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Atmospheric Chemistry: New Insights Define Current Research

When one reviews the knowledge of atmospheric chemical composition as it existed two or three decades ago, one is struck by the primitive state of the science. . . . The atmosphere near the earth was viewed as a fluid in motion, transporting moisture and heat. It also transported pollutants arising from cities, factories, and fires. The chemical species in the air were regarded as essentially inert and for good reason—most of the components that were known were inert gases. . . . It is now understood that the atmosphere is a reactive environment.

Global Tropospheric Chemistry—A Plan for Action,
National Academy of Sciences, 1984

In spite of the relatively primitive state of the science of atmospheric chemistry in the late 1950s, the authors of the UCAR "Blue Book" proposed chemistry as one of the disciplines that should be included in a broad and vigorous attack on scientific problems of the atmosphere. They recognized that some chemical species in the atmosphere are far from inert and suggested some scientific issues that the new institute should address:

For example, the photochemical processes by which the ionization and recombination of gaseous components is [sic] achieved are only partly known. The chemical equilibrium of ozone in the upper atmosphere has a profound influence on biological processes on the ground as well as being a possible link between fluctuations in the emission of ultra- violet energy from the sun and atmospheric motion at lower levels. The distribution and decay of natural and artificial radioisotopes, the carbon dioxide and nitrogen cycles in the atmosphere, and the processes by which particulate materials are diffused and removed from the atmosphere are problems which have assumed ecological significance.

The current thrust in atmospheric chemistry research is guided by three relatively recent insights that have become basic axioms:

For more than 200 years, since the time of Cavendish, Lavoisier, and Priestley, scientists have known that the mixture we call air is about 79% nitrogen and 20% oxygen. Knowledge of the composition of the remainder has come more slowly. In 1892, Lord Rayleigh and Sir William Ramsey discovered that most of that last 1% is argon, an extremely inert gas. Four other inert "rare gases"—helium, krypton, neon, and xenon—were soon identified in much lower concentrations.

Carbon dioxide, which is exhaled by humans and other animals and released by the combustion of fossil fuels such as coal and oil and the decay of plant and animal matter, makes up about 0.3% of the air. During the first half of this century, as new analytical techniques were developed, traces of other gases such as carbon monoxide, hydrogen, methane, and ozone were detected. By contrast with the slow pace of this early progress, the past decade or so has seen what one atmospheric chemist, Douglas Davis of the Georgia Institute of Technology, calls "an explosion of knowledge" about the composition of the atmosphere. "We have found that the atmosphere is a reservoir for a myriad of trace gases and aerosol species with concentrations well below one part per million per volume of air," Davis says. "In spite of their extremely low concentrations, some of these species are highly reactive and often have major impacts on the environment." For example, atmospheric chemists know now that many important chemical reactions in the troposphere—the lower atmosphere—as well as the stratosphere involve the hydroxyl radical, an unstable and highly reactive combination of hydrogen and oxygen with the chemical formula OH.

Ralph Cicerone, director of NCAR's Atmospheric Chemistry Division, tells what happened when James Anderson, an atmospheric chemist at Harvard University, came up with a plan in the mid-1970s to measure concentrations of the OH radical in the troposphere. "He wrote a proposal and sent it to a federal funding agency," Cicerone recalls. "The proposal was promptly returned, without the customary scientific review, with the comment that everybody knows there aren't any free radicals in the troposphere."

"That's how it happened," Anderson confirms. "I was so enraged I wadded the letter up and threw it in the wastebasket. I should have saved it—it would be a historic document now."

Along with the recognition of the chemical reactivity of the multitude of trace gases that exist in the atmosphere has come a realization that the chemical composition of the atmosphere is controlled largely by biological processes. Cicerone is convinced that the chemistry of the atmosphere cannot be understood without a much deeper understanding of the constant and complex interactions of the atmosphere and biosphere. "The very existence of some things that we're finding in the atmosphere is completely due to biological processes," he says. "If you calculate how much methane and nitrous oxide should be in the atmosphere on a purely physical basis, assuming a nonbiological system in which nitrogen, oxygen, carbon dioxide, and water vapor interact until they reach chemical equilibrium, the result will be twelve orders of magnitude off for methane and thirty orders of magnitude off for nitrous oxide. Much of the concentration of these gases in the atmosphere is influenced by biological processes in the soil, the oceans, and other elements of the biosphere."

