Title: Ozone Layer Depletion
In Investigations Lessons 1 and 2 students were introduced to the composition and structure of the atmosphere and the electromagnetic spectrum. This lesson links the atmospheric composition concepts discussed in Investigations Lesson 1 and the EMS concepts discussed in Investigations Lesson 2 with depletion of the ozone layer (Stratospheric Ozone). Investigations Lesson 4 highlights for students the harmful effects of human produced ozone tropospheric ozone and helps illustrate the distinction between good (stratospheric) ozone and bad (tropospheric) ozone. Investigations Lesson 7 deals with ozone as a greenhouse gas. The three faces of ozone are presented across Investigations Lessons 3, 4 and 7 -- ozone as a protector (Investigations Lesson 3), ozone as a poison or toxin (Investigations Lesson 4), and ozone as a greenhouse gas (Investigations Lesson 7).
This lesson helps learners:
The concept map and concept map (unlike other investigations lessons this lesson contains two concept maps) show model relationships among concepts this lesson seeks to develop. The second of these two maps is devoted solely to the formation and destruction of ozone. Concepts introduced in this lesson are bolded on that concept map and concepts from other lessons are in plain text (not bolded).
Content. The video Ozone: The Hole Story and the booklet Reports to the Nation on Our Changing Planet: Our Ozone Shield which are used as student resources in this lesson, are excellent basic content resources for teachers. Teachers should become familiar with these.
One area that these resource materials do not deal with is how CFCs are connected with insulating plastic foams, known generally as "expandable polystyrenes." Foam insulated cups, sheets of foam insulating board, and the foam insulation that is in the walls of refrigerators are common examples of items made of expandable polystyrenes. "Styrofoam" is the brand name of an expandable polystyrene made by Dow Chemical Co. CFCs are involved in the manufacture of styrofoam. Other companies make similar expandable polystyenes that involve CFCs that have different brand names but we commonly refer to them as styrofoam. In fact, we commonly refer to foam products that look similar to real "Styrofoam" as styrofoam.
CFCs are used as "blowing agents" in the manufacture of "Styrofoam" and styrofoam-like expandable polystyrenes made by companies other than Dow. That is, CFCs are one of the components that are mixed with the plastic resin. (Think of the plastic resin as bread dough.) The CFCs are mixed with the plastic resin using different techniques (technologies). Sometimes it is mixed in with the other components that make up the resin; sometimes it is injected into the resin. Irrespective of how it is done, the CFCs cause the plastic resin to expand as bubbles of CFCs are formed (the resin expansion is comparable in many ways to the rising of bread dough caused by CO2 bubbles from yeast that is part of the dough mixture, except it is much faster). The CFCs bubbles in the resulting "Styrofoam" may be open, in which case the CFCs escape during manufacturing, or the bubbles can be closed, in which case the CFCs are retained in the plastic foam.
Closed and open bubbles and the gases in closed bubbles give expandable polystyrenes different insulating properties. Expandable polystyrenes with open bubbles have very good insulation properties, but those with closed bubbles are better insulators. Expandable polystyrenes with CFC bubbles (i.e., "Styrofoam") are especially good because the CFC bubbles work better as insulators than do bubbles of some other gases that are used to make expandable polystyrene. CFCs retain heat. Eventually, however, the CFCs in the closed bubbles escape into the atmosphere as products made with "Styrofoam" break-down (the bubbles containing CFCs break), e.g., older refrigerators and styrofoam cups in open dumps, junk yards and landfills. Hence, CFCs are released in the production of "Styrofoam" and when it deteriorates. It takes about eight to ten years for CFCs to migrate to the Stratosphere, where one CFC molecule can break down thousands of molecules of ozone.
We need to be careful about thinking that all expandable polystyrenes involve CFCs. Because the chemical industry has been seeking substitutes for CFCs, most expandable polystyrenes manufactured today do not contain CFCs or involve CFCs in the manufacturing process. Plastic foam cups found at fast-food restaurants or sold in grocery stores, sheets of foam insulation, and the expandable polystyrene insulation in today's refrigerators are not made with CFCs. CFC substitutes, generally referred to was H-CFCs and HFCs are used. Pentane is a common H-CFC. While these H-CFCs and HFCs do not deplete the Ozone Layer, many of them retain heat and so are greenhouse gases.
