Charles Fabry’s quantification of the amount of ozone in an atmospheric column led to the discovery of the ozone layer.
Experiments performed in the eighteenth century showed that air is a mixture composed of different substances rather than a single element as had long been supposed. Early study of the major reactive gases of the atmosphere—oxygen, nitrogen, and carbon dioxide—was followed by investigations of other components.
Ozone is a notable gas because of its sharp odor. Impure ozone is produced easily by a spark in oxygen and can collect in the atmosphere near large electric motors. Curious about the possible concentration of ozone at the earth’s surface, Walter Noel Hartley
The science of spectroscopy was at an early stage of development in the late nineteenth and early twentieth centuries. No good light detectors were available other than photographic film. Although film images were all black-and-white, scientists recognized that the dispersion of light into different colors caused by a prism made the different colors fall onto different places on a film. The relative attenuation of light at each wavelength—each position on a film—is characteristic of a particular substance. It can be used to identify the presence and quantity of a substance in a mixture such as air.
By studying films, researchers learned that ozone absorbs both visible and ultraviolet light. Virtually complete atmospheric absorption of sunlight at wavelengths below 300 nanometers in the ultraviolet was noticed first by Alfred Cornu. Hartley identified this with ozone, thus absorptions in this range are termed the Hartley bands. Other examples of absorptions include Chappuis bands, in the region of visible light, and Huggins bands, in the ultraviolet above 300 nanometers. The Chappuis bands are responsible for the blue color of ozone.
Because surface measurements at high and low altitudes showed little difference in the wavelength of cutoff of sunlight, Hartley suggested that much ozone existed high in the atmosphere. Charles Fabry determined how much ozone existed at that level. Fabry was a physicist whose life’s work involved studies of optics. With Alfred Pérot,
The intensity of the radiation reaching the earth’s surface is a function of the intensity of the sunlight reaching the top of the atmosphere, the path length of the light through the atmosphere and thus the angle from the zenith (vertical), the amount of dust and molecular scattering of the light on its way through the atmosphere, and the absorption of light by gas molecules such as ozone. All of these except dust scattering and path length vary with wavelength.
Working with Henri Buisson,
In 1920, Fabry and Buisson modified their instrument to reduce the amount of stray sunlight reaching the photographic film. They made repeated measurements of ozone and checked several pairs of absorptions to ensure that it was indeed ozone they were measuring. They found its concentration steady at 3 millimeters. In discussing their results, Fabry and Buisson showed remarkable insight. They noted that ozone probably forms in the upper atmosphere because of absorption of solar radiation and that the maximum concentration of ozone might be at a height of 40 kilometers (about 24.9 miles).
From his own study of meteor tracks in 1921, Frederick Alexander Lindemann (who later became Viscount Cherwell) suggested that the atmosphere above 30 kilometers (about 18.6 miles) is much warmer than had been supposed. He believed that absorption of solar radiation by ozone is responsible for the warmth. As it was already known that the temperature of the stratosphere changes with weather patterns, there was some speculation that changes in ozone concentrations might be responsible for these patterns.
Soon thereafter, G. M. B. Dobson established a program at Oxford University for the monitoring of atmospheric ozone. Using a spectrograph he built based on the design of Fabry and Buisson, Dobson collected a series of measurements from a hill outside Oxford, England, in 1925. From these the annual variation in atmospheric ozone was first seen: a maximum in the spring and a minimum in the fall. In the next year and a half, Dobson distributed a set of six spectrographs throughout Europe to investigate the relation between ozone and weather patterns. His finding of higher concentrations of ozone behind cyclones (large wind systems rotating around regions of low pressure) and ahead of anticyclones (systems around regions of high pressure) further reinforced interest in ozone measurement.
In 1928 and 1929, the same instruments were sent to sites throughout the world to examine gross variations in ozone with latitude and location. Concentrations were found to be higher and far more variable at higher latitudes. Later measurements suggest a maximum at roughly 60 degrees latitude, but with considerable variation.