The third basic insight about atmospheric chemistry is such as truism today that it's hard to remember that it hasn't always been known and accepted. It is the demonstrable fact that many human activities are perturbing the natural chemical cycles of the atmosphere in ways that, although not fully understood, can have some serious consequences. Acid precipitation, formed through reactions involving pollutants that contain sulfur and nitrogen, is disrupting the reproductive cycles of trout in mountain lakes and may be damaging other ecosystems, including agricultural ones. The so-called greenhouse effect of rising levels of carbon dioxide and other gases in the atmosphere is expected to cause a gradual global warming that eventually could melt the polar ice caps, raise the level of the oceans, and change large-scale weather patterns. Destruction of ozone in the stratosphere through reactions involving gases from human sources allows increased levels of ultraviolet radiation to reach the earth's surface, with potentially harmful effects on human health as well as on climate.

Sulfur and nitrogen entering the atmosphere from power-plant smokestacks undergo chemical reactions that produce acid rain and other forms of acid deposition, which can damage ecosystems hundreds or thousands of miles from the sources of the pollution.

Cicerone was vice chair of the National Academy of Sciences (NAS) panel that drafted the plan for a global tropospheric chemistry research program quoted at the beginning of this chapter. Chaired by Robert Duce of the University of Rhode Island, the panel included six scientists from universities, four from federal agencies, one from industry, and three from NCAR. Their plans for a major international research program aimed at measuring, understanding, and predicting the changes in global chemistry during the coming century will involve decades of study by many hundreds of scientists.

The plan points out that much recent research in atmospheric chemistry has resulted from a need to develop public policy that responds to potential problems such as acid rain, the greenhouse effect, and the destruction of stratospheric ozone. "Research carried out in a crisis- response mode attempts to obtain, as quickly as possible, the minimum amount of information needed for policy formulation," the authors assert. While acknowledging that such research has made considerable progress in quantifying the atmospheric effects of many human activities, the panel concluded that broader research is needed to prepare for future problems and to establish a solid base of fundamental knowledge of atmospheric chemistry from which effective responses can be formulated.

A central issue in atmospheric chemistry to be addressed by the Global Tropospheric Chemistry Plan concerns the ability of the atmosphere to oxidize substances that enter it from both natural and human sources, transforming their chemical state by combining them with oxygen. "There are many unanswered questions," Cicerone says. "How does the atmosphere oxidize these substances? What are the rates of oxidation and what controls them? What are the limits of the atmosphere's ability to oxidize substances such as methane, which is building up rapidly in the atmosphere? We believe that the most important agents of oxidation are ozone, which is an unstable form of oxygen, and the OH radical. But there is a lot we don't understand about how they do it."

Much of the current explosion in knowledge about the chemistry of the atmosphere is the result of dramatic advances in the chemists' ability to measure trace gases such as ozone and OH. Harvard's James Anderson says: "Twenty years ago the science was primitive in terms of pure chemistry, but the technology that was available to observe what was going on in the atmosphere was even more primitive."

Anderson's primary research interest is stratospheric chemistry. "The stratosphere is unique," he says. "We can carry out experiments there that are comparable in quality to laboratory experiments. The chemistry of the troposphere is chaotic. But in the stratosphere, where characteristic response times for chemical reactions are very fast, we can break off a piece of this huge reactive network and study it by itself. The stratosphere is a scientific bridge between isolated laboratory experiments and the chaotic processes in the troposphere."

Anderson's work in the stratosphere started in 1970, after he received his doctorate from the University of Colorado and joined the university's Laboratory for Atmospheric and Space Physics (LASP). The laboratory used rocket-borne instruments to measure free radicals in the stratosphere. "Rockets can get the instrument well up into the stratosphere," Anderson says, "but they are very brutal—the launch produces a terrible impact on delicate instrumentation."

Anderson decided to try a gentler instrument platform. In 1974, he launched his first balloon-borne experiment at UCAR's National Scientific Balloon Facility (NSBF) at Palestine, Texas. "Balloons really have no competition for this kind of research," he says. "Our instruments use lasers in various combinations, which can add up to a heavy payload. In addition to being kind to the instruments, a helium balloon can lift tons of them."

By the late 1970s, Anderson was head of a research group at Harvard that had made a series of balloon flights from the NSBF to measure gases in the stratosphere. "We would lift the instruments to the top of the stratosphere and drop them with a parachute. The atmosphere is practically a vacuum at that altitude, so the experiments descended at about half the speed of sound," he recalls.

