Richard S. Stolarski

NASA Goddard Space Flight Center

Greenbelt, MD 20771

USA

Ozone is a substance that touches on our modern society in many ways. Ozone is used in industrial processes and as a disinfectant. Ozone is a strong oxidizing agent that is present in photochemical smog. Ozone in the stratosphere screens the surface of the earth from biologically-damaging ultraviolet radiation. All of these uses emerge from our basic understanding of ozone that began with the recognition by Schönbein in 1840 that ozone was a unique substance produced in a variety of processes.

In this paper I review some of the history behind our present understanding of atmospheric ozone. This history will be given in mostly in broad outlines, from point of view of someone who has been studying stratospheric ozone for the last 25 years. There are parallel historical threads that lead to other aspects of current ozone research.

I will emphasize four broad time periods in atmospheric ozone research

1. Discovery and determination of properties (1840 - ~1880)
2. Solar spectrum cutoff and location in stratosphere (~1880 - ~1930)
3. Theory and quantification of atmospheric distribution (~1930 - ~1965)
4. Catalytic ozone loss and connection to atmospheric chemistry (~1965 — present).

My recounting of the history of ozone research is limited. The limited reference list will, I hope, provide a starting point for others who can expound more fully on aspects of the development of atmospheric ozone research.

Like most areas of science, the discovery of ozone did not come as a bolt from the blue. In the 1770s, Priestley, in England, and Scheele, in Sweden, were studying the properties of air. They were able to separate air into two parts, one that supported combustion at an increased rate over normal air, and one that did not support combustion. They both tried to interpret their experiments in terms of the existing phlogiston theory and didn’t realize that they had discovered a new element. In 1776, Lavoisier repeated many of their experiments and correctly recognized the elementary nature of the gas that he named oxygen.

In 1785 Van Marum passed electric sparks through oxygen and noted a peculiar smell and that the resulting gas reacted strongly with mercury. The peculiar odor in the air after a lightning strike had been known for centuries, including references in the Odyssey and Iliad. Van Marum and others attributed the odor to the electricity, calling it the "electrical odor". It was Schönbein in 1840 who recognized that the odor was not due to the electricity, but was due to the properties of a substance produced during the electrical process. He named this substance ozone (from ozein, Greek for "to smell"). I quote from Albert Leeds (1) writing in 1880:

For the few decades immediately following Schönbein’s discovery of ozone, many studies were carried out regarding the identity of ozone, its properties, and its possible uses. In 1845 Marignac and De la Rive interpreted their experiments as indicating that ozone was just oxygen, modified by its passage through a peculiar electrical state. But in 1848, T. Sterry Hunt put forward a hypothesis that is close to our present understanding. Quoting him from Leeds (1):

By the middle of the 1870s, ozone was established as a potentially important component of the normal atmosphere. Several tests had been developed to measure the amount of ozone in the air. Debates raged over the accuracy of these measurements that used litmus-type papers. Did they measure just ozone, or was there a significant contribution from hydrogen peroxide in the air? Despite these uncertainties there were some 300 or so locations at which measurements were routinely being made. Ozone was a very important and lively topic for research at this time. In his book "Ozone and Antozone", Cornelius Fox (2) wrote in 1873:

Fox went on to discuss a large number of "facts" concerning ozone as they were known. Among these were:

Clearly, a lot had been learned about ozone. A lot remained to be discovered and the interest level was high.

In 1801, Johann Ritter used a prism to spread the electromagnetic spectrum into its component wavelengths. He discovered that there existed a portion of the spectrum beyond the violet that would break silver chloride down to metallic silver. This became known as the ultraviolet. In 1879, Cornú (3) used newly-developed techniques for ultraviolet spectroscopy to measure the sun’s spectrum. To his surprise, the intensity of the sun’s radiation dropped off rapidly at wavelengths below about 300 nm. He demonstrated that the wavelength of the "cutoff" increased as the sun set and the path through the atmosphere increased. He correctly interpreted that the cutoff was the result of an absorbing substance in the atmosphere.

