BEYOND DISCOVERY T H E PAT H F R O M R E S E A R C H T O H U M A N B E N E F I TTHE OZONE DEPLETION PHENOMENONL ike an infection that grows more and more virulent, the continent-size hole in Earth’s ozone layer keeps getting bigger and bigger. The Problem Each year since the late 1970s, much of the protec-tive layer of stratospheric ozone above Antarctica has For four months of every year, Antarctica’sdisappeared during September, creating what is popu- McMurdo Research Station lies shrouded in dark-larly known as the ozone hole. The Antarctic hole now ness. Then the first rays of light peek out over themeasures about 9 million square miles, nearly the size horizon. Each day, the sun lingers in the sky just aof North America. Less dramatic, but still significant, little longer and the harsh polar winter slowly givesdepletion of ozone levels has been recorded around the way to spring.globe. With less ozone in the atmosphere, more ultravio- Spring also brings another type of light to thelet radiation strikes Earth, causing more skin cancer, Antarctic, a light that harms instead of nurtures. Ineye damage, and possible harm to crops. this season of new beginnings, the hole in the What is ozone? How did researchers discover its role ozone layer reforms, allowing lethal ultraviolet radi-in Earth’s atmosphere and the devastating consequences ation to stream through Earth’s atmosphere.of its depletion? The fol- The hole lasts forlowing article, adapted only two months, butfrom an account by Dr. F. its timing could not beSherwood Rowland, a pio- worse. Just as sunlightneering researcher in the awakens activity in dor-field who shared the 1995 mant plants and ani-Nobel Prize in Chemistry mals, it also delivers afor his work, attempts to dose of harmful ultravi-answer these and other olet radiation. Afterquestions. In doing so, it eight weeks, the holedramatically illustrates leaves Antarctica, onlyhow science works and, in to pass over more popu-particular, how basic lated areas, includingresearch—motivated by a New Zealand anddesire to understand Australia. This biologi-nature—often leads to cally damaging, high-practical results of energy radiation canimmense societal benefit A satellite image of thethat could not have been ozone hole (pink area)anticipated when the over Antarctica taken onresearch first began. September 25, 1995.N A T I O N A L A C A D E M Y O F S C I E N C E S
cause skin cancer, injure eyes, harm the immune system, are mainly released at northern latitudes—mostly and upset the fragile balance of an entire ecosystem. from Europe, Russia, Japan, and North America— Although, two decades ago, most scientists and play a leading role in lowering ozone concentra- would have scoffed at the notion that industrial tions around the globe. chemicals could destroy ozone high up in the Worldwide monitoring has Although, two atmosphere, researchers now know that chlorine shown that stratospheric decades ago, most creates the hole by devouring ozone molecules. ozone has declined for at least Years of study on the ground, in aircraft, and from two decades, with losses of scientists would satellites has conclusively identified the source of about 10 percent in the win- have scoffed at the chlorine: human-made chemicals called chloro- ter and spring and 5 percent the notion that fluorocarbons (CFCs) that have been used in spray in the summer and autumn in industrial chemi- cans, foam packaging, and refrigeration materials. such diverse locations as cals could destroy Europe, North America, and Australia. Researchers now ozone high up in find depletion over the North the atmosphere, All About Ozone Pole as well, and the problem researchers now seems to be getting worse know that chlo- Ozone is a relatively simple molecule, consisting each year. According to a United Nations report, the rine creates the of three oxygen atoms bound together. Yet it has dramatically different effects depending upon its annual dose of harmful ultra- hole by devouring location. Near Earth’s surface, where ozone comes violet radiation striking the ozone molecules. into direct contact with life forms, it primarily dis- northern hemisphere rose by plays a destructive side. Because it reacts strongly 5 percent during the past decade. with other molecules, large concentrations of ozone During the past 40 years, the world has seen an near the ground prove toxic to living things. At alarming increase in the incidence of malignant skin higher altitudes, where 90 percent of our planet’s