Ozone Layer Depletion

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Ozone layer depletion

Destroying the ozone shield

Ozone , a form of oxygen consisting of three atoms of oxygen instead of two, is considered an air pollutant when found at ground levels and is a major component of smog . It is formed by the reaction of various air pollutants in the presence of sunlight. Ozone is also used commercially as a bleaching agent and to purify municipal water supplies. Since ozone is toxic, the gas is harmful to health when generated near the earth's surface. Because of its high rate of breakdown, such ozone never reaches the upper atmosphere .

But the ozone that shields the earth from the sun's radiation is found in the stratosphere , a layer of the upper atmosphere found 9–30 mi (15–50 km) above ground. This ozone layer is maintained as follows: the action of ultraviolet light breaks O₂ molecules into atoms of elemental oxygen (O). The elemental oxygen then attaches to other O₂ molecules to form O₃. When it absorbs ultraviolet radiation that would otherwise reach the earth, ozone is, in turn, broken down into O₂ + O. The elemental oxygen generated then finds another O₂ molecule to become O₃ once again.

In 1974, chemists F. Sherwood Rowland and Mario J. Molina realized that chlorine from chlorofluorocarbon (CFC) molecules was capable of breaking down ozone in the stratosphere. In time, evidence began accumulate that the ozone layer was indeed being broken apart by these industrial chemicals , and to a lesser extent by nitrogen oxide emissions from jet airplanes as well as hydrogen chloride emissions from large volcanic eruptions.

When released into the environment , CFCs slowly rise into the upper atmosphere, where they are broken apart by solar radiation. This releases chlorine atoms that act as catalysts, breaking up molecules of ozone by stripping away one of their oxygen atoms. The chlorine atoms, unaltered by the reaction, are each capable of destroying ozone molecules repeatedly. Without a sufficient quantity of ozone to block its way, ultraviolet radiation from the sun passes through the upper atmosphere and reaches the surface of the earth.

When damage to the ozone layer first became apparent in 1974, propellants in aerosol spray cans were a major source of CFC emissions, and CFC aerosols were banned in the United States in 1978. However, CFCs have since remained in widespread use in thermal insulation, as cooling agents in refrigerators and air conditioners, as cleaning solvents, and as foaming agents in plastics , resulting in continued and accelerating depletion of stratospheric ozone.

The Antarctic ozone hole

The most dramatic evidence of the destruction of the ozone layer has occurred over Antarctica , where a massive "hole" in the ozone layer appears each winter and spring, apparently exacerbated by the area's unique and violent climatological conditions. The destruction of ozone molecules begins during the long, completely dark, and extremely cold Antarctic winter, when swirling winds and ice clouds begin to form in the lower stratosphere. This ice reacts with chlorine compounds in the stratosphere (such as hydrogen chloride and chlorine nitrate) that come from the breakdown of CFCs, creating molecules of chlorine.

When spring returns in August and September, a seasonal vortex—a rotating air mass—causes the ozone to mix with certain chemicals in the presence of sunlight. This helps break down the chlorine molecules into chlorine atoms, which, in turn, react with and break up the molecules of ozone. A single chlorine, bromine , or nitrogen molecule can break up literally thousands of ozone molecules.

During December, the ozone-depleted air can move out of the Antarctic area, as happened in 1987, when levels of ozone over southern Australia and New Zealand sank by 10% over a three week period, causing as much as a 20% increase in ultraviolet radiation reaching the earth. This may have been responsible for a reported increase in skin cancers and damage to some food crops.

The seasonal hole in the ozone layer over Antarctica has been monitored by scientists at the National Aeronautics and Space Administration's (NASA) Goddard Space Flight Center outside Washington, D.C. NASA's NIMBUS-7 satellite first discovered drastically reduced ozone levels over the Southern Hemisphere in 1985, and measurements are also being conducted with instruments on aircraft and balloons. Some of the data that has been gathered is alarming.

In October 1987, ozone levels within the Antarctic ozone hole were found to be 45% below normal, and similar reductions occurred in October 1989. A 1988 study revealed that since 1969, ozone levels had declined by 2% worldwide, and by as much as 3% or more over highly populated areas of North America, Europe, South America, Australia and New Zealand.