A year later in 1880, W. N. Hartley (4) suggested that the atmospheric absorber was ozone. This conclusion was based on his laboratory studies of the ultraviolet absorption by ozone. He compared the wavelength of the edge of the cutoff in the spectrum to the cutoff wavelengths observed in the laboratory for a variety of substances. He even asked the question:

He thus anticipated the later focus on the location of the ozone layer in the stratosphere.

Quantitative measurements of the radiation near the cutoff were very difficult to make because the low intensity of radiation at these wavelengths. Fabry designed a double spectrograph to cut down on stray light from longer wavelengths and overcome the problem. In 1913, Fabry and Buisson (5) used this instrument to make accurate measurements of the cutoff in the solar spectrum. From these measurements they deduced that the total amount of ozone in the atmosphere was equivalent to a layer at normal temperature and pressure which would be only 5 mm in thickness.

Fowler and Strutt (who became Lord Rayleigh) showed in 1917 (6) that near the edge of the cutoff in the solar spectrum, a number of absorption bands could be observed. These were consistent with the ozone absorption bands observed in the laboratory, further proving that ozone is the absorber in the atmosphere is ozone. In the next year, Strutt (7) attempted to measure the absorption by ozone from a light source located four miles across a valley. He could detect no absorption and concluded that:

By this time it was firmly established that ozone was responsible for the cutoff in the solar spectrum. It was further established that this ozone was not near the ground, but was in the upper atmosphere, and a quantitative estimate of the amount of ozone had been made which was reasonably close to the amount now known to be in the atmosphere.

In the middle 1920s, G. M. B. Dobson (8) invented a new spectrometer that is still being used around the world today. He used this spectrometer to institute the first regular measurements of the total amount of ozone in the atmosphere. He discovered that there were day-to-day fluctuations in the ozone amount over Oxford, England and that there was a regular seasonal variation. He hypothesized that the variations in ozone might be related to variations in atmospheric pressure. To test this idea he had several more spectrometers constructed and distributed throughout Europe (9). These measurements demonstrated regular variations in ozone with the passage of weather systems. One of these spectrometers was installed in the town of Arosa in the Swiss Alps. Measurements have continued to be made at that station since 1926 (10). The record is now 73 years long.

While these developments were occurring, laboratory chemists were busily engaged in studies designed to understand the structure and properties of the ozone molecule. In particular, the photochemical and thermal decomposition of ozone was examined under a variety of conditions. The earliest study of the thermal decomposition of ozone was that of Warburg in 1902. Early studies of the decomposition of ozone using ultraviolet light included those of Regener in 1906 and of Eva von Bahr in 1910. These early studies were summarized by Griffith and McKeown (11).

It was quickly recognized that these studies were crucially dependent on the purity of the ozone/oxygen mixture used. Small amounts of impurities could radically change the results. Wiegert discovered in 1907 that the photo-decomposition of ozone under blue light was greatly accelerated by the presence of chlorine gas. In 1915, Wiegert and Böhm noted the catalyzing effect of hydrogen on the decomposition of ozone under the action of ultraviolet light. In 1925, Griffith and McKeown (11) mention in passing, that bromine speeds the decomposition of ozone greatly. All of these studies together laid the foundation for later discoveries of the importance of chemical acceleration of ozone loss.

The next era of atmospheric ozone research might be said to start with the classic theory paper of Chapman, published in 1930. For several decades, laboratory measurements had been carried out to determine the mechanism responsible for ozone decomposition, but arguments persisted as to the correct mechanism. In 1930, Schumacher (12) published a review of the laboratory work on ozone decomposition and concluded that the key intermediary formed in the initial absorption by ozone was atomic oxygen. Chapman (13), in the same year, applied this knowledge to the first model of the distribution of ozone as a function of altitude in the atmosphere.

Chapman’s mechanism had ultraviolet radiation from the sun being absorbed by molecular oxygen, breaking it into two oxygen atoms. These atoms then attached themselves to O₂ to form ozone, O₃. The O₃ would absorb a uv photon to regenerate the atomic oxygen. The O atom would alternately stick to an O₂ and be driven off by a uv photon until it happened to find an ozone molecule with which it would react to reform two O₂ molecules.

