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Xref: info.physics.utoronto.ca news.answers:30603 sci.answers:1672 sci.environment:48626 Path: spot.Colorado.EDU!rparson From: rparson@spot.colorado.edu (Robert Parson) Newsgroups: sci.environment,sci.answers,news.answers Subject: Ozone Depletion FAQ Part I: Introduction to the Ozone Layer Followup-To: sci.environment Date: 10 Oct 1994 03:15:06 GMT Organization: University of Colorado, Boulder Lines: 1196 Approved: news-answers-request@MIT.Edu Message-ID: <37abjq$gec@CUBoulder.Colorado.EDU> Reply-To: rparson@spot.colorado.edu NNTP-Posting-Host: spot.colorado.edu Summary: This is the first of four files dealing with stratospheric ozone depletion. It provides scientific background for the more detailed questions in the other three parts. Keywords: ozone layer cfc stratosphere depletion Originator: rparson@spot.Colorado.EDU Archive-name: ozone-depletion/intro Last-modified: 9 October 1994 Version: 4.7 ------------------------------ Subject: How to get this FAQ These files are posted monthly, usually in the third week of the month. The current versions are also stored on several archives: A. World-Wide Web A hypertext version is located at: http://www.cis.ohio-state.edu/hypertext/faq/usenet/ozone-depletion/top.html Plaintext versions can be found at: ftp://rtfm.mit.edu/pub/usenet/news.answers/ozone-depletion/ ftp://ftp.uu.net/usenet/news.answers/ozone-depletion/ ---- B. Anonynmous ftp To rtfm.mit.edu, in the directory /pub/usenet/news.answers/ozone-depletion To ftp.uu.net, in the directory /usenet/news.answers/ozone-depletion Look for the four files named intro, stratcl, antarctic, and uv. ---- C. Regular email Send the following messages 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". ----------------------------- Subject: Copyright Statement *********************************************************************** * 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 * * ask you to keep your archive up to date; in the case of world-wide * * web pages, this is most easily done by linking to the master at the * * ohio-state http URL instead of storing local copies. Finally, I * * 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. * *********************************************************************** ----------------------------- Subject: General remarks This is the first of four FAQ files dealing with stratospheric ozone depletion. This part deals with basic scientific questions about the ozone layer, and serves as an introduction to the remaining parts which are more specialized. Part II deals with sources of stratospheric chlorine and bromine, part III with the Antarctic Ozone Hole, and Part IV with the properties and effects of ultraviolet radiation. The later parts are mostly independent of each other, but they all refer back. to Part I. I emphasize physical and chemical mechanisms rather than biological effects, although I make a few remarks about the latter in part IV. I have little to say about legal and policy issues other than a very brief summary at the end of part I. The overall approach I take is conservative. I concentrate on what is known and on most probable, rather than worst-case, scenarios. For example, I have relatively little to say about the effects of UV radiation on terrestrial plants - this does not mean that the effects are small, it means that they are as yet not well quantified (and moreover, I am not well qualified to interpret the literature.) Policy decisions must take into account not only the most probable scenario, but also a range of less probable ones. There have been surprises, mostly unpleasant, in this field in the past, and there are sure to be more in the future. ----------------------------- Subject: Caveats, Disclaimers, and Contact Information | _Caveat_: I am not a specialist. In fact, I am not an atmospheric | chemist at all - I am a physical chemist studying gas-phase | reactions who talks to atmospheric chemists. These files are an | outgrowth of my own efforts to educate myself about this subject | I have discussed some of these issues with specialists but I am | solely responsible for everything written here, including all errors. | On the other hand, if you find this document in an online archive | somewhere, I am not responsible for any *other* information that | may happen to reside in that archive. 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@spot.colorado.edu Robert.Parson@colorado.edu ----------------------------- Subject: TABLE OF CONTENTS How to get this FAQ Copyright Statement General remarks Caveats, Disclaimers, and Contact Information TABLE OF CONTENTS 1. THE STRATOSPHERE 1.1) What is the stratosphere? 1.2) How is the composition of air described? 1.3) How does the composition of the atmosphere change with 2. THE OZONE LAYER 2.1) How is ozone created? 2.2) How much ozone is in the layer, and what is a 2.3) How is ozone distributed in the stratosphere? 2.4) How does the ozone layer work? 2.5) What sorts of natural variations does the ozone layer show? 2.5.a) Regional and Seasonal Variation 2.5.b) Year-to-year variations. 2.6) What are CFC's? 2.7) How do CFC's destroy ozone? 2.8) What is an "Ozone Depletion Potential?" 2.9) What about HCFC's and HFC's? Do they destroy ozone? 2.10) *IS* the ozone layer getting thinner? 2.11) Is the middle-latitude ozone loss due to CFC emissions? 2.12) If the ozone is lost, won't the UV light just penetrate 2.13) Do Space Shuttle launches damage the ozone layer? 2.14) Will commercial supersonic aircraft damage the ozone layer? 2.15) What is being done about ozone depletion? 3. REFERENCES FOR PART I Introductory Reading Books and Review Articles More Specialized References ----------------------------- Subject: 1. THE STRATOSPHERE ----------------------------- Subject: 1.1) What is the stratosphere? The stratosphere extends from about 15 km to 50 km. In the stratosphere temperature _increases_ with altitude, due to the absorption of UV light by oxygen and ozone. This creates a global "inversion layer" which impedes vertical motion into and within the stratosphere - since warmer air lies above colder air, convection is inhibited. The word "stratosphere" is related to the word "stratification" or layering. The stratosphere is often compared to the "troposphere", which is the atmosphere below about 15 km. The boundary - called the "tropopause" - between these regions is quite sharp, but its precise location varies between ~10 and ~17 km, depending upon latitude and season. The prefix "tropo" refers to change: the troposphere is the part of the atmosphere in which weather occurs. This results in relatively rapid mixing of tropospheric air. [Wayne] [Wallace and Hobbs] Above the stratosphere lie the "mesosphere", ranging