Teachers should be alert to the alternative conception that will be common among students, that foam plastic cups and foam insulation that are made today contain CFCs or involved CFCs in the manufacturing process. While that is not the case in the recent past, that was the case. Many of these materials were used in the building of our homes are in older models of refrigerators, are in items, such as refrigerators, found in open dumps and landfills. These items continue to leak CFCs into the atmosphere. Given CFCs take on average eight years to migrate to the Stratosphere, we can expect the levels of CFCs in the upper atmosphere to continue to increase. And, given that one CFC molecule reacts with ozone (changing it to oxygen) thousands of times, CFCs embody a significant risk to the ozone layer.
The following is a report on the status of the Ozone Layer taken off the EcoNet list-serve on Internet:
Date: Thu, 29 Dec 1994 08:17:29 -0800
From: Tom Gray <tgray@IGC.APC.ORG>
To: Multiple recipients of list BIOSPH-L <BIOSPH-L@UBVM.cc.buffalo.edu>Subject:
Ozone Update
/* Written 7: 10 PM Nov 27, 1994 by larris in igc:climate.news*/
Title: OZONE LOSS CONTINUES
THERE IS good news and bad news in the latest assessment of the ozone depletion problem. International curbs on the chemicals responsible are affecting atmospheric concentrations but high-atmosphere ozone levels continue to deteriorate over much of the planet.
Details of the latest scientific assessment of the ozone depletion problem were released in August 1994. The assessment is in preparation for the 1995 review of the Montreal Protocol on Substances that Deplete the Ozone Layer. It has been organized under the auspices of the World Meteorological Organization and the United Nations Environment Programme and represents the consensus of an international panel of experts and reviewers.
The Montreal Protocol is the international agreement intended to control releases of ozone-depleting chemicals. Agreed in 1987, the Protocol has been strengthened on two occasions in response to accumulating scientific evidence regarding the seriousness of the threat to the ozone layer. Chemicals such as CFC-11 and CFC-12 destroy ozone in the upper atmosphere as a result of the chlorine they contain, reducing protection from the effects of harmful ultraviolet radiation at the Earth's surface.
According to the 1994 Scientific Assessment of Ozone Depletion, atmospheric growth rates of several of the major ozone-depleting chemicals have slowed down. This is in response to the control measures adopted as a result of the Montreal Protocol and later amendments. In fact, the latest figures suggest that emissions may well be lower than the maximum limit set by the international agreement.
Because of the long atmospheric lifetime of many of the ozone-depleting substances, increasing losses in global ozone are predicted for the remainder of the 1990s with significant recovery not likely until well into the 21st century. Consistent with this prediction, data for the past two years show that record low global ozone levels have occurred notwithstanding the curbs of releases.
The eruption of Mount Pinatubo in June 1991 amplified the decreasing trend caused by releases of ozone-depleting chemicals. Sulphate aerosols from the eruption contributed to the chemical reactions responsible for ozone depletion and, for a short period, accelerated the downward trend. Changes in upper atmosphere temperature and circulation induced by the volcanic pollution also affected the spatial distribution of ozone.
Ozone depletion is most marked over the South Pole in the southern spring owing to the particular characteristics of the upper atmosphere in this region. Extreme cold and the reappearance of sunlight conspire to accelerate ozone losses. The Antarctic ozone "holes" of 1992 and 1993 were the most severe on record with the greatest areal extent and lowest ozone levels observed in the past two decades. Locally, the amount of ozone was depleted by more than 99 per cent in each October of the two years.
Jonathan Shanklin of the British Antarctic Survey (BAS) has predicted that spring ozone over Antarctica may disappear completely by the year 2005 if present trends continue. This assessment is based on BAS data collected at the Halley Research Station over the past 30 years.
Over middle latitudes of both hemispheres, ozone levels are decreasing at a rate of around four to five per cent a decade. These losses are greatest in winter and spring. There is no evidence of significant trends in tropical ozone levels.
There is a rather complex link between the ozone depletion problem and the threat of global warming. On the one hand, the ozone-depleting chemicals are also greenhouse gases. So action to reduce one problem will have benefits in limiting the other. On the other hand, ozone loss has a cooling effect on the atmosphere and this means that reducing ozone depletion will warm the atmosphere.