cancer; the rate today is tenfold higher than in the ozone resides, it does a remarkable job of absorbing 1950s. Although the entire increase cannot be blamed ultraviolet radiation. In the absence of this gaseous on ozone loss and increased exposure to ultraviolet shield in the stratosphere, the harmful radiation has radiation, there is evidence of a relationship. Scientists a perfect portal through which to strike Earth. estimate that for each 1 percent decline in ozone lev- Although a combination of weather conditions els, humans will suffer as much as a 2 to 3 percent and CFC chemistry conspire to create the thinnest increase in the incidence of certain skin cancers. ozone levels in the sky above the South Pole, CFCs UV-B radiation Exploring Earth’s Atmosphere Like many lines of scientific inquiry, research lead- km Stratospheric ing to the prediction and discovery of global ozone 50 ozone depletion and the damaging effects of CFCs followed sphere a path full of twists and turns. Investigators did not set Strato Mt. Everest out to determine whether human activity affects our sphere environment nor did they know much about chemical Tropo pollutants. Instead, they began with basic questions 10 km about the nature of Earth’s atmosphere—its composi- tion, density, and temperature distribution. Stratospheric ozone occupies the region of the atmosphere between 10 and 50 kilometers from Earths surface and provides a shield against damaging ultraviolet radiation.2 B E Y O N D D I S C O V E R Y
The composition of our planet’s atmosphere fas- Frequency Wavelength (in hertz) (in centimeters)cinated humans long before chemistry became aformal science. “The storm thundered and light- 1025 10-15ened, and the air was filled with sulfur,” Homer Gammawrote in the Odyssey, referring to the sharp odor, rayscreated during thunderstorms, of what later became 1020 10-10 high frequencyknown as ozone. short wavelength By the late 1800s, atmospheric scientists had iso- X–rays Ultravioletlated carbon monoxide and inferred the existence of light 1015 VISIBLE LIGHT 10 -5a second combustible gas in the air, which they ten-tatively identified as methane, the simplest hydrocar- Infrared raysbon. But in attempting to further analyze the com- 1010 Radar waves 1position of the atmosphere, researchers at the turn Microwavesof the century faced a major stumbling block: virtu- Television and FM radio wavesally all gases, except for molecular nitrogen and oxy- AM radio waves 105 105gen, exist in such minute concentrations that avail-able equipment could not detect them. Help, however, was on the way. During the low frequency 10101880s, scientists had begun perfecting a new, highly long wavelength 1precise method of identifying a compound by Cancer-causing ultraviolet radiation occupies the regionrecording a special kind of chemical fingerprint—the of the spectrum between visible light and even higherparticular pattern of wavelengths of light it emits or frequency radiation such as x-rays and gamma rays.absorbs. Scientists call this pattern a spectrum. In the 1920s, G.M.B. Dobson developed a spec- had begun developing a cavalcade of new instru-trometer that could measure small concentrations of ments and methods to measure minute quantities ofozone. By measuring the spectrum of air, the compounds in the laboratory.Belgian scientist M.V. Migeotte demonstrated in Such research spurred1948 that methane is a common constituent of the advances on two fronts: The hole lasts foratmosphere with a concentration of about one part a substantial increase in the only two months,per million by volume. Soon, scientists had the tools precision and accuracy of mea-to detect other atmospheric gases that occur in con- but its timing surements of atmosphericcentrations one-tenth to one-hundredth as great as gases and a striking decrease could not bethat. By the 1950s, researchers had identified 14 in the minimum concentration worse. Just asatmospheric chemical constituents. of a compound that must be sunlight awakens Despite this progress, researchers were still miss- present to be detected. activity in dor-ing a major piece of the atmospheric puzzle. All of As a result, the number ofthe compounds detected possessed an even number mant plants and atmospheric compounds iden-of electrons, a characteristic which typically gives tified by scientists has animals, it alsochemical stability. Other less common compounds increased from 14 in the early delivers a dose ofwith an odd number of electrons—known