Starting at the top of the atmosphere, Chapman’s theory predicts that the production rate, and hence concentration, of ozone increases as the total density of the atmosphere and of O₂ increases. It further predicts that the uv light available for O₂ photodissociation will be absorbed by O₂ and O₃ before it can reach lower altitudes. This will lead to a decrease in the number of uv photons available for production of atomic oxygen and a decrease in the production rate of ozone at lower altitudes. The result is a peak in the production rate and in the concentration of ozone at an altitude in the stratosphere, just as had been deduced from measurements.

F. W. P. Götz made the measurements with Dobson’s instrument at Arosa, Switzerland. He made measurements of the ratio the intensity of two wavelengths looking at the zenith sky throughout the day and noticed that the ratio of the intensities decreased as the sun set and then just as the sun was near the horizon, the ratio turned around and increased. He called this the Umkehr (turn-around) effect. In 1934, Götz, Meetham, and Dobson (14) published an interpretation of this phenomenon, pointing out that the shape of the turnaround was dependent on the shape of the altitude profile of the ozone concentration. They were thus able to provide experimental confirmation of the basic Chapman theory of ozone formation and loss.

Measurements of the total column amount of ozone using Dobson’s spectrometers were made over the next few decades at a growing number of locations. These made it possible to put together a climatology of the variation of ozone with latitude and season. This climatology showed that the amount of ozone was a minimum at the equator. The amount of ozone increased towards the pole and began exhibiting a strong seasonal variation with a maximum in the spring and a minimum in the late fall or early winter. This distribution of ozone is in direct conflict with the predictions of the Chapman theory that would predict ozone to be a maximum at the equator decreasing towards the pole.

An answer to this puzzle was put forward in 1949 by A. W. Brewer and Dobson (15). They suggested that there was a basic circulation through the stratosphere that moved ozone around and modified the concentrations that would be predicted by the purely photochemical Chapman theory. This circulation consisted of slow upward motion into the stratosphere in the tropics; a slow downward and poleward motion at middle latitudes; and return of air to the troposphere at middle and high latitudes.

This circulation would bring tropospheric air with small concentrations of ozone into the stratosphere in the tropics. It would take air from the ozone production region high in the low-latitude stratosphere downward and poleward. There, the ozone at low altitude would be shielded from the oblique ultraviolet rays of the sun. It would not be rapidly destroyed and could accumulate to much greater concentrations. This circulation would pump ozone towards the high latitudes throughout the winter when the sun was low, resulting in a maximum concentration at the end of winter or in early spring. The summer sun would then initiate chemical loss processes that would decrease ozone throughout the summer leading to the minimum at the end of fall. Thus the general behavior of the seasonal and latitudinal distribution of ozone had been described in a theory which was a merging of the photochemical theory of Chapman and the dynamical theory of Brewer and Dobson.

Explaining ozone was not the only motivation for Brewer and Dobson in the development of their concept of the circulation. They had recently developed a frost-point hygrometer for water vapor measurements and launched it on a balloon into the stratosphere (16). They discovered that the stratosphere was a very dry place; less than 10 parts per million of water compared to concentrations near the surface that could be as high as a few percent. The upward motion in the tropics would remove water from the air such that air entering the stratosphere would have water concentrations equal to or less than the saturation mixing ratio at the cold temperatures found in the tropical upper troposphere. The details of how this drying process works and how it might respond to global change is an active field of research today.

As the understanding of the distribution of stratospheric ozone continued to develop, work in the laboratory evolved into a much more detailed understanding of chemical properties and reactions. Methods were developed to study individual fast reactions occurring in laboratory systems and to measure extremely small concentrations of highly-reactive radical species. Eigen, Norrish, and Porter received the Nobel Prize in Chemistry for their studies, in the 1950s, of extremely fast reactions. These developments lead to a quantitative understanding of reaction kinetics that eventually showed the need for a modification to the Chapman photochemical theory.

Measurements of emissions of specific spectral lines became possible with the development of high-resolution spectrometers during the 1940s and 1950s. These instruments were pointed up into the sky to measure the so-called dayglow and nightglow of the atmosphere. Specific bands of molecules, such as nitric oxide (NO) and hydroxyl (OH), were measured. These measurements lead to the development of a scheme describing the chemical composition of many of the minor constituents of the atmosphere. In 1950 Bates and Nicolet (17) wrote their exposition on the chemistry of the hydrogen oxides in the upper atmosphere. Earlier, Nicolet (18) had written several papers spelling out details of the expected nitrogen oxide chemistry of the upper atmosphere. These papers laid the foundation for later developments but did not make the key connection of catalytic ozone loss.

