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ARTICLES
The Ozone Hole. The Ozone Hoax. Pollution. Skin Cancer. The topic of ozone makes headlines on a regular basis, but why does a single molecule merit such media coverage? How important is the ozone in our atmosphere and why are scientists so concerned about its disappearance?
First things first: Ozone is a molecular form of oxygen containing three oxygen atoms; when enough molecules are present it forms a pale blue gas. It is an unstable molecule that readily combines with other atoms, changing its chemical composition. Ozone is found in both the troposphere (the lower atmosphere) and the stratosphere, some 10-30 miles above us. Ozone in the troposphere is a primary component of smog, damaging human and plant tissue and oxidizing metal and paint. Ozone in the stratosphere, however, shields us from damaging ultraviolet (UV) light, allowing life as we know it to exist on earth.
The ozone "shield" as a descriptor is a bit misleading, however. The molecule does not form an impermeable sphere of atmospheric armor. Ozone continuously breaks apart into its oxygen atoms and reforms as ozone molecules, so a particular ozone molecule doesn't last very long. The "shield" changes constantly, but the atmospheric chemical processes maintain a dynamic equilibrium that keeps the overall amount of ozone constant-that is, until humans began contributing to the chemical processes.
While some social commentators and scientists claim that the ozone hole is a hoax or a naturally occurring phenomenon over which man has little control, the scientists who spend their days analyzing satellite data, flying over the Arctic and Antarctic and conducting laboratory experiments find inarguable evidence that human-produced chlorofluorocarbons (CFCs) have been interacting with geography, seasonal conditions, and perhaps even volcanic debris to deplete ozone well beyond natural levels.
TOMS provides global maps of ozone distribution in the atmosphere between the satellite and the earth's surface in Dobson Units. The satellite data give an overall measurement of ozone in a column of atmosphere. A Dobson Unit equals approximately one part per billion of ozone. G.M.B. Dobson invented the spectrophotometer that is still used today. (Satellite Image from the NASA Meteor 3 TOMS)
The debate over the existence of an ozone problem breeds media coverage; however, the real story is not whether ozone levels are decreasing, but what those decreases may mean for life on the surface. As the percentage of ozone in the atmosphere decreases, the amount of UV-B reaching the surface increases. It's the UV-B, not the ozone itself, that concerns scientists, because the invisible wavelengths are linked to skin cancers and other biological damage. As one of the handful of world experts on UV-B and its relation to stratospheric ozone, Sasha Madronich of the National Center for Atmospheric Research's Atmospheric Chemistry Division studies the potential for widespread increases in skin cancer due to ozone depletion.
His work is not easy because measuring UV-B is tricky. Levels are affected by time of day, day of the year, latitude, weather conditions, and the amount of ozone aloft. (Ozone levels fluctuate during the year.) UV is the part of the electromagnetic spectrum made up of wavelengths between 280 and 400 nanometers (billionths of a meter). Most of this is UV-A light, only mildly associated with sunburn and DNA damage and relatively benign to most plant life. But the ill effects increase more than a thousandfold in the shorter wavelengths referred to as UV-B (280-350 nanometers). Engineers face enormous challenges when designing instruments that can measure individual wavelengths, yet such precision is necessary to determine the amount of dangerous light entering the atmosphere. Below 300 nanometers the rays are sparse but very damaging; near 315 nanometers they're more numerous but much less destructive. Close to 310 nanometers is the middle ground, where the number and impact of rays combine to produce the most overall effect on humans and plants.
Madronich recently published the first paper that quantifies possible increases in skin-cancer rates at various latitudes due to the global ozone depletion measured by TOMS, the Total Ozone Mapping Spectrometer. From the TOMS data, Madronich calculated the biological effects using data from laboratory studies of three different UV impacts: reddening of the skin (erythema), skin cancer in mice, and damage to pure DNA. (Researchers cannot use human subjects.)
The results are significant. At the latitude of Toronto and Milwaukee, for example, Madronich predicts that future squamous cell cancers based on 1992 exposure could be about 20 percent more frequent than from 1979 exposure, due to the UV increase. Toward the equator, the percentage increases are lower, but actual skin cancer totals will tend to be higher due to the higher sun. Further ozone loss would only raise Madronich's percentages.
