Ozone layer depletion

Ozone is a gas that occurs naturally in relatively large concentrations in the upper-atmospheric layer known as the stratosphere. The stratosphere is between 5–10.6 mi (8–17 km) to about 31 mi (50 km) above the earth's surface. Stratospheric ozone is very important to life on the surface of Earth because it absorbs much of the incoming solar ultraviolet radiation , and thereby shields organisms from its deleterious effects. Since the mid-1980s, there has been evidence that concentrations of stratospheric ozone are diminishing as a result of complex photochemical reactions involving chlorofluorocarbons (CFCs) . These persistent chemicals are synthesized by humans and then emitted to the lower atmosphere, from where they eventually reach the stratosphere and deplete ozone.

Stratospheric ozone

Typically, stratospheric ozone (O3) concentrations are about 0.2–0.4 ppm (parts per million), compared with about 0.03 ppm in unpolluted situations close to ground level in the troposphere. Stratospheric ozone concentrations are also measured in Dobson units (DU). A Dobson unit is equivalent to the amount of ozone that, if accumulated from the entire atmosphere and spread evenly over the surface of the earth at a pressure of one atmosphere and a temperature of about 68°F (20°C), would occupy a thickness of 10 mm (0.01 m or 0.4 in). Typically, stratospheric zone occurs at a concentration of about 350 DU, equivalent to a layer of only 3.5 mm (0.14 in).

Stratospheric ozone is formed and consumed naturally by photochemical reactions involving ultraviolet radiation.

Molecular oxygen (O2) interacts with ultraviolet radiation and splits into oxygen atoms (O) (reaction 1), which either recombine to form O2 (reaction 2), or combine with O2 to form O3 (reaction 3). Once formed, the ozone can be consumed by various reactions, including a photodissociation involving ultraviolet radiation (reaction 4), or reactions with trace gases such as nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), or with simple molecules or ions of the halogens , including chlorine (reaction 5), bromine, and fluorine. At any time, the formation and consumption of ozone proceed simultaneously. The actual concentration of ozone is a net function of the rates of reactions by which it is formed, and the rates of the reactions that consume this gas. The halogens catalyze the dissociation reaction; in other words, they increase the rate of ozone degradation without themselves being consumed in the chemical reaction. This means that they are available after one reaction to catalyze thousands of other such reactions. The increase of halogens present in the upper atmosphere has caused a shift in the equilibrium between the reactions toward an increased rate of ozone depletion.

The concentration of ozone in the stratosphere naturally varies with latitude and with time. Rates of ozone formation are largest over the equatorial regions of Earth because solar radiation is most intense over those latitudes. However, stratospheric winds carry tropical ozone to polar latitudes, where it tends to accumulate. On average, ozone concentrations are about 450 DU over sub-polar regions, and 250 DU over the tropics. Ozone concentrations can be as large as 600 DU during the wintertime maximum over the Antarctic. Daily variation in ozone can change by as much as 60 DU at higher latitudes. Seasonal variation at high latitudes can also be great, as much as 125 DU between spring and summer. Because ozone formation is driven by UV light , there is a small effect of the 11-year solar sunspot cycle on ozone. Intensified solar activity can also affect ozone concentration, and although these fluctuations can be intense, they usually do not persist. Although volcanic eruptions were once thought to contribute to ozone depletion, they are now considered a minor influence. Eruptions cause changes in the concentration of ozone by injecting aerosols into the atmosphere, which provide a surface on which ozone destruction reactions can occur quickly. These effects, however, are small and very short-lived. For example, the eruption of Mount Pinatubo in the Phillipines in 1991 injected 30 million tons of aerosol into the atmosphere, all of which was depleted in 11 months.

Human activities have resulted in large increases in emissions of some ozone consuming substances or their precursors into the atmosphere. As a result, there are concerns about potential changes in the dynamic equilibria among the stratospheric ozone reactions, which could result in decreases in ozone concentration.