The 1994 Assessment concludes that, on balance, ozone depletion may have offset about a fifth of the global warming expected since 1980 due to the rise in greenhouse gas levels alone. The beneficial by-product of the ozone depletion problem will be lost as ozone levels in the upper atmosphere gradually return to normal, increasing the rate of global temperature rise.
The report spells out the policy implications of the latest scientific findings.
The review also notes that many of the substitutes for the major ozone-depleting chemicals now being adopted are relatively powerful greenhouse gases. The implications of increased use should be considered during the Framework Convention on Climate Change negotiations.
The importance of incorporating the implications of ozone change when assessing the problem of global warming is emphasized: "Changes in ozone since pre-industrial times as a result of human activity are believed to have been a significant influence on radiative forcing: this human influence is expected to continue into the foreseeable future." The Intergovernmental Panel on Climate Change will release new estimates of the role of ozone in the greenhouse problem as part of its 1995 scientific assessment.
Tiempo, a bulletin on global warming and the Third World, is published by
the International Institute for Environment and Development (London, UK) and the University of East Anglia (Norwich, UK) with support from the Swedish International Development Authority in association with the Stockholm Environment Institute. Editorial office: TIEMPO, c/o Mick Kelly, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK (email crunorwich@gn.apc.org)
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The video Ozone: The Hole Story
A molecular model sets or other items that can be used in the same way, e.g., marshmallows and toothpicks.
Reports to the Nation on Our Changing Planet: Our Ozone Shield or What's the Big Deal About Ozone? (selection will be determined by which age group you are working with).
Establish five cooperative groups. Assign each the task of designing a short (e.g., 2-4 minute) skit on one of the follow topics:
Restrict the students to using simple props (e.g., a piece of paper with the word "ozone" molecule written on it). Provide each group with copies of Reports to the Nation on Our Changing Planet: Our Ozone Shield or What's the Big Deal About Ozone? to use as a resource Also, show the first section of Ozone: The Hole Story .
As an intermediate step, the teacher may want to review the major ideas that each group will focus on in their skit. Have each group present their skit before the class. Review/discuss the major ideas presented in each skit after it is presented.
Show the second section of the video Ozone: The Hole Story . Pose the question, Where do CFCs come from? to the class and ask the students to brainstorm sources of CFCs. Keep a running list on the chalkboard or overhead, ask the class to check over the list for non-sources of CFCs. Enter into a discussion with the class in each case. (Aerosol cans and "Styrofoam" or foam insulation most likely will appear on the brainstormed list given most students believe aerosol cans contain CFCs as propellants. Even if aerosol cans and "Styrofoam"/foam insulation do not appear on the list, but especially if they do, its is advised that teachers deal directly with these as CFC source.) Lastly, note CFC sources on the list for which substitutes are being sought, e.g., refrigerants/coolants, expanable polystyrenes. Ask students why substitutes are being sought for these. Let this lead to a discussion of ozone layer depletion and the consequences for plants and animals.
End the lesson by asking the students to again individually draw a concept map using as many of the concepts used in the initial c-map as they can. As before, it is recommended that students place the concepts that they did not use in their concept maps in a box at the bottom of their papers so you can easily check which concepts they cannot relate. Also, give the students the freedom to select the terminal concept or use "ozone layer depletion" as the terminal concept, whichever was done before. You also may allow students to add 2-3 additional concepts as before. When everyone is finished, ask the students to compare their two c-maps and write a short analysis in which they discuss the similarities and differences. Save the analysis and c-maps or ask the student to save these for possible inclusion in their portfolios-the analysis could be done as an assignment.
For nearly a billion years, ozone molecules in the atmosphere have protected this planet from dangerous rays in sunlight. But over the past half century, humans have placed the ozone layer in danger. We have polluted the air with chemicals that threaten to destroy ozone molecules.
Although ozone molecules play such a important role in the atmosphere, they are extremely rare; in every million molecules in the air, fewer than ten are ozone. Nitrogen and oxygen make up the majority of the molecules of the air we breathe. Ozone is similar to a sprinkle of salt in a pot of soup.
