Atoms in the Atmosphere

Extending 22,000 miles above every inch of the earth, the atmosphere is our most immense resource. But infinitesimally small changes--a matter of a few molecules among millions-- endanger our invisible life support system

Minnesota, July/August 1984

by William Hoffman


Sections

Why is there air? By his own account, comedian Bill Cosby faced this profound question when he was a self-conscious physical education student at Temple University in 1960. His college sweetheart, a philosophy major with an IQ of "three-hundred thousand," used to walk around asking, "Why is there air?"

Cosby was baffled because the answer is so easy: "Any phys ed major knows why there's air. There's air to blow up volleyballs, blow up basketballs. Guys call me dumb, for crying out loud. Walking around asking why there's air!"

All right. Suppose there were no air, no atmosphere. Of course, you couldn't breathe, but let's say you could adapt to that. If you walked outdoors, you might think you were on a stage: the sun would be like a brilliant spotlight against a coal-black background. Before the meteor showers forced you back indoors, your skin would be zapped with an instant tan courtesy of unmitigated ultraviolet light. From the windows of your house, you would witness dramatic changes in the scenery. Trees, grass, animals-literally all life as we know it-would cease to exist.

Of course, the atmosphere isn't just going to float off into space someday, gravity is not so easily denied. Nonetheless scientists have been posing some frightening scenarios: a disrupting warming trend caused by the greenhouse effect of carbon dioxide build-up; a new ice age brought on by the cooling effect of increased particulate matter in the atmosphere; threats to plant and human life from increased ultraviolet light due to depletion of the earth's ozone. They see lakes and rivers dead, forests decimated by acid rain.

What is happening to the air we breathe?

The atmosphere is constantly changing, but since the industrial revolution it has been changing in ways nature is only partly responsible for. We are beginning to realize that the atmosphere is as precious a resource as water, wood, and soil. It has been compared to the cell membrane: Both its protective and permeable qualities are vital to life.

If we were as conscious of our atmosphere as deep-sea divers are of their air supply, perhaps we would do a better job of shepherding this global life-support system. Only now, when human activities have begun to threaten the atmosphere, have we begun to recognize the atmosphere as a finite resource that must be conserved for future generations. University researchers, along with scientists across the country, are examining the ways we are endangering the air we breathe and, perhaps, threatening the survival of future generations.

Studying the atmosphere

Scientists have been studying air for as long as there have been scientists, that is, for about 7,000 years. Air would be easier to study if it weren't so dynamic, if it weren't constantly in flux with the changes in the weather. At least it is stable in its makeup: Clean dry air at sea level contains nitrogen (78 percent), oxygen (21 percent), and argon (1 percent).

But some of air's trace gases-elements so rare they are measured in parts per million rather than percentages-are changing in their atmospheric concentrations. Although they compose only a minute proportion of the atmosphere, even slight changes in these trace gases can upset the delicate balance in the atmosphere. Because these gases are produced partly by human activity, we need to understand what impact our actions will have.

Three trace gases are of special concern: ozone (O3), carbon dioxide (CO2), and sulfur dioxide (SO2). Understanding how they are formed and what effect they will have on the environment is the business of both basic and applied science. A number of University researchers are playing key roles in nationwide studies aimed at learning more about the behavior of these gases in a changing atmosphere.

Searching for the ozone factor

Oxygen is the most abundant element on earth, making up about half of the earth's surface material and 90 percent of water, in addition to nearly one-fourth of air. When molecular oxygen (O2) is separated by solar energy high in the stratosphere (the layer of air between six and 15 miles above the earth's surface), it can recombine with elemental oxygen to form ozone (O3). Farther out, at 30 miles into the stratosphere, ozone concentration is only 12 parts per million. That may not seem like much, but it is vitally important. Ozone is the only atmospheric gas that absorbs ultraviolet light, shielding the earth's surface, and us, from harmful and potentially lethal radiation.

Several trace substances are interfering with the ozone shield, and all of them are increasing their atmospheric levels. The most critical are chlorofluorocarbons-CFCs-often know by the trade name Freon. Molecules of CFC drift into the stratosphere from the earth's surface, where their inertness has made them useful as refrigerants and as spray propellants. They are broken down by sunlight into chlorine and chlorine oxide that in turn react with ozone, weakening the shield.

