composition of the atmosphere were underway and going largely unnoticed. As shown in Figure 1, this was an era of rapidly rising total chlorine concentrations, largely a result of increased emissions of CFCh and CF?Cl?. also known as fluorocarbons 11 and 12 (6). Tiese syntietl';: compounds were first produced for use as cooline fluids in refrigerators and air conditioners. but they find broader applicahkty today. For example, they now also serve as hlowine aeents in the manufacture of the structural polyurethane foams in furniture cushions and, when retained within the foam, as insulating material. They have long been the gases of choice in these areas because chemically they are peculiarly inert and so are safe for use in 388
Journal of Chemical Education
and the remainder as the radical fragments photo-oxidize (6).Chlorine atoms then interact with odd oxygen.
Reactions 11and 12 are again precisely analogous with reactions 5 and 6, or reactions 7 and 8, and add to the equivalent of reaction 4. Chlorofluorocarbon emissions represent the addition of vet another familv of radicals with ozone-controlling capabilities, the odd chlorine or chlorine oxide family. Here again was potential for anthropogenic influence over the ozone layer. However, odd chlorine was different in two frightening ways. First of all, CFC emissions were already an ongoing reality when the problem was uncovered and were known to he rising rapidly. Secondly, the chlorofluorocarbon-ozone relationship is characterized by a series of insidious time lags, detailed in the next section. The urgency of the CFC threat to the ozone layer prompted a great deal of noblic concern. and in the late 1970's lecrislation was passed in several countries banning one of the more nonessential oro~ellants in aerosol sprav cans. This uses of 11 and 12,. as . . provided a temporary solution. World totai ~ ~ o d u c t i oofn CFC's 11and 12 dropped for several years. In the meantime, however, demand for the other uses described has taken up the slack, and today production is higher than a t peak levels for the 1970's (8). Time Lags In CFC-Ozone Relatlonshlps
Tropospheric stability translates to extremely long atmosnheric lifetimes for CFC's 11 and 12. Assume that the troposphere is instantaneously well mixed (in fact, mixing requires only a year or two). In reality, and also in 2-dimensional models of the stratosphere, the CFC's rise through the tropopause a t low latitudes, with air warmed in the tropics. For purposes of calculating time scales for global removal, it is sometimes more convenient to parameterize this motion by averaging it over the entire globe and assigning it a "diffusion coefficient" that has been determined using tracers such as N20 or CHa. The transport is actually accomplished in the real atmosphere largely by winds and so is much more advective than diffusive in nature. But the nrohlem was originally tackled by atmospheric modellers in one dimension for the sake of mathematical simplicity, and in a single dimension, transport is necessarily diffusive. In one particularlv simnle t r a n s ~ o rmodel. t upward motions in the tropics a n d the corresponding downward motions a t higher iati-
tudes are represented by globally averaged diffusive processes with a vertical constant K = lo4cm2s-', independent of altitude below 50 km (6). According to Ficke's second law, the time scale for vertical movement over a distanced is t = d2/K. For an atmospheric scale height, a convenient and meaningful reference distance, t 2 years (5 X 107 s). The CFC's will not he removed in this diffusive situation until they reach an altitude where photolysis becomes competitive with upward movement. The photolysis rate constant is given exactly by
-
where F is a photon flux. For simplicity, here, we note that CFC's 11and 12 are transparent a t wavelengthsgreater than 300 nm. I n the lower stratosphere, they photolyze mainly in the so-called "window" region centered a t 200 nm, between major 0 2 and 0 3 absorptions (6). T o zeroth order, J(z) = F(z)wAX in this window, if F and u are averaged over the wavelength range 190-210 nm (note, Ah = 20 nm). Table 1 shows Jvalues estimated in this fashion. Fluxes a t z = are adapted from ref 7. The altitudinally dependent F was first calculated for a zenith anele of 45O in order to renresent " roughly a global and seasonal average for the noon maximum. then adiusted downward hv a factor of 113 to account for diurnal variarims in i d n r intensity. W e harr used o = 7 X 1 0 "nnd A X IU-' c m n ~ d e r u l u -for ~ CFCI? and