The measurements showed some apparent fluctuations of free-radical concentrations that were very interesting but somewhat questionable, because the speed of the instruments' descent introduced potential sources of contamination and instrumental error. "We needed to be able to do the experiment five or six times with the same instruments in the same day," Anderson says. "We decided to develop a way to lower the instruments from the balloon and pull them back up again, over and over."

Harvard's James Anderson prepares his pioneering balloon-borne reel-down experiment for a dawn launch at the National Scientific Balloon Facility (NSBF).

Anderson received funding in 1980 to build and fly a "reel-down" experiment that would use a winch to lower instruments at the end of a lightweight plastic filament. "We flew the experiment in September 1982," Anderson says. "It worked beautifully, with remarkable stability that exceeded our expectations. Now we have a valuable new capability—using a series of precisely controlled descents and ascents to obtain reliable profiles of the fluctuations of free radicals in the stratosphere. We flew the reel-down experiment again in September 1984, and we'll do it again in 1986."

Douglas Davis and his research group at Georgia Tech have done a great deal to advance the technology used in atmospheric chemistry measurements. "Ten or eleven years ago, we set out to develop techniques for measuring the OH radical because it is at the heart of atmospheric chemistry," Davis recalls. "Measuring parts per quadrillion was something new, and it turned out to be a far more difficult task than any of us realized it was going to be. We've gone through two generations of instrumentation based on a technique called laser-induced fluorescence, and we've done the prototype work on the third generation.

"My feeling, which is shared by a lot of other atmospheric chemists, is that the future progress of the field will depend on the availability of appropriate instrumentation," Davis declares. "We don't have any way to make a lot of measurements we need right now. Thus, our group develops high-tech instruments that have great sensitivity and selectivity—in the part-per-trillion to part-per-quadrillion range. Then we take the instruments out on field sampling programs where we develop techniques to use the instruments to address some of the big scientific problems in atmospheric chemistry."

Because atmospheric trace gases are continually interacting with each other and with the physical processes in the atmosphere, it is important to make many measurements simultaneously. As Davis puts it, "If you measure one thing and see a change, you may come up with three or four possible explanations that are difficult to test. But if you measure other chemical species at the same time and place, you can narrow down the number of hypotheses."

Davis directed a pioneering field program conducted in 1977 and 1978 under the acronym GAMETAG, for Global Atmospheric Measurements Experiment on Tropospheric Aerosols and Gases. Davis describes it as an effort to take a "chemical snapshot" of the atmosphere.

"The idea was that, if we're really going to understand the chemistry of the atmosphere, we must make simultaneous measurements of many variables. We tried to collect the most advanced instrumentation available on a single platform—NCAR's turboprop Electra research aircraft—and we made a series of flights that took us to the Arctic Circle and nearly to the Antarctic. We were looking at the latitudinal dependence of oxidizing species, because solar radiation plays such an important role in their formation. You would expect the highest levels of species such as OH near the equator, where the solar flux is highest. That is one of the things that the models predicted and GAMETAG confirmed."

GAMETAG involved scientists from seven universities, a private research institute, a private company, and NCAR. During the sampling missions, 13 to 14 scientists were on board the Electra in addition to six or seven aircraft operations people including pilots, flight engineer, and computer technicians.

"We flew all over North America and the Pacific," Davis recalls. "We had to have food and a place to lay our heads at night between flights, and NCAR set that up. As project director, I was able to concentrate on the science and the instrumentation. NCAR worried about the aircraft and logistics, provided on-board meteorologists, and archived the data. Obviously, a university can do all this for itself, but it requires setting up an infrastructure. Unless a university research group is going to operate a field program continuously for a number of years, it's much more efficient to work with NCAR support than to recruit staff and set it all up yourself within the university. GAMETAG involved major interaction between Georgia Tech and NCAR, in terms of both scientific collaboration and operational support."

Volcanic eruptions are one natural source of many particles and liquid droplets that are involved in the chemistry of the atmosphere and that may have serious long-term impacts on climate.

A number of the more than two dozen trace gases that were measured in GAMETAG and subsequent Georgia Tech aircraft missions in the troposphere are playing a major role in determining the earth's future climate. Ralph Cicerone recently collaborated with NCAR colleagues, V. Ramanathan and Jeffrey Kiehl and with H.B. Singh of SRI International in a study of the contribution of about 20 trace gases to the gradual warming that the earth apparently is experiencing. Carbon dioxide (CO2) and certain other trace gases, which come partially from fossil fuel combustion and other human sources, allow short-wave solar radiation to pass freely into the atmosphere but hold back the long-wave heat energy radiated upward by the earth's surface. Thus, as the concentration of these gases in the atmosphere increases, the average temperature goes up. This phenomenon is popularly known as the greenhouse effect.