The use of CFCs in aerosol sprays was banned in the United States in 1977 after widespread concern that they were depleting the ozone layer. A drop of just a few percent could decrease crop productivity, especially of ultraviolet-sensitive crops like soybeans, and could increase skin cancer. CFCs in parts per trillion of surface air could spell trouble for stratospheric ozone.

Since 1978, University physics professor Konrad Mauersberger has been sending balloons into the stratosphere with an instrument aboard that measures ozone concentration. The instrument he uses is a mass spectrometer, invented by University Regents' Professor of Physics Alfred O.C. Nier in the 1940s and modified by Mauersberger to operate at high altitudes.

Mauersberger's balloons are launched from the NASA station in Palestine, Texas. Each flight obtains "a single profile, a snapshot" of stratospheric gas particles, Mauersberger said. His research complements other NASA projects that involve satellite and ground-based measurements of ozone.

Because his instrument is extremely precise, Mauersberger is able to obtain data crucial for calibrating other instruments in operation constantly. Information from these instruments is used to build large-scale mathematical models "that can simulate the pace of chemical reactions that are occurring in the stratosphere," Mauersberger said, explaining that there are more than 150 such reactions.

"Personally, I think the models are getting better and better," he said, adding that, with better data, scientists should be able to predict ozone change "a hundred years from now." His launchings are being coordinated with NASA satellite launchings scheduled for the next three years.

Last winter, the National Research Council, the research arm of the National Academy of Sciences, reported that new estimates for ozone depletion were reduced from the range of 5 to 9 percent to the range of 2 to 4 percent, based on better information and more sophisticated models. The NRC concluded that ozone depletion in the stratosphere is being accompanied by ozone increase in the troposphere (the layer of air extending from the earth to a height of 6 to 15 miles).

"There appears to be a large decrease at the higher levels of the stratosphere and an increase where jet aircraft fly," Mauersberger said. Subsonic jets release certain oxides of nitrogen (NO and NO2) that result in increased ozone. Supersonic jets, on the other hand, inject nitrous oxide (N2O) into the stratosphere, which depletes ozone much in the same way CFCs do, locking it in a "catalytic cycle," he said.

Even though current estimates of ozone depletion are somewhat lower than previous estimates, long-term prospects are not encouraging. CFCs have no efficient substitutes in refrigeration and air conditioning, and they have excellent insulating properties. According to one report, industrial production of CFCs in the west has started to climb once again after dropping significantly between 1975 and 1980. Production has never been curtailed in the east, and, as Mauersberger noted, developing nations will probably use more CFCs as they modernize.

Ironically, at the same time that less ozone in the stratosphere threatens our well-being, excess ozone in surface air is creating a different set of problems. Normally present in parts per billion of surface air, ozone is also a major constituent of urban smog. It is produced by the action of sunlight on automobile emissions. Humans have a defense mechanism that neutralizes ozone when it is inhaled, but plants are vulnerable. In 1980, the Environmental Protection Agency established a program to estimate crop loss nationally that can be traced to air pollution. Since then, ozone has emerged as the leading crop polluter.

Sagar Krupa, associate professor of plant pathology, has studied the effects of ozone on cash crops in Minnesota. He sets up open-top growing chambers downwind of urban areas and uses filters to compare plant growth in purified and unpurified or ambient air.

"Air quality affects plant productivity," Krupa said. An "immense" plume form the Twin Cities moves westward, where ozone attacks plant chloroplasts that are exposed on the underside of the leaf. Soybeans and alfalfa are especially sensitive to ozone, he said.

In a recent study, Krupa and his University colleagues estimated that Wright County west of the Twin Cities lost 14,000 tons of alfalfa to ozone damage in 1979, compared to no loss in Nobles County in the southwestern corner of the state. Loss estimates of alfalfa statewide jumped from 35,000 tons for 1979 to 415,000 tons for 1980. (Corn and wheat also showed some loss in 1980 after no loss in 1979). The drastic difference in alfalfa production may be that in 1980 alfalfa plants tended to have their stomata (minute openings in the leaves) open at the time of high ozone concentration, according to Krupa.

Examining the effects of ozone on cash crops is a fairly new field of study. Krupa's group was the first to show evidence of ozone injury on Minnesota crops. He calls it a "unique problem" that requires an understanding of atmospheric chemistry, meteorology, and plant pathology and the aid of a computer to model crop loss trends.

CO2: From the icehouse to the greenhouse

Most of the carbon in nature is locked up in rocks and marine sediments. Only a tiny fraction circulates in the atmosphere in the form of carbon dioxide (CO2), where it acts as a thermostat for surface temperature through the so-called greenhouse effect.

