Ozone Hole

The so-called ozone hole sometimes is confused with the problem of global warming. Even though there is a connection between the two environmental issues, because ozone contributes to the greenhouse effect, the ozone hole is a separate issue. This article briefly addresses how ozone depletion is measured.

Ozone in Earth's Atmosphere

Ozone is a colorless, gaseous form of oxygen found in the Earth's atmosphere, primarily in the upper region known as the stratosphere, where it is naturally produced and destroyed. The chemical element oxygen normally forms a molecule containing two atoms (O₂). But in the presence of ultra-violet light or an electrical spark in the air, oxygen can form a molecule containing three atoms (O₃). The molecule of three oxygen atoms is called ozone.

Within the stratosphere is a layer between 20 and 40 kilometers (km) above Earth's surface that is known as the ozone layer. Here ozone takes up a greater proportion of the atmospheric column than at any other height. In the stratosphere, the concentration of ozone is 1,000 times greater than in the lower region of Earth's atmosphere known as the troposphere. Ozone in the stratosphere is beneficial because it protects Earth's inhabitants from the Sun's harmful ultraviolet radiation.

Measuring Ozone Levels

Scientists assess ozone by calculating how much there would be if all the ozone over a particular spot on Earth were compressed to a standard atmosphere of pressure—that is, the average pressure of air at sea level. On average, this would result in a column of ozone no more than 3 millimeters (mm) thick.

The unit of measure used to represent the amount of ozone above a particular position on the surface is the Dobson unit (DU), with one unitrepresenting 0.01 mm of ozone compressed to one standard atmosphere. Therefore, there is typically 300 DU in a column of the normal atmosphere.

G. M. B. Dobson was a British physicist who initiated the first regular monitoring of atmospheric ozone using spectrographic instruments in the 1920s. He was able to derive the vertical distribution of ozone from a series of measurements of the relative intensities of two particular wavelengths of light scattered in the zenith sky. One wavelength is more strongly absorbed by ozone than the other wavelength. As the Sun's zenith angle varies, a reversal occurs in the variation of the ratio of the intensities. Dobson compared the intensities of these two wavelengths with an instrument he constructed using a photomultiplier and an optical wedge; this instrument is now known as a Dobson spectrophotometer.

Dobson found that the ozone in the atmosphere is far from uniformly spread. The lowest concentrations, around 250 DU, were consistently found at the equator, although the polar winters resulted in periods where their concentrations might fall below the equatorial level. The highest concentrations were found in higher latitudes, where the variation fluctuated from as high as 460 DU to 290 DU in the upper latitudes of the Northern Hemisphere and between 400 DU and 300 DU in the Southern Hemisphere.

The International Geophysical Year* of 1957–1958 witnessed the World Meteorological Organization (WMO) take responsibility for establishing uniform and high quality ozone measurement world wide. The WMO subsequently established 160 ground-based ozone observation stations.

*The International Geophysical Year (July, 1957 through December, 1958) consists of eighteen months of a period of maximum sunspot activity. It was designated for cooperative study of the solar-terrestrial environment by the scientists of sixty-seven nations.

From the 1920s to the 1970s ozone was measured from the ground. Since the late 1970s scientists have used satellites, aircraft, and balloons to measure ozone levels from above Earth. The National Aeronautics and Space Administration (NASA) has also launched many scientific studies to investigate ozone. The figure below is one example of the results of this data monitoring.

Recording Low Ozone Levels

In the 1970s a research group with the British Antarctic Survey (BAS) was monitoring the atmosphere above Antarctica when the scientists first noticed a loss of ozone in the lower stratosphere. At first they believed their instruments to be faulty, and new instruments were sent to ensure that the readings were accurate.

By 1985 the BAS was reporting a dramatic decline of 50 percent in springtime ozone levels above Halley Bay Station when compared to the previous decade. At the most affected altitude, 14 to 19 km above the surface, more than 99 percent was lost. This was an unsettling discovery because NASA had been monitoring ozone levels globally since 1979 with the Total Ozone Mapping Spectrometer (TOMS) aboard the Nimbus 7 satellite.