In September 1992, the NIMBUS-7 satellite found that the depleted ozone area over the southern polar region had grown 15% from the previous year, to a size three times bigger than the area of the United States, and was 80% thinner than usual. The ozone hole over Antarctica was measured at approximately 8.9 million mi² (23 million km²), as compared to its usual size of 6.5 million mi² (17 km²). The contiguous 48 states is, by comparison, about 3 million mi² (7.8 million km²), and all of North America covers 9.4 million mi² (24.3 million km²). Researchers attributed the increased thinning not only to industrial chemicals but also to the 1991 volcanic eruptions of Mount Pinatubo in the Philippines and Mount Hudson in Chile, which emitted large amounts of sulfur dioxide into the atmosphere.

Dangers Of ultraviolet radiation

The major consequence of the thinning of the ozone layer is the penetration of more solar radiation, especially Ultraviolet-B (UV-B) rays, the most dangerous type, which can be extremely damaging to plants, wildlife , and human health. Because UV-B can penetrate the ocean's surface, it is potentially harmful to marine life forms and indeed to the entire chain of life in the seas as well.

UV-B can kill and affect the reproduction of fish, larvae, and other plants and animals, especially those found in shallow waters, including phytoplankton , which forms the basis of the oceanic food chain/web . The National Science Foundation reported in February 1992 that its research ship, on a six week Antarctic cruise, found that the production of phytoplankton decreases at least 6–12% during the period of greatest ozone layer depletion, and that the destructive effects of UV radiation could extend to depths of 90 ft (27 m).

A decrease in phytoplankton would affect all other creatures higher on the food chain and dependent on them, including zooplankton , microscopic ocean creatures that feed on phytoplankton and are also an essential part of the ocean food chain. And marine phytoplankton are the main food source for krill , tiny Antarctic shrimp that are the major food source for fish, squid, penguins, seals , whales , and other creatures in the Southern Hemisphere.

Moreover, phytoplankton are responsible for absorbing, through photosynthesis , great amounts of carbon dioxide (CO₂) and releasing oxygen. It is not known how a depletion of phytoplankton would affect the planet's supply of life-giving oxygen, but more CO₂ in the atmosphere would exacerbate the critical problem of global warming, the so-called greenhouse effect .

There are numerous reports, largely unconfirmed, of animals in the southern polar region being harmed by ultraviolet radiation. Rumors abound in Chile, for example, of pets, livestock, sheep, rabbits, and other wildlife getting cataracts, suffering reproductive irregularities, or even being blinded by solar radiation. Many residents of Chile and Antarctica believe these stories, and wear sunglasses, protective clothing, and sun-blocking lotion in the summer, or even stay indoors much of the day when the sun is out. If the ozone layer's thinning continues to spread, the lifestyles of people across the globe could be similarly disrupted for generations to come.

Particularly frightening have been incidents reported to have taken place in Punta Arenas, Chile's southernmost city, at the tip of Patagonia. After several days of record low levels of ozone were recorded in October 1992, people reported severe burns from short exposure to sunlight. Sheep and cattle became blind, and some starved because they could not find food. Trees wilted and died, and melanomatype skin cancers seem to have increased dramatically. Similar stories have been reported from other areas of the southern hemisphere. And malignant melanoma, once a rare disorder, is now the fastest rising cancer in the world.

Ozone thinning spreads

Indeed, ozone layer depletion is spreading at an alarming rate. In the 1980s, scientists discovered that an ozone hole was also appearing over the Arctic region in the late winter months, and concern was expressed that similar thinning might begin to occur over, and threaten, heavily populated areas of the globe. These fears were confirmed in April 1991, when the Environmental Protection Agency (EPA) announced that satellite measurements had recorded an ominous decrease in atmospheric ozone, amounting to an average of 5% over the mid-latitudes (including the United States), almost double the loss previously thought to be occurring.

The data showed that ozone levels measured in the late fall, winter, and early spring over large areas of the United States, Europe, and the mid-latitudes of the Northern and Southern Hemisphere had dropped by 4–6% over the last decade—twice the amount estimated in earlier years. The greatest area of ozone thinning in the United States was found north of a line stretching from Philadelphia to Denver to Reno, Nevada. One of the most alarming aspects of the new findings was that the ozone depletion was continuing into April and May, a time when people spend more time outside, and crops are beginning to sprout, making both more vulnerable to ultraviolet radiation.