Ozone molecules are spread throughout the atmosphere. But about 90% of the ozone is located in an area between 10 and 40 kilometers (6 and 25 miles) above the Earth's surface in a region of the atmosphere called the Stratosphere. Ozone there plays a helpful role by absorbing dangerous ultraviolet rays from the sun. This good ozone is threatened by some of the chemical pollutants that we have released into the atmosphere.
Close to the planet's surface in the Troposphere, however, ozone is destructive. This bad ozone is a part of the smog that hangs over many major cities across the world. It can severely damage the living tissue of plants and animals.
What is ozone and where does it originate? The term ozone comes from the Greek word meaning "smell," which refers to ozone's distinctively strong odor. Each molecule contains three oxygen atoms bonded together in the shape of a wide triangle. In the stratosphere, new ozone molecules are constantly created in chemical reactions using energy from the sun and lightning.
The recipe for making ozone starts off with oxygen molecules (O2). When struck by the sun's rays, the molecules split apart into single oxygen atoms (O), which want to join with other molecules. Within a fraction of a second, the atoms bond with nearby oxygen molecules to form triatomic molecules of ozone (O3).
Ozone gas molecules are destroyed by natural compounds containing nitrogen, hydrogen, and chlorine. These chemicals have been present in the stratosphere-in small amounts-long before humans began polluting the air. Nitrogen comes from soil and the ocean, hydrogen comes mainly from atmospheric water vapor, and chlorine comes from the ocean.
A balance has been established over the ages between the creation and destruction of ozone. So, the total amount of ozone in the stratosphere remains fairly constant.
Sometimes the balance changes from natural processes such as the seasonal cycle, volcanic eruptions, and changes in the sun's intensity.
For about a billion years, the natural ozone creation and destruction was in balance. But now, human beings have upset the balance by polluting the atmosphere with additional chlorine-containing chemicals. These chemicals destroy ozone.
No one thought human activity would threaten the ozone layer until the early to mid-1970's, when scientists discovered two potential problems: One was with commercial aircraft called supersonic transports (SST). These planes could fly faster than the speed of sound and promised to trim hours off long journeys. In the 1970's the United States and other nations began considering whether to build large fleets of such ultrafast jets. SSTs must fly high up in the atmosphere in order to reach fast speeds. Several researchers suspected that the nitrogen compounds from SST exhaust might reach the stratosphere, causing ozone destruction there.
In 1974, news of another possible threat to the ozone layer made national headlines. This time scientists pointed to chemicals known as chlorofluorocarbons (CFCs), which are man-made chemicals that were used as the propellant in spray cans. CFCs contain chlorine, fluorine, and carbon atoms.
CFCs were the ideal compounds for many purposes. Because of their special properties, they make excellent propellants for aerosol cans, cleaners for electronic equipment, and coolants for refrigerators and air conditioners. CFCs also trap heat well, so manufacturers put them into foam products such as cups and insulation for houses.
Most scientists did not think CFCs would effect the atmosphere. But two chemists, F. Sherwood Rowland and Mario Molina, discovered that CFCs could drift up to the stratosphere, where sunlight broke them apart, releasing millions of tons of chlorine atoms into the stratosphere. This added to the stratosphere much more than the amount of chlorine supplied naturally.
Rowland and Molina hypothesized that the extra chlorine from CFCs would destroy the ozone layer. According to their predictions, each chlorine atom could destroy 100,000 ozone molecules.
Any thinning of the ozone shield, whether from SSTs or CFCs , would allow more ultraviolet light to reach the Earth's surface. This could hold severe consequences for life on the planet. Exposure to ultraviolet light raises an individual's risk for skin cancer and cataracts, so an increase in this radiation could lead to more cases of such diseases. Ultraviolet light also harms food crops and other plants, as well as many species of animals.
Thus, in the early 1970s, the world faced two ozone-related environmental issues. Policy makers had to decide whether to build SST planes, and they had to decide whether to limit the production and use of CFCs.
The United States had plans for the largest fleet of SSTs. The U.S. government decided against the proposed fleet.
Political leaders faced a tougher decision on limiting CFCs. In the United States, CFCs were a multi-billion-dollar industry. Though the Rowland and Molina warned that CFCs might endanger the health of the planet's inhabitants, some officials feared that a ban on CFCs would disrupt society and create economic hardships.
Because the ozone layer belonged to the entire world, all countries would have to address the problem.