The standard TOMS data-processing procedure was to automatically neglect ozone levels below a fixed value of 180 DU, considering such data to be unreliable. Hence, the Antarctic springtime data had been ignored. Only after the British survey team's report were the TOMS data reprocessed; the ozone depletion was verified and the geographical extent of the hole was determined. This lowering of the amount of ozone over the Antarctic became known as the ozone hole.

Phillip Nissen

Bibliography

Kondratyev, Kirill Y., and Costas Varatsos. Atmospheric Ozone Variability: Implications for Climate Change, Human Health and Ecosystems. Chichester, U.K.: Praxis Publishing Ltd., 2000.

Makhijani, Arjun, and Kevin R. Gurney. Mending the Ozone Hole: Science, Technology, and Policy. Cambridge, MA.: The MIT Press, 1995.

Internet Resources

Measure of Atmospheric Ozone. Atmospheric Chemistry and Data Resources. <http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/ATM_CHEM/ozone_measurements.html>.

The Ozone Hole Tour. Center for Atmospheric Studies. <http://www.atm.ch.cam.ac.uk/tour/index.html>.

OZONE CLOSE TO EARTH

In Earth's lower atmospheric layer known as the troposphere, the concentration of ozone is usually between 0.02 and 0.03 parts per million (ppm). Under smog conditions, the impurities in the air can act as catalysts and allow sunlight to form ozone. In the troposphere, ozone is harmful and can damage lung tissue and plants.

OZONE DEPLETION

OZONE DEPLETION became a serious concern in the 1980s and has prompted international agreements and changes in manufacturing processes in an attempt to slow depletion and minimize health and environmental problems. Ozone is a denser form of oxygen that shields the Earth from excessive ultraviolet radiation from the sun; without it, the earth's inhabitants and environment are exposed to damaging UV-B rays. Scientists detected substantial seasonal fluctuations in stratospheric ozone levels over Antarctica as early as the 1950s. In the 1970s the chemists Sherwood Rowland and Mario Molina of the University of California (in findings later confirmed by the National Academy of Sciences) blamed the lower wintertime level of ozone over Antarctica on the rapidly increasing use of chlorofluorocarbons (CFCs) as refrigerants and as propellants in aerosol cans and in the manufacture of plastic foam products. CFC molecules deplete the ozone layer because they migrate to the stratosphere, collect over the Antarctic ice cap during the cold winter months, and become fixed on polar stratospheric clouds, isolated from the normal atmospheric circulation. When sunlight returns to Antarctica in early spring, its ultraviolet rays trigger a chemical reaction that releases a chlorine-oxide free radical, which precipitates another reaction that breaks up the oxygen molecules that form the ozone layer. A world that has been producing and releasing

into the atmosphere 1 million tons of CFCs per year has seen CFC levels in the atmosphere rise from 0.8 parts per billion by volume in 1950 to at least 4 parts per billion at the close of the century.

In 1985 the discovery of an ozone "hole" over wintertime Antarctica prompted international action. In 1987 all major industrial nations signed the Montreal Protocol, agreeing to deadlines for ending the use of CFCs; eighty nations signed amendments calling for the almost total elimination of CFCs, methyl chloroform, and carbon tetrachloride by 1996. By the 1990s, 173 countries, including the United States, had signed.

However, the danger is far from over. Alarm bells rang in October 2000, when for the first time ever, a major ozone hole opened over a populated city: Punta Arenas, Chile. Not all countries and industries are complying with the ban on ozone-depleting substances; for instance, U.S. companies have been fighting efforts to cut use of methyl bromides, claiming that scientists exaggerate their effects on the ozone layer. Furthermore, a black market in CFCs is thriving. Many nations lack the resources to monitor production of ozone-depleting chemicals, and some consumers in more industrialized nations buy smuggled-in compounds to avoid retrofitting the many appliances made before the CFC-phaseout. Without major cuts in CFC and methyl bromide emissions, continuing thinning of the ozone layer may bring quadrupled levels of skin cancer by 2100, increases in cataracts, suppression of the immune system, and increasing rates of non-Hodgkin's lymphoma. Scientific findings in the early 2000s suggest far-reaching ecological disruption may also ensue, including genetic mutations and accelerated species extinctions.

