The Science of Ozone Depletion
No environmental issue as vividly demonstrates the impact human-produced chemicals can have on nature as the destruction of stratospheric ozone.
The Importance of Stratospheric Ozone
The stratosphere is the uppermost portion of the earth's atmosphere and overlays the lowest part -- the troposphere -- where we live and breathe. The stratosphere is generally considered to extend from approximately 10 km to 50 km (6.2 to 31 miles) above the earth, but it can start as low as 7 km in polar regions and as high as 17 km above the equator. Unlike the troposphere, which cools with altitude, the stratosphere warms with altitude. The fact that the stratosphere warms as it rises means the stratosphere exists as a permanent "temperature inversion" region that traps trace gases and small particles for long periods of time -- often several years.
What Is Ozone?
Ozone is a relatively unstable form of molecular oxygen containing three oxygen atoms (O3). Radiation from the sun continuously bombards the Earth's atmosphere, causing molecules to break apart into component elements that form into new chemical compounds. Ozone is produced when upper-atmosphere oxygen molecules (O2) are broken apart by ultraviolet light. Most of the freed oxygen atoms immediately bond with nearby oxygen molecules to form ozone (O + O2 = O3).
Ozone near the earth's surface is regarded negatively. It is created from pollutants from industrial, transportation, and some natural sources. It is the most noxious component of smog. At high concentrations, O3 is known to reduce human lung capacity, as well damage the cells of many plants, animals, and other organisms. For these reasons, ozone is treated as an air pollutant in most industrial countries. Furthermore, O3 in the upper troposphere is a powerful greenhouse gas and is believed to play a role in global warming.
On the other hand, ozone in the stratosphere is necessary. It serves as a protective radiation shield that intercepts solar ultraviolet light harmful to living organisms. Ultraviolet light splits the relatively unstable O3 molecules into O2 and atomic O. Most of the time, the O atom created by ozone breakup recombines with one of the plentiful O2 molecules to again form O3. This ozone-creation process is constantly at work producing more ozone.
The Stratospheric Ozone Layer
Stratospheric ozone is found in a broad band, generally extending from about 15 to 35 kilometers (9 to 22 miles) above the earth. The profile and concentration of the stratospheric ozone layer over a particular part of the earth's surface depends on the dynamics of the stratospheric winds, as well as the sunlight and trace pollutants active in that part of the stratosphere. Although the amount and distribution of stratospheric ozone varies, the layer is surprisingly thin. If the total ozone from a normal stratospheric layer were to be filtered out of the background air and brought to the pressure and temperature of air at the earth's surface, it would form a gaseous layer only 2 to 3 millimeters (approximately 0.1 inch) thick. As negligible as this seems, it is sufficient to filter out the bulk of harmful ultraviolet radiation that would otherwise reach the earth's surface.
Importance of the Stratospheric Ozone Layer
Scientists cannot predict with certainty the consequences for life on earth if the stratospheric ozone layer weakens. In general, biologists and health professionals recognize that life on earth evolved under the protection of an ozone layer thick enough to remove much of the UV-B solar radiation known to damage cellular DNA. Accordingly, various organisms -- including humans -- may have difficulty adjusting to the higher UV-B levels resulting from a thinner ozone layer.
Medical studies have quantified some of the expected effects of increased UV-B levels, based on information from people exposed to higher than average UV-B levels, such as populations living at high altitudes and in the tropics, where the average ozone layer is thinner and the sunlight more direct. The most serious medical effects include increased incidence of cataracts and skin cancer, as well as evidence of weakened immune-system response. In addition, ecological research indicates that some crop yields will decrease and disruptions in marine food chains may occur.
A weakened ozone layer may also cause climatological effects. The stratosphere warms with altitude because the splitting of stratospheric ozone is caused by ultraviolet photons, which contain much more energy than that required to break the O-O2 bond. This extra energy is converted to heat. Less stratospheric ozone means less local heating, but it also means that more ultraviolet light is transmitted to heat the lower atmosphere and the earth's surface.