from ~50 to ~100 km, in which temperature decreases with altitude; the "thermosphere", ~100-400 km, in which temperature increases with altitude again, and the "exosphere", beyond ~400 km, which fades into the background of interplanetary space. In the upper mesosphere and thermosphere electrons and ions are abundant, so these regions are also referred to as the "ionosphere". In technical literature the term "lower atmosphere" is synonymous with the troposphere, "middle atmosphere" refers to the stratosphere and mesosphere, while "upper atmosphere" is usually reserved for the thermosphere and exosphere. This usage is not universal, however, and one occasionally sees the term "upper atmosphere" used to describe everything above the troposphere (for example, in NASA's Upper Atmosphere Research Satellite, UARS.) ----------------------------- Subject: 1.2) How is the composition of air described? (Or, what is a 'mixing ratio'?) The density of the air in the atmosphere depends upon altitude, and in a complicated way because the temperature also varies with altitude. It is therefore awkward to report concentrations of atmospheric species in units like g/cc or molecules/cc. Instead, it is convenient to report the "mole fraction", the relative number of molecules of a given type in an air sample. Atmospheric scientists usually call a mole fraction a "mixing ratio". Typical units for mixing ratios are parts-per-million, billion, or trillion by volume, designated as "ppmv", "ppbv", and "pptv" respectively. (The expression "by volume" reflects Avogadro's Law - for an ideal gas mixture, equal volumes contain equal numbers of molecules - and serves to distinguish mixing ratios from "mass fractions" which are given as parts-per-million by weight.) Thus when it is said that the mixing ratio of hydrogen chloride at 3 km is 0.1 ppbv, it means that 1 out of every 10 billion molecules in an air sample collected at that altitude will be an HCl molecule. [Wayne] [Graedel and Crutzen] ----------------------------- Subject: 1.3) How does the composition of the atmosphere change with altitude? (Or, how can CFC's get up to the stratosphere when they are heavier than air?) In the earth's troposphere and stratosphere, most _stable_ chemical species are "well-mixed" - their mixing ratios are independent of altitude. If a species' mixing ratio changes with altitude, some kind of physical or chemical transformation is taking place. That last statement may seem surprising - one might expect the heavier molecules to dominate at lower altitudes. The mixing ratio of Krypton (mass 84), then, would decrease with altitude, while that of Helium (mass 4) would increase. In reality, however, molecules do not segregate by weight in the troposphere or stratosphere. The relative proportions of Helium, Nitrogen, and Krypton are unchanged up to about 100 km. Why is this? Vertical transport in the troposphere takes place by convection and turbulent mixing. In the stratosphere and in the mesosphere, it takes place by "eddy diffusion" - the gradual mechanical mixing of gas by motions on small scales. These mechanisms do not distinguish molecular masses. Only at much higher altitudes do mean free paths become so large that _molecular_ diffusion dominates and gravity is able to separate the different species, bringing hydrogen and helium atoms to the top. [Chamberlain and Hunten] [Wayne] [Wallace and Hobbs] Experimental measurements of the fluorocarbon CF4 verify this homogeneous mixing. CF4 has an extremely long lifetime in the stratosphere - probably many thousands of years. The mixing ratio of CF4 in the stratosphere was found to be 0.056-0.060 ppbv from 10-50 km, with no overall trend. [Zander et al. 1992] An important trace gas that is *not* well-mixed is water vapor. The lower troposphere contains a great deal of water - as much as 30,000 ppmv in humid tropical latitudes. High in the troposphere, however, the water condenses and falls to the earth as rain or snow, so that the stratosphere is extremely dry, typical mixing ratios being about 5 ppmv. Indeed, the transport of water vapor from troposphere to stratosphere is even less efficient than this would suggest, since much of the small amount of water in the stratosphere is actually produced _in situ_ by the oxidation of methane. [SAGE II] Sometimes that part of the atmosphere in which the chemical composition of stable species does not change with altitude is called the "homosphere". The homosphere includes the troposphere, stratosphere, and mesosphere. The upper regions of the atmosphere - the "thermosphere" and the "exosphere" - are then referred to as the "heterosphere". [Wayne] [Wallace and Hobbs] ----------------------------- Subject: 2. THE OZONE LAYER ----------------------------- Subject: 2.1) How is ozone created? Ozone is formed naturally in the upper stratosphere by short wavelength ultraviolet radiation. Wavelengths less than ~240 nanometers are absorbed by oxygen molecules (O2), which dissociate to give O atoms. The O atoms combine with other oxygen molecules to make ozone: O2 + hv -> O + O (wavelength < 240 nm) O + O2 -> O3 ----------------------------- Subject: 2.2) How much ozone is in the layer, and what is a "Dobson Unit" ? A Dobson Unit (DU) is a convenient scale for measuring the total amount of ozone occupying a column overhead. If the ozone layer over the US were compressed to 0 degrees Celsius and 1 atmosphere pressure, it would be about 3 mm thick. So, 0.01 mm thickness at 0 C and 1 at is defined to be 1 DU; this makes the ozone layer over the US come out to ~300 DU. In absolute terms, 1 DU is about 2.7 x 10^16 molecules/cm^2. In all, there are about 3 billion metric tons, or 3x10^15 grams, of ozone in the earth's atmosphere; about 90% of this is in the stratosphere. The unit is named after G.M.B. Dobson, who carried out pioneering studies of atmospheric ozone between ~1920-1960. Dobson designed the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity solar UV radiation at four wavelengths, two of which are absorbed by ozone and two of which are not. These instruments are still in use in many places, although they are gradually being replaced by the more elaborate Brewer spectrometers. Today ozone is measured in many ways, from aircraft, balloons, satellites, and space shuttle missions, but the worldwide Dobson network is the only source of long-term data. A station at Arosa in Switzerland has been measuring ozone since the 1920's, and some other stations have records that go back nearly as long (although many were interrupted during World War II). The present worldwide network went into operation in 1956-57. ----------------------------- Subject: 2.3) How is ozone distributed in the stratosphere? In absolute terms: about 10^12 molecules/cm^3 