The earth absorbs solar energy and emits infrared radiation, which is in turn absorbed by, among other things, carbon dioxide in the air. CO2 then scatters radiation in all directions. Some escapes into space.

As the amount of an infrared absorber such as carbon dioxide increases, more radiation is absorbed by the earth's surface and temperatures rise.

If scientists perfectly understood the carbon cycle and if the burning of fossil fuels remained constant, they could calculate what part of the expected rise in temperature is due to the carbon dioxide released when fossil fuels are burned. But carbon is keeping some secrets as it cycles through the environment.

In geological time, we are emerging from an ice age when the average temperature was two to three degrees Celsius cooler than it is today. The last warming episode was 1,000 years ago and helped the Vikings to sail as far as Newfoundland. A century from now, average global temperatures could be from five to eight degrees C higher if nothing is done to curtail the burning of fossil fuels such as coal, oil, and natural gas.

That's the view of Peter Ciborowski, a research fellow at the University's Humphrey Institute of Public Affairs. Ciborowski has studied the greenhouse effect since 1980 as part of the institute's Global Environmental Policy Project under the direction of institute professor Dean Abrahamson. He is a walking compendium of facts and figures surrounding the greenhouse problem.

Ciborowski concedes that a two- to four-degree C increase in average global temperature is probably inevitable, but without concerted action now the increase could be twice that, with catastrophic consequences for agriculture. Most crop varieties are not bred to withstand changes in climate. Soil moisture would be depleted and erosion intensified. Yields would be drastically reduced, and massive starvation would ensue in countries dependent on U.S. food production.

"We have to seek a limit" to fossil fuel combustion, Ciborowski said. Waiting 30 years to begin changing from carbon-based to noncarbon-based power generation would have enormous ramifications, in his view.

The U.S. could take the lead because it burns more fossil fuel than any other nation. About a ton of carbon as carbon dioxide is released every year for every person on earth. (Breathing releases only about a quarter tone of CO2 per person per year.) Of the worldwide total, Americans contribute about five tons per person per year, according to one report.

Last fall, the EPA and the NRC released reports on carbon dioxide and world climate. The reports concluded that the level of CO2 will probably double sometime after the middle of the next century. A doubling will be accompanied by a global warming of three degrees C, depending on the rate of fossil fuel combustion.

The EPA report, in Ciborowski's opinion "arbitrary" in its prediction, did take into consideration the many trace gases-methane, ozone, CFCs, oxides of nitrogen, and carbon monoxide-that add to the effects of carbon dioxide. But the agency used what Ciborowski considers "very conservative numbers" in its estimate: the trace gases were thought to increase warming by at least 70 percent over the CO2 effects. Yet a recent study indicates that these trace gases may equal, or even double, the warming effects of CO2, Ciborowski said.

The preindustrial level of carbon dioxide in the atmosphere is estimated to have been about 270 parts per million of air. Direct atmospheric sampling of CO2 began in 1958 and registered 318 ppm compared to 340 ppm today. But scientists are just beginning to analyze the concentration of other infrared-absorbing "greenhouse" gases and include them in their computer models of climactic change. Indirect contributions to atmospheric CO2 by the clearing of the rain forests, though much smaller, also may have to be reassessed.

Once of the biggest problems is detecting a definite sign of climate warming amidst the "noise" of a dynamic atmosphere and shifting oceanic currents. Since 1940, average global temperatures have exhibited a cooling trend except for 1981, which was the warmest year on record in the northern hemisphere. In 1982, the eruption of the Mexican volcano El Chicon is believed to have had a cooling effect on global temperatures.

University agricultural climatologist Don Baker doesn't think there has been a clear signal yet. In any event, the expected temperature increase probably has been exaggerated in its predicted effects, he said.

Baker thinks there should be room for disagreement on the greenhouse effect. He cites the example of Sherwood Idso, head of the Institute for Biospheric Research in Tempe, Ariz., and a 1964 graduate of the University's Institute of Technology. Idso is regarded by many scientists as an outlaw on the CO2 matter because he has questioned the reliability of current climate models and the motives of the National Academy of Sciences, which he charges is promoting "science by decree."

In Idso's view, the greenhouse effect, rather than melting polar ice, raising ocean levels, flooding coastal lowlands, and disrupting agriculture, will benefit a hungry world. Higher concentrations of atmospheric CO2 will aid plant photosynthesis, translating into abundant yields.