The first concerns about depletion of stratospheric ozone were raised in the 1960s. At that time, a number of scientists suggested that emission of water vapor and various other chemicals from high-flying military jets and rockets might cause a consumption of stratospheric ozone. These discussions intensified during the early 1970s, when there were proposals to develop fleets of supersonic aircraft flying in the stratosphere. (Mostly for economic reasons, this capital-expensive commercial venture did not materialize.) Some scientists additionally suggested that emissions of oxides of nitrogen from vehicles and agricultural practices might also have some effect on the ozone layer, as could emissions associated with launchings of space shuttles and other spacecraft.

Since about the mid-to-late 1970s, there has been evidence of large decreases in the concentrations of stratospheric ozone at polar latitudes during the late winter to early springtime. The term used to describe these phenomena is ozone "holes." These seasonal occurrences are most noticeable over the Antarctic, where the ozone holes develop between September and November when the stratosphere is intensely cold, but sunlight is intense. The first convincing evidence of ozone holes was obtained over Antarctica in 1984, when the average ozone concentration in October was 180 DU, compared with 300 DU in the early 1970s. In November 1999, the ozone concentration was down to 165 DU. These sorts of observations stimulated a re-examination of earlier data from satellites and other observation systems, which suggested that the ozone holes have existed since at least the 1970s.

The Antarctic ozone holes typically develop at altitudes of 7.4–16 mi (12–25 km). The average decreases in springtime stratospheric ozone concentrations over Antarctica have been 30–40%. However, in some years the decrease in ozone has been over 60%. In the worst years, the ozone concentration over Antarctica was only 120 DU. In October 1999, the ozone concentrations were less than 50% of what they were in the 1960s.

The immediate cause of the depletions of stratospheric ozone is thought to involve atoms of chlorine or simple compounds such as chlorine monoxide (ClO). However, these chemicals are thought to have an indirect origin through human activities, especially the emission of CFCs to the atmosphere. Once formed in the stratosphere by the degradation of a CFC molecule , a single chlorine atom is capable of destroying as many as 100,000 ozone molecules before it is removed from the upper atmosphere.

The occurrence of seasonal ozone holes is restricted to high-latitude regions, especially over Antarctica, and to a lesser degree over the Arctic. However, concentrations of stratospheric ozone can also be affected at lower latitudes, although the ozone depletion is relatively small. This happens during the late springtime, when the normal lower-latitude ozone concentrations are diluted by ozone-depleted polar air that becomes widely dispersed as the ozone holes break up and dissipate. Ozone layer depletions have been observed over North America , Europe , Asia , most of Africa , Australia , and South America . Ozone levels over the United States have fallen 5–10%, depending on the season. One study estimated that seasonal concentrations of stratospheric ozone over mid-latitudes of the Southern Hemisphere may have decreased by 3–8%. It has been hypothesized that global warming might be one cause of the seasonal ozone loss in the Southern Hemisphere.

The importance of stratospheric ozone

Stratospheric ozone is biologically important because it selectively absorbs much of the incoming solar electromagnetic radiation within the ultraviolet (UV) range. Ozone is very effective within the UV-C wavelength range of 200–280 nm, somewhat less so in the UV-B range of 280–320 nm, and it is rather ineffective in absorbing UV-A at 320–400 nm. However, UV-A is not very damaging to organisms. Although UV-C is extremely damaging, virtually none of this radiation penetrates through Earth's upper atmosphere. Therefore, the greatest anxiety in terms of biological damages caused by ultraviolet radiation concerns the relatively variable exposures to UV-B, which are directly influenced by concentrations of stratospheric ozone. Note, however, that fluxes of UV-B to Earth's surface are also related to certain conditions in the troposphere, such as the thickness of cloud cover, concentrations of particulates and certain chemicals, and changes in the angle of the sun , which influences the thickness of atmosphere that must be penetrated by UV-B rays before the earth's surface is reached.