A key development in the history of atmospheric ozone research was the International Geophysical Year (IGY) in 1957. In preparation for the IGY, the British Antarctic Survey station at Halley Bay was set up. This station would later become important for its long series of measurements leading up to the discovery of the Antarctic ozone hole. The network of stations making total ozone measurements using the Dobson spectrometers was greatly expanded during the IGY and the first international protocols for instrument inter-calibration were begun.

By the middle 1960s chemical evidence was accumulating that the production-loss balance in the Chapman theory of photochemistry of ozone was quantitatively incorrect. In 1964, J. Hampson (19) suggested that the reactions of the hydrogen oxides with ozone would result in a catalytic cycle that would enhance ozone loss in the normal Chapman photochemical model. In 1965, B. Hunt (20) put these reactions into a mathematical model of stratospheric ozone and demonstrated that these reactions reduced the calculated ozone concentrations to bring them into closer agreement with measurements.

In 1970, Crutzen (21) showed the importance of catalytic loss of ozone by the reaction of nitrogen oxides. In 1971, he published his model with hydrogen and nitrogen oxide catalytic reactions contributing to ozone loss. In 1971, Johnston (22) showed that the nitrogen oxides produced in the high-temperature exhaust of a proposed fleet of supersonic aircraft could contribute significantly to the nitrogen oxide budget and ozone loss. The new paradigm was set: ozone production is balanced by ozone loss due to catalytic reactions of the nitrogen and hydrogen oxides and human activities could influence this balance and affect ozone concentrations.

Instrumental techniques for atmospheric measurements became available at about this same time. Nitric acid vapor was detected in the stratosphere by Murcray (23). Hydrochloric acid vapor was detected by Farmer (24). The concentrations of chlorine monoxide molecules and of hydroxyl radicals were measured by Anderson and colleagues (25,26). The results of these and other measurements began to confirm that the new paradigm describing stratospheric ozone was on the right track.

It was at about this time that I was fortunate enough to get involved in research in this area. Ralph Cicerone and I began studying the possible effects of the space shuttle exhaust on the atmosphere. The shuttle boosters burned ammonium perchlorate and released hydrochloric acid during their passage through the stratosphere. We started by studying the effects of the shuttle, but ended up focussing on chlorine from volcanoes which appeared to be a large, albeit sporadic source for chlorine (27). We had a conversation with Don Stedman in which he said:

This was in 1972 before the chlorine issue had come to the forefront of the field. Stedman had done his Ph.D. in Michael Clyne’s laboratory in England. He had been using all of the latest laboratory techniques to study chlorine reactions in the laboratory and knew that the history of chlorine enhancing the recombination of ozone went back to the early 1900s.

Another colleague asked me why we were studying the perturbation of ozone by chlorine from the shuttle since it was clearly a negligible source on a global scale. I answered that someday, someone would come up with a larger source and then maybe our work on chlorine chemistry would be significant. Little did I know that someone already had. Mario Molina and Sherry Rowland were looking into the fate of the chlorofluorocarbons which were being ubiquitously used in air conditioning, aerosol spray cans, and other applications.

Molina and Rowland (28) developed what became known as the fluorocarbon-ozone theory in 1974. Briefly, the theory goes as follows. The fluorocarbons are long-lived compounds. They are not soluble in water; they are unreactive; and they do not absorb visible light. They will accumulate and become well mixed by small-scale motions in the troposphere. They will be slowly transported to the stratosphere by small and large-scale motions. There they will encounter ultraviolet light that does not reach the ground because of ozone absorption. This ultraviolet light will dissociate the molecules leaving a chlorine atom. The remaining radical will chemically react until all of the chlorine atoms have been released. These chlorine atoms will then participate in the catalytic cycles that destroy ozone until they are removed from the stratosphere by transport back down into the troposphere.