at 15 km, rising to nearly 10^13 at 25 km, then falling to 10^11 at 45 km. In relative terms: ~0.5 parts per million by volume (ppmv) at 15 km, rising to ~8 ppmv at ~35 km, falling to ~3 ppmv at 45 km. Even in the thickest part of the layer, ozone is a trace gas. ----------------------------- Subject: 2.4) How does the ozone layer work? UV light with wavelengths between 240 and 320 nm is absorbed by ozone, which then falls apart to give an O atom and an O2 molecule. The O atom soon encounters another O2 molecule, however (at all times, the concentration of O2 far exceeds that of O3), and recreates O3: O3 + hv -> O2 + O O + O2 -> O3 Thus _ozone absorbs UV radiation without itself being consumed_; the net result is to convert UV light into heat. Indeed, this is what causes the temperature of the stratosphere to increase with altitude, giving rise to the inversion layer that traps molecules in the troposphere. The ozone layer isn't just _in_ the stratosphere; the ozone layer is responsible for the _existence_ of the stratosphere. Ozone _is_ destroyed if an O atom and an O3 molecule meet: O + O3 -> 2 O2 ("recombination"). This reaction is slow, however, and if it were the only mechanism for ozone loss, the ozone layer would be about twice as thick as it is. Certain trace species, such as the oxides of Nitrogen (NO and NO2), Hydrogen (H, OH, and HO2) and chlorine (Cl, ClO and ClO2) can catalyze the recombination. The present ozone layer is a result of a competition between photolysis and recombination; increasing the recombination rate, by increasing the concentration of catalysts, results in a thinner ozone layer. Putting the pieces together, we have the set of reactions proposed in the 1930's by Sidney Chapman: O2 + hv -> O + O (wavelength < 240 nm) : creation of oxygen atoms O + O2 -> O3 : formation of ozone O3 + hv -> O2 + O (wavelength < 320 nm) : absorption of UV by ozone O + O3 -> 2 O2 : recombination . Since the photolysis of O2 requires UV radiation while recombination does not, one might guess that ozone should increase during the day and decrease at night. This has led some people to suggest that the "antarctic ozone hole" is merely a result of the long antarctic winter nights. This inference is incorrect, because the recombination reaction requires oxygen atoms which are also produced by photolysis. Throughout the stratosphere the concentration of O atoms is orders of magnitude smaller than the concentration of O3 molecules, so both the production and the destruction of ozone by the above mechanisms shut down at night. In fact, the thickness of the ozone layer varies very little from day to night, and above 70 km ozone concentrations actually _increase_ at night. (The unusual catalytic cycles that operate in the antarctic ozone hole do not require O atoms; however, they still require light to operate because they also include photolytic steps. See Part III.) ----------------------------- Subject: 2.5) What sorts of natural variations does the ozone layer show? There are substantial variations from place to place, and from season to season. There are smaller variations on time scales of years and more. [Wayne] [Rowland 1991] We discuss these in turn. ----------------------------- Subject: 2.5.a) Regional and Seasonal Variation Since solar radiation makes ozone, one expects to see the thickness of the ozone layer vary during the year. This is so, although the details do not depend simply upon the amount of solar radiation received at a given latitude and season - one must also take atmospheric motions into account. (Remember that both production and destruction of ozone require solar radiation.) The ozone layer is thinnest in the tropics, about 260 DU, almost independent of season. Away from the tropics seasonal variations become important, but in no case (outside the Antarctic ozone hole) does the layer become appreciably thinner than in the tropics. For example: Location Column thickness, Dobson Units Jan Apr Jul Oct Huancayo, Peru (12 degrees S) : 255 255 260 260 Aspendale, Australia (38 deg. S): 300 280 335 360 Arosa, Switzerland (47 deg. N): 335 375 320 280 St. Petersburg, Russia (60 deg. N): 360 425 345 300 These are monthly averages. Interannual standard deviations amount to ~5 DU for Huancayo, 25 DU for St. Petersburg. [Rowland 1991]. Notice that the highest ozone levels are found in the _spring_, not, as one might guess, in summer, and the lowest in the fall, not winter. Indeed, at high latitudes in the Northern Hemisphere there is more ozone in January than in July! Most of the ozone is created over the tropics, and then is carried to higher latitudes by prevailing winds (the general circulation of the stratosphere.) [Dobson] [Garcia] [Salby and Garcia] [Brasseur and Solomon] The antarctic ozone hole, discussed in detail in Part III, falls *far outside* this range of natural variation. Mean October ozone at Halley Bay on the Antarctic coast was 117 DU in 1993, down from 321 DU in 1956. ----------------------------- Subject: 2.5.b) Year-to-year variations. Since ozone is created by solar UV radiation, one expects to see some correlation with the 11-year solar sunspot cycle. Higher sunspot activity corresponds to more solar UV and hence more rapid ozone production. This correlation has been verified, although its effect is small, about 2% from peak to trough averaged over the earth, about 4% in polar regions. [Stolarski et al.] Another natural cycle is connected with the "quasibiennial oscillation", in which tropical winds in the lower stratosphere switch from easterly to westerly every 26 months. This leads to variations of the order of 3% at a given latitude, although the effect tends to cancel when one averages over the entire globe. Episodes of unusual solar activity ("solar proton events") can also affect ozone levels, as can major volcanic eruptions such as Agung in 1963, El Chichon in 1982, and Pinatubo in 1991. (The principal mechanism for this is _not_ injection of chlorine into the stratosphere, as discussed in Part II, but rather the injection of sulfate aerosols which change the radiation balance in the stratosphere by scattering light, and which convert inactive chlorine compounds to active, ozone-destroying forms.) These are all small effects, however, (a few % at most in a global average), and persist for short periods, 3 years or less. ----------------------------- Subject: 2.6) What are CFC's? CFC's - ChloroFluoroCarbons - are a class of volatile organic compounds that have been used as refrigerants, aerosol propellants, foam blowing agents, and as solvents in the electronic industry. They are chemically very unreactive, and hence safe to work with. In fact, they are so inert that the natural reagents that remove most atmospheric pollutants