as free 1950s to more than 3,000 harmful ultravio-radicals—readily undergo chemical reactions and do today. Detectors today rou-not survive for long. These compounds play crucial let radiation. tinely measure compounds atroles in such phenomena as urban smog, the loss of concentrations below one partstratospheric ozone, and the global removal of per trillion, and some can record gases that occur inatmospheric impurities. concentrations one-thousandth as great at that. Scientists had not detected free radicals because Even in some of the most remote locations onthey reside in the atmosphere at concentrations well Earth, scientists have detected hundreds of com-below the part-per-million level that state-of-the-art pounds. Curiously, some of the substances that occurequipment in 1948 could detect. But unrelated in the smallest concentrations rank as some of theresearch in an entirely different field, analytical chem- biggest players in altering the atmosphere. A case inistry, soon came to the rescue. Analytical chemists point: the group of chemicals known as CFCs. N A T I O N A L A C A D E M Y O F S C I E N C E S 3
20 Ozone partial pressure (mPa) 15 Enter the CFCs CFCs were invented about 65 years ago during a search for a new, nontoxic substance that could serve 10 as a safe refrigerant. One of these new substances, often known by the DuPont trademark Freon, soon replaced ammonia as the standard cooling fluid in 5 home refrigerators. It later became the main coolant in automobile air conditioners. The 1950s and 1960s saw CFCs used in a variety 0 0 5 10 15 20 25 30 35 of other applications: as a propellant in aerosol sprays, Altitude (km) in manufacturing plastics, and as a cleanser for elec- Ozone loss over the South Pole in 1995 (in green) compared tronic components. All this activity doubled the world- with 1993 (in red). The blue line shows values before ozone wide use of CFCs every six to seven years. By the early destruction began. 1970s, industry used about a million tons every year. SOURCE: National Oceanic and Atmospheric Administration. Yet as recently as the late 1960s, scientists remained unaware that CFCs could affect the concentration of about 60 parts per trillion. To put atmosphere. Their ignorance was not from lack of that measurement in perspective, the concentration interest, but from lack of tools. Detecting the of methane (natural gas) is 25,000 times greater. minuscule concentrations of these compounds in Twenty years earlier, merely detecting methane had the atmosphere would require a new generation of been considered a major feat. sensitive detectors. Lovelock found CFC-11 in every air sample that After developing such a detector, the British sci- passed over Ireland from the direction of London. entist James Lovelock, in 1970, became the first to That was not surprising, because most major cities, detect CFCs in the air. He reported that one of including London, widely used CFCs. However, these compounds, CFC-11, had an atmospheric Lovelock also detected CFC-11 from air samples Advances in Atmospheric Science and Policy Decisions Through 1996 This timeline shows the chain of events leading to prediction of the ozone depletion phenomenon, recognition of its consequences, and eventual actions to avert a threatened disaster. It is rich in examples of how basic research often contributes to unanticipated outcomes of immense societal benefit.1840 1913–1932 1957Christian Friedrich C. Fabry and M. Buisson show 1973 As part of theSchönbein identifies that the total amount of ozone Richard Stolarski Internationalozone as a compo- in a vertical column of the and Ralph Geophysical Year, four 1970nent of the lower atmosphere can be measured Cicerone discover to five research sta- The Nimbusatmosphere and and that it equals (in modern stratospheric tions in Antarctica series of satel-names it. units) 300 Dobson units. chlorine chain begin making regular lites begins reaction. ozone measurements. making ozone measurements. 1881 W.N. Hartley identifies 1924 ozone as the substance G.M.B. Dobson sets that absorbs ultraviolet 1930 1970 up a regular program radiation from the sun Sydney Chapman James Lovelock of ozone measure- at wavelengths below explains how sun- uses his electron ments at Oxford with 290 nanometers. He light striking mol- capture detector his newly developed also shows that ozone ecular oxygen in to measure chloro- spectrophotometer. resides primarily at the atmosphere fluorocarbons(CFCs). high altitudes. generates ozone.