The theory clearly illustrates a principle that had been recently developed for the nitrogen oxides. Fast-reacting radicals are not effectively transported from the ground to the stratosphere. They react and become soluble compounds that are rapidly rained out. To get to the stratosphere, they need to be tied up in a long-lived, unreactive compound. For nitrogen oxides, this compound is nitrous oxide (N₂O). For chlorine, the only significant natural source compound is methyl chloride (CH₃Cl). The industrially-produced chlorofluorocarbons (e.g. CFC-11 which is CF₃Cl or CFC-12 which is CF₂Cl₂) quickly became the primary chlorine sources to the stratosphere as there industrial production increased.

The fluorocarbon ozone theory predicted that there would be a decline in ozone which would be confined to mostly the upper stratosphere around 40 km altitude. By the middle 1980s this should have led to a small decline in total ozone which would have not been detectable given the variability observed in total ozone.

In 1985, Farman, Gardiner, and Shanklin, (29) who worked for the British Antarctic Survey, reported on their nearly 30 year record of ozone measurements over their station at Halley Bay in Antarctica. They found a clear decline in the Antarctic spring (October) which had reached 40% by 1984. The data from the SBUV and TOMS instruments on the Nimbus 7 satellite confirmed these findings and showed that the decline was a continental scale phenomenon (30).

In quick succession, Solomon et al.(31), Wofsy and McElroy (32), and Crutzen and Arnold (33) published variants of the same theory. This theory was an extension of the fluorocarbon-ozone theory to include surface reactions on polar stratospheric cloud particles. These heterogeneous reactions take place preferentially in the southern polar region because the temperatures there are significantly colder than in the northern polar region, leading to more consistent formation of polar stratospheric clouds.

So, at this point, we were unable to find trends on a global scale, but we had a trend in the Antarctic spring that was much greater than any thing that had been predicted. In 1988, a group of 30 or so scientists was brought together to conduct a year-long study on trends in ozone. This group was called the Ozone Trends Panel (34). The initial reason for the study was a report by Don Heath (35) that the satellite data from the SBUV instrument on the Nimbus 7 satellite indicated a rapid downward trend in ozone. This assertion was met with general disbelief because of the difficulty in demonstrating that the results were any thing but degradation in orbit of the satellite instrument.

The study concluded that the satellite results were primarily satellite degradation. But, during the course of the study, Sherry Rowland and his student Neil Harris took a new look at the ground station data from the Dobson spectrometers, many of which had been taking regular measurements since the IGY in 1957. They found that, if the data were separated into seasons, the winter data did indeed show a significant trend at many northern high latitude stations. After several more years of data and a new technique for internal calibration of the satellite data, the trends began to show up in that satellite data on a global scale.

In 1989, Hofmann and Solomon (36) pointed out that similar heterogeneous reactions might be occurring on sulfate aerosols that existed in the normal stratosphere well outside the cold polar regions. In this case, the primary reaction converted nitrogen oxides to a more stable nitric acid. The decrease in nitrogen oxides reduced their interference in the catalytic cycle of chlorine oxides making the effect of the chlorine added to the atmosphere by chlorofluorocarbons stronger. The inclusion of heterogeneous chemistry on sulfate particles lead to prediction of a significant reduction in ozone in the lower stratosphere, where most of the ozone is located. The new predictions were more in line with the significant winter and spring trends observed in ozone concentration in the northern middle latitudes.

Another implication of heterogeneous chemistry on sulfate particles comes about because explosive volcanic eruptions inject large amounts of sulfur into the stratosphere. El Chichon in 1982 and Mt. Pinatubo in 1991 made significant perturbations to stratospheric sulfate particles. These would have a fairly large effect on stratospheric chemistry for about two years following the eruption. They probably also significantly alter the dynamics of the stratosphere because of the absorption of solar radiation by the aerosol particles.

The confounding of many influences over the last decade including the increase in chlorine, the injection of volcanic sulfur, year-to-year variations in the dynamics of the stratosphere, and also possibly the 11-year solar sunspot cycle, make the determination of trends in ozone an interesting and difficult problem.

Some obvious, important questions face the atmospheric ozone research community today. Will ozone respond to the leveling off and decrease in chlorine occurring over the next decade or so? Assuming that ozone does respond to this decrease in chlorine, will it respond in the way we expect? If the response is not exactly as expected, why is it not?