do not react with them, so after many years they drift up to the stratosphere where short-wave UV light dissociates them. CFC's were invented in 1928, but only came into large-scale production after ~1950. Since that year, the total amount of chlorine in the stratosphere has increased by a factor of 4. [Solomon] The most important CFC's for ozone depletion are: CF2Cl2 (CFC-12), CFCl3 (CFC-11), and CF2ClCFCl2 (CFC-113). In discussing ozone depletion, "CFC" is occasionally used to refer to a somewhat broader class of chlorine-containing organic compounds that have similar properties - unreactive in the troposphere, but readily photolyzed in the stratosphere. These include: HydroChloroFluoroCarbons such as CHClF2 (HCFC-22), Carbon Tetrachloride, CCl4, Methyl Chloroform, CH3CCl3, and Methyl Chloride, CH3Cl. (The more careful publications always use phrases like "CFC's and related compounds", but this gets tedious.) Only methyl chloride has a large natural source; it is produced biologically in the oceans and chemically from biomass burning. The CFC's and CCl4 are nearly inert in the troposphere, and have lifetimes of 50-200+ years. Their major "sink" is photolysis by UV radiation. [Rowland 1989, 1991] The hydrogen-containing halocarbons are more reactive, and are removed in the troposphere by reactions with OH radicals. This process is slow, however, and they live long enough (1-20 years) for a large fraction to reach the stratosphere. Most of Part II is devoted to stratospheric chlorine chemistry; look there for more detail. ----------------------------- Subject: 2.7) How do CFC's destroy ozone? CFC's themselves do not destroy ozone; certain of their decay products do. After CFC's are photolyzed, most of the chlorine eventually ends up as Hydrogen Chloride, HCl, or Chlorine Nitrate, ClONO2. These are called "reservoir species" - they do not themselves react with ozone. However, they do decompose to some extent, giving, among other things, a small amount of atomic chlorine, Cl, and Chlorine Monoxide, ClO, which can catalyze the destruction of ozone by a number of mechanisms. The simplest is: Cl + O3 -> ClO + O2 ClO + O -> Cl + O2 Net effect: O3 + O -> 2 O2 Note that the Cl atom is a _catalyst_ - it is not consumed by the reaction. Each Cl atom introduced into the stratosphere can destroy thousands of ozone molecules before it is removed. The process is even more dramatic for Bromine - it has no stable "reservoirs", so the Br atom is always available to destroy ozone. On a per-atom basis, Br is 10-100 times as destructive as Cl. On the other hand, chlorine and bromine concentrations in the stratosphere are very small in absolute terms. The mixing ratio of chlorine from all sources in the stratosphere is about 3 parts per billion, (most of which is in the form of CFC's that have not yet fully decomposed) whereas ozone mixing ratios are measured in parts per million. Bromine concentrations are about 100 times smaller still. (See Part II.) The complete chemistry is very complicated - more than 100 distinct species are involved. The rate of ozone destruction at any given time and place depends strongly upon how much Cl is present as Cl or ClO, and thus upon the rate at which Cl is released from its reservoirs. This makes quantitative _predictions_ of future ozone depletion difficult. [Rowland 1989, 1991] [Wayne] ----------------------------- Subject: 2.8) What is an "Ozone Depletion Potential?" The ozone depletion potential (ODP) of a compound is a simple measure of its ability to destroy stratospheric ozone. It is a relative measure: the ODP of CFC-11 is defined to be 1.0, and the ODP's of other compounds are calculated with respect to this reference point. Thus a compound with an ODP of 0.2 is, roughly speaking, one-fifth as "bad" as CFC-11. More precisely, the ODP of a compound "x" is defined as the ratio of the total amount of ozone destroyed by a fixed amount of compound x to the amount of ozone destroyed by the same mass of CFC-11: Global loss of Ozone due to x ODP(x) == --------------------------------- Global loss of ozone due to CFC-11. Thus the ODP of CFC-11 =1.0 by definition. The right-hand side of the equation is calculated by combining information from laboratory and field measurements with atmospheric chemistry and tranport models. Since the ODP is a relative measure, it is fairly "robust", not overly sensitive to changes in the input data or to the details of the model calculations. That is, there are many uncertainties in calculating the numerator or the denominator of the expression, but most of these cancel out when the ratio is calculated. The ODP of a compound will be affected by: The nature of the halogen (bromine-containing halocarbons usually have much higher ODPs than chlorocarbons, because atom for atom Br is a more effective ozone-destruction catalyst than Cl.) The number of chlorine or bromine atoms in a molecule. Molecular Mass (since ODP is defined by comparing equal masses rather than equal numbers of moles.) Atmospheric lifetime (CH3CCl3 has a lower ODP than CFC-11, because much of the CH3CCl3 is destroyed in the troposphere.) The ODP as defined above is a steady-state or long-term property. As such it can be misleading when one considers the possible effects of CFC replacements. Many of the proposed replacements have short atmospheric lifetimes, which in general is good; however, if a compound has a short _stratospheric_ lifetime, it will release its chlorine or bromine atoms more quickly than a compound with a longer stratospheric lifetime. Thus the short term effect of such a compound on the ozone layer is larger than would be predicted from the ODP alone (and the long-term effect correspondingly smaller.)(The ideal combination would be a short tropospheric lifetime, since those molecules which are destroyed in the troposphere don't get a chance to destroy any stratospheric ozone, combined with a long stratospheric lifetime.) To get around this, the concept of a Time-Dependent Ozone Depletion Potential has been introduced [Solomon and Albritton] [WMO 1991]: Loss of ozone due to X over time period T ODP(x,T) == ---------------------------------------------- Loss of ozone due to CFC-11 over time period T As T->infinity, this converges to the steady-state ODP defined previously. The following table lists time-dependent and steady-state ODP's for several halocarbons [Solomon and Albritton] [WMO 1991] Compound Formula Ozone Depletion Potential 10 yr 30 yr 100 yr Steady State CFC-113 CF2ClCFCl2 0.56 0.62 0.78 1.10 carbon tetrachloride CCl4 1.25 1.22 1.14 1.08 methyl chloroform CH3CCl3 0.75 0.32 0.15 0.12 HCFC-22 CHF2Cl 0.17 0.12 0.07 0.05 Halon - 1301 CF3Br 10.4 10.7 11.5 12.5 ----------------------------- Subject: 2.9) What about HCFC's and HFC's? Do