Because ozone selectively absorbs these deleterious wavelengths of solar radiation, it serves as an ultraviolet shield. As such, stratospheric ozone helps to protect humans and other organisms on Earth's surface from some of the harmful effects of exposure to this high-energy electromagnetic radiation. In fact, without the protective action of the stratospheric ozone layer, it is likely that terrestrial life would not be possible on Earth, and that oceanic life would be restricted to relatively greater depths than those at which it can now comfortably occur.

If not intercepted, ultraviolet radiation is capable of damaging genetic material. The genetic molecules deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) and many proteins and other biochemicals are effective absorbers of ultraviolet radiation. DNA and RNA are especially efficient at absorbing wavelengths shorter than 320 nm, but these important chemicals are damaged by this absorption. Damages to genetic materials could result in an increased incidence of skin cancers. Basal carcinomas account for about 75% of human skin cancers, and squamous cell carcinomas about 20%. These are both serious diseases, but they can usually be successfully treated if detected early enough. The other skin cancer is malignant melanoma, a deadly disease that accounts for about 5% of total skin carcinomas, and which is often fatal soon after it is diagnosed.

It is well known that people living in relatively sunny places have increased risks of all of these skin cancers, and that individual behaviors that increase exposures to UV-B also carry higher risks of developing these diseases (for example, sunbathing, or occupation exposures related to working outdoors, such as in agriculture, fishing, or construction). Compared with light-skinned people, individuals with relatively dark skin are much more tolerant of exposure to UV-B, because they are protected by the skin pigment melanin.

Within skin-color types, there are well-established, statistical relationships between exposures to UV-B and risks of developing skin cancers, most notably malignant melanoma. Using these relationships, predictions of increased rates of skin cancers that could be caused by depletions of stratospheric ozone have been made. For example, the U.S. Environmental Protection Agency has suggested that a 1% decrease in stratospheric ozone could result in a 2% increase in exposure to UV-B, and a 3–6% increase in skin cancers. In fact, many countries have reported increased incidences of all skin cancers. Usually this phenomenon is attributed to human behaviors that influence exposure to UV-B, such as sunbathing. However, increased exposures related to depletions of stratospheric ozone may also be important. Because melanoma takes 10–20 years to develop, there has not been enough time for ozone depletion to play a significant role in the rate of occurrence of this cancer. Research over the next decade will indicate whether the incidence of this cancer is rising and whether this rise may be due to the depletion of the ozone layer.

Other human-health effects of ultraviolet exposure include increased risks of developing cataracts and other damage to the cornea, damage to the retina, a suppressed immune system , sunburns of exposed skin, skin allergies, and an accelerated aging of the skin.

Domestic and wild animals are subject to the same sorts of increased risks of diseases and damages associated with increased UV-B exposure as are humans. Unlike humans, however, these animals do not wear protection to diminish those risks.

Other potential ecological damages associated with increased ultraviolet exposures include decreases of plant productivity in regions stressed by UV-B radiation, caused by the degradation of photosynthetic pigments. All terrestrial plants are at risk, as are plants occurring in shallow waters. The most exposed plants occur at high altitudes, for example in alpine tundra , or at high latitudes, such as polar seas and tundras. However, few data are now available that allow a general evaluation of the importance of these potential decreases in plant productivity.

In addition, some stratospheric ozone makes its way to the lower atmosphere, where it contributes to ozone pollution . Ozone is an important pollutant in the lower troposphere where it damages agricultural and wild plants, weakens synthetic materials, and causes discomfort to humans. During events of great turbulence in the upper atmosphere, such as thunderstorms, stratospheric ozone may enter the troposphere. Usually this only affects the upper troposphere, although observations have been made of stratospheric ozone reaching ground level for short intervals of time. On average, stratospheric incursions account for about 18% of the ozone in the troposphere, while photochemical reactions within the lower atmosphere itself account for the remaining 82% of tropospheric ozone.