Other scientists also have argued that there are serious discrepancies between how the atmosphere actually works and how mathematical models show it works. One of the unknowns is whether there is a natural self-regulating mechanism in the atmosphere that would compensate for increased surface temperatures. If there is, it may be in the clouds.

Clouds reflect some of the incoming solar radiation back into space. If warmer temperatures create more cloud cover, this bouncing off might offset the greenhouse effect. Though there is a flurry of activity now in cloud physics, such a mechanism has not been described so far, Ciborowski said. He, for one, wouldn't count on it.

Strong rain and sulfurous vistas

Besides altering the equilibrium of stratospheric ozone and the carbon cycle, we may be upsetting the natural sulfur cycle by loading the air with sulfur compounds from fossil fuel emissions. According to one estimate, nearly half of the 500 million tons of sulfur dioxide released annually is thought to be the result of human activity, and the percentage is growing.

Whatever effect we are having on the sulfur cycle, we are clearly upsetting delicate aquatic ecosystems in Scandinavia and the northeastern United States, and now forests in West Germany and along the Atlantic seaboard are showing the damaging effects of acid rain. Indeed, it is estimated that two-thirds of the land area of North America receives acid precipitation.

The power plant emissions mainly responsible for acid rain contain large amounts of sulfur dioxide (SO2) gas. SO2 is transformed into sulfuric acid in the atmosphere and returns to the earth's surface in acidified rain or snow, sometimes hundreds of miles from its source. If the ground soil where it falls has poor neutralizing capacity, lakes and streams in the area will eventually become acidified. In a manner of speaking, they die.

Ecology professor Eville Gorham is an internationally recognized pioneer in the study of acid rain. In the 1950s, while investigating peat bogs in northern England, Gorham identified sulfuric acid in rain when the wind blew in from the industrialized regions to the south and east. He also happened to be living in London in 1952 when sulfur dioxide smog killed several thousand people in a week.

Despite the difficulty in tracing environmental damage to specific sources, there is "no doubt" that air pollution is the culprit behind acid rain, Gorham said. Acid rain, which can contain nitric and hydrochloric acid, in addition to sulfuric acid, destroys lakes and forests and is a major factor in pipeline and building corrosion, he said. In addition, acid sulfate particles that contribute to acid rain "are in the size range that penetrates deep into the lung," and may well be a factor in lung diseases, according to Gorham.

Soil scientists have suggested that acid rain can have beneficial effects on croplands low in sulfur, but Gorham argues that farmers already know very well how to treat sulfur-depleted soil. "They apply fertilizer."

Recently, scientists reported in the journal Atmospheric Environment that a tracing method involving the element selenium, which accompanies other smoke stack emissions in certain concentrations, successfully linked a sulfate haze in the Shenandoah Valley to the Midwestern coal-fired power plants. Gorham believes that for years there has been enough evidence on which to base legislation to reduce emissions.

Minnesota representative Gerry Sikorski introduced a bill in Congress last year that would tax industries using nonnuclear-powered plants to generate electricity, with the aim of reducing sulfur dioxide by 10 million tons. Senator David Durenburger also proposed legislation to control emissions.

Sikorski's bill failed in committee in early May and Durenburger's was never seriously considered, according to David Thornton, acid-rain coordinator of the state Pollution Control Agency. Acid rain legislation "appears to be dead this session," he said.

In Minnesota, no serious damage from acid rain has occurred yet. _"We haven't seen any clear evidence of lake acidification," Gorham said, adding, however, that the poor buffering capacity of soils in northeastern Minnesota makes that area especially vulnerable.

The PCA announced in May that it would publish a weekly index comparing the acidity of normal rainfall with rain collected at nine sampling stations across the state. The agency estimates that 2,500 lakes and 3.5 million acres of forests are sensitive to acid rain damage.

In a three-year analysis of rain chemistry downwind of the Northern States Power coal-fired plant in Sherburne County, Sagar Krupa, Gregory Pratt, and Michael Coscio of the University's plant pathology department found no definite trend in the ionic components of rain that might indicate increasing acidity. Their findings confirm other studies' findings that sulfur dioxide emissions are distributed in the atmosphere in a highly complex and as yet poorly understood manner and may have longer-than-expected residence time.

This long life would come as no surprise to Peter McMurry and James C. Wilson of the Particle Technology Laboratory in the Institute of Technology. They study the formation of aerosols-fine solid or liquid particles suspended in the atmosphere. They are mainly interested in sulfate aerosols, both near the earth's surface and in the stratosphere.

