Ozone in the stratosphere Continuation of the Pimntel report The possibility of polluting the stratosphere to the point of partially depleting the protective ozone layer was first raised only about a dozen years ago. This seemingly improbable notion found much scientific support, and it is now one of the best examples of a potentially serious environmental problem of global extent. It is a problem, furthermore, that exemplifies chemistry’s central role in its understanding, analysis, and solution. Why do we need to worry about stratospheric chemistry? Ozone in the stratosphere is the natural filter that absorbs and blocks the sun’s short wavelength ultraviolet radiation that is harmful to life. The air in the stratosphere-a cloudless, dry, cold region at altitudes between about IO to SO kmmixes slowly in the vertical direction, but rapidly in the horizontal. Consequently, harmful pollutants, once introduced into the stratosphere, might remain there for periods as long as years, and, if so, they will rapidly be distributed around the earth across borders and oceans, making the problem truly global. A large reduction of our ozone shield would result in an increase of potentially dangerous ultraviolet radiation at the earth’s surface. To understand how easily the ozone layer might be perturbed, it is useful to recognize that ozone is actually only a trace constituent of the stratosphere; at its maximum concentration ozone makes up only a few parts per million of the air molecules. If the diffuse ozone layer were concentrated into a thin shell of pure ozone gas surrounding the earth at atmospheric pressure, it would measure only about 3 millimeters ( % inch) in thickness. Furthermore, ozone destruction mechanisms are based on chain reactions in which one pollutant molecule may destroy many thousands of ozone molecules before being transported to the lower atmosphere, chemically transformed, and removed by rain. Chemistry’s crucial role in understanding this problem has emerged 328 Envimn. Sci. Technol.. MI. 20. NO.4. 1986
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through the identification and measurement of several ozonedestroyinn chain processes. Fifty years ago, ;he Tormation of an ozone layer in the midstratosphere was qualitatively described in terms of four chemical and photochemical reactions involving pure oxygen species (0, 02,and 0,). Today, we know that the rates of at least IS0 chemical reactions must be considered in order to approach a quantitative model for simulating the present strato-
+
711 torr
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sphere and predicting changes resulting from the introduction of various pollutants. The chemistry begins with absorption of solar ultraviolet radiation by O2 molecules in the stratosphere. Chemical bond rupture occurs, and ozone, 0,.and oxygen atoms, 0, are produced. Then, if nitric oxide, NO, is somehow introduced into the stratosphere, an important chemical chain reaction takes place. The NO and NO2 reactions together
opportunmes In Chemllsny @ 1985 by the National Academy of Sciences
furnish a true catalytic cycle in which NO and N q are the catalysts. Neither species is consumed, because each is regenerated in a complete cycle. Each cycle has the net effect of destroying one oxygen atom and one ozone molecule (collectively called “odd oxygen”). This catalytic cycle is now believed to be the major mechanism of ozone destruction in the stratosphere. In the natural atmosphere, the oxides of nitrogen are provided by biogenic emissions at the Earth’s surface by soil and sea bacteria of nitrous oxide, NzO. This relatively inert molecule slowly mixes into the stratosphere where it can absorb ultraviolet light and then react to form NO and NOz. Of course, oxides of nitrogen directly introduced to the stratosphere are expected to destroy ozone as well, and this was the basis of the first perceived threat to the ozone layer-larger fleets of supersonic aircraft flying in the stratosphere and depositing oxides of nitrogen via their engine exhausts. Nuclear explosions also produce copious quantities of oxides of nitrogen, which are camed into the stratosphere by the buoyancy of the hot fireballs. A significant depletion of the ozone layer in the event of a major nuclear war was forecast in a 1975 study by the National Academy of Sciences, although this environmental effect of nuclear war may pale in comparison with the recently suggested potential of a “nuclear winter.” Both effects underscore the delicacy of the atmosphere and its sensitivity to chemical transformations. Then, in 1974, just as the possible impact of stratospheric planes was reaching the analysis stage, concern was raised about other man-made atmospheric pollutants. Halocarbons, such as CFC13 and CFzCIz (chlorofluoromethanes, or CFMs), had become popular as spray-can propellants and refrigerant fluids, mainly because of their chemical inertness. The absence of reactivity meant absence of toxicity or other harmful effects on terrestrial life. Ironically, this meant that there was no place for the CFMs to go but u p u p into the stratosphere where ultraviolet photolysis could occur. Chemists then recognized that if this occurred, the resultant chlorine species, CI and CIO, could enter into their own catalytic cycle, destroying ozone in a manner exactly analogous to the destruction caused by the oxides of nitrogen. Once this possibility had been recognized, analysis of the whole strato spheric ozone chemistry began in earnest. An international committee of scientific experts assembled by the National Academy of Sciences examined in detail the state of our knowledge of
HOC1
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Reactions key to stratospheric protection
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every aspect of the problem. It became clear that the additional chemistry introduced to the stratosphere added not just these two catalytic chemical reactions to the roster, but a total of about 40 new reactions involving such species as CI, CIO, HCI,HOCI, CIONOz, the halocarbons, and several others. Most of these reactions had never before been studied in the laboratory. Laboratory kineticists and photochemists responded to the challenge by providing reliable rate constants and absorbances for the proposed processes using the growing arsenal of modern experimental methods. Recent progress in the experimental accomplishments of this field has been remarkable. It has become possible, for example, to generate nearly any desired reactive molecular species in the laboratory in a variety of ways, to bring them together with other reactive species, and to measure their rates of reaction under known, controlled conditions. Such direct measurements of these extremely rapid reactions were only a distant goal a decade ago, but they are now a reality. Finally, field measurements of minor atmospheric species have been revolutionized by some of the recent advances in analytical cheniistry. Methods originally developed for ultrasensitive detection of extremely reactive species in laboratory studies have been modified and adapted to measure such constituents as 0, OH, CI, CIO, and others at parts-per-trillion concentrations in the natural stratosphere. This has been accomplished recently in experiments in which a helium-filled balloon carries an elaborate instrument package to the top of the stratosphere where the package is dropped while suspended by a parachute. As the instrument traverses the stratosphere, it measures concentrations of several important trace chemical species and telemeters the information to a ground station. Very recently,
the first successful reeldown experiment was performed in which the instrument package was lowered IO to 15 km from a stationary balloon platform and reeled back up again like a giant yo-yo. This method results in a huge increase in the amount of data that can be obtained in a single balloon flight. It will also allow for the first time a study of the time evolution and variability of the stratosphere. Much has been accomplished in the past IO years. Many of the needed 100 to 150 photochemical and rate processes have been measured in the laboratory, and many of the trace species measured in the atmosphere. Yet, research remains to be done. For example, two of the important chemical species containing chlorine, HOC1 and CIONOz, have yet to be measured anywhere in the stratosphere. Refinements in the reaction rates for many of the important processes are still required, and exact product distributions for many of the reactions are still lacking. Nevertheless, the original NAS study, the research programs it spawned, and the subsequent follow-up studies p r o vided a firm and timely basis for legislative decisions about regulation of CFM use. Industrial chemists produced alternative, more readily degradable substances to replace the CFMs in some applications. Monitoring programs are in place so that trends in the stratospheric composition can be watched. The stratospheric ozone issue provides a showcase example of how science can examine, clarify, and point to solutions for a potential environmental disturbance. Premature initiation of regulation was avoided because the problem was recognized early enough to permit deliberate, objective analysis and focused research to narrow the uncertainty ranges. From first recognition on, chemists played a lead role. 7his is the rhird in a seven-parr series. Enviton. Sci. Technol.. Vol. 20. NO. 4. 1986 329