At the same time, less lower stratospheric ozone would be available to trap outgoing infrared radiation from the surface and the lower atmosphere. The net effect is calculated to be a slight cooling at the surface, but a more significant cooling in the lower stratosphere. Scientists remain uncertain about the impact this changing stratospheric temperature profile will have.
Stratospheric Ozone Destruction
Ozone can be destroyed by chemicals that react with it directly, or by those that react with the oxygen atom temporarily freed whenever an ozone molecule breaks apart. However, since ozone concentrations are higher than those of most reactive chemicals in the stratosphere, the only ozone destroyers of concern are those that can participate in a "catalytic cycle" -- that is, where one trace catalytic chemical can be responsible for destroying tens or even hundreds of thousands of ozone molecules.
Atmospheric chemists have discovered a dozen or more catalytic cycles that influence stratospheric ozone at various altitudes. The most effective cycles involve chlorine (Cl), bromine (Br), nitrogen oxides (NOx), and hydrogen oxide radicals (HOx). Natural catalytic cycles involving NOx and HOx have historically helped keep ozone levels in the atmosphere stable.
In recent decades, various human activities have released ozone-destroying chemicals into the atmosphere. Of particular importance are halogen atoms -- chlorine and bromine. Chemicals released into the atmosphere by industrial practices include chlorocarbon compounds (such as CCl4 and CH3Cl3), chlorofluorocarbon compounds, CFCs, (such as CFCl3 and CF2Cl2), and halon compounds (such as CF3Br and CF2ClBr).
Chlorocarbon compounds are used primarily as industrial solvents, degreasing compounds, and CFC precursors. The CFCs are used as working fluids in refrigeration and air conditioning systems, as foam-blowing agents, and -- through the 1970s -- as aerosol propellants. The halons are used as fire suppressants.
Once in the stratosphere, all of these chlorine- or bromine-containing halocarbon compounds are broken apart by solar ultraviolet radiation, releasing their chlorine or bromine atoms to initiate ozone destruction. Each chlorine atom can initiate a cycle that can destroy up to 100,000 ozone molecules. Bromine atoms are even more efficient destroyers.
In the early 1970s, the detection of significant concentrations of chlorocarbons and CFCs in the lower atmosphere was coupled with the realization that photochemical and rainout processes that usually remove most pollutants from the lower atmosphere were not working for these compounds.
It became clear that many halocarbons would remain in the atmosphere for a long time and might build up to high enough atmospheric levels to cause real mischief. It is now known that many CFCs and halons are extremely stable and reach the stratosphere intact. They have atmospheric lifetimes between 50 and several hundred years, so that significant fractions of their peak concentrations will persist in the atmosphere long after emissions of these chemicals have stopped.
The realization that CFCs and related chlorocarbons tend to release their chlorine in the stratosphere -- and that this chlorine could catalytically destroy stratospheric ozone -- was first published, in 1975, by Sherwood Rowland and Mario Molina of the University of California, Irvine. Their early-warning paper was written well before measurable stratospheric ozone loss actually occurred. Work by Steven Wofsy and Michael McElroy of Harvard University showing that catalytic cycles involving bromine oxide compounds would also destroy stratospheric ozone was published a little later.
The past decade has witnessed significant levels of stratospheric ozone loss, and careful atmospheric measurements have clearly indicted chlorine and bromine catalytic cycles as the culprits. Even though the actual chemistry leading to most ozone depletion is more complex than the original papers by Rowland and Molina outlined, their concern has proven well founded. In 1995 Molina and Rowland, along with Paul Crutzen, received the Nobel Prize in Chemistry for their research on the human threat to the ozone layer.
