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OZONE DEPLETION FAQ Part I: Basic Questions about ozone. Copyright 1993 Robert Parson. 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 the remaining parts which are more specialized. Part II deals 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. I emphasize physical and chemical mechanisms rather than biological effects, although I make a few remarks about the latter. For completeness, some questions have been included in more than one part. 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 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, just as in warfare one needs to consider not only what the enemy will probably do, but also the worst that he could possibly do. There have been surprises, mostly unpleasant, in this field in the past, and there are sure to be more in the future. _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 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 parson_r@cubldr.colorado.edu CONTENTS 1. THE STRATOSPHERE 1.1) What is the stratosphere? 1.2) How is the composition of air described? 2. THE OZONE LAYER 2.1) How is ozone created? 2.2) How much ozone is in the ozone layer, and what is a "Dobson Unit"? 2.3) What is the concentration of ozone in the stratosphere? 2.4) How does the ozone layer work? 2.5) What sorts of natural variations does the ozone layer show? 2.6) What are CFC's? [See Part II for more detail] 2.7) How do CFC's destroy ozone? 2.8) What about HCFC's and HFCs? Do they destroy ozone? 2.9) *IS* the ozone layer getting thinner (outside antarctica) ? 2.10) Is middle-latitude ozone loss due to CFC emissions? 2.11) Do Space Shuttle launches damage the ozone layer? 2.12) Will commercial supersonic aircraft damage the ozone layer? 2.13) What is being done about ozone depletion, and what can we expect to see? 3. REFERENCES _________________________________________________________________ 1. THE STRATOSPHERE 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": the density of air decreases particularly rapidly with height, which impedes vertical motion within the stratosphere. 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 precise location of the boundary between these regions, called the tropopause, 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. 1.2) How is the composition of air described? (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 relative number of molecules - the number of molecules of a given component in a small volume, relative to the total number of molecules in that volume. Chemists usually call this a mole fraction, but atmospheric scientists have taken to calling it a "mixing ratio". Typical units for trace species are parts-per-billion by volume (ppbv); we will abbreviate this to ppb. (The 'by volume' reflects "Avogadro's Law": for an ideal gas mixture, equal volumes contain equal numbers of molecules.) 2. THE OZONE LAYER 2.1) How is ozone created? Ozone is formed naturally in the upper stratosphere by short-wavelength UV radiation. Wavelengths less than ~240 nanometers are absorbed by oxygen molecules, which dissociate to give O atoms. The O atoms combine with other O2 molecules to form ozone: O2 + hv -> O + O (lambda < 240 nm) O + O2 -> O3 2.2) How much ozone is in the ozone layer? (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 other terms, 1 DU is about 2.7 x 10^16 molecules/cm^2. 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 ratio of 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). Really global coverage began in 1956-57. 2.3) What is the concentration of ozone 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 at 15 km, rising to 8 ppm at ~35 km, falling to ~3 ppm at 45 km. Even in the thickest part of the layer, ozone is a trace gas. 2.4) How does the ozone layer work? UV light with wavelengths between 240 and 320 nm is absorbed by ozone, giving an O atom and and O2 molecule. The O atom almost immediately 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. In a sense, not only is the ozone layer _in_ the stratosphere, the ozone layer is responsible for the existence of the stratosphere. Ozone _is_ removed 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 three times 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 of O2 and recombination of O and O3; 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 (lambda < 240 nm) : creation of oxygen atoms O + O2 -> O3 : formation of ozone O3 + hv -> O2 + O (lambda < 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. (The special 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). 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. a. Regional and Seasonal Variation Since solar radiation makes ozone, one expects to see the thickness of the ozone layer depend upon the season. 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 into account the dynamics of the atmosphere. (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, Aus. (38 degrees S): 300 280 335 360 Arosa, Switz. (47 degrees N): 335 375 320 280 St. Petersburg, FSU (60 degrees 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. Most of the ozone is created over the tropics, and then flows to higher latitudes. 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. [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 also convert inactive chlorine compounds to active, ozone-destroying forms.) These are all small effects, however, (a few % at most), and persist for short periods, 1-2 years or less. 2.6) What are CFC's? CFC's - chlorofluorocarbons - are a class of volatile organic compounds that have been used for refrigeration, aerosol propellants, foam blowing, 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 the 1920's, 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 organic chlorine-containing 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 organochlorine 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. 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 of these cycles 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 tens of 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. Each Br atom can destroy _millions_ of ozone molecules before it is removed. 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] 2.8) 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. 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. The impact of a compound on stratospheric ozone is usually measured by its "ozone depletion potential", defined as the steady-state limit of the amount of ozone destroyed by the halocarbon, relative to the amount destroyed by CFC-12. HCFC's generally have ozone depletion potentials around 0.01-0.1, so that in the long time limit a typical HCFC will have destroyed 1-10% as much ozone as CFC-12. This measure can sometimes be misleading, however. 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 their chlorine more quickly. Just as short-lived radioisotopes are, other things being equal, more intensely radioactive than long-lived ones, HCFC's deplete ozone more quickly than CFC's. The short-term effects are therefore larger than one would predict from the ozone depletion potential alone, and the long-term effects correspondingly smaller. This must be taken into account when substituting HCFC's for CFC's. [Solomon and Albritton] HFC's, hydrofluorocarbons, contain no chlorine at all, and hence have an ozone depletion potential of zero. The most 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 less so than the CFC's). Some engineers have argued that propane-butane mixtures make better refrigerants than HFC's. 2.9) *IS* the ozone layer getting thinner (outside antarctica) ? So it seems, although so far the effects 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 remains. 