BIBLIOGRAPHY

Cogan, Douglas G. Stones in a Glass House: CFCs and Ozone Depletion. Washington, D.C.: Investor Responsibility Research Center, 1988.

de Gruijl, Frank R., and Jan C. van der Leun. "Environment and Health: 3. Ozone Depletion and Ultraviolet Radiation." Canadian Medical Association Journal 163 (2000): 851–855.

Grundmann, Reiner. Transnational Environmental Policy: The Ozone Layer. New York: Routledge, 2001.

Rowland, F. Sherwood. "Stratospheric Ozone in the Twenty-First Century: The Chlorofluorocarbon Problem." Environmental Science and Technology 25 (1991): 622–628.

Nancy M.Gordon/d. b.

See alsoChemical Industry ; Climate ; Environmental Movement ; Epidemics and Public Health .

Ozone layer depletion

The ozone layer is a part of the atmosphere between 18.6 mi and 55.8 mi (30 and 90 km) above the ground. The ozone present is responsible for blocking potentially harmful ultraviolet radiation reaching the surface of the earth. During the last twenty years, evidence has accumulated that human activity may be the cause of a generalized depletion of the ozone layer. This phenomena is global and distinct from the natural factors that induce annual ozone layer hole formation over Antarctica .

Ozone is constantly created and destroyed in natural processes (manufactured by the action of lightning on oxygen and destroyed by the action of ultraviolet radiation), however the amounts normally balance each other out so there is no net increase or decrease due to natural processes. In 1970, Paul Crutzen showed that naturally occurring oxides of nitrogen can catalytically destroy ozone. In 1974, F. Sherwood Rowland and Mario Molina demonstrated that chlorofluorcarbons (CFCs) could also destroy ozone. In 1995, all three were jointly awarded the Nobel Prize for chemistry.

The CFCs that were observed as being damaging included Freon 11 (CFCl₃) and Freon 12 (CF₂Cl₂). These chemicals are widely used in industry and the home. They have uses as propellants in aerosol spray cans, refrigerant gases, and foaming agents for blown plastics. One problem associated with these gases is their relative lack of reactivity. When released there is very little that will break them down and, as they are not soluble in water , they are not removed from the atmosphere by rain. As a consequence, once released they tend to concentrate in the upper regions of the atmosphere. It is estimated that some several million tons of CFCs are present in the atmosphere.

Once in the upper atmosphere the CFCs are exposed to high energy radiation that can cause disassociation of the molecule, producing free chlorine atoms. This atomic chlorine reacts readily with ozone to produce chlorine monoxide and molecular oxygen. The chlorine monoxide can further react to produce molecular oxygen and more atomic chlorine. This all accelerates the destruction of ozone beyond its natural ability to regenerate. Overall, there is a net reduction in the amount of ozone present in the upper atmosphere. This has led to a thinning of the ozone layer. The majority of this loss is at an altitude between 7.44 mi and 18.6 mi (12 and 30 km) and in the late 1990s evidence was seen that suggested losses were also occurring at other altitudes. In addition to the annual holes in the ozone layer now detected over Antarctica, in the late 1990s, holes were detected over Australia and atmospheric sampling indicated a dramatic thinning of the ozone layer in the Northern Hemisphere during the winter months. In the Northern Hemisphere losses of some 30% have been recorded at an altitude of 12.4 mi (20 km).

In 1987, the Montreal Protocol was signed with the appropriate countries agreeing to reduce CFC production. By 1996, more than 100 countries agreed to cease widespread commercial use of CFCs and to stop or curtail production of CFCs.

In the absence of the ozone layer, harmful ultraviolet radiation is able to reach the surface of the earth in higher doses. This can lead to increases in skin cancers.