The Ozone Hole
Starting most clearly in the austral (southern hemisphere) spring of 1980, a massive, continental-sized hole -- accounting for one-quarter to one-half of the ozone -- appeared over the largely unpopulated core of Antarctica. For the past 18 years, this hole has grown in size. Recently, ozone depletion over the South Pole has became so severe that less than one-third of the average October levels (measured from the 1950s through the 1970s) remained on many days of that same month in 1993. The 1995 antarctic ozone hole opened up before mid-September and was the largest ever recorded that early in the austral spring.
The antarctic ozone hole took most atmospheric chemists by surprise. The original Rowland/Molina theory predicted that the most severe ozone loss would occur relatively high in the stratosphere (above 30 km). In fact, the largest depletions over Antarctica occurred in the middle range between 13 and 21 km.
What atmospheric scientists had failed to factor in was the powerful "polar vortex." The winter stratospheric air over the antarctic routinely gets colder than the air anywhere else. Because the winter stratospheric air over most of Antarctica is isolated from warmer, lower-altitude air by a strong set of confining winds known as the polar vortex, this air gets so cold that polar stratospheric clouds regularly form between the altitudes of approximately 12 to 25 km. Particle surfaces in these clouds cause chemical reactions that create even more reactive halogen chemicals. Thus, when the first light of austral spring reaches these molecules, they break apart and release massive levels of the chlorine and bromine atoms that drive catalytic destruction cycles. Additional chemical reactions also related to the surface particles in the stratospheric clouds actually lengthen ozone-destroying cycles.
Over the past decade, ground aircraft and satellite-based measurements have shown direct, quantitative relationships between the occurrence of polar stratospheric clouds, the creation of high levels of reactive halogen chemicals, and ozone destruction. In recent years, these halogen-driven destruction cycles have been vigorous enough to destroy almost all of the ozone in the core of the polar vortex -- between 14 and 18 km -- and much of the ozone several kilometers above and below these altitudes.
Amount of Stratospheric Ozone Loss
When the antarctic polar vortex breaks up in the late spring, the ozone-poor stratospheric air resulting from the ozone hole spreads northward over populated regions of Australia, New Zealand, and South America. This effect has been strong enough in recent years to cause significantly increased levels of UV-B to fall on portions of these regions.
In the northern hemisphere, the arctic polar vortex is much weaker that the antarctic vortex, so the arctic stratosphere does not get as cold for as long. Polar stratospheric clouds do form sporadically, however. After the resulting air is exposed to sunlight, high levels of reactive halogen chemicals form immediately. These patches of stratospheric air often experience significant ozone depletion, sometimes as they are driven south over populated regions of Europe, Asia, and North America.
It is not clear whether current midlatitude ozone losses are due to polar ozone-depleted air mixing down to lower latitudes, or to increased on-site halogen-catalyzed ozone destruction. This uncertainty is being investigated by ongoing atmospheric measurement and modeling programs. It is abundantly clear, however, that recent years have shown a disturbing level of ozone loss at middle as well as high latitudes.
Starting with ozone levels measured in the late 1960s and early 1970s -- well before atmospheric chlorine and bromine levels were high enough to cause significant ozone depletion -- satellite and ground-launched ozone measurements have documented northern hemisphere winter/spring downward trends of about 6 percent per decade, and summer/fall downward trends of 3 percent per decade for 1979-1994. Seasonal differences are less pronounced in the southern hemisphere, but yearly averaged downward trends of 4 to 5 percent per decade have been recorded. Only in the tropics (20° N - 20° S) is there no statistically significant loss of ozone.
Expected Ozone Loss in the Future
Atmospheric chlorine and bromine levels are expected to peak around 1998, if current international agreements limiting the production and release of halocarbons are implemented and enforced. This suggests that maximum northern hemisphere midlatitude winter/spring ozone losses of 12 to 13 percent of historic background levels will probably occur in that time frame (about 2.5 percent more loss than in 1994).