CFICI?, respectively (6). The photodissociation timescale is J - ~ . Photolysis of CFC13becomes competitive with transport hetween 20 and 25 km, and of CF2C12,between 25 and 30 km. Because atmospheric pressure falls exponentially with z, only a small fraction of the CFC molecules are absorbing at any instant, and those with lifetimes of several years, so that overall, atmospheric removal must require many times several years. T o semiquantify this notion, assume that the CFCln ~hotodissociationlifetime is constant from 20 to 25 km and that the molecule is removed completely within this altitude hand. Aoolving .. . - integration of n = noe-~'~,the number of molecules between 20and 25 km is 5-x loi3 ~ m - or ~, 3% of the total column. We might guess that CFC13 resides for around 50-100 years in the atmosphere as a whole (2 years/3%). A similar calculation on CF& gives about 100150 years (2 years/l.5%). The generally accepted lifetimes from more rigorous models are 70 and 120 years. Production of CFC's 11 and 12 has usually risen rapidly as a function of time, except for the few years following the oridnal CFC-ozone controversv in the mid-1970's. The and Corporation projects futuie production increases (8) as shown in Figure 2, essentially tracking increases in demand for CFC-related products, which in turn track population growth. Two solution scenarios that are often discussed regarding the chlorofluorocarbon-ozone dilemma are production caps and production halts. I t might be imagined, for example, that the-two could he combined in sequence-that a worldwide cap could he instituted until chlorine related ozone depletions become apparent, and then a halt could he enacted. In a simple 1-box model of the atmosphere
wherr p i i thv rmissioc ratr. /I the atmospheric burden, and irl1ryl11hallifetimr. A solutinn tu rhedifit.rentinl is H = p r ( 1 - ,,-t 'I. This is. 01' course, a standnrd first-ordrr kinetic expression describing the approach to a steady state. For convenience, we now make the approximation that emissions begin in the year in which they are capped. This can he justified to some extent by rapid growth in production over the last few decades. In the simple first-order system, steady state is not actually reached for several lifetimes, or, in the cases of CFC's 11 and 12, not for 100-200 years. When
YEAR Figure 2. Past and projected future production levels for CFCI.
and CF2CI2.
Table 1. Photolysls Rate Calculatlons CFCla
CF2C12
production is capped, then, the worst effects are always several human lifetimes and manv human eenerations awav. If the results at or on the way toward stead; state turn out he unacceptable and a decision is made to bar production at that the kinetics of first-order decay tell us that full recovery is, likewise, several hundred years away. Burden at steady state is given by B = pr. For acap a t 1985 production levels, as taken from Figure 2, pr = 25 X 1012g for CFCh, and 60 X 1012g for CF&. Current concentrations of CFC decomposition products in the stratosphere reflect tropospheric concentrations from about 1980; it will he remembered that in the one-dimensional sense, transport into the stratos~hererequires about 2 vears for one scale height, 10 years'for 2H. two-dimensi&al models, advection times from low to high latitude are comparable. Measured concentrations of C ~ C ' S11 and 12 in the 1980 troposphere were around 200 and 300 pptv (parts per trillion by volume), respectively. With a total of 1.1 X 10" molecules in the atmosphere (Hnoe-'IH X A, where A is the earth's surface area), these values translate to 5 X 10'2 and 7 X 10'2 g. Even if production levels are capped immediately, CFC contrihutions to the atmosnheric chlorine content will eventuallv stand to rise by a factor of 5 to 10. The Rand projections Figure 2 roughly double by the year 2000. If production were not capped until the turn of the millenium, the chlorine content of the atmosphere could rise hv a factor of 10 to 20. Note that even if production were to he-halted completely in the near future, several years elapse before maximum chlorine reaches the stratosphere and, even after stratospheric concentrations have peaked, return to normalcy requires many decades. These calculations also underscore the reasons for focussing on CFC's 11 and 12 initially from among the suite of chlorinated compounds in Figure 1. Other contributors to chlorine increases are either shorter lived (CHzCC13) so that they are easier to control, or have been in production longer so that they are closer to steady state, or both (CCl4).