See also Atmospheric chemistry; Greenhouse gases and greenhouse effect; Ozone layer and hole dynamics; Ultraviolet rays and radiation

ozone layer or ozonosphere, region of the stratosphere containing relatively high concentrations of ozone , located at altitudes of 12–30 mi (19–48 km) above the earth's surface. Ozone in the ozone layer is formed by the action of solar ultraviolet light on oxygen.

The ozone layer prevents most ultraviolet (UV) and other high-energy radiation from penetrating to the earth's surface but does allow through sufficient ultraviolet rays to support the activation of vitamin D in humans. The full radiation, if unhindered by this filtering effect, would destroy animal tissue. Higher levels of radiation resulting from the depletion of the ozone layer have been linked with increases in skin cancers and cataracts and have been implicated in the decline of certain amphibian species.

In 1974 scientists warned that certain industrial chemicals, e.g., chlorofluorocarbons (CFCs) and to a lesser extent, halons and carbon tetrachloride, could migrate to the stratosphere. There, sunlight could free the chlorine or bromine atoms to form chlorine monoxide or other chemicals, which would deplete upper-atmospheric ozone. A seasonal decrease, or "hole," discovered in 1985 in the ozone layer above Antarctica was the first confirmation of a thinning of the layer. The hole occurs over Antarctica because the extreme cold helps the very high clouds characteristic of that area form tiny ice particles of water and nitric acid, which facilitate the chemical reactions involved. In addition, the polar winds, which follow a swirling pattern, create a confined vortex, trapping the chemicals. When the Antarctic spring sun rises in August or September and hits the trapped chemicals, a chain reaction begins in which chlorine, bromine (from the halons), and ice crystals react with the ozone and destroy it very quickly. The effect usually lasts through November. There is a corresponding hole over the Arctic that similarly appears in the spring, although in some years warmer winters there do not result in a major depletion of the ozone layer. A global thinning of the ozone layer results as ozone-rich air from the remaining ozone layer flows into the ozone-poor areas.

Minimum ozone levels in the Antarctic decreased steadily throughout the 1990s, and less dramatic decreases have been found above other areas of the world. In 2000 (and again in 2003 and 2006) the hole reached a record size, extending over more than 10.5 million sq mi (27 million sq km), an area greater than that of North America. In 1987 an international agreement, the Montreal Protocol , was reached on reducing the production of ozone-depleting compounds. Revisions in 1992 called for an end to the production of the worst of such compounds by 1996, and CFC emissions dropped dramatically by 1993. Recovery of the ozone layer, however, is expected to take 50 to 100 years. Damage to the ozone layer can also be caused by sulfuric acid droplets produced by volcanic eruptions.

ozone layer (ozonosphere) A layer of the earth's atmosphere in which most of the atmosphere's ozone is concentrated. It occurs 15–50 km above the earth's surface and is virtually synonymous with the stratosphere. In this layer most of the sun's ultraviolet radiation is absorbed by the ozone molecules, causing a rise in the temperature of the stratosphere and preventing vertical mixing so that the stratosphere forms a stable layer. By absorbing most of the solar ultraviolet radiation the ozone layer protects living organisms on earth. The fact that the ozone layer is thinnest at the equator is believed to account for the high equatorial incidence of skin cancer as a result of exposure to unabsorbed solar ultraviolet radiation. In the 1980s it was found that depletion of the ozone layer was occurring over both the poles, creating ozone holes. This is thought to have been caused by a series of complex photochemical reactions involving nitrogen oxides produced from aircraft and, more seriously, chlorofluorocarbons (CFCs) and halons. CFCs rise to the stratosphere, where they react with ultraviolet light to release chlorine atoms; these atoms, which are highly reactive, catalyse the destruction of ozone. Use of CFCs is now much reduced in an effort to reverse this human-induced damage to the ozone layer. See also air pollution.