Stratosphere and chlorofluorocarbons

Chlorofluorocarbons (CFCs) are compounds that contain atoms of carbon , chlorine, and fluorine. CFCs are very stable chemicals, and they are easily liquefied and gasified, are non-flammable, and are of low toxicity. CFCs have had many industrial uses, especially in refrigeration, as propellants in aerosol sprays, as blowing agents used to manufacture synthetic foams and insulation, as cleaning agents for electronic components, as carrier gases for sterilizers of medical instruments, and as dry-cleaning fluids. After most of these uses, CFCs are emitted to the lower atmosphere, where they are very persistent. The CFCs slowly penetrate into the stratosphere, where exposures to highly energetic, short wave solar radiation are intense, causing the CFCs to degrade. This releases chlorine and fluorine atoms, which are then available to consume ozone molecules in secondary reactions. It has been estimated that CFCs account for at least 80% of the depletion of stratospheric ozone worldwide. It has been estimated that the ozone hole over Antarctica is almost 100% a result of CFCs.

As a result of widespread awareness and concerns about the role of CFCs in the depletion of stratospheric ozone, the uses and emissions of these chemicals are being rapidly diminished. Some uses were widely banned as early as the 1980s, for example, the use of CFCs as propellants in aerosol spray cans. A conference sponsored by the United Nations Environment Programme in 1987 resulted in the so-called Montreal Protocol, which was subsequently revised and made more stringent in 1990, when it called for a complete phase out of global CFC use by the year 2000. One hundred and sixty five nations signed this document. The United States, Australia, and other developed countries have completely phased out the production of CFCs. According to the Montreal Protocol, developing countries have until the year 2010 to complete their phase out. Since the Montreal Protocol, there has been a 3% overall decline in ozone-depleting substances, including CFCs as well as methyl chloroform and halons from fire extinguishers. There has been a relatively rapid and effective international response to CFC emissions.

Other substances that can find their way into the stratosphere can increase the rate of ozone depletion as well. Halons, which are compounds consisting of bromine, fluorine, and carbon, can end up in the upper atmosphere where the halogens found in the compounds catalyze the ozone consuming reactions. Methyl bromide provides the catalyst bromine. Hydrochlorofluorocarbons (HCFCs, compounds consisting of hydrogen , chlorine, fluorine, and carbon) and carbon tetrachloride release chlorine and fluorine into the upper atmosphere.

Recent data from both ground-based and satellite measurements indicate that after decades of continual increase, concentrations of chlorine in the stratosphere are starting to plateau. The concentrations of bromide, however are continuing to increase and the depletion of ozone as a result of halides has been as great as 30% over the last decade. However, if halide and CFC concentrations continue to fall as directed by the Montreal Protocol, computer models predict that Antartic ozone concentrations should begin to increase in 2010.

See also Greenhouse effect.

Resources

books

Benedick, R.E. Ozone Diplomacy: New Directions in Safeguarding the Planet. Washington, DC: World Wildlife Fund, 1991.

periodicals

Zurer, P.S. "Ozone Depletion's Recurring Surprises Challenge Atmospheric Scientists." Chemical and Engineering News (May 24, 1993): 8–18.

other

Executive Summary. Scientific Assessment of Ozone Depletion: 2002. July, 31 2002.

Bill Freedman

KEY TERMS

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Electromagnetic radiation

—The energy of photons, having properties of both particles and waves. The major wavelength bands are, from short to long: cosmic, ultraviolet, visible or "light," infrared, and radio. Solar electromagnetic radiation is emitted by the Sun, and is the major external source of energy to the earth.

Ozone holes

—Decreased concentrations of stratospheric ozone, occurring at high latitudes during the early springtime. Ozone holes are most apparent over Antarctica, where they develop under intensely cold conditions during September and November, allowing a greater penetration of deleterious solar ultraviolet radiation to Earth's surface.

Stratosphere

—A layer of the upper atmosphere above an altitude of 5–10.6 mi (8–17 km) and extending to about 31 mi (50 km), depending on season and latitude. Within the stratosphere, air temperature changes little with altitude, and there are few convective air currents.

Troposphere

—The layer of air up to 15 mi (24 km) above the surface of the earth, also known as the lower atmosphere.