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Ozone Depletion Estimates A simple perturbation technique can he used to convey semiquantitatively the potential for chlorine-catalyzed ozone depletions. Although in reality the NO, and C10, svstems are closelv couded. imaeine a hvvothetical stratosphere in which chemikry is unaffeited by changes in total chlorine. Also assume for simolicitv . " that the NO. and C10, ozone depleting cycles given ahove are the only oddoxygen-removing processes. Their rate-determining steps are reactions 8 and 12, respectively. Oxygen atoms are scarce relative to ozone, and the NO2 and CIO produced by 0 abstraction tend to he consumed along other pathways. The rate of odd oxygen removal in this situation is 2kaNOn0 2k12C10 0. The rate constants ks and klz are well established and to zeroth order, temperature independent, with values of 9.3 X 10-l2 and 3.7 X cm3 molecule-' s-'. The ratio of rates for reactions 12 and 8 is then -4 C10/N02, so that removal can he re-expressed as 2kaNO?O(1 4ClO/NO?). Where odd oxygen is determined b; a dhotochemical eqGlihrium, and with J 1 . 0 7 oroduction held constant. ozone might he expected totrack the ratio of removal rates as C10 concentrations change, or
NO,
+
+
O,(O/O,(i)
= (I
+ 4ClO(i)/NOJ/(l + 4CIO(O/NO2)
(14)
where f and i signify final and initial states. There are several wavs of estimatine current NO* and the CIO levels. As of a deiade ago, concekrations hadto he modelled, with NOz as one of several ~hotodecomoositionoroducts from N90. another example-being nit& acid and ClO along witi other CFC decomposition products such as HCI and CIONOz. At lower altitudes in particular, HN03, HC1, and CIONOz act as reservoirs, withholding NO, and CIO, from their catalvtic rhams. In some nrcas, phutorhem~ralequilibrium p r w ~ d e as rcntyh appruximat~mto the HKOiINO, and HCIICIO or CIONO. CIO relationshios. Toduv. ., howrver. measurements are available, and for our purposes here it is advantageous to avoid photochemical modelling. The values in Table 2 are adapted from data in ref 7, and are meant to represent midlatitude or, rounhlv eauivalentlv. - . elohal averaees. Now suopose the st'ratos~hkric~chloriue content rises h i a factor of's, to steady state with production capped a t 1985 levels, and that the relationship between C10 and total chlorine also remains unaffected. Ratios of rates for reactions 12 and 8 are given in Table 2 for the present and at steady state (indicated P (1985) 7 ) . Using eq 14, photochemical equilibrium ozone depletions are 7, 23, and 53% at 20, 30, and 40 km, respectively. The potential for mischief is greater in the upper stratosphere because the CIO/N02 ratio is larger there. Odd oxveen is in fact in ~hotochemicaleouilihrium at 30 and 40 k i t h r o u g h much bf the current st;atosphere, and our estimates of localized Oa deoletion a t these altitudes are nimilsr ro [hose made through siphkticared computer modcll~nr.At tropical and middle lut~tudrs,lower stratusoheric air hBs risenfrom the troposphere with little ozon; Odd oxygen lifetimes are longer than the residence lifetime of air in this area, and, furthermore, the CFC's have not fully photolyzed, so that lower stratospheric ozone levels are more a reflection of integrated production than of removal rates. This complicates the calculation of depletions in column abundance. hut roneh estimates are still ~ossihle.A tvoical mid-latitude verticz ozone profile might show conceutramolecules throughout the troposphere, 4 X tions of loL2 loL2between 15 and 25 km, and roughly constant mole fraction ahove 25 km, or n = 4 X loLZe-(z-2s)'H. If changes of -25% are applied a t 25-35 km, and -50% a t all altitudes ahove 35, the total column deoeletion is on the order of 10%. Sophisticated models predict that large depletions at high altitudes should increase the ultraviolet flux in the lower stratosphere, u,hich in turn enhances ndd oxygen prnduction rhnmph reaction 1 and nctually leads to a few percent in-
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Table 2. NO, and CIO Concentrations (as Mole Fraction) Z (km) 20 30 40
NO2
lo-' 5X 5X
lo-* lo-'
current CIO 5X 1OPo 5X
current 4CIO(i)/N02
P(1985)r CIO
P(1985)r 4C10(f)/N02
0.02 0.08 0.4
25 X 10V2 5X lOP0 25 X
0.1 0.4 7