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, being somewhat larger in the Southern Hemisphere. There is no significant depletion in the tropics; over the US, Europe, and Australia 4%/decade is typical. 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, FSU 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.A. 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 Since 1991 these trends have accelerated. Satellite and ground-based measurements now show 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. At 60 degrees North the spring ozone levels were 14% below normal, and there is evidence that this depletion is persistig into the summer months. This decline overwhelms the effect of the solar cycle; 1991 was a solar maximum, while the 1992 results are already below those for the 1986 solar minimum. Sulfate aerosols from the July 1991 eruption of Mt. Pinatubo may be the cause of this latest spike; 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. 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.] 2.10) Is middle-latitude ozone loss due to CFC emissions? That's the majority opinion, although not everyone agrees. The present effects are too small to allow a watertight case to be made (as _has_ been made for the far larger, but localized, depletions in the Antarctic Ozone hole; see Part III.). Other possible causes are being investigated. To quote from [WMO 1991], p. 4.1: "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." The recent UARS measurements of ozone and ClO in the Northern Hemisphere find a correlation between enhanced ClO and depleted ozone, which further supports this hypothesis. [Waters et al.] 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 '60's and '70's. Present controls on CFC emissions are designed to avoid possibly large amounts of ozone depletion 30 years from now, not to remediate the small degree of depletion that has taken place up to now. 2.11) Do Space Shuttle launches damage the ozone layer? No. In the early 1970's, when very 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 quickly shown that the effect was negligible, and this has been repeatedly demonstrated since. Each shuttle launch produces about 68 metric tons of chlorine as HCl; a full year's worth of shuttle and solid- rocket launches produces about 725 tons. 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). [Prather et al.] [WMO 1991] [Johnston 1992] 2.12) 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 model calculations indicate that a fleet of high-speed civil transports would deplete the ozone layer by <1%. [WMO 1991] 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 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, essentially the same 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. (The direct anthropogenic contribution is small, although changes in land use and fertilizers may influence the rate of biological production.) N2O, unlike NOx, is very unreactive - it has an atmospheric lifetime of more than 150 years - so it rises to the stratosphere, where most of it is converted to nitrogen and oxygen by UV photolysis. A small fraction of the N2O in the stratosphere reacts instead with oxygen atoms (to be precise, with the electronically excited singlet-D oxygen atoms), and this is the major natural source of NOx in the stratosphere. About 1.2 megatons are produced each year by this mechanism. 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 40% 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] (Aside: subsonic aircraft do sometimes enter the stratosphere; however they stay very low and do not appreciably affect its chemistry.) In 1970-71, Paul Crutzen and Hal Johnston independently 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 Previously, 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 of these radicals. (The importance of chlorine radicals, Cl, ClO, and ClO2, referred to as - you guessed it - "ClOx", was only discovered two years later.) It had been shown - 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 increasing ozone loss, on the other it suppresses the action of the 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 less than 1%. 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] .......................................... _Aside_: One sometimes hears that the US government killed the SST project in 1971 because of concerns raised by 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 debate then centered around economics and the effects of noise, especially sonic booms, although there were some vague remarks about "pollution" and references to the possible effects of water vapor on ozone, remarks that do not seem to have been regarded seriously. (This is the sense I get from reading Johnston's reminiscences [Johnston 1992] and old issues of _Time_ and _Newsweek_.) About 6 weeks after both houses 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.) ............................ 2.13) What is being done about ozone depletion? The 1988 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.) The phase-out schedule was accelerated by four years by the 1992 Copenhagen agreements. A great deal of effort has also been devoted to recovering and recycling CFC's that are currently being used in closed-cycle systems. Model calculations predict that ozone levels, averaged over the year and over the Northern hemisphere, will fall to about 4% below 1980 levels in the first decade of the 21st century if the protocols are obeyed. Very little depletion is expected in the tropics, so correspondingly larger losses - more than 6% - are expected at middle and high latitudes. These models have systematically _underestimated_ ozone depletion in the past, so somewhat larger losses may be expected. In fact, 4% global year-averaged ozone depletion was already measured in 1993 [Gleason et al.] although this may be a transient caused by Mt. Pinatubo's eruption in July 1991. After 2010 the ozone layer will slowly recover over a period of 20 years or so, although the form of the recovery is strongly model-dependent. [WMO 1991] I have no results at hand for the southern hemisphere; if current trends continue ozone depletion will be more serious there. The antarctic ozone hole is expected to last until 2050 or so. This does not take into account the possibility of global warming, which by cooling the stratosphere could make ozone depletion more serious both at mid latitudes and in polar regions. Some scientists are investigating ways to replenish stratospheric ozone, either by removing CFC's from the troposphere or by tying the chlorine up in inactive compounds. This is discussed in Part III. ___________________________ 3. REFERENCES FOR PART I 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 journals such as _J. Geophys. Res._) and (2) directly related to the "frequently asked questions". 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. Introductory Reading: [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. ---------------------------- Books and Review Articles: [Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of the Middle Atmosphere_, 2nd. Edition, D. Reidel, 1986 [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. [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. [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. [WMO 1988] World Meteorological Organization, _Report of the International Ozone Trends Panel_, Global Ozone Research and Monitoring Project - Report #18. [WMO 1991] World Meteorological Organization, _Scientific Assessment of Ozone Depletion: 1991_ Global Ozone Research and Monitoring Project - Report #25. ----------------------------------- More Specialized: [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. [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. [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.