Reproduced, with permission, from: Kowalok, M. E. 1993. Common threads: Research lessons from acid rain, ozone depletion, and global warming. Environment 35 (6): 12-20, 35-38.

Common Threads: Research Lessons from Acid Rain, Ozone Depletion, and Global Warming

Michael E. Kowalok

Stratospheric Ozone Depletion

Unlike the issue of acid rain, which has long been associated with human activities and documented by environmental observations, scientific concern about human-induced destruction of the ozone layer dates only to about 1970. Ozone is a molecule of oxygen (O3) that absorbs certain wavelengths of biologically damaging ultraviolet light. It is the only gas in the atmosphere that does so and, therefore, is an essential part of the Earth's ecological balance. The evolution of land life is believed to be tied closely to the formation of the protective ozone layer.[20]

Studies of the upper atmosphere started in the late 1800s and stemmed from curiosity about temperature gradients in the air. In 1902, a Frenchman named Teisserenc de Bort reported that the air temperature decreases with altitude up to a height of about 10 kilometers, after which the temperature starts to increase with height. Later studies showed that the temperature continues to rise with altitude up to a height of about 50 kilometers. The region of air between 10 and 50 kilometers of height became known as the stratosphere.

Most of the world's ozone is found in the stratosphere, and its daily creation and destruction is a natural process. The first person to explain the dynamics of ozone was Sidney Chapman, then a scientist at Oxford University. In 1930, he suggested that oxygen molecules (O2) are split by cosmic rays into two oxygen atoms.[21] Each of these atoms then combines with another oxygen molecule to form ozone. He also suggested, however, that the single oxygen atoms can also destroy ozone by randomly colliding with O3 molecules and creating two O2 molecules again. The rate of ozone destruction increases with the amount of ozone present and continues to increase until ozone is being destroyed and created at the same rate.

Chapman was successful in explaining why ozone existed in the stratosphere, but his theory was soon found to be incomplete. Measurements showed that oxygen atoms were destroying ozone at only one-fifth the rate of creation and that the total amount of ozone present was much lower than what Chapman's model would predict. Some unknown process was destroying ozone, and scientists knew that it was not related to any of the other major atmospheric gases (nitrogen, carbon dioxide, and water vapor) because none of them reacts with ozone. The only other possible sinks were atmospheric trace gases, but scientists could not explain how gases of such small concentrations could destroy large amounts of ozone.

The realization that trace gases in the atmosphere could react catalytically with ozone provided an explanation. A catalytic chain is a series of two or more chemical reactions in which one chemical (the catalyst) destroys another chemical without itself being destroyed. The chain can be repeated indefinitely until all of the chemical is destroyed or until the catalyst is removed by some competing process.

In 1950, David R. Bates, an applied mathematician at the University of Belfast, and Marcel Nicolet, an atmospheric scientist at the Institut d'Aeronomie Spatiale de Belgique in Brussels, were the first to suggest that naturally occurring oxides of hydrogen (HOx) were effective catalysts for ozone destruction.[22] In 1965, John Hampson, a scientist working at the Canadian Armaments Research and Development Establishment in Quebec, was the first to show how HOx could catalytically destroy ozone in the stratosphere.[23] And in 1966, B. G. Hunt, a scientist with the Australian Weapons Research Establishment, proposed rate constants for Hampson's reactions to obtain agreement between predicted and observed amounts of ozone. (Both Hampson and Hunt were studying the upper atmosphere because their respective institutions were interested in the problem of re-entry of ballistic missiles into the atmosphere).[24]

In 1970, Paul J. Crutzen---an atmospheric chemist at the University of Stockholm who also worked at research institutes in Germany and the United States---was the first to suggest that oxides of nitrogen (NOX) were another natural catalyst of crucial importance in determining the ozone budget.[25] Crutzen also recognized that oxides of chlorine, sodium, and bromine were able to destroy ozone, but he discounted their roles because, at that time, scientists knew of no significant natural sources of them in the stratosphere.[26]