Summer/fall midlatitude losses of 6 to 7 percent are also anticipated. Such losses would be accompanied by increases in levels of harmful ultraviolet solar radiation of approximately 15 percent in winter/spring and approximately 8 percent in summer/fall. Because of the long life of many halocarbons, particularly CFCs, it will be a long time before stratospheric ozone levels are completely restored. Current projections envision a significant annual antarctic ozone hole for several decades into the next century.
Other Potential Threats to the Ozone Layer
In addition to the release of the industrially produced halocarbons now restricted by international treaty, several other societal activities are potential threats to stratospheric ozone and require careful, ongoing assessments, as described below.
CFC and Halon Replacements
Several replacement compounds for various CFC and halon uses also contain chlorine or bromine, but have much shorter atmospheric lifetimes and therefore deliver much less halogen to the stratosphere. Although their current use represents a distinct improvement, future use/release levels must be carefully controlled to assure that future atmospheric chlorine and bromine levels are safe.
Methyl Bromide Fumigants
Methyl bromide (CH3Br) is currently used as a soil and crop fumigant. It can significantly increase crop yields by killing soil pathogens and is required as an insect treatment before fruits and vegetables can be imported into Japan and other countries. Unlike CFCs and halons, there are large natural sources and tropospheric sinks for methyl bromide. Since bromine is far more effective at destroying ozone than is chlorine (by a factor of 30-60), however, even a small amount of stratospheric bromine can be important. Current international regulations call for a cap on methyl bromide production, while US law calls for it to be phased out. Ongoing research will have to determine the significance of the release of industrially produced methyl bromide, given the large natural sources and sinks.
Stratospheric Aircraft
Original concerns about the stability of the ozone layer in the early 1970s focused on the impact of nitrogen oxide exhaust gas (NOx) emitted by a proposed fleet of supersonic transport (SST) aircraft that were to cruise in the lower stratosphere. Because the US SST project was canceled, and the British/French consortium built only a handful of Concordes, the issue moved to the back burner.
Since 1990, NASA and the European Community have mounted a major effort to define the probable impact of a fleet of modern SSTs using novel, low-NOx jet engines now in development. Preliminary results suggest that it may be possible to operate a large fleet of low-NOx engine-equipped aircraft with minimal damage to the ozone layer. The final judgment about the environmental impact on the stratosphere may hinge on how low NOx levels in engine exhaust can be pushed, how high the planes cruise (probably 18 - 20 km), and how well scientists can predict the altitudes and latitudes where the exhaust NOx will be transported by stratospheric winds.
Solid Rocket Motor Exhaust
The solid rocket strap-on motors used in the most powerful space launch systems -- the US space shuttle and the Titan IV, as well as the European Ariane V -- produce copious amounts of HCl and possibly other reactive chlorine-containing exhaust products. Since these strap-on motors burn well into the stratosphere, a significant fraction of their exhaust gases is deposited there. The plume from each launch causes a temporary "mini" ozone hole, although since space launch trajectories are slant paths, the ozone depletion is not stacked up over a single surface point. Current launch levels are so low that the stratospheric chlorine injected by space launches is only a few tenths of a percent of that due to halocarbon decomposition. But if more frequent space launches occur in the future, care should be taken to design more stratospherically benign rocket propulsion systems for both US and foreign launch systems.
For Further Information
Jonathan P.D. Abbott and Mario J. Molina, "Status of Stratospheric Ozone Depletion," Annual Reviews of Energy and Environment, vol. 18 (1993) pp. 1-29.
Seth Cagin and Philip Dray, Between Earth and Sky (New York: Pantheon Books, 1993).
Owen B. Toon and Richard P. Turco, "Polar Stratospheric Clouds and Ozone Depletion," Scientific American (June 1991).
World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1991 (Global Ozone Research and Monitoring Project - Report No. 25, Geneva, Switzerland, 1991).
World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1994 (Global Ozone Research and Monitoring Project - Report No. 37, Geneva, Switzerland, 1995).