crease in ozone in this region, moderating the 10% figure somewhat. This has come to he known as the "chemical selfhealing" effect (7). It should he clear that, from the point of view of the atmosphere, production of CFC's 11 and 12 cannot be allowed to rise, and further, that production caps are not sufficient or acceptable solutions to the CFC dilemma, no matter how soon they can be instituted. An immediate and complete halt is the safest answer, and the more closely this ideal can he approached, the better. This would he the case even if there were no such thing as an "ozone hole". The Ozone Hole The chemistry outlined in the previous section represents the consensus among atmospheric scientists over the period 1975-1985. If in fact chlorine were ever to cause ozone losses, a gradual erosion of the entire global ozone layer was expected. Predictions of average column deoletion at steadv state ranged from a few percent to 20%, hut generally fell between 5 and 10%.There are well-known natural ozone variations of similar magnitude that have time scales of days, seasons, and many years. This meant that chlorine-catalyzed change would not become measurable until the atmosphere was well advanced toward steady state, or, not for a t least several decades. Gradual erosion may still lie ahead, hut for the mome'nt, the attention of the atmospheric community is focussed on a distinctly different and completely unanticipated phenomenon. A huge. localized rio in the ozone laver is onenine over ~ntarctica(1-4).~ e ~ i n iini about n ~ 1978, ozon; has &appeared raoidlv . .from the lower stratosohere everv.sorine. . ". in a column of air covering roughly the entire continent. This socalled "ozone hole" has deeoened svstematicallv and should reach 50% depletion in totai column abundance during austral spring in the next few years (see the cover of this issue). Such loss levels are completely unprecedented in the history of ozone observations. Several theories have already attempted to describe the ozone hole as part of natural atmospheric cycles, hut they fail to explain the timing of its onset. I t has been proposed, for example, that the hole is linked to the recent maximum in solar activity. However, such maxima return every 11 years, and there were no Antarctic ozone losses in the 1960's or 1970's. Referring aeain to Fieure 1. note that ozone hole development closei\, &inrides with the rise in atmospheric chlorine runtent induced hv CFC em~ssinn.;.Because chlw rine is a potential ozone-removing agent, this is currently taken as strong circumstantial evidence that the CFC's and the hole are connected, or, in other words, that chlorine causes the hole. The Antarctic ozone losses may he seen as an indication that the potential for chlorine-catalyzed ozone depletion has been greatly underestimated. Anv mechanism relatine" chlorine and the ozone hole must he unique to the Antarctic. No corresponding losses occur in the northern hemisphere. Over Antarctica is the only place in the stratosphere where temperatures drop low enough for clouds to form. Meteorological conditions prevent this over the Arctic. Clouds provide surfaces and a condensed phase, both of which are traditionally neglected in stratospheric chemistry because they are relatively poorly understood. This could be the reason the localized Antarctic losses came as such a surprise. In all the chlorine-related mechanisms so far advanced, the presence of clouds has been invoked to
destabilize the reservoir molecules HCI and CION02in some way, shifting the distribution of total chlorine toward catalytically active C10. High C10 concentrations, then, would constitute additional evidence that chlorine is actually the culprit. A team of brave American scientists spent austral spring of 1986 a t McMurdo Sound, 78' S latitude, attempting to measure 'I0 and several other key species. They he returning again this year. The chlorine mechanisms so far differ greatly in the de. tails of reservoir destabilization. Until the matter is resolved, we are left with a rather disturbing question. There are cloudlike aerosol particles throughout the stratosphere, Will ozone hole-related chemistry lead to acceleration or
enhancement of depletions elsewhere? Or, to put this less accurately hut more compactly, will the ozone hole spread? Unfortunately, the possibility exists that other accelerated depletions may also he upon us long before we are able to foresee them. Literature 1 . Farrnan,J. C.:Gsrdiner,B. G.:Shanklin,J.DNolure 1985,316,207. 2. s ~ I ~ ~ ~ ~ R. ~.:~ow~and. . s . : GF.~s.;w ~ ~~ ~ b . ~b ~~o ~t u,1986,321. r~e . 755. 3. McRlroy. M. R.;Salewiuh.R.J.;Wof8y.S. C.:Logan. J.A.Noture 1986.321.759. 4. crutzen. P. C.; Arnold, F.,submitted far publication in Nalure.
,,Wnvne.R.~.~hemis~ryo~~fmospher~a;~~s.endon:oxfor~.~~s~. owla and.^. s.: ~ o t i n a , ~ . ~ C . IOP~YS.SP~~P~Y~.
6. R~U. 1975,13(1).I. 7. Rwssew',G.: Solomon, Aeronomy olthe Middle Afmospham; Reidel: Boaton. 1984. 8. Mot,., W. E.: Wnll. K. A ; Camm, Potential Constminis on Curnvlafivr Global ~ ~ ~of ~ ~ hd ~~ ~ ~Rand: t ~&"a i ~~ o n i~el sCA, , 1986. ~ ~ ~ ~
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