Scientific concern about human-induced destruction of the ozone layer arose with debate about the environmental impacts of supersonic transports (SSTs). Combustion products of the SST included nitrogen oxides, sulfate particles, and a substantial amount of water vapor. Some environmentalists feared that the water vapor injected into the stratosphere would cause ozone depletion or excess cloud cover. In July 1970, a study group of 70 environmental scientists met at a conference entitled "Study of Critical Environmental Problems" (SCEP) to determine what was known about global environmental problems and what further research was needed. The discussion on the effects of the SST received the most media attention. The group concluded that the amount of ozone depletion caused by water vapor would be insignificant and that any problems associated with nitrogen oxide could be neglected.[27]

In 1971, the SCEP conclusions were attacked by Harold Johnston, a physical chemist at the University of California at Berkeley. Johnston showed that the SCEP report incorrectly discounted NOx and that the SST did pose a serious threat to stratospheric ozone. Johnston was not a member of the SCEP study group but became involved in the SST/ozone issue by working within a government-sponsored program to study the stratospheric effects of SSTs. This program was a 3-year, $21-million effort known as the Climatic Impact Assessment Program (CIAP) and involved the efforts of more than 1,000 scientists from 10 different countries.[28]

Research attention turned to the role of chlorine in the stratosphere in 1972, when scientists at the U.S. National Aeronautics and Space Administration (NASA) recognized that the space shuttle's solid rocket boosters would inject chlorine directly into the stratosphere. Until that time, scientists thought that volcanic eruptions were the only source of stratospheric chlorine and that that source was negligibly small.[29]

In July 1972, the NASA environmental impact statement for the space shuttle revealed that chlorine in the form of hydrogen chloride (HCI) would be spread as an exhaust product along the shuttle's trajectory and that a large amount would be deposited directly into the stratosphere. Despite this finding, and in the absence of scientific concern about significant amounts of chlorine in the stratosphere, the statement initially concluded that the shuttle was expected to have no negative environmental impacts on the stratosphere.

To check its work, NASA awarded a contract to Richard S. Stolarski and Ralph J. Cicerone, then at the University of Michigan, to examine the statement for weaknesses. Although they were not atmospheric chemists, the two had been working on the dynamics of the ionosphere and saw the contract as a way to break into a new field of research. Stolarski was a physicist by training and Cicerone was an electrical engineer. They thought that the HCI might dissociate to free chlorine, and, by late 1972, they learned of a catalytic chain of ClOX reactions that could destroy ozone. In the spring of 1973, Stolarski and Cicerone made a formal presentation to NASA that concluded that chlorine compounds from the shuttle could be a significant destroyer of ozone.

Stolarski and Cicerone were not the only scientists to see the shuttle's impact statement and focus on the chlorine problem. After they read the impact statement, Michael E. McElroy and Steven C. Wofsy of Harvard University began to consider chlorine dynamics independently from Stolarski and Cicerone. McElroy was an expert in planetary atmospheres and had studied the chlorine chemistry of Venus as part of NASA's planetary exploration program. He already knew that chlorine could deplete ozone, and he began to apply his previous findings to the Earth's atmosphere. Within a year, McElroy and Wofsy had designed models to predict how much ozone would be destroyed for a given input of chlorine, regardless of its source.

The activities of the Michigan and Harvard researchers crossed paths for the first time in September 1973 at a meeting of the International Association of Geomagnetism and Aeronomy (IAGA) in Kyoto, Japan. In his presentation, McElroy focused on the role of NOx in atmospheric photochemistry. Stolarski, therefore, was the first at the meeting to speak about chlorine chemistry in the Earth's atmosphere. He gave a brief summary of his team's work but did not name the space shuttle as a source of stratospheric chlorine. He spoke instead about volcanic eruptions as a source. After the presentation, McElroy attacked Stolarski's talk, charging that the chemistry was incomplete and that volcanoes were not a significant source of chlorine. Others in the audience agreed that chlorine could indeed destroy ozone but felt that the topic was moot for lack of a major source. They wondered why McElroy and Stolarski were pressing the issue.[30]