The answers to these questions have important implications for near-term research agendas. Carefully calibrated long-term measurement records are critical. Better understanding of the underlying phenomena responsible for ozone production, loss, and transport are necessary. Our knowledge must be incorporated into global models so that we can test our understanding of how the entire system will respond to changes.

Our understanding is, at this point, based on the present-day atmosphere for which we have measurements. This atmosphere will change. Atmospheric concentrations of methane, nitrous oxide, carbon dioxide, and other chemicals are increasing. Many of these increases will continue. Some of these will modify the stratosphere. Increased water vapor will result from methane. Stratospheric temperatures will cool because carbon dioxide will increase the rate of radiation to space. Both of these effects could lead to an increase in the occurrence of polar stratospheric clouds in the Arctic. This, in turn, could lead to a larger decrease in ozone, which will further cool the stratosphere. There are likely other, more subtle feedbacks through dynamics that could enhance or mitigate ozone effects. The decline in chlorine should allow ozone to begin to recover. That recovery might be slowed by some of these feedbacks. Other subtle, climate-chemistry interactions might occur that we have not yet conceived.

There is clearly a rich field of research on how the stratosphere will respond to future changes in climate and on how the stratosphere might interact with those future changes of climate.

(1) A. R. LEEDS, "Lines of Discovery in the History of Ozone", Annals of the New York Academy of Sciences 1 (3): 363-391 (1880).

(2) C. B. FOX, Ozone and Antozone: Their History and Nature, (London, England: J & A Churchill, 1873).

(3) M. A. CORNU, "Sur la limite ultra-violette du spectre solaire", Comptes Rendus 88: 1101-1108 (1879).

(4) W. N. HARTLEY, "On the Probable Absorption of the Solar Ray by Atmospheric Ozone", Chem. News, Nov. 26, p. 268 (1880).

(5) C. FABRY and H. BUISSON, "L’absorption de l’ultra-violet par l’ozone et la limite du spectre solaire", J. de Phys. 3 (5): 196-206 (1913).

(6) A. FOWLER and R. J. STRUTT, Proc. Roy. Soc. London A, 93: 577 (1917).

(7) R. J. STRUTT, "Ultra-violet Transparency of the Lower Atmosphere, and its Relative Poverty in Ozone", Proc. Roy. Soc. London A, 94: 260-268 (1918).

(8) G. M. B. DOBSON and D. N. HARRISON, "Measurements of the Amount of Ozone in the Earth’s Atmosphere and its Relation to the Other Geophysical Conditions", Proc.Roy. Soc. London A, 110: 660-693 (1926).

(9) G. M. B. DOBSON, D. N. HARRISON, and J. LAWRENCE, "Measurements of the Amount of Ozone in the Earth’s Atmosphere and its Relation to other Geophysical Conditions-Part II", Proc. Roy. Soc. London A, 114: 521-541 (1927).

(10) J. STAEHELIN, R. KEGEL, and N. R. P. HARRIS, "Trend analysis of the homogenized total ozone series of Arosa (Switzerland), 1926-1996", J. Geophys. Res. 103: 8389-8399 (1998).

(11) R. O. GRIFFITH and A. M. McKeown, "The photochemical and thermal decomposition of ozone", Trans. Faraday Soc. 21: 597-602 (1925).

(12) H. J. SCHUMACHER, "The Mechanism of the Photochemical Decomposition of Ozone", J. Amer. Chem. Soc., 52: 2377-2391 (1930).

(13) S. CHAPMAN, "On Ozone and Atomic Oxygen in the Upper Atmosphere", Phil. Mag. 10: 369-383 (1930).

(14) F. W. P. GÖTZ, A. R. MEETHAM, and G. M. B. DOBSON, "The vertical distribution of ozone in the atmosphere", Proc. Roy. Soc. London A, 145: 416-446 (1934).

(15) A. W. BREWER, "Evidence for a World Circulation Provided by the Measurements of Helium and Water Vapour Distribution in the Stratosphere", Quart. J. Roy. Met. Soc., 75: 351-363 (1949).