directly off the North Atlantic, uncontaminated by a lecture describing Lovelock’s work. Like other recent urban pollution. researchers at the time, Rowland had no inkling that This unexpected discovery prompted Lovelock CFCs could harm the environment, but the injection to do further studies. Accordingly, he asked the into the atmosphere of large quantities of previously British government for a modest sum of money to unknown compounds piqued his interest. What would place his apparatus on board a ship traveling from be the ultimate fate of these compounds? Rowland, England to Antarctica. His request was rejected; joined by Mario Molina, a colleague at the University one reviewer commented that even if such a mea- of California, Irvine, decided to find out. surement succeeded, he could not imagine a more The scientists showed that CFCs remained useless bit of knowledge than finding the atmos- undisturbed in the lower atmosphere for decades. pheric concentration of CFC-11. Invulnerable to visible sunlight, nearly insoluble in But Lovelock persisted. Using his own money, water, and resistant to oxidation, CFCs display an he put his experiment aboard the research vessel impressive durability in the atmosphere’s lower Shackleton in 1971. Two years later the British depths. But at altitudes above 18 miles, with 99 researcher reported that his shipboard apparatus had percent of all air molecules lying beneath them, detected CFC-11 in every one of the more than 50 CFCs show their vulnerability. At this height, the air samples collected in the North and South harsh, high-energy ultraviolet radiation from the Atlantic. Lovelock correctly concluded that the gas sun impinges directly on the CFC molecules, break- was carried by large-scale wind motions. He also ing them apart into chlorine atoms and residual stated that CFCs were not hazardous to the envi- fragments. ronment, a conclusion soon to be proven wrong. If Rowland and Molina had ended their CFC study with these findings, no one other than atmos- pheric scientists would ever have heard about it. However, scientific completeness required that the Ozone Loss: The Chemical researchers explore not only the fate of the CFCs, Culprits but also of the highly reactive atomic and molecular fragments generated by the ultraviolet radiation. In 1972, the life of atmospheric scientist F. In examining these fragments, Rowland and Sherwood Rowland took a critical turn when he heard Molina were aided by prior basic research on chemical 1984 1996 A British research 1988 group led by Joseph 1987 Complete ban 1976 The Montreal The United on industrial Farman detects a The National Academy of Protocol is States ratifies production of 40 percent ozone Sciences releases its signed, calling the Montreal CFCs goes loss over Antarctica report verifying the for eventual Protocol in a into effect. during spring in Rowland-Molina finding. worldwide CFC unanimous vote. the southern hemi- sphere. reduction by 50 percent. 1976 1988 1995 974 The Food and Drug 1978 Scientists present F. Sherwood Sherwood Administration and CFCs used in 1985 preliminary find- Rowland, Mario owland and the Environmental aerosols are ings of a hole in Molina, and NASA satellite dataMario Molina Protection Agency banned in the the ozone layer Paul Crutzen confirm the existence iscover that announce a phase- United States. over the Arctic. awarded the of the ozone hole FCs can out of CFCs in Nobel prize for over the Antarctic. estroy ozone aerosols. their work inn the stratos- atmospheric here. chemistry.
kinetics—the study of how quickly molecules react with one another and how such reactions take place. Scientists had demonstrated that a simple lab- CFCs CCl3F + UV Light Cl + CCl2F + oratory experiment will show how rapidly a particu- Sunlight Cl Cl lar reaction takes place, even if the reaction involves Cl C C the interaction of a chlorine atom with methane at Chlorine F Cl Cl Cl F an altitude of 18 miles and a temperature of -60 Atoms degrees Fahrenheit. Rowland and Molina did not have to carry out O2 + UV Light 2O even a single laboratory experiment on the reaction rates of chlorine atoms. They had only to look up 2Cl + 2O3 2ClO + 2O2 Chlorine the rates already measured by other scientists. Basic 2ClO + 2O 2Cl + 2O2 Atoms research into chemical kinetics had reduced a Destroy Net Reaction: 2O3 3O2 Ozone decade’s worth of work to two or three days. Molecules 2 O 3 O O After reviewing the pertinent reactions, the two O O researchers determined that most of the chlorine atoms combine with ozone, the form of oxygen that protects Earth from ultraviolet radiation. When Chlorine atoms induce the decomposition of two ozone mole- cules into three oxygen molecules in a net chain reaction in chlorine and ozone react, they form the free radical which the chlorine atoms are regenerated so that decomposi- chlorine oxide, which in turn becomes part of a tion of ozone continues. chain reaction. As a result of that chain reaction, a single chlorine atom can Even in some of remove as many as 100,000 molecules of ozone. research and that of Harold Johnston also focused the most remote Unknown to Rowland attention on the effect of nitrogen oxides spewed locations on and Molina, the same chlo- by high-altitude aircraft. These emissions may also Earth, scientists rine atom chain reaction reduce ozone levels in the stratosphere. have detected had been discovered a few Earlier studies, which had investigated whether months earlier by Richard exhaust emissions from the supersonic transport hundreds of com- Stolarski and Ralph and other high-speed aircraft could damage the