In the presence of radiant energy, sulfur dioxide combines with water vapor to form sulfuric acid aerosol, a gas-phase chemical reaction. Sulfur dioxide can also dissolve in a suspended water droplet, again forming sulfuric acid aerosol, this time in a liquid-phase reaction. The later reaction, which tends to occur more frequently at high humidity, is the reaction McMurry is currently trying to decipher.

In the laboratory, McMurry has been able to trace a particle from its molecular beginning. Data from his field and laboratory experiments are then used to construct numerical models of aerosol behavior in the atmosphere, a problem of baffling complexity. (Aerosols are in constant motion, Brownian motion.)

Particles formed by the chemical reactions of aerosols have certain optical qualities that produce a characteristic haze. These particles are about one-half micron in size (a human hair is about 50 microns in diameter), and they scatter light. Larger particles do not produce a haze, McMurry said.

Wilson is currently analyzing the concentration of stratospheric aerosols and how they are formed. He and two undergraduate students developed in an instrument-a condensation nucleus counter-to measure the concentration of submicron particles. This information helps Wilson to estimate the rate at which sulfur dioxide is converted to sulfuric acid in the stratosphere.

After the eruption of the Mexican volcano El Chichon in 1982, Wilson took his instrument to California where it was placed aboard a NASA U-2 airplane along with other instruments and flown into the stratosphere to do its work. Once data collected by the instrument are analyzed, they will be compared with measurements taken from satellite, balloon, and ground-based instruments and used to test computer models of gas-to-particle conversion rates.

It has been suggested that stratospheric sulfuric acid may be responsible for pitting aircraft windows, but some scientists think it also may influence the earth's climate. Stratospheric aerosols both absorb and scatter radiation. A warming of the stratosphere has been confirmed, Wilson said. He would like to know if the increased concentration of aerosols is cyclic or steadily building.

Scientist Carl Sagan has written that stratospheric aerosols may be generated by the incomplete burning of fossil fuels, but volcanic activity is considered to be a more important factor. The eruption of El Chichon ejected millions of tons of gaseous sulfur dioxide into the stratosphere and produced a cloud 100 times denser than that of Mount St. Helens in 1980. Wilson is in the process of analyzing data taken after the eruption and comparing them with those from balloon instruments to get a picture of sulfuric aerosol formation in regions of the stratosphere sampled by the U-2.

The information should deepen our understanding of the ways volcanoes can alter global climate. El Chichon is estimated to have had a maximum effect of .2 degree C two months after the eruption, when a stratospheric cloud of sulfuric acid aerosol was reported to be fully formed. Some scientists have suggested that El Chichon also contributed to El Nino, the sudden change in atmospheric and oceanic circulation in the equatorial Pacific during the winter of 1982-83.

Mathematical models and supercomputers: An answer for the future?

Ozone, carbon dioxide, and sulfur dioxide are not the only ingredients in the air that are drawing the attention of scientists, policy makers, and the public. They are the most important ones, however, because their effects are global. Together, they compose only a tiny fraction of ambient air, but they have the potential to change living conditions for future generations.

Atmospheric research, as we have seen, relies heavily on mathematical modeling. Large-scale computation allows the scientist to simulate the activity of complex and dynamic physical systems like the atmosphere. Theoretical, experimental, and computer scientists interact in this problem-solving process.

Because the problems are so complex, the fastest and most powerful computers are required. In this respect, the University is at least a step ahead of most research institutions. (See "The Number Crunchers" in the May/June 1984 Minnesota). But the complexity of current mathematical models is limited by the capacity of computers to execute them. As fast as current machines are (the University's Cray-1 supercomputer can make a million calculations a second), they are not yet fast enough to address satisfactorily many of the problems of the magnitude posed by the atmosphere.

On the other hand, scientific computing is only about 25 years old and has already proven an indispensable tool to researchers studying the air. Perhaps the biggest uncertainties will not be about how the atmosphere works but whether we are willing and able to protect it from ourselves.

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Aerosols May Drive a Significant Portion of Arctic Warming - NASA 04.08.09

Aerosols can influence climate directly by either reflecting or absorbing the sun's radiation as it moves through the atmosphere. The tiny airborne particles enter the atmosphere from sources such as industrial pollution, volcanoes and residential cooking stoves. Credit: NASA Goddard's Scientific Visualization Studio