In Dobson’s early work, all exposed photographic plates were returned to Oxford University for development and analysis. This practice removed a potential source of inconsistency. After 1930, the measurement program continued until World War II, with specially designed spectrophotometers using photomultipliers as detectors in place of film. These were particularly advantageous for measurements at low light levels, such as on cloudy days and early or late in a day. These took on great importance after it was found that information about the altitude distribution of the ozone could be gleaned from such measurements.
One of the people who produced exposures for Dobson was F. W. P. Götz, who was working with Dobson to establish the vertical distribution of ozone. Based on variation of their measurements with zenith angle, they tentatively concluded that ozone might be concentrated in a layer near 50 kilometers (about 31.1 miles) above the earth’s surface, in rough agreement with the suggestion of Fabry and Buisson.
Because Götz’s measurements were made in summer at the far northern island of Spitsbergen, Norway, the Sun remained close to the horizon for relatively long periods. Light intensity drops steadily at all wavelengths as the angle of the Sun from the zenith increases, because beams must pass through a greater thickness of atmosphere. Götz noted, however, that the ratio of intensity at a short versus a long wavelength went through a minimum near 85 degrees, then, contrary to expectation, increased for observations made overhead while the Sun was approaching the horizon.
Light measured in this way has all been scattered. The higher the altitude of scattering, the smaller the opportunity to be absorbed. The longer-wavelength light is effectively scattered from lower altitudes, so its intensity keeps decreasing rapidly as the Sun sets. Shorter-wavelength light is scattered above the ozone layer, and so its intensity decreases only slowly. One can determine the altitude range of the ozone layer by measuring this Umkehr, or reversal effect. A concentration maximum occurs at roughly 25 kilometers (15.5 miles), decreasing rather sharply at higher altitudes and more slowly toward the earth’s surface.
The simplicity of the measurement, and the ability to use the instruments already in service for determining total ozone in a column, made this the most common approach to locating the ozone layer until much later in the twentieth century, when satellite data became widely available. A scattering of balloon and rocket measurements confirmed Götz’s conclusion and suggested some variations of ozone concentration that ground-based measurements could not detect.
One instructive improvement on Dobson’s work was based on measurement of the changes in absorption of light by ozone with change in temperature. At the low temperatures of the upper atmosphere, background absorption is reduced, thus one can use wavelengths at absorption bands of ozone to determine ozone concentration; measurement of background absorption gives ozone temperatures. Such data confirm a region of high temperatures above 35 kilometers (21.7 miles), which is caused by ozone absorption.
Sidney Chapman
Research concerning ozone in the late twentieth century focused more on its consumption than on its formation. In 1950, Marcel Nicolet examined the role of radical hydroxyl in the reaction chemistry of ozone. In 1970, Paul J. Crutzen further modified the picture to include the catalytic and stoichiometric reactions with nitrogen oxides. In the late 1970’s and 1980’s, researchers raised concerns about the catalytic destruction of ozone by chlorine radicals, especially at high latitudes. Such destruction has the potential to increase the intensity of ultraviolet radiation reaching the earth, changing the temperature patterns of the stratosphere and thus changing weather patterns around the world.
Fabry and Buisson’s publication of their findings on January 17, 1913, stimulated little other work regarding ozone, and such research was curtailed sharply during World War I. The experiment was clever, but the result was initially merely a curiosity. The first quantitative determination of ozone in the atmosphere was not Fabry’s most notable achievement as a physicist.
The paper that Fabry and Buisson published in 1921 had a greater effect. The results were more reliable because of improvements on the instrument and more repetitions of measurements. More important, Lindemann and Dobson could apply the conclusion and discussion immediately to explain their own meteor data. Fabry and Buisson’s paper gave Dobson a basis for believing that an extended series of measurements of atmospheric ozone was not only possible but also potentially useful.
Once started on a program of global monitoring for ozone, Dobson found that he could use his early results to convince granting bodies that money invested in such measurements was well spent. The monitoring results that Dobson reported in 1930 served as a framework on which all later measurements were based. Dobson’s conclusions proved to be erroneous only in a few points, demonstrating his careful attention to equipment and experimental design.