The changes from a greenhouse warming could be serious. Large-scale rainfall patterns could shift enough to turn major food-producing regions into deserts. The west Antarctic ice sheet could slide into the sea and raise the level of the oceans some 15 feet, enough to flood many coastal cities and other low-lying areas.

The role of carbon dioxide in influencing the earth's climate has been recognized for a long time. The first calculations and predictions of a CO2-induced global warming were made in 1896 by the Swedish chemist Svante Arrhenius, who estimated that a doubling of atmospheric CO2 would produce a global warming of four to six degrees Celsius. However, the work of Cicerone and his colleagues has revealed that many other trace gases may play an even more important role than CO2 in a future greenhouse warming.

"We're saying that you now have to think of a much larger change than we attributed to CO2 alone," Cicerone says. "We're tending toward saturation as far as the greenhouse effect of CO2 is concerned—that is, each successive amount of CO2 that we add to the atmosphere has a slightly smaller effect than the previous equal amount. We're adding the other gases at a much faster rate than CO2, and some of them absorb even more infrared radiation than CO2 does. In the scenarios we've put together—and we've tried to be conservative—these other greenhouse gases could double or triple the warming effect caused by CO2 alone."

There are many unknowns, even about CO2. The question of how much CO2 is absorbed by the ocean, for example, and how long it will stay there, is still open. The role of tropical rain forests has not been clearly defined—plants take in carbon dioxide and give off oxygen, reversing the pattern of humans and other animals. As tropical forests are cleared, how will the atmospheric CO2 cycle be affected?

The cycles of other greenhouse gases are equally uncertain. Methane, which has a significant greenhouse role, is produced mainly by anaerobic bacteria—microorganisms that live where oxygen is not present. Anaerobic bacteria flourish in the digestive tracts of animals, and for many years the presence of atmospheric methane was attributed mainly to the flatulence of cattle. It is also produced by the decay of plant matter underwater in flooded rice paddies. As both cattle and rice production have increased in the developing countries of the Third World, an increase in atmospheric methane could be expected, and it has been increasing by 50 or 60 million tons a year. But the answer may not be simply that the cows and rice paddies are putting out that much more of the gas.

"Another explanation is that it is not just that the sources are increasing, but that the sinks that consume the methane are decreasing. Because of some chemical changes in the atmosphere, its power to oxidize methane and change it into something else may be decreasing," Cicerone explains. "Trying to understand what's happening is leading us into some very fundamental and very difficult research."

It seems clear that the premise of the NAS panel that drafted the plan for a global tropospheric chemistry program—that the best way to respond to future crises involving human perturbations of the chemistry of the atmosphere is to build a solid base of fundamental knowledge—is a sound principle for guiding the direction of future atmospheric chemistry research in the UCAR community.

Here, Johanna Darlington of the National Museums of Kenya and Nairobi poses beside a giant termite mound. The digestive processes of termites that live beneath mounds like these in many tropical regions of the world produce sufficient methane to play a significant role in the chemistry of the atmosphere.

The role of atmosphere/biosphere interactions in determining the chemistry of the atmosphere clearly must be defined as accurately and precisely as possible. Some scientists have proposed that much of our chemical and physical environment is controlled by a sort of conglomerate organism composed of many terrestrial life forms. England's James Lovelock has postulated that the biosphere regulates the physical environment to benefit itself through a pervasive living feedback system that he calls Gaia.

"One shouldn't look at the earth as a living being, in a formal sense. That's carrying things much too far," Lovelock cautions. "But Gaia has collective properties just as living entities do. You and I are assemblies of billions of cells. We have properties, which are quite remarkable, when you compare them with those of the constituents of which we're made. This is a characteristic of living systems—their collectives are very powerful. I look at Gaia as a kind of collective of all the living species on the planet, having properties that are more than the sum of the properties of its parts."

Other scientists, while not necessarily embracing Lovelock's concept of Gaia in all its details, agree that atmospheric chemistry and biology are closely linked as elements in a global system. "It's really hard to think of a question in atmospheric chemistry today that doesn't have biological implications," Ralph Cicerone observes, "either in the origin of the substances that are involved or in their effects."