Why haven't doctors been inundated with increases in skin cancer already? It apparently takes sevemI decades to induce most kinds of skin cancer. "Current rates of skin cancer have nothing to do with ozone depletion," says Madronich. "We didn't have midlatitude ozone depletion in the 1960s and 1970s, when today's skin cancer cases were being generated."
Isn't where you live, or your lifestyle, more important than these changes in ozone? After all, someone in Arizona should get more UV-B than someone in Maine, no matter how much stratospheric ozone is lost. Precisely the point, says Madronich. "People have said that the effect of ozone depletion on cancer rates is the equivalent of moving from New York to Washington, D.C. Yes, that's true -- and again, you would see more skin cancer." Madronich also points out that, while other variations may come and go, the impact of ozone depletion, however small, is ever-present. It's as if a few pennies were withdrawn from your bank account every day witnout your knowledge. Your average balance would still rise and fall, but the high points would be a little lower and the chances of bouncing a check in a tight month would increase.
It's unfortunate that ozone in the troposphere is not as beneficial as ozone in the stratosphere, because with increasing populations, more automobiles, and more industry there's more ozone in the lower atmosphere. The molecule that shields us from damaging rays of the sun is a harsh pollutant at ground level. Since 1900 the amount of ozone near the earth's surface has more than doubled. While ozone shields us from ultraviolet radiation, this irritating, reactive molecule also damages forests and crops; destroys nylon, rubber, andothermaterials; and injures orkills living tissue. It is a particular threat to people who exercise outdoors or who suffer from respiratory problems.
Tropospheric ozone is formed by the interaction of sunlight, particularly ultraviolet light, with hydrocarbons and nitrogen oxides (emitted by automobiles and power plants). Declining ozone concentrations in the stratosphere may actually increase the amount of ozone formed near the planet's surface because more ultraviolet light will come through, thus creating more ozone.
Ozone affects plants in several ways. When exposed to high concentrations of ozone, plants are damaged because their stomata (pores on the leaf surface that carbon dioxide and water molecules diffuse through) close. Closure of stomata causes a decrease in photosynthesis. Ozone may also affect plants by entering through the stomata and directly damaging internal cells. The volatile molecule even reacts with hydrocarbons produced by plants, producing hydrogen peroxides and free radicals that cause damage to plant cells.
So why can't we take all of this "bad" ozone and blast it up into the stratosphere? The answer lies in the vast quantities needed and ozone's instability in the dynamic atmosphere. Ozone molecules don't last very long, with or without human intervention. The vehicle necessary to transport such enormous amounts of ozone does not exist, and if it did it would require so much fuel that the resulting pollution might undo any positive effort. Rather than seek such grandiose solutions we need to decrease the production of chemicals that break down ozone in the stratosphere and help create ozone in the troposphere.
The "ultraviolet outlook" is now part of the daily forecast issued by the National Weather Service. The reports should help society get accustomed to the idea that UV-B is dangerous and variable, and steer energy away from the debate over ozone depletion toward the creation of products and practices that will help us deal with increasing levels of UV-B.
The appearance of a hole in Earth's ozone layer over Antarctica, first detected in 1976, was so unexpected that scientists didn't pay attention to what their instruments were telling them; they thought their instruments were malfunctioning. When that explanation proved to be erroneous, they decided they were simply recording natural variations in the amount of ozone. It wasn't until 1985 that scientists were certain they were seeing a major problem.
Why did it take scientists so long to solve this mystery?
To begin with, observations that challenge preconceived ideas don't always get taken seriously -- even in science. Two decades ago, scientists did not suspect the importance of the chemical processes that rapidly destroy ozone in the Antarctic stratosphere. When they saw dramatic fluctuations in ozone levels, they assumed their instruments were in error, or that whatever was happening was due to natural processes like sunspot activity or volcanic eruptions.
What they didn't realize was that chlorine was the main culprit, and that most of the chlorine in the stratosphere comes from human activity. The largest source is a class of chemical compounds known as chlorofluorocarbons (CFCs). Because of their chemical stability, low toxicity, and valuable physical properties, these chemicals-versatile and stable in the lower atmosphere, at least-have been extensively used since the 1960s as propellants in aerosol spray cans, refrigerants, industrial cleaning solvents, and to make Styrofoam.