Would CFCs cause great harm to the ozone layer? This was the question politicians were asking in 1974. Scientists set out to find the answer over the next few years.
In 1979 the U.S. and some other countries banned the sale of aerosol cans containing CFCs. In 1985 countries signed an international agreement called the Vienna Convention. The convention called for a plan for worldwide action on the issue. Many governments thought it critically important to limit the use of the chemicals as soon as possible.
Then in May of 1985 shocking news spread throughout the scientific community. British researchers reported finding dramatic declines in ozone value over Antarctica each spring-actual "holes" in the ozone layer. This motivated countries to act. In September 1987 representatives from around the world met in Montreal and wrote a treaty like no other in the history of international negotiations.
Environmental ministers from 24 nations, representing most of the industrialized world, agreed to set sharp limits on the use of CFCs, and by mid-1989 countries would freeze their production and use of CFCs at 1986 levels. Then over the next ten years, they would cut CFC production and use in half.
The Montreal Protocol established a new way of viewing environmental problems. In the past, the world had addressed such issues only after damage grew noticable. The Montreal agreement tackled the ozone issue before it was a large problem.
Scientists found that the ozone hole was actually born in the late 1970s, long before the Montreal Protocol was signed. Like a leak in the roof over the distant part of a house, the hole at first grew very slowly.
Each spring the amount of ozone in the atmosphere over Antarctica dropped below normal and then rose gradually toward normal amounts in summer. And each year the springtime losses grew worse.
The British team which had measured ozone levels over the Antarctic coast since 1956, first began noting the phenomenon in the early 1980s. But it was hard to believe the evidence at first. Was the ozone hole real, or were the instruments malfunctioning? The scientists wondered. After checking and rechecking the instruments, the British researchers grew confident of their discovery. In 1985 they announced their startling news to the rest of the world.
Atmospheric experts moved quickly to determine whether the ozone depletion was real. Consulting measurements made by instruments in satellites and sent up by balloon, they found evidence confirming the springtime ozone depletion. Even more staggering, measurements showed it extending over the entire Antarctic continent.
The discovery of the ozone depletion blindsided the scientific community, catching it totally off guard and without a suitable explanation. But within a few months, theoretical scientists came up with three conflicting ideas that could explain why, what was being referred to as the "ozone hole" had developed over Antarctica.
One group of scientists focused on the solar cycle-the periodic increase and decrease of the sun's energy output. Noting that solar radiation had grown particularly strong in the 1980s, some researchers proposed the intense solar radiation had created above normal levels of reactive nitrogen chemicals in the stratosphere. These compounds could then concentrate over Antarctica and destroy ozone there.
A second group suggested that natural changes in stratospheric winds were responsible. According to this theory, the ozone hole resulted from changes in the system of air motions that caried ozone away from the polar regions.
Both the solar cycle and the dynamical theories suggested natural processes as a cause for the depletion. A third theory held that human-made chemicals were to blame. According to this idea, the cold conditions above Antarctica strengthened the ozone-destroying power of CFCs speeding up the loss in this region.
These three separate theories held very different indications for the world. If halocarbon pollution created the hole, then scientists had greatly underestimated the chemicals' destructive power, and the ozone layer faced even more danger than previously thought. But if the hole formed because of natural processes, then humans could breathe a sigh of relief.
With very little known about Antarctic ozone losses, atmospheric researchers could not tell which theory was correct. Yet they recognized that political leaders would need an answer as soon as possible
The scientific community started working on the problem, launching several field expeditions aimed at solving the riddle of the ozone depletion. In October 1987,
researchers came back from the Southern Hemisphere with a dark message for the world: blame for the ozone hole falls on human shoulders.
Evidence gathered during these expeditions and new data from laboratories enabled scientists to construct a consistent theory to explain the hole. Ice particles form during the polar night, when several months of darkness descend on Antarctica and temperatures fall below -80oC (-112oF) in the stratosphere. On these floating ice particles, reactions convert chlorine from the "safe" to the "destructive" form. The real action begins when the sun returns to this part of the world in the springtime, energizing the chemical that destroys ozone. Wind patterns during winter and spring contribute by isolating the Antarctic stratosphere from warmer air to the north.