OZONE LAYER

Ozone is a poisonous colorless gas with an acrid odor. Chemically, it is a variant of normal oxygen, except that ozone has three oxygen atoms per molecule rather than the two found in normal oxygen. There exists a layer of ozone occurring naturally six to thirty-one miles above the earth. This layer of ozone gas surrounding the earth protects living organisms at the earth's surface from the dangerous ultraviolet radiation of the sun. The ozone layer normally absorbs about 98 percent of the ultraviolet rays that continually shower the earth. In small amounts, ozone can be useful as a water disinfectant and a purifier. If, however, ultraviolet rays came to ground level through the shield of the ozone layer, there would be massive lethal consequences for wildlife, crops, vegetation, and profound life-threatening problems for human beings, including cancer and immune system damage.

In 1974 chemists F. Sherwood Rowland and Marle Molina found that chlorine from chlorofluorocarbon (CFC) molecules was capable of breaking down ozone in the ozone layer above the earth. There was evidence that industrial chemicals and chemical exhaust from jet airplanes, as well as large volcanic eruptions, severely threatened the upper atmosphere and the ozone layer. In 1974, when damage to the ozone layer first became apparent, the propellants in common aerosol spray cans were a major source of CFC emissions. CFC aerosols were banned in the United States by 1978, but CFC chemicals remained in widespread use as coolant agents in refrigerators and in air-conditioners as well as in cleaning solvents. During the last decades of the twentieth century there was only a gradual move to ban CFC chemicals from all refrigerant systems, forcing modern industry to deal with alternative systems to stabilize the ozone layer. The question of how much protection is necessary has continued to be a controversial political issue because necessary changes in industrial systems proved to be profoundly expensive. At some level, however, such expenditure is crucial to the existence of human life.

See also: Environmentalism

ozone layer The atmospheric layer at 15–30 km altitude, in which ozone (O₃) is concentrated at 1–10 parts per million. Ozone also occurs in very low concentration at altitudes of 10–15 km and 30–50 km. Generally, atmospheric ozone is produced by the photochemical dissociation of oxygen (O₂), resulting from absorption of ultraviolet solar radiation, to form atoms of oxygen (O). These atoms collide with molecular oxygen (O₂) to form ozone (O₃), which in turn absorbs solar radiation for further dissociation to O and O₂. The ozone layer limits the amount of ultraviolet radiation reaching the ground surface. See also ATMOSPHERIC STRUCTURE.

ozone layer The atmospheric layer at 15—30 km altitude, in which ozone (O₃) is concentrated at 1—10 parts per million. Ozone also occurs in very low concentration at altitudes of 10—15 km and 30—50 km. Generally, atmospheric ozone is produced by the photochemical dissociation of oxygen (O₂), resulting from absorption of ultraviolet solar radiation, to form atoms of oxygen (O). These atoms collide with molecular oxygen (O₂) to form ozone (O₃), which in turn absorbs solar radiation for further dissociation to O and O₂. The ozone layer limits the amount of ultraviolet radiation reaching the ground surface.

ozone layer Region of Earth's atmosphere in which ozone (O₃) is concentrated. It is densest at altitudes of 21–26km (13–16mi). Produced by ultraviolet radiation in incoming sunlight, the ozone layer absorbs much of the ultraviolet, thereby shielding the Earth's surface. Aircraft, nuclear weapons and some aerosol sprays and refrigerants yield chemical agents that can break down high-altitude ozone, which could lead to an increase in the amount of harmful ultraviolet radiation reaching the Earth's surface. See also chlorofluorocarbon (CFC)

http://www.cmdl.noaa.gov

o·zone hole • n. a region of marked thinning of the ozone layer in high latitudes, chiefly in winter, attributed to the chemical action of chlorofluorcarbons and other atmospheric pollutants. The resulting increase in ultraviolet light at ground level gives rise to an increased risk of skin cancer.

o·zone lay·er • n. a layer in the earth's stratosphere at an altitude of about 10 km (6.2 miles) containing a high concentration of ozone, which absorbs most of the ultraviolet radiation reaching the earth from the sun.

ozone hole See ozone layer.