The new findings led the EPA to project that over the next 50 years, thinning of the ozone layer could cause Americans to suffer some 12 million cases of skin cancer, 200,000 of which would be fatal. Several years earlier, the agency had calculated that over the next century, there could be an additional 155 million cases of skin cancers and 3.2 million deaths if the ozone layer continued to thin at the then current rate. Another EPA projection made in the 1980s was that the increase in radiation could cause Americans to suffer 40 million cases of skin cancer and 800,000 deaths in the following 88 years, plus some 12 million eye cataracts.

No one can say how accurate such varying projections will turn out to be, but evidence of ozone layer thinning is well-documented. In October 1991, additional data of spreading ozone layer destruction were made public. Dr. Robert Watson, a NASA scientist who co-chairs an 80-member panel of scientists from 80 countries, called the situation "extremely serious," saying that "we now see a significant decrease of ozone both in the Northern and Southern Hemispheres, not only in winter but in spring and summer, the time when people sunbathe, putting them at risk for skin cancer, and the time when we grow crops."

In February 1992, a team of NASA scientists announced that they had found record high levels of ozone-depleting chlorine over the Northern Hemisphere. This could, in turn, lead to an ozone "hole" similar to the one that appears over Antarctica developing over populated areas of the United States, Canada, and England. The areas over which increased levels of chlorine monoxide were found extended as far south as New England, France, Britain, and Scandinavia.

Action to protect The ozone layer

As evidence of the critical threats posed by ozone layer depletion has increased, the world community has begun to take steps to address the problem. In 1987, the United States and 22 other nations signed the Montreal Protocol, agreeing, by the year 2000, to cut CFC production in half, and to phase out two ozone-destroying gases, Halon 1301 and Halon 1211. Halons are man-made bromine compounds used mainly in fire extinguishers, and can destroy ozone at a rate 10 to 40 times more rapidly than CFCs. Fortunately, these restrictions appear to already be having an impact. In 1992, it was found that the rate at which these two Halon gases were accumulating in the atmosphere had fallen significantly since 1987. The rate of increase of levels of Halon 1301 was about 8% a year from 1989 to 1992, about half of the average annual rate of growth over previous years. Similarly, Halon 1211 was increasing at only 3% annually, much less than the previous growth of 15% a year.

Since the Montreal Protocol, other international treaties have been signed limiting the production and use of ozone-destroying chemicals. When alarming new evidence on the destruction of stratospheric ozone became available in 1988, the world's industrialized nations convened a series of conferences to plan remedial action. In March 1989, the 12-member European Economic Community (EEC) announced plans to end the use of CFCs by the turn of the century, and the United States agreed to join in the ban. A week later, 123 nations met in London to discuss ways to speed the CFC phase-out. The industrial nations agreed to cut their own domestic CFC production in half, while continuing to allow exports of CFCs, in order to accommodate third world nations.

Ironically, the large industrial nations, which have created the CFC problem, are now much more willing to take effective action to ban the compounds than are many developing nations, such as India and China. The latter nations resist restrictions on CFCs on the grounds that the chemicals are necessary for their own economic development.

After the meeting in London, leaders and representatives from 24 countries met in an environmental summit at The Hague, and agreed that the United Nations' authority to protect the world's ozone layer should be strengthened.

In May 1989, members of the EEC and 81 other nations that had signed the 1987 Montreal Protocol decided at a meeting in Helsinki to try to achieve a total phase-out of CFCs by the year 2000, as well as phase-outs as soon as possible of other ozone-damaging chemicals like carbon tetrachloride, halons, and methyl chloroform. In London in June 1990, most of the Montreal Protocol's signatory nations formally adopted a deadline of the year 2000 for industrial nations to phase out the major ozone-destroying chemicals, with 2010 being the goal for developing countries.

Finally, in November 1992, 87 nations meeting in Copenhagen decided to strengthen the action agreed to under the Montreal Protocol and move up the phase-out deadline from 2000 to January 1, 1996, for CFCs, and to January 1, 1994, for halons. A timetable was also agreed to for eliminating hydrochlorofluorocarbons (HCFCs) by the year 2030. HCFCs are being used as substitutes for CFCs even though they also deplete ozone, albeit on a far lesser scale than CFCs. The conference failed to ban the production of the pesticide methyl bromide, which may account for 15% of ozone depletion by the year 2000, but did freeze production at 1991 levels.