At the turn of the century, chlorine levels in the stratosphere were much lower than at present. As the use of CFCs has increased, however, so has their concentration in the atmosphere. Scientists could detect 100 parts per trillion (ppt) of CFC-12 in the atmosphere by the 1960s, 200 ppt by 1975, and more than 400 ppt by 1987. By 1990, they detected more than 750 ppt of CFC-ll and CFC-12, the two most destructive and persistent CFCs.
Once in the atmosphere, CFCs drift slowly upward to the stratosphere, where they are destroyed by ultraviolet radiation, releasing the chlorine that catalytically destroys ozone. Since 1974, scientists have known that chlorine can destroy ozone, but no one thought the destruction would be very rapid. Events in the Antarctic region proved them wrong.
The ozone hole story began at Halley Bay in Antarctica, where British scientists had been measuring ozone in the atmosphere since 1957. In 1976, they detected a 10% drop in ozone levels during September, October, and November -- the Antarctic spring. Since ozone concentrations over this region often vary from season to season, the researchers weren't concerned, even as the springtime drops occurred again and again. It wasn't until their instruments registered record low levels of ozone in 1983 that they realized something important was happening. By then, springtime ozone declines had occurred during seven of the previous eight years.
Within two years, scientists determined that the ozone hole over Antarctica is created when high levels of chlorine catalytically destroy ozone. The high levels of active chlorine are formed in the cold, dark winter stratosphere when reactions on the surface of cloud particles release chlorine from harmless (to ozone) chemical compounds into types that react with ozone. When the sunlight returns to the polar region in the austral spring, the active chlorine rapidly begins to destroy ozone. The extremely cold ice clouds can form over both poles during winter, but they are more common over the Antarctic region. During winter, atmospheric circulation creates a vortex of air above both poles. Very low temperatures occur inside a polar vortex, which is isolated from the rest of the atmosphere. The extreme cold fosters the formation of ice clouds during the winter and paves the way for the ozone destruction when light returns in spring. Scientists documented this mechanism in a series of field experiments in 1987.
The Arctic region is typically spared the worst of the ozone destruction because its vortex normally breaks down several weeks before the sun returns, dissipating the ice clouds. The larger percentage of land masses in the northern latitudes, particularly mountains, prevent an excessive buildup of ice clouds. Geography isn't always enough to prevent the vortex, however. The north pole's vortex was unusually strong and long-lived during the winter of 1992-1993, for example. When sunlight appeared, this drove down Arctic ozone levels well into March. Because there is more ozone over the north pole to begin with, this decline didn't create a hole. However, it did send ozone-depleted air over populated areas of the northern hemisphere when the vortex broke up.
The loss of ozone over populous regions underscores the importance of following up on the 1987 Montreal Protocol. This agreement, now signed by more than 70 countries, set goals of reducing CFC production by 20 percent (relative to 1986 levels) by 1993 and by 50 percent by 1998. These targets have since been strengthened to call for the elimination of the most dangerous CFCs by 1996 and for regulation of other ozone depleting chemicals. The United States and other nations are well on their way to meeting these goals. In 1993, global CFC production was already down 40% compared to 1986 levels. That's fortunate, since CFCs in circulation will pose a threat to Earth's ozone layer for another hundred years.
ACTIVITY
Concepts:
* Ozone is present in the air that surrounds us. This tropospheric ozone is
often called "bad" ozone because it damages living material. However,
some tropospheric ozone is natural and is found near lightning and static discharges.
* Ozone will break down certain materials.
* The concentrations of ozone are not uniform.
* The longer the exposure to ozone the greater the effect.
Background Information:
This activity is based on the high oxidation capacity of ozone. Due to this
characteristic, items of rubber and plastic beak down after relatively short
exposure. By observing which rubber band begins to develop cracks or pits first,
the relative ozone levels at different locations can be determined. Some rubber
bands may demonstrate faster deterioration than others. The time duration of
this activity can be extended to allow more damage to the rubber bands.
Materials: (per group of students)
3 glass jars or beakers
3 medium size rubber bands
hand lens
Procedure:
1. Have students work in groups of 2-3. Each group should get three glass jars
or beakers, a hand lens, and 3 rubber bands.
Students:
2. Write the starting date and location on a piece of paper and place it on
the beaker or jar.