The ozone hole forms only in Antarctica because this region has a unique combination of weather conditions; it is the coldest and most isolated spot on Earth. But somewhat similar conditions exist in the Arctic, and scientists wondered whether the North also suffered from ozone loss. Even small depletions in this region would be cause for concern, because many people live in northern latitudes potentially affected by Arctic ozone loss. So in 1988, two small teams traveled to Greenland and Canada to gather data.
The northern expeditions revealed that during wintertime, the Arctic stratosphere has the same types of destructive chlorine compounds that cause the problems in the Antarctic. Therefore they reached this verdict: global ozone levels had declined over the past 17 years, mainly in the winter. Normal processes such as the solar cycle had caused part of the drop, but natural effects could not explain the entire ozone loss.
The news grew even worse. An international panel announced that ozone levels had dropped by measurable amounts not only in winter and spring but also in summer. Because people spend far more time outdoors during summer, ozone loss at this time of year poses the greatest threat to the health of humans.
Scientists suspect that CFCs were to blame for much of the ozone decline, which has reached several percent .
The fast-paced research of the 1980s revealed that the original Montreal Protocol would not go far enough toward protecting the fragile ozone layer. Even with the 50% cuts mandated by the treaty, levels of chlorine and bromine would still rise in the stratosphere, meaning that ozone loss would only worsen with time.
In June 1990, diplomats met in London and voted to significantly strengthen the Montreal Protocol. The revised treaty calls for a complete phaseout of CFCs by the year 2000 (except for essential uses) and a rapid phaseout of other ozone-destroying chlorine compounds (carbon tetrachloride by 2000 and methyl chloroform by 2005).
The treaty also attempted to make the phaseout fair for developing countries, which cannot easily afford the higher-priced substitutes that will replace banned compounds. The revised agreement establishes an environmental fund-paid for by developed nations-to help developing nations switch over to more "ozone friendly" chemicals. It also placed tighter restrictions on the exports of controlled substances. This revised treaty is known as the London Amendment.
The Protocol was further strengthened in November of 1992 when international delegations once again met-this time in Copenhagen. The Copenhagen Amendment moves up the date for the phaseout of CFCs, to 1996. It also placed three more ozone-depleting substances under the Montreal Protocol controls: hydrochlorofluorocarbons (HCFCs), hydrobromofluorocarbons (HBFCs), and methyl bromide.
Many pieces of the ozone puzzle remain missing, and scientists wonder whether new ozone problems will develop in the near future. Experts are exploring several unanswered questions, including:
Decision makers will need answers to such questions as they continue to revisit their international agreements in the future and ask if these are adequate in light of new research findings.
The Montreal Protocol provides a dramatic example of science in the service of humankind. By quickly piecing together the ozone puzzle, atmospheric researchers revealed the true danger of halocarbons, allowing world leaders to take action to protect the ozone layer.
The international agreement represents a critical step towards saving the world's ozone layer. But perhaps more importantly, it has taught scientists and policy makers an invaluable lesson about addressing environmental problems. Negotiations on this issue mark the first time the nations of the world have joined forces to protect the Earth for future generations.
The treaty can serve as an important lesson for world leaders and scientists, who now face an even more frightening environmental matter-the threat of global greenhouse warming that looms over the future of this planet. The successful ozone agreement offers hope that scientific understanding can provide for responsible action by the international community.
Has humankind learned that the activities of of modern industrialized economies driven by the demands of consumers and rapidly growing populations can change the delicate balances of nature? We can no longer pretend that there will be no consequences as we continue to bombard our planet with billions of tons of pollutants.
Finding new substitute chemicals and developing new technologies may turn out to be less difficult than getting rid of old appliances that contain CFCs. In the United States alone can be found 45 million home air conditioners, 90 million car air conditioners, 160 million refrigerators, and 30 million freezers that operate with CFCs. If the coolants in these appliances are not maintained properly, or if the appliances are simply dumped without recycling their contents, their loads of potentially destructive CFCs will head straight for the stratosphere.
We need to remember that regulations limiting the production and use of ozone-destroying substances will not mean the end to this environmental problem. Because CFCs have a long lifetime in the atmosphere (60 - 100 years) and take approximately 8 - 10 years to drift up to the stratosphere, they will continue to attack the ozone shield long after agreements have been signed.
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