they destroy ozone? HCFC's (hydrochlorofluorocarbons) differ from CFC's in that only some, rather than all, of the hydrogen in the parent hydrocarbon has been replaced by chlorine or fluorine. The most familiar example is CHClF2, known as "HCFC-22", used as a refrigerant and in many home air conditioners (auto air conditioners use CFC-12). The hydrogen atom makes the molecule susceptible to attack by the hydroxyl (OH) radical, so a large fraction of the HCFC's are destroyed before they reach the stratosphere. Molecule for molecule, then, HCFC's destroy much less ozone than CFC's, and they were suggested as CFC substitutes as long ago as 1976. Mos HCFC's have ozone depletion potentials around 0.01-0.1, so that during its lifetime a typical HCFC will have destroyed 1-10% as much ozone as the same amount of CFC-12. Since the HCFC's are more reactive in the troposphere, fewer of them reach the stratosphere. However, they are also more reactive in the stratosphere, so they release chlorine more quickly. The short-term effects are therefore larger than one would predict from the steady-state ozone depletion potential. In deciding upon substitutes for CFC's, the "time-dependent ozone depletion potential", discussed in the preceding section, is more informative than the steady-state ODP. [Solomon and Albritton] HFC's, hydrofluorocarbons, contain no chlorine at all, and hence have an ozone depletion potential of zero. (In 1993 there were tentative reports that the fluorocarbon radicals produced by photolysis of HFC's could catalyze ozone loss, but this has now been shown to be negligible [Ravishankara et al.]) A familiar example is CF3CH2F, known as HFC-134a, which is being used in some automobile air conditioners and refrigerators. HFC-134a is more expensive and more difficult to work with than CFC's, and while it has no effect on stratospheric ozone it is a greenhouse gas (though somewhat less potent than the CFC's). Some engineers have argued that non-CFC fluids, such as propane-isobutane mixtures, are better substitutes for CFC-12 in auto air conditioners than HFC-134a. ----------------------------- Subject: 2.10) *IS* the ozone layer getting thinner? There is no question that the ozone layer over antarctica has thinned dramatically over the past 15 years (see part III). However, most of us are more interested in whether this is also taking place at middle latitudes. The answer seems to be yes, although so far the effect are small. After carefully accounting for all of the known natural variations, a net decrease of about 3% per decade for the period 1978-1991 was found. This is a global average over latitudes from 66 degrees S to 66 degrees N (i.e. the arctic and antarctic are excluded in calculating the average). The depletion increases with latitude, and is somewhat larger in the Southern Hemisphere. Over the US, Europe and Australia 4% per decade is typical; on the other hand there is no significant ozone loss in the tropics. The depletion is larger in the winter months, smaller in the summer. [Stolarski et al.] The following table, extracted from a much more detailed one in [Herman et al.], illustrates the seasonal and regional trends in _percent per decade_ for the period 1979-1990: Latitude Jan Apr Jul Oct Example 65 N -3.0 -6.6 -3.8 -5.6 Iceland 55 N -4.6 -6.7 -3.1 -4.4 Moscow, Russia 45 N -7.0 -6.8 -2.4 -3.1 Minneapolis, USA 35 N -7.3 -4.7 -1.9 -1.6 Tokyo 25 N -4.2 -2.9 -1.0 -0.8 Miami, FL, USA 5 N -0.1 +1.0 -0.1 +1.3 Somalia 5 S +0.2 +1.0 -0.2 +1.3 New Guinea 25 S -2.1 -1.6 -1.6 -1.1 Pretoria, S. Africa 35 S -3.6 -3.2 -4.5 -2.6 Buenos Aires 45 S -4.8 -4.2 -7.7 -4.4 New Zealand 55 S -6.1 -5.6 -9.8 -9.7 Tierra del Fuego 65 S -6.0 -8.6 -13.1 -19.5 Palmer Peninsula (These are longitudinally averaged satellite data, not individual measurements at the places listed in the right-hand column. There are longitudinal trends as well.) Between 1991 and 1993 these trends accelerated. Satellite and ground-based measurements showed a remarkable decline for 1992 and early 1993, a full 4% below the average value for the preceding twelve years and 2-3% below the _lowest_ values observed in the earlier period. In Canada the spring ozone levels were 11-17% below normal [Kerr et al.]. However, by February 1994 ozone over the United States had recovered to levels similar to 1991. [Hofmann et al. 1994b] Sulfate aerosols from the July 1991 eruption of Mt. Pinatubo are the most likely cause of this transient; these aerosols can convert inactive "reservoir" chlorine into active ozone-destroying forms, and can also interfere with the production and transport of ozone by changing the solar radiation balance in the stratosphere. [Brasseur and Granier] [Hofmann and Solomon] [Hofmann et al. 1994a] Another cause may be the unusually strong arctic polar vortex in 1992-93, which made the arctic stratosphere more like the antarctic than is usually the case. [Gleason et al.] [Waters et al.] In any event, the rapid ozone loss in 1992 and 1993 appears to have been a transient phenomenon, superimposed upon the much slower downward trend identified for 1970-1991. ----------------------------- Subject: 2.11) Is the middle-latitude ozone loss due to CFC emissions? That's the majority opinion, although not everyone agrees. The present trends are too small to allow a watertight case to be made (as _has_ been made for the far larger, but localized, depletion in the Antarctic Ozone hole; see Part III.). Other possible causes are being investigated. To quote from the 1991 Scientific Assessment published by the World Meteorological Organization, p. 4.1 [WMO 1991]: "The primary cause of the Antarctic ozone hole is firmly established to be halogen chemistry....There is not a full accounting of the observed downward trend in _global ozone_. Plausible mechanisms include heterogeneous chemistry on sulfate aerosols [which convert reservoir chlorine to active chlorine - R.P.] and the transport of chemically perturbed polar air to middle latitudes. Although other mechanisms cannot be ruled out, those involving the catalytic destruction of ozone by chlorine and bromine appear to be largely responsible for the ozone loss and _are the only ones for which direct evidence exists_." (emphases mine - RP) The Executive Summary of the subsequent 1994 scientific assessment states: "Direct in-situ meaurements of radical species in the lower stratosphere, coupled with model calculations, have quantitatively shown that the in-situ photochemical loss of ozone due to (largely natural) reactive nitrogen (NOx) compounds is smaller than that predicted from gas-phase chemistry, while that due to (largely natural) HOx compounds and (largely anthropogenic) chlorine and bromine compounds is larger than that predicted by gas-phase chemistry. This