In July 1985, UCAR and the Ecological Society of America (ESA) cosponsored a cross-disciplinary workshop on atmosphere/biosphere interactions. This workshop, held in Boulder, was designed to explore needs and mechanisms for collaborative research between the atmospheric and oceanic sciences and biological science, especially ecology. More than two dozen physical and biological scientists from five nations participated. The workshop was organized by John Eddy of UCAR and biologist Harold Mooney of Stanford University.

The workshop report pointed out that, in the past, atmospheric scientists and biologists have cooperated mainly because each field needed data from the other. The atmospheric scientists needed information on the effects of vegetative cover on radiation and moisture to make their climate models more realistic, and the biologists needed meteorological information to assess the effects of such things as radiation and moisture on plants and ecosystems. These motives for cooperation were rooted in disciplinary self-interest—each group needed technical information from the other, but there was no strong thrust for real scientific collaboration. In fact, there probably was a certain amount of resistance based on fear that one discipline might be viewed as in service to the other.

An NCAR student visitor works with laboratory apparatus used to simulate chemical processes that occur in the atmosphere.

"This explains in part," the workshop report suggests, "why communication between atmospheric and biological scientists has been largely restricted to the explorations of a few generalists in subfields of climatology and ecology, and limited to collaborations that were specific and of short term, and why the main streams of the two disciplines, though in contact, have remained to this day essentially immiscible."

The report went on to make a strong argument against disciplinary chauvinism: "Major research questions are now recognized that transcend traditional disciplinary labels and ask for more than superficial exchanges. Comprehension of the cycling of elements and compounds essential to life such as carbon, or carbon dioxide, for example, is a global problem that includes ecology as well as atmospheric science; the intrinsic processes involved are interrelated and even synergistic. Similarly, the development of coupled models of the Earth is as much an extension of ecological models as of global climate models; each strives to understand a system broader than itself."

UCAR will encourage cooperation at the atmosphere/biosphere research interface, as well as other boundaries between traditional disciplines, through its newly created Office for Interdisciplinary Earth Studies, headed by John Eddy. "Increasingly, we are impressed with the holistic, global nature of our science," UCAR President Murino says. "We encounter scientific obstacles we cannot surmount without corresponding advances in closely related disciplines, such as chemistry, oceanography, biology, hydrology, geology, geophysics, and numerous other sciences. We have begun to establish new areas for fruitful collaboration with scientists working in these disciplines worldwide, and much more needs to be done."

Balloons: The Oldest Tool

Of all the tools that scientists use to reach up into the atmosphere to measure and sample it, the balloon is the oldest. It antedates the airplane by more than 100 years and radar by more than 150. Yet balloons are still basic tools of atmospheric research. Radiosondes, or weather balloons, are launched by researchers, as well as weather forecasters, to provide vertical profiles of atmospheric conditions as they ascend. Global horizontal sounding balloons developed at NCAR have flown for three months or more, circling the earth to track large-scale wind patterns.

Large scientific balloons are heavy, carrying payloads to higher and higher altitudes.

Perhaps the most impressive balloons used in scientific research are flown from the National Scientific Balloon Facility (NSBF) near Palestine, Texas. This field station was established by NCAR in 1963 as its first joint-use facility. Each year, NSBF's ballooning experts launch 50 to 100 flights from Palestine or from remote sites in the United States, Canada, Brazil, Italy, and countries such as New Zealand and Australia.

NSBF's huge scientific balloons can carry payloads weighing thousands of pounds to altitudes of 30 km (100,000 feet) or higher. They can take telescopes and radiation sensors above most of the atmosphere to make astronomical observations. They can lift measuring instruments and sampling devices to critical regions of the stratosphere above the levels at which airplanes can operate. NSBF and the National Aeronautics and Space Administration (NASA) are conducting a program to push large balloons to the limits of their capabilities, and launching increasingly heavy payloads to higher and higher altitudes. Balloons are used to study such questions as the origins of the universe, the birth and evolution of stars, and the origin and propagation of cosmic rays. The balloon is an essential tool for studying chemistry, dynamics, and radiation in the upper atmosphere. Balloon measurements have been extremely valuable in studying ozone depletion and stratospheric carbon dioxide concentrations.

Until 1982, NCAR managed and operated NSBF. Since then, the facility has been managed by UCAR under a contract with NASA. Many instruments flown from NSBF on balloons are prototypes for satellite instruments.

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