3. Place one rubber band around each container near the center.
4. Use a hand lens to observe a section of the rubber band. Mark this section.
5. Draw this section to show the condition of the rubber band.
6. Place one jar outside away from direct sunlight, one jar in the classroom,
and one jar near a photo copy machine.
7. Observe and record (draw) changes within the marked section daily for the
next week.
Observations and Questions:
1. Make drawings and a table describing the observable changes in the rubber
bands. (Cracking or pitting of the rubber bands should be observed in some locations.)
2. Which location showed the greatest changes? Which location showed the least
changes? (Answers will vary. Usually the sample near the copy machine will show
more changes.)
3. On which day did you first see noticeable changes? (Answers vary depending
upon ozone concentration.)
4. Did all the rubber bands change on the same day? (Probably not.)
5. What do you think may have caused the change in the rubber bands? (Ozone
will cause a deterioration of the rubber bands depending upon the ozone levels.)
6. What might explain why you observed different degrees of change in various
locations? (Hopefully students will relate the amount of change to different
ozone levels.)
7. What do you think the effect on the rubber bands might sifggest about any
possible effect on living tissue such as your lungs or plants? (Hard to make
direct comparisons, but there might be a parallel between ozone levels and amount
of damage to biological materials.)
8. Describe in a short paragraph why your data might suggest possible hazards
to people who work in copy rooms. (Higher concentrations of ozone might be harmful
to tissues of people working for longer periods of time near copying machines.)
Extensions:
1. Have students try different sizes of rubber bands, nylon stockings, different
types of plastics, etc. at different locations, and for different amounts of
time. Students can examine differences in ozone levels at different street intersections
since ozone is formed by the interaction of hydrocarbons and nitrogen oxides
with sunlight. Students should keep a record of weather conditions, particularly
sunlight, and level of air pollution during the time of recording. The health
department in some cities records pollution levels, including ozone levels.
2. Have students determine the economic impact of changes in tropospheric ozone.
Things to consider: Cost of ozone damage to materials such as tires, rubber,
and plastics; cost benefit analysis of pollution mitigation.
RESOURCES
Why were aerosol sprays banned? Why is ultraviolet light bad for your skin? Does the ozone hole hurt Antarctic penguins? Answers to these questions are graphically presented in SIRS Photo Essays. The "Ozone Depletion" unit consists of eight posters, each with a photograph and brief explanation. The posters can be used for bulletin board displays or to stimulate group discussion. The units are designed to help develop students' inquiry and analytical skills. A study guide offers worksheets, exercises and selected bibliography.
For a catalog, contact SIRS Customer Service at 1-800-232-SIRS or request via e-mail: custserve@sirs.com.
Project LEARN teachers refined a technique for measuring tropospheric ozone using Schoenbein paper. The method uses a cornstarch mixture spread on filter paper. For a detailed lesson plan, write to Caroline Hanson, Project LEARN, P.O. Box 3000, Boulder, CO 80307 or send your request via e-mail to chanson@ncar.ucar.edu.
If you have an Internet account, you can access the Frequently Asked Ouestions About Ozone file. The information is updated the middle of each month. It may be obtained by anonymous ftp from rtfm.mit. edu: /pub/usenet/news.answers/ozone-depletion/intro, or from mail server@rtfm.mit.edu by sending a mail message containing the following: send usenet/news.answers/ozone-depletion/intro
Science
Now
is jointly published by the Walter
Orr Roberts Institute at the University
Corporation for Atmospheric Research
and SIRS Publishing, Inc.
(Social Issues Resources Series.) Science
Now is published three times during the
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Editor:
Caroline Hanson
Scientific
Editor:
Pat Kennedy
Contributors:
Bob Henson, NCAR Outreach and Information Staff;
Sasha Madronich, Atmospheric Chemistry Division
UCAR is a consortium of over 60 universities in the U.S. and Canada with doctoral programs in atmospheric and related sciences. UCAR manages and operates the National Center for Atmospheric Research under the sponsorship of the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Anyone who undertakes any of the activities described herein shall do so at their own risk; UCAR and SIRS Publishing, Inc. assume no liability, whatsoever, for any injury or harm, which may result therefrom.
© COPYRIGHT 1994 UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH. ALL RIGHTS RESERVED.
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