confirms the key role of chemical reactions on sulfate aerosols in controlling the chemical balance of the lower stratosphere. These and other recent scientific findings strengthen the conclusion of the previous assessment that the weight of scientific evidence suggests that the observed middle- and high-latitude ozone losses are largely due to anthropogenic chlorine and bromine compounds." [WMO 1994] A legal analogy might be useful here - the connection between _antarctic_ ozone depletion and CFC emissions has been proved beyond a reasonable doubt, while at _middle latitudes_ there is only probable cause for such a connection. One must remember that there is a natural 10-20 year time lag between CFC emissions and ozone depletion. Ozone depletion today is (probably) due to CFC emissions in the 1970's. Present controls on CFC emissions are designed to avoid possibly large amounts of ozone depletion 30 years from now, not to repair the depletion that has taken place up to now. ----------------------------- Subject: 2.12) If the ozone is lost, won't the UV light just penetrate deeper into the atmosphere and make more ozone? This does happen to some extent - it's called "self-healing" - and has the effect of moving ozone from the upper to the lower stratosphere. Recall that ozone is _created_ by UV with wavelengths less than 240 nm, but functions by _absorbing_ UV with wavelengths greater than 240 nm. The peak of the ozone absorption band is at ~250 nm, and the cross-section falls off at shorter wavelengths. The O2 and O3 absorption bands do overlap, though, and UV radiation between 200 and 240 nm has a good chance of being absorbed by _either_ O2 or O3. (Below 200 nm the O2 absorption cross-section increases dramatically, and O3 absorption is insignificant in comparison.) Since there is some overlap, a decrease in ozone does lead to a small increase in absorption by O2. This is a weak feedback, however, and it does not compensate for the ozone destroyed. Negative feedback need not imply stability, just as positive feedback need not imply instability. Numerical calculations of ozone depletion take the "self-healing" phenomenon into account, by letting the perturbed ozone layer come into equilibrium with the exciting radiation. ----------------------------- Subject: 2.13) Do Space Shuttle launches damage the ozone layer? Very little. In the early 1970's, when little was known about the role of chlorine radicals in ozone depletion, it was suggested that HCl from solid rocket motors might have a significant effect upon the ozone layer - if not globally, perhaps in the immediate vicinity of the launch. It was immediately shown that the effect was negligible, and this has been repeatedly demonstrated since. Each shuttle launch produces about 200 metric tons of chlorine as HCl, of which about one-third, or 68 tons, is injected into the stratosphere. Its residence time there is about three years. A full year's schedule of shuttle and solid rocket launches injects 725 tons of chlorine into the stratosphere. This is negligible compared to chlorine emissions in the form of CFC's and related compounds (1.2 million tons/yr in the 1980's, of which ~0.3 Mt reach the stratosphere each year). It is also small in comparison to natural sources of stratospheric chlorine, which amount to about 75,000 tons per year. [Prather et al.] [WMO 1991] [Ko et al.] See also the sci.space FAQ, Part 10, "Controversial Questions", available by anonymous ftp from rtfm.mit.edu in the directory pub/usenet/news.answers/space/controversy, and on the world-wide web at: http://www.cis.ohio-state.edu/hypertext/faq/usenet/space/controversy/faq.html ----------------------------- Subject: 2.14) Will commercial supersonic aircraft damage the ozone layer? Short answer: Probably not. This problem is very complicated, and a definite answer will not be available for several years, but present model calculations indicate that a fleet of high-speed civil transports would deplete the ozone layer by < 2%. [WMO 1991, 1994] Long answer (this is a tough one): Supersonic aircraft fly in the stratosphere. Since vertical transport in the stratosphere is slow, the exhaust gases from a supersonic jet can stay there for two years or more. The most important exhaust gases are the nitrogen oxides, NO and NO2, collectively referred to as "NOx". NOx is produced from ordinary nitrogen and oxygen by electrical discharges (e.g. lightning) and by high-temperature combustion (e.g. in automobile and aircraft engines). The relationship between NOx and ozone is complicated. In the troposphere, NOx _makes_ ozone, a phenomenon well known to residents of Los Angeles and other cities beset by photochemical smog. At high altitudes in the troposphere, similar chemical reactions produce ozone as a byproduct of the oxidation of methane; for this reason ordinary subsonic aircraft actually increase the thickness of the ozone layer by a very small amount. Things are very different in the stratosphere. Here the principal source of NOx is nitrous oxide, N2O ("laughing gas"). Most of the N2O in the atmosphere comes from bacteriological decomposition of organic matter - reduction of nitrate ions or oxidation of ammonium ions. (It was once assumed that anthropogenic sources were negligible in comparison, but this is now known to be false. The total anthropogenic contribution is now estimated at 8 Tg (teragrams)/yr, compared to a natural source of 18 Tg/yr. [Khalil and Rasmussen].) N2O, unlike NOx, is very unreactive - it has an atmospheric lifetime of more than 150 years - so it reaches the stratosphere, where most of it is converted to nitrogen and oxygen by UV photolysis. However, a small fraction of the N2O that reaches the stratosphere reacts instead with oxygen atoms (to be precise, with the very rare electronically excited singlet-D oxygen atoms), and this is the major natural source of NOx in the stratosphere. About 1.2 million tons are produced each year in this way. This source strength would be matched by 500 of the SST's designed by Boeing in the late 1960's, each spending 5 hours per day in the stratosphere. (Boeing was intending to sell 800 of these aircraft.) The Concorde, a slower plane, produces less than half as much NOx and flies at a lower altitude; since the Concorde fleet is small, its contribution to stratospheric NOx is not significant. Before sending large fleets of high-speed aircraft into the stratosphere, however, one should certainly consider the possible effects of increasing the rate of production of an important stratospheric trace gas by as much as a factor of two. [CIC 1975] In 1969, Paul Crutzen discovered that NOx could be an efficient catalyst for the destruction of stratospheric ozone: NO + O3 -> NO2 + O2 NO2 + O -> NO + O2 ------------------------------- net: O3 + O -> 2 O2 This sequence was rediscovered two years later by H. S. Johnston, who made the connection to SST emissions. Until then it had been thought that the radicals H, OH, and HO2 (referred to collectively as "HOx") were the principal catalysts for ozone loss; thus, investigations of the impact of aircraft exhaust on stratospheric ozone had focussed on emissions of water vapor, a possible source for these radicals. (The importance of chlorine radicals, Cl, ClO, and ClO2, referred to as - you guessed it - "ClOx", was not discovered until 1973.) It had been argued - correctly, as it turns out - that water vapor injection was unimportant for determining the ozone balance. The discovery of the NOx cycle threw the question open again. Beginning in 1972, the U.S. National Academies of Science and Engineering and the Department of Transportation sponsored an intensive program of stratospheric research. [CIC 1975] It soon became clear that the relationship between NOx emissions and the ozone layer was very complicated. The stratospheric lifetime of NOx is comparable to the timescale for transport from North to South, so its concentration depends strongly upon latitude. Much of the NOx is injected near the tropopause, a region where quantitative modelling is very difficult, and the results of calculations depend sensitively upon how troposphere-stratosphere exchange is treated. Stratospheric NOx chemistry is _extremely_ complicated, much worse than chlorine chemistry. Among other things, NO2 reacts rapidly with ClO, forming the inactive chlorine reservoir ClONO2 - so while on the one hand increasing NOx leads directly to ozone loss, on the other it suppresses the action of the more potent chlorine catalyst. And on top of all of this, the SST's always spend part of their time in the troposphere, where NOx emissions cause ozone increases. Estimates of long-term ozone changes due to large-scale NOx emissions varied markedly from year to year, going from -10% in 1974, to +2% (i.e. a net ozone _gain_) in 1979, to -8% in 1982. (In contrast, while the estimates of the effects of CFC emissions on ozone also varied a great deal in these early years, they always gave a net loss of ozone.) [Wayne] The discovery of the Antarctic ozone hole added a new piece to the puzzle. As described in Part III, the ozone hole is caused by heterogeneous chemistry on the surfaces of stratospheric cloud particles. While these clouds are only found in polar regions, similar chemical reactions take place on sulfate aerosols which are found throughout the lower stratosphere. The most important of the aerosol reactions is the conversion of N2O5 to nitric acid: N2O5 + H2O -> 2 HNO3 (catalyzed by aerosol surfaces) N2O5 is in equilibrium with NOx, so removal of N2O5 by this reaction lowers the NOx concentration. The result is that in the lower stratosphere the NOx catalytic cycle contributes much less to overall ozone loss than the HOx and ClOx cycles. Ironically, the same processes that makes chlorine-catalyzed ozone depletion so much more important than was believed 10 years ago, also make NOx-catalyzed ozone loss less important. In the meantime, there has been a great deal of progress in developing jet engines that will produce much less NOx - up to a factor of 10 - than the old Boeing SST. The most recent model calculations indicate that a fleet of the new "high-speed civil transports" would deplete the ozone layer by 0.3-1.8%. Caution is still required, since the experiment has not been done - we have not yet tried adding large amounts of NOx to the stratosphere. The forecasts, however, are good. [WMO 1991, Ch. 10] [WMO 1994] .................................................................. _Aside_: One sometimes hears that the US government killed the SST project in 1971 because of concerns raised by H. S. Johnston's work on NOx. This is not true. The US House of Representatives had already voted to cut off Federal funding for the SST when Johnston began his calculations. The House debate had centered around economics and the effects of noise, especially sonic booms, although there were some vague remarks about "pollution" and one physicist had testified about the possible effects of water vapor on ozone. About 6 weeks after both houses had voted to cancel the SST, its supporters succeeded in reviving the project in the House. In the meantime, Johnston had sent a preliminary report to several professional colleagues and submitted a paper to _Science_. A preprint of Johnston's report leaked to a small California newspaper which published a highly sensationalized account. The story hit the press a few days before the Senate voted, 58-37, not to revive the SST. (The previous Senate vote had been 51-46 to cancel the project. The reason for the larger majority in the second vote was probably the statement by Boeing's chairman that at least $500 million more would be needed to revive the program.) .................................................................... ----------------------------- Subject: 2.15) What is being done about ozone depletion? The 1987 Montreal Protocol specified that CFC emissions should be reduced by 50% by the year 2000 (they had been _increasing_ by 3% per year.) This agreement was amended in London in 1990, to state that production of CFC's, CCl4, and halons should cease entirely by the year 2000. Restrictions have also been applied to other Cl sources such as methylchloroform. (The details of the protocols are complicated, involving different schedules for different compounds, delays for developing nations, etc. See the book by [Benedick].) The phase-out schedule was accelerated by four years by the 1992 Copenhagen agreements. A great deal of effort has been devoted to recovering and recycling CFC's that are currently being used in closed-cycle systems. Recent NOAA measurements [Elkins et al.] show that the _rate of increase_ of halocarbon concentrations in the atmosphere has decreased markedly since 1987, by a factor of 4 for CFC-11 and a factor of 2 for CFC-12. It appears that the Protocols are being observed. Under these conditions total stratospheric chlorine is predicted to peak at 3.8 ppbv in the year 1998, 0.2 ppbv above current levels, and to slowly decline thereafter. [WMO 1994] Extrapolation of current trends suggests that the maximum ozone losses will be [WMO 1994]: Northern Mid-latitudes in winter/Spring: 12-13% below late 1960's levels, ~2.5% below current levels. Northern mid-latitudes in summer/fall: 6-7% below late 1960's levels, ~1.5% below current levels. Southern mid-latitudes, year-round: ~ 11% below late 1960's levels, ~2.5% below current levels. Very little depletion has been seen in the tropics and very little is expected