pounds. Curiously, Cicerone. In 1974, environment, had already begun to document the some of the sub- Rowland and Molina made effects of ozone loss. Compiled because of the per- stances that occur a disturbing prediction: If ceived threat from these aircraft, the data were in the smallest industry continued to brought to bear on the very real threat from CFCs release a million tons of and nitrogen oxides. concentrations CFCs into the atmosphere With less ozone in the atmosphere, more ultra- rank as some of each year, atmospheric violet radiation reaches Earth. Scientists estimated the biggest play- ozone would eventually that increased exposure would lead to a higher inci- ers in altering the drop by 7 to 13 percent. dence of skin cancer, cataracts, and damage to the To make matters worse, immune system and to slowed plant growth. atmosphere. other scientists had Because some CFCs persist in the atmosphere for demonstrated that an more than 100 years, these effects would last entirely different group of compounds could fur- throughout the twenty-first century. ther reduce ozone levels. Paul Crutzen first showed Concluding that such long-term hazards were in 1970 that nitrogen oxides react catalytically with unacceptable, Rowland and Molina called for a ban ozone, playing an important role in the natural on further release of CFCs. Alerted to this clear ozone balance. Soil-borne microorganisms produce and present danger, the United States, Canada, nitrogen oxides as a decay product, and Crutzen’s Norway, and Sweden in the late 1970s banned the work spotlighted how microbe-rich agricultural fer- use of CFCs in aerosol sprays. tilizers might lead to reduced ozone levels. His6 B E Y O N D D I S C O V E R Y
But in 1978 and 1979, the British scientists found something different. In October, the begin-The Ozone Hole Emerges ning of spring in the southern hemisphere, the researchers detected less ozone than had been As it turned out, the ozone problem was far detected during the past 20 years. During the nextworse than Rowland and Molina could have imag- several years, October ozone levels continued toined. The first warning signs of a bigger crisis did decline.not appear until the late 1970s, but the studies that In 1984, when the British first reported theiruncovered these findings had their roots in research disturbing findings, October ozone levels weredating back nearly a century. about 35 percent lower than the average for the In the 1880s, W.N. Hartley discovered that a 1960s. The U.S. satellite Nimbus-7 quickly con-broad band of ultraviolet light reaches Earth firmed the results, and the term Antarctic ozonealmost unimpeded. This band, known as UV-A, hole entered popular language.has wavelengths just slightly shorter than ordinaryvisible light. The ozone layer partly absorbs anoth-er ultraviolet band, known as UV-B, before it canreach Earth. During the 1920s, G.M.B. Dobson The Evidence Mountsmanaged to measure the ratio of UV-A to UV-B inincoming sunlight. By doing so, he determined for By the mid 1980s, scientists had become expertthe first time the total amount of ozone in the in measuring the concentration of chlorine-con-atmosphere. taining compounds in the stratosphere. Some mon- Dobson had hoped his study would lead to a itored the compounds from the ground; othersnew method of predicting the weather. Instead, he used balloons or aircraft. In 1986 and 1987, these became interested in the scientists, including Susan Solomon and James seasonal variations in Anderson, established that the unprecedented When chlorine and ozone concentrations. An ozone loss over Antarctica involved atomic chlorine ozone react, they instrument that he devel- and chlorine oxide radicals. form the free radi- oped, the Dobson spec- At the same time, measurements in the lower cal chlorine oxide, trometer, has become the atmosphere established that CFC levels had standard for monitoring increased steadily and dramatically since the first which in turn ozone from the ground. recordings taken by Lovelock in 1970. The conclu- becomes part of a sion was clear: The prime sources of the ozone- The rapid development chain reaction. As of new scientific tools after devouring chlorine atoms over Antarctica were the a result of that World War II—many of CFCs and two other pollutants, the industrial sol- them based on wartime vents carbon tetrachloride and methylchloroform. chain reaction, a instrumentation—led to a A satellite operated by the National Aeronautics single chlorine and Space Administration appears to have removed flowering of studies in atom can remove earth science. In any possible doubt about the role of CFCs. Data as many as 1957–1958, this led to a collected over the past three years by the Upper 100,000 molecules worldwide scientific effort Atmosphere Research Satellite revealed these com- known as the International pounds in the stratosphere. Moreover, the satellite of ozone. Geophysical Year (IGY). has traced the worldwide accumulation of stratos- IGY sparked an interna- pheric fluorine gases, a direct breakdown producttional outpouring of research on the oceans, the of CFCs. The quantitative balance of CFCs and itsatmosphere, and unexplored land areas of the planet. products eliminates the possibility that chlorine Monitoring ozone levels in the south polar from volcanic eruptions or other natural sourcesregion, researchers found them to be consistently created the ozone hole.about 35 percent higher in late spring than in win-ter. Annual monitoring showed the same seasonalpattern through the late 1970s. N A T I O N A L A C A D E M Y O F S C I E N C E S 7