(16) G. M. B. DOBSON, A. W. BREWER, and B. M. CWILONG, "Meteorology of the lower stratosphere", Proc. Roy. Soc. London A, 185: 144-175 (1946).

(17) D. R. BATES and M. NICOLET, "The photochemistry of atmospheric water vapor", J. Geophys. Res., 55: 301-327 (1950).

(18) M. NICOLET, "The aeronomic problem of nitrogen oxides", J. Atmos. Terr. Phys. 7: 152-169 (1955).

(19) J. HAMPSON, "Chemical Instability of the Stratosphere", paper presented at the International Association of Meteorology and Atmospheric Physics (IUGG) Symposium on Atmospheric Radiation, Leningrad, USSR. (1964).

(20) B. G. HUNT, "The need for a modified photochemical theory of the ozonesphere", J. Atmos. Sci. 23: 88-95 (1966).

(21) P. J. CRUTZEN, "Ozone production rates in an oxygen, hydrogen, nitrogen-oxide atmosphere", J. Geophys. Res., 76: 7311-7327 (1971).

(22) H. S. JOHNSTON, "Reduction of stratospheric ozone by nitrogen oxide catalysts from supersonic transport exhaust", Science, 173: 517-522 (1971).

(23) D. G. MURCRAY, T. G. KYLE, F. H. MURCRAY, and W. J. WILLIAMS, "nitric acid and nitric oxide in the lower stratosphere", Nature, 218: 78-79 (1968).

(24) C. B. FARMER, O. F. RAPER, and R. H. NORTON, "Spectroscopic detection and vertical distribution of HCl in the troposphere and stratosphere", Geophys. Res. Lett., 3: 13-16 (1976).

(25) J. G. ANDERSON, J. J. MARGITAN, and D. H. STEDMAN, "Atomic chlorine and the chlorine monoxide radical in the stratosphere: Three in-situ observations", Science, 198: 501-503 (1977).

(26) J. G. ANDERSON, "The absolute concentration of OH (X^2p ) in the Earth’s stratosphere", Geophys. Res. Lett., 3: 165-168 (1976).

(27) R. S. STOLARSKI and R. J. CICERONE, "Stratospheric chlorine: A possible sink for ozone", Can. J. Chem, 52: 1610-1615 (1974).

(28) M. J. MOLINA and F. S. ROWLAND, "Stratospheric sink for chlorofluoromethanes: chlorine atom catalyzed destruction of ozone", Nature, 249: 810-814 (1974).

(29) J. C. FARMAN, B. G.GARDINER, and J. D. SHANKLIN, "Large losses of ozone in Antarctica reveal seasonal ClO_x/NO_x interaction", Nature, 315: 207-210 (1985).

(30) R. S. STOLARSKI, A. J. KRUEGER, M. R. SCHOEBERL, R. D. McPeters, P. A. NEWMAN, and J. C. ALPERT, "Nimbus 7 SBUV/TOMS measurements of the springtime Antarctic ozone decrease", Nature, 322: 808-811 (1986).

(31) S. SOLOMON, R. R. GARCIA, F. S. ROWLAND, and D. J. WUEBBLES, "On the depletion of Antarctic ozone", Nature, 321: 755-758 (1986).

(32) M. B. McELROY, R. J. SALAWITCH, S. C. WOFSY, and J. A. LOGAN, "Reductions of Antarctic ozone due to synergistic interactions of chlorine and bromine", Nature, 321: 759-762 (1986).

(33) P. J. CRUTZEN and F. ARNOLD, "Nitric acid cloud formation in the cold Antarctic stratosphere: A major cause for the springtime ‘ozone hole’", Nature, 324: 651-655 (1986).

(34) R. T. WATSON, chair, Report of the International Ozone Trends Panel 1988, (Geneva, Switzerland: World Meteorological Organization, Global Ozone Research and Monitoring Project-Report No. 18, 1990).

(35) D. F. HEATH, "Non-seasonal changes in total column ozone from satellite observations, 1970-1986", Nature, 332: 219-227 (1988).

(36) D. J. HOFMANN and S. SOLOMON, "Ozone destruction through heterogeneous chemistry following the eruption of El Chichón", J. Geophys. Res., 94: 5029-5041 (1989).