there. After the year 2000, the ozone layer will slowly recover over a period of 50 years or so. The antarctic ozone hole is expected to last until about 2045. [WMO 1991,1994] Some scientists are investigating ways to replenish stratospheric ozone, either by removing CFC's from the troposphere or by tying up the chlorine in inactive compounds. This is discussed in Part III. ----------------------------- Subject: 3. REFERENCES FOR PART I A remark on references: they are neither representative nor comprehensive. There are _hundreds_ of people working on these problems. Where possible I have limited myself to papers that are (1) available outside of University libraries (e.g. _Science_ or _Nature_ rather than archival journals such as _J. Geophys. Res._) and (2) directly related to the "frequently asked questions". I have not listed papers whose importance is primarily historical. Readers who want to see "who did what" should consult the review articles listed below, or, if they can get them, the WMO reports which are extensively documented. ----------------------------- Subject: Introductory Reading [Garcia] R. R. Garcia, _Causes of Ozone Depletion_, _Physics World_ April 1994 pp 49-55. [Graedel and Crutzen] T. E. Graedel and P. J. Crutzen, _Atmospheric Change: an Earth System Perspective_, Freeman, NY 1993. [Rowland 1989] F.S. Rowland, "Chlorofluorocarbons and the depletion of stratospheric ozone", _American Scientist_ _77_, 36, 1989. [Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises Challenge Atmospheric Scientists", _Chemical and Engineering News_, 24 May 1993, pp. 9-18. ----------------------------- Subject: Books and Review Articles [Benedick] R. Benedick, _Ozone Diplomacy_, Harvard, 1991. [Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of the Middle Atmosphere_, 2nd. Edition, D. Reidel, 1986 [Chamberlain and Hunten] J. W. Chamberlain and D. M. Hunten, _Theory of Planetary Atmospheres_, 2nd Edition, Academic Press, 1987 [Dobson] G.M.B. Dobson, _Exploring the Atmosphere_, 2nd Edition, Oxford, 1968. [CIC 1975] Climate Impact Committee, National Research Council, _Environmental Impact of Stratospheric Flight_, National Academy of Sciences, 1975. [Johnston 1992] H. S. Johnston, "Atmospheric Ozone", _Annu. Rev. Phys. Chem._ _43_, 1, 1992. [Ko et al.] M. K. W. Ko, N.-D. Sze, and M. J. Prather, "Better Protection of the Ozone Layer", _Nature_ _367_, 505, 1994. [McElroy and Salawich] M. McElroy and R. Salawich, "Changing Composition of the Global Stratosphere", _Science_ _243, 763, 1989. [Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion", _Ann. Rev. Phys. Chem._ _42_, 731, 1991. [Salby and Garcia] M. L. Salby and R. R. Garcia, "Dynamical Perturbations to the Ozone Layer", _Physics Today_ _43_, 38, March 1990. [Solomon] S. Solomon, "Progress towards a quantitative understanding of Antarctic ozone depletion", _Nature_ _347_, 347, 1990. [Wallace and Hobbs] J. M. Wallace and P. V. Hobbs, _Atmospheric Science: an Introductory Survey_, Academic Press, 1977. [Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd. Ed., Oxford, 1991. [WMO 1988] World Meteorological Organization, _Report of the International Ozone Trends Panel_, Global Ozone Research and Monitoring Project - Report #18. [WMO 1989] World Meteorological Organization, _Scientific Assessment of Stratospheric Ozone: 1991_ Global Ozone Research and Monitoring Project - Report #20. [WMO 1991] World Meteorological Organization, _Scientific Assessment of Ozone Depletion: 1991_ Global Ozone Research and Monitoring Project - Report #25. [WMO 1994] World Meteorological Organization, _Scientific Assessment of Ozone Depletion: 1994_ Global Ozone Research and Monitoring Project - Report #25. (Executive Summary; the full report is to be published in November 1994). ----------------------------- Subject: More Specialized References [Brasseur and Granier] G. Brasseur and C. Granier, "Mt. Pinatubo aerosols, chlorofluorocarbons, and ozone depletion", _Science_ _257_, 1239,1992. [Elkins et al.] J. W. Elkins, T. M. Thompson, T. H. Swanson, J. H. Butler, B. D. Hall, S. O. Cummings, D. A. Fisher, and A. G. Raffo, "Decrease in Growth Rates of Atmospheric Chlorofluorocarbons 11 and 12", _Nature_ _364_, 780, 1993. [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. [Herman et al.] J. R. Herman, R. McPeters, and D. Larko, "Ozone depletion at northern and southern latitudes derived from January 1979 to December 1991 TOMS data", J. Geophys. Res. _98_, 12783, 1993. [Hofmann and Solomon] D. J. Hofmann and S. Solomon, "Ozone destruction through heterogeneous chemistry following the eruption of El Chichon", J. Geophys. Res. _94_, 5029, 1989. [Hofmann et al. 1994a] D. J. Hofmann, S. J. Oltmans, W. D. Komhyr, J. M. Harris, J. A. Lathrop, A. O. Langford, T. Deshler, B. J. Johnson, A. Torres, and W. A. Matthews, "Ozone Loss in the lower stratosphere over the United States in 1992-1993: Evidence for heterogeneous chemistry on the Pinatubo aerosol", Geophys. Res. Lett. _21_, 65, 1994. [Hofmann et al. 1994b] D. J. Hofmann, S. J. Oltmans, J. M. Harris, J. A. Lathrop, G. L. Koenig, W. D. Komhyr, R. D. Evans, D. M. Quincy, T. Deshler, and B. J. Johnson, "Recovery of stratospheric ozone over the United States in the winter of 1993-94", Geophys. Res. Lett. _21_, 1779, 1994. [Kerr et al.] J. B. Kerr, D. I. Wardle, and P. W. Towsick, "Record low ozone values over Canada in early 1993", Geophys. Res. Lett. _20_, 1979, 1993. [Khalil and Rasmussen] M.A.K. Khalil and R. Rasmussen, "The Global Sources of Nitrous Oxide", _J. Geophys. Res._ _97_, 14651, 1992. [Prather et al. ] M. J. Prather, M.M. Garcia, A.R. Douglass, C.H. Jackman, M.K.W. Ko, and N.D. Sze, "The Space Shuttle's impact on the stratosphere", J. Geophys. Res. _95_, 18583, 1990. [Ravishankara et al.] A. R. Ravishankara, A. A. Turnipseed, N. R. Jensen, S. Barone, M. Mills, C. J. Howard, and S. Solomon, "Do Hydrofluorocarbons Destroy Stratospheric Ozone?", _Science_ _263_, 71, 1994. [SAGE II] Special Section on the Stratospheric Aerosol and Gas Experiment II, _J. Geophys. Res._ _98_, 4835-4897, 1993. [Solomon and Albritton] S. Solomon and D.L. Albritton, "Time-dependent ozone depletion potentials for short- and long-term forecasts", _Nature_ _357_, 33, 1992. [Stolarski et al.] R. Stolarski, R. Bojkov, L. Bishop, C. Zerefos, J. Staehelin, and J. Zawodny, "Measured Trends in Stratospheric Ozone", Science _256_, 342 (17 April 1992) [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. [Zander et al. 1992] R. Zander, M. R. Gunson, C. B. Farmer, C. P. Rinsland, F. W. Irion, and E. Mahieu, "The 1985 chlorine and fluorine inventories in the stratosphere based on ATMOS observations at 30 degrees North latitude", J. Atmos. Chem. _15_, 171, 1992.

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