All of the papers presented at the IAGA meeting were published in a special issue of the Canadian Journal of Chemistry in April 1974. McElroy and Wofsy updated their paper to include their work on chlorine's role in ozone reduction and explicitly stated that they had initiated this work because of concern about the space shuttle as a source of stratospheric chlorine. Stolarski and Cicerone also mentioned the space shuttle and showed that oxides of chlorine are even more efficient in destroying ozone than are oxides of nitrogen.[31]

Like the SST, the space shuttle forced scientists to consider the effects of anthropogenic pollutants on the stratosphere. However, this shuttle-induced work on chlorine was just half of the story. Any doubts that oxides of chlorine could be a major destroyer of ozone were dispelled soon after the IAGA meeting when two chemists at the University of California at Irvine, Mario J. Molina and F. Sherwood Rowland, identified chlorofluorocarbons (CFCs) as plentiful sources of stratospheric chlorine. These chemicals are entirely synthetic and have a variety of industrial uses, such as refrigerants, insecticides, and propellants for aerosol cans.

Rowland had become interested in the dynamics of fluorocarbons in 1972, when he learned of the work of James E. Lovelock, an independent scientist working at the University of Reading in England. Out of curiosity, Lovelock had been measuring the concentrations of fluorocarbons in the lower atmosphere. He took measurements in both the Northern and Southern Hemispheres in 1970, 1971, and 1972 and found that the gases were in the air and sea "wherever and whenever they were sought."[32] In his 1973 Nature article reporting his findings, Lovelock suggested that the gases could be used as atmospheric tracers of air movements because they were chemically and physically inert. His work was not motivated by any possible link between fluorocarbons and the environment, and, to his later regret, he stated in 1973 that the compounds were of "no conceivable hazard" to the environment.[33]

By 1972, Rowland knew that Lovelock had compared his measurements of atmospheric CFC concentrations with annual amounts of industrial production. Lovelock found that the amount in the atmosphere was very close to the total amount that had ever been produced.[34] In other words, nothing in the atmosphere was destroying CFCs after their release. Rowland, intrigued by Lovelock's findings, began to study CFC dynamics in the summer of 1973. His rationale was that, because scientists were interested in using fluorocarbons as atmospheric tracers, it would be interesting to try to predict their chemical interactions. Molina joined him in October 1973. Like Rowland, Molina was an "outsider" to stratospheric chemistry and chose the issue out of a desire to do something different. Rowland and Molina soon found that CFCs could induce catalytic destruction of ozone and that the current annual amounts of CFC production could cause large rates of ozone destruction. Alarmed about these findings, Rowland met with Harold Johnston in December 1973 to discuss their plausibility. Johnston informed him that the catalytic chlorine chain that Rowland and Molina suggested was, indeed, plausible and had just been discussed at the September IAGA meeting in Kyoto.

In June 1974, Molina and Rowland proposed an alarming hypothesis in Nature that the use of chlorofluorocarbons added chlorine to the environment in steadily increasing amounts.[35] They suggested that, once released into the environment, CFCs had lifetimes of between 40 and 150 years and had no obvious sinks other than photodissociation in the stratosphere. Photodissociation would produce chlorine atoms, which would then catalytically destroy ozone. Molina and Rowland concluded that the stratosphere had only a finite capacity to absorb chlorine atoms and that, even if CFC production were reduced, a lengthy amount of time "of the order of calculated atmospheric lifetimes" would be required for natural moderation. Rowland's and Molina's work suggested that fluorocarbons were indeed an ominous threat to the environment because they were a significant source of stratospheric chlorine and had already been released in sufficient quantities to begin ozone depletion.

News of the fluorocarbon threat hit the popular press in September 1974, when a meeting of the American Chemical Society prompted the New York Times to run a front-page story about the work of McElroy and Wofsy.[36] The article stated that the common aerosol can was a major source of fluorocarbons in the environment and that such gases could lead to ozone depletion. The day after the Times ran this story, another paper by Stolarski and Cicerone appeared in Science and concluded that the chlorine derived from CFCs in the atmosphere would become the dominant factor in ozone depletion.[37] The Times article signaled the beginning of public concern over CFCs and their use in aerosol cans and refrigerators. The story was soon picked up by Walter Cronkite of CBS-TV and by the major news magazines.[38] Much new research was initiated, and international collaboration was stimulated.