Environmentalists were disappointed that stronger action was not taken to protect the ozone layer. But Environmental Protection Agency (EPA) Administrator William K. Reilly, who headed the U.S. delegation, estimated that the reductions agreed to could, by the year 2075, prevent a million cases of cancer and 20,000 deaths.

Although the restrictions apply to developed nations, which produce most of the ozone-damaging chemicals, it was also agreed to consider moving up a phase-out of such compounds by developing nations from 2010 to 1995. A month after the Copenhagen conference, the nations of the European Community agreed to push bans on the use of CFCs and carbon tetrachloride to 1995 and to cut CFC emissions by 85% by the end of 1993.

The private sector has also taken action to reduce CFC production. The world's largest manufacturer of the chemicals, DuPont Chemical Company, announced in 1988 that it was working on a variety of substitutes for CFCs, would phase out production of them by 1996, and would partially replace them with HCFCs. Environmentalists charge that DuPont has been moving too slowly to eliminate production of these chemicals.

There are many ways that individuals can help reduce the release of CFCs into the atmosphere, mainly by avoiding products that contain or are made from CFCs, and by recycling CFCs whenever possible. Although CFCs have not generally been used in spray cans in the United States since 1978, they are still used in many consumer and industrial products, such as styrofoam. Other products manufactured using CFCs include solvents and cleaning liquids used on electrical equipment, polystyrene foam products, and fire extinguishers that use halons.

Refrigerants in cars and home air conditioning units contain CFCs and must be poured into closed containers to be cleaned or recycled, or they will evaporate into the atmosphere. Using foam insulation to seal homes also releases CFCs. Many alternatives to foam insulation exist, such as cellulose fiber, gypsum, fiberboard, and fiberglass.

Unfortunately, whatever steps are taken in the next few years, the problem of ozone layer depletion will continue even after the release of ozone-destroying chemicals is limited or halted. It takes six to eight years for some of these compounds to reach the upper atmosphere, and once there, they will destroy ozone for another 20–25 years. Thus, even if all emissions of destructive chemicals were stopped, compounds already released would continue to damage the ozone layer for another quarter century.

Understanding ozone depletion

As detailed collection of data about interactions in the stratosphere progresses, the observational support for the ozone depletion theory continues to grow more compelling. Yet atmospheric scientists are beginning to realize that their understanding of the upper atmosphere is still quite crude. While certain key reactions which maintain and destroy ozone are theoretically and observationally supported, scientists will have to comprehend the interaction of dozens, if not hundreds, of reactions between natural and artificial species of hydrogen, nitrogen, bromine, chlorine and oxygen before a complete picture of ozone-layer dynamics emerges. The recent eruption of Mount Pinatubo , for example, made scientists aware that heterogenous processes —those reactions which require cloud surfaces to take place—may play a far greater role in causing ozone depletion than originally believed. Such reactions had previously been observed taking place only at the earth's poles, where stratospheric clouds form during the long winter darkness, but it is now thought that sulfur aerosols ejected by Pinatubo may be serving as a catalyst to speed ozone depletion at nonpolar latitudes.

Ironically, ozone-depleting reactions are best understood around the thinly inhabited polar regions, where stable and isolated conditions over the winter allow scientists to understand stratospheric changes most easily. In contrast, at the temperate latitudes where constantly moving air masses undergo no seasonal isolation, it is difficult to determine whether a fluctuation in a given chemical's density is a result of local reactions or atmospheric turbulence. In 1995, the Nobel Prize for Chemistry was awarded to F. Sherwood Roland, Mario Molina and Paul Crutzen for their work on the formation and destruction of the ozone layer. It is hoped that increasingly detailed measurements using a new generation of equipment (such as NASA's Perseus remote-control aircraft) will begin to shed more light on the processes occurring away from the poles. Joe Waters of NASA's Jet Propulsion Laboratory summarizes the urgent task: "We must be able to lay out the catalytic cycles that are destroying ozone at all altitudes all over the globe—from its production region in the tropics to the higher latitudes and the polar regions."

See also Photochemical reaction

[Lewis G. Regenstein ]