The studies on ozone helped form or correct meteorologists’ views of the atmosphere and thus helped keep the fields of atmospheric physics and meteorology on a sound footing. For example, the study of ozone offered a means to measure gas temperatures above the altitudes reached by most balloons. The most impressive feature of ground-based ozone study is the long-term and global nature of the information. It was not until rocket observations were made in the late 1950’s and 1960’s that detailed investigation of the stratosphere was possible. Without a comprehensive database already in place, sensible choices and interpretations of experiments would not have been possible.
Because investigation of matter in the upper atmosphere is so difficult, scientists have applied a great deal of effort to the development of mathematical models of atmospheric chemistry and physics. These models consider all the different kinds of molecules and their reactions in attempting to understand what is happening. This work requires actual measurements with which to check the models. Although data gathered by satellites and high-altitude airplane flights have improved researchers’ understanding of ozone distribution, the scale provided is not fine enough to allow the examination of ozone movements near the tropopause. Further improvement of atmospheric models requires even more ozone monitoring, so that scientists can understand local variations as well as differences between widely scattered points.
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citation-type="booksimple" xlink:type="simple">Christie, Maureen. The Ozone Layer: A Philosophy of Science Perspective. New York: Cambridge University Press, 2001. Presents the history of human knowledge about stratospheric ozone in a manner accessible to lay readers. Addresses basic issues of both real-world science and the philosophy of science. Includes figures, references, and index. -
citation-type="booksimple" xlink:type="simple">Craig, Richard A. The Edge of Space: Exploring the Upper Atmosphere. Garden City, N.Y.: Doubleday, 1968. Discusses the various methods of measuring atmospheric ozone. Written at a time when understanding of ozone chemistry was far less developed than it is in the twenty-first century, but still useful. Based on an important book in aeronomy, The Upper Atmosphere: Meteorology and Physics, which Craig published in 1965, this volume is aimed at a general audience. -
citation-type="booksimple" xlink:type="simple">Dobson, G. M. B. “Forty Years’ Research on Atmospheric Ozone at Oxford: A History.” Applied Optics 7 (March, 1968): 387-405. Dobson’s personal recollection of the development of the global network for ozone monitoring presents a fascinating account of the scientific work. Notes the unexpected observation in 1956 of low concentrations of ozone until late in the Antarctic spring—the phenomenon, intensified in subsequent years by the presence of radical chlorine from chlorofluorocarbons, now known as the ozone hole. -
citation-type="booksimple" xlink:type="simple">Gribbin, John. The Hole in the Sky: Man’s Threat to the Ozone Layer. Rev ed. New York: Bantam Books, 1988. Details concerns about the destruction of the ozone layer. Probably the most balanced of the books published on this topic in the late 1980’s. -
citation-type="booksimple" xlink:type="simple">_______, ed. The Breathing Planet. New York: Basil Blackwell, 1986. Collection of short articles originally published in the English journal New Scientist includes a section on ozone. Chapter titled “Monitoring Halocarbons in the Atmosphere” suggests the difficulties and uncertainties of monitoring substances in the atmosphere. -
citation-type="booksimple" xlink:type="simple">Kerr, J. B., I. A. Asbridge, and W. F J. Evans. “Intercomparison of Total Ozone Measured by the Brewer and Dobson Spectrophotometers at Toronto.” Journal of Geophysical Research 93 (September, 1988): 11129-11140. Plots of careful ozone measurements and discussion of factors recognized as affecting such measurements over long periods at one site demonstrate, to the seriously suspicious, the variability that makes spotting trends in stratospheric ozone concentration so difficult. -
citation-type="booksimple" xlink:type="simple">Parson, Edward A. Protecting the Ozone Layer: Science and Strategy. New York: Oxford University Press, 2003. Comprehensive technical discussion of efforts to protect the ozone layer undertaken through international cooperation. Chapter 2 is devoted to a review of early stratospheric science. Includes notes, references, and index.
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