Stratospheric Ozone, the Protector

This section provides an overview of the ozone layer, that is, the layer of life-protecting ozone found at the top of the stratosphere. A brief history of the discovery of the ozone 'hole' is included. The general concepts found in this section include the following:

  • Concentrations of stratospheric ozone represent a balance, established over eons, between creative and destructive forces and this balance, or dynamic equilibrium, has been changed by human activity.

  • Ozone is formed in the earth's stratosphere and is critical to life on earth as we know it.

  • There is compelling scientific evidence that ozone is destroyed in the stratosphere and that some human-released chemicals are speeding up the breakdown of ozone in the atmosphere.

  • CFCs, a human-developed compound, are particularly destructive to the breakdown of ozone in the atmosphere.

  • Ultraviolet radiation is present in natural outdoor light and can be blocked or filtered by various substances.

This section includes three classroom activities.

Introduction

The debate over the existence of an ozone problem breeds media coverage. However, the real story is not whether stratospheric ozone levels are decreasing, but what those decreases may mean for life on earth. As the percentage of ozone in the atmosphere decreases, the amount of UV-B radiation reaching the surface increases. It's the UV-B radiation, not the ozone itself that concerns scientists, because the invisible wavelengths are linked to skin cancers and other biological damage.

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. 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. 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 lies the middle ground, where the number and impact of rays combine to cause the greatest harm to humans and plants. 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.

The Story of the Ozone Hole

Although often referred to as the ozone 'hole', it is really not a hole but rather a thinning of the ozone layer in the stratosphere. We will use the term 'hole' in reference to the seasonal thinning of the ozone layer.

The appearance of a hole in the 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.

They didn't realize 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 refrigerants, industrial cleaning solvents, propellants in aerosol spray cans, 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-11 and CFC-12, the two most destructive and persistent CFCs.

Once in the atmosphere, CFCs drift slowly upward to the stratosphere, where they are broken up by ultraviolet radiation, releasing the chlorine that catalytically destroys ozone. In the graphic below, the destructive cycle of a chlorine atom is shown.

  1. UV radiation breaks off a chlorine atom from a CFC molecule.

  2. The chlorine atom attacks an ozone molecule (), breaking it apart and destroying the ozone.

  3. The result is an ordinary oxygen molecule () and a chlorine monoxide molecule (ClO).

  4. The chlorine monoxide molecule (ClO) is attacked by a free oxygen atom releasing the chlorine atom and forming an ordinary oxygen molecule ().

  5. The chlorine atom is now free to attack and destroy another ozone molecule (). One chlorine atom can repeat this destructive cycle thousands of times.

The following animation shows the destruction of an ozone molecule by a chlorine atom.


Since 1974 scientists have known that chlorine can destroy ozone, but no one thought the destruction would be very rapid. Events over 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 declines occurred repeatedly. It wasn't until their instruments registered record low levels of ozone in 1983 that they realized something important was happening. By then, record springtime ozone declines had occurred during seven of the previous eight years.

Within two years, scientists determined that the ozone hole over Antarctica occurs 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 icy cloud particles release chlorine from harmless (to ozone) chemical compounds into an active form that reacts 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 whirlpool, or 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 destruction of ozone when the light returns during spring. Scientists documented this mechanism in a series of field experiments in 1987. The graphic below compares the ozone averages (measured in Dobson Units) over Antarctica for the periods 1970-72, 1979, and 1992-95.



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, prevents an excessive build-up of ice clouds. Geography isn't always enough to dissipate 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, it 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% (relative to 1986 levels) by 1993 and by 50% 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 the CFCs already in circulation will continue to pose a threat to the earth's ozone layer for another hundred years. There is good news to this story. The graph below shows the skyrocketing path of CFC-11 from the 1950s until the mid-1990s. Recent measurements have shown a clear decline in CFC-11.


Concluding Thoughts

While the stratospheric ozone issue is a serious one, in many ways it can be thought of as an environmental success story. Scientists detected the developing problem, and collected the evidence that convinced governments around the world to take regulatory action. Although the global elimination of ozone-depleting chemicals from the atmosphere will take decades yet, we have made a strong and positive beginning. For the first time in our species' history, we have tackled a global environmental issue on a global scale. As you and your students explore the scientific issues associated with ozone depletion in the activities below, you may wish to emphasize the "success story" aspect.

Activities

The following activities will help your students better understand the concepts covered in this section.

To proceed, either click on Activities in the menu at the top or click on another unit to switch units.