Bad, Ground-Level Ozone, A Harmful Pollutant

Near the Earth's surface, in the air that we breathe, ozone is not so beneficial to life, and it is a main ingredient of eye-burning smog that can damage lung tissue and plants. During smog alerts in large cities, such as Los Angeles and Mexico City, children, elderly people, and jogging enthusiasts are advised to stay indoors because of potential ozone damage to their lungs. Children are especially at risk, because, for their body size, they inhale several times more air than adults, and also since they tend to spend more time outdoors.

In contrast to the "good" naturally occurring ozone in the stratosphere, which is created by incoming solar radiation, the "bad", ground level ozone is produced by humans, primarily from compounds emitted in the exhaust of cars and trucks. Fires have also caused ground-level ozone and smog across fields and grasslands of Brazil and the savannas of southern Africa, with an intensity comparable to the pollution over industrialized regions in Europe, Asia, and the United States, "tte "bad" ground-level ozone can also be carried hundreds of kilometers by the wind, so the potential damage is not limited to densely populated and heavily industrialized cities.

ricane. Each year the gaping hole opens up during Antarctic spring when the sunlight triggers ozone-destroying chemical reactions, tte hole then begins to close up in the early polar fall when the long sunless winter begins. Ozone-depleted air is dispersed globally, and the ozone is slowly restored to fill in the hole until the cycle repeats in the following year.

tte ozone layer is itself invisible, so you can't see the ozone hole. Instruments measure the amount of ultraviolet radiation getting through the atmosphere at specific wavelengths or spectral lines, tte absorption lines are strengthened when there is more ozone, and weakened if there are lesser amounts.

British scientists first detected a springtime decrease of ozone above the South Pole in 1957-58. ttey had previously found that the amount of ozone varies annually throughout Europe. When this annual variation was compared to that measured at the Halley Bay station in Antarctica, assuming a six-month difference in the seasons, the ozone values for polar spring were much lower than was expected. It was initially thought that some large mistake had been made, or that the instrument had developed some major fault, but in the polar fall the ozone values jumped to those expected from extrapolation of the European results. According to Gordon Miller Bourne "G.M.B." Dobson (1889-1976):

It was not until a year later [in 1958-59], when the same type of annual variation was repeated, that we realized that the early results were indeed correct and that Halley Bay showed a most interesting difference from other parts of the world. It was clear that the winter vortex over the South Pole was maintained late into the spring and that this kept the ozone values low. When it suddenly broke up in November both the ozone values and the stratosphere temperatures suddenly rose.43

tte total ozone concentration was measured above Antarctica for several decades, always showing an anomalous springtime loss that became steadily larger as the years

FIG. 9.8 Hole in the sky A satellite map showing an exceptionally low concentration of ozone, called the ozone hole, that forms above the South Pole in the local spring. In October 1990 it had an area larger than the Antarctic continent, shown in outline below the hole. Eventually spring warming breaks up the polar vortex and disperses the ozone-deficient air over the rest of the planet. (Courtesy of NASA.)

FIG. 9.8 Hole in the sky A satellite map showing an exceptionally low concentration of ozone, called the ozone hole, that forms above the South Pole in the local spring. In October 1990 it had an area larger than the Antarctic continent, shown in outline below the hole. Eventually spring warming breaks up the polar vortex and disperses the ozone-deficient air over the rest of the planet. (Courtesy of NASA.)

went on. By 1985 the thinning of the ozone layer above Antarctica had become so large and dominant in the measurements that it could not be ignored, and the British Antarctic Survey announced their discovery of the so-called ozone hole that is largest and deepest in September and early October each year.

Astounded scientists, who had been monitoring the ozone layer from a satellite, rechecked their data, finding that they had unwittingly recorded the missing ozone for several years, tte embarrassed experts had programmed their computers to reject very low values of ozone as bad data, so the now-famous ozone hole had been discarded as an anomaly. Later analysis of the raw satellite data confirmed the discovery of the ozone hole, and demonstrated that it covered most of the Antarctica continent. Up to 70 percent of the ozone normally found over Antarctica was destroyed, resulting in a significant thinning of the ozone layer but not a completely open and empty hole.

Destroying Ozone

What's consuming the ozone layer that protects life on Earth from excessive ultraviolet radiation? In 1974, Mario Molina (1943- ) and F. Sherwood Rowland (1927- ), two chemists at the University of California at Irvine, showed that man-made chemicals are destroying the protective ozone layer, making it thinner and wasting it away. As they expressed it:

Chlorofluoromethanes are being added to the environment in steadily increasing amounts, "ttese compounds are chemically inert and may remain in the atmosphere for 40 to 150 years, and concentrations can be expected to reach 10 to 30 times present levels. Photodissociation of the chlorofluoromethanes in the stratosphere produces significant amounts of chlorine atoms, and leads to the destruction of atmospheric ozone.44

tte ozone-destroyers are man-made gases, invented about a half-century ago and now given the euphonious name "chlorofluorocarbons". Ms name is a giveaway to their composition, chlorine, fluorine, and carbon, and is often abbreviated as CFC. A number sometimes follows the shorthand CFC notation, providing a complex description of the number of atoms in each molecule, the most widely used being CFC-11 and CFC-12.

tte CFCs are synthetic chemicals entirely of human industrial origin with no counterparts in nature, and for several decades they were hailed as wonder chemicals, tteir molecules are assembled using the strongest chemical bonds permitted by nature, making them almost impervious to breakage, ttey are very stable, nontoxic, noncor-rosive, nonflammable, and relatively easy to manufacture, tte CFCs were once widely used as coolants in refrigerators, freezers and air conditioners, as foaming agents for insulation found in coffee cups and packaging, as propellants in hair spray and deodorants, and as cleaning solvents in the process of manufacturing computer chips.

When initially released in the lower atmosphere, the hardy chemicals don't interact chemically to form other substances that would get removed from the atmosphere naturally. And they're not soluble in water, so they don't get rained out. ttey are so inert and stable that once released into the atmosphere the CFC molecules can survive for more than a century.

In the lower atmosphere, CFCs are protected from the Sun's intense ultraviolet radiation by the ozone layer, enabling them to migrate intact into the stratosphere. Once there, however, the energetic solar ultraviolet tears the sturdy CFC molecules apart, producing free chlorine atoms, which in turn destroy the ozone. As Molina and Rowland theorized more than three decades ago, the chlorine is the real agent of ozone destruction, being able to destroy thousands of times its own weight in ozone.

Once freed in the stratosphere, a chlorine atom can react with an ozone molecule, taking one oxygen atom to form chlorine monoxide; the ozone is thereby returned to a normal oxygen molecule and its ultraviolet absorbing capability is largely removed. If this were all that happened, there wouldn't be much concern. However, when the chlorine monoxide encounters a free oxygen atom, the chlorine is set free to strike again and again. Because the cycle repeats over and over, a little chlorine goes a long way. In technical terms, the chlorine acts as a catalyst, initiating a series of reactions that destroy ozone, while surviving the process.

A single chlorine atom can disrupt as many as 100,000 ozone molecules before it is captured and locked away in some other molecule. A small amount of chlorine can therefore produce significant changes in the ozone layer, which is itself very rarefied. ttis discovery that the chlorine in man-made chemicals will destroy enormous amounts of ozone in the atmosphere was so important that Molina and Rowland were awarded the 1995 Nobel Prize in Chemistry, together with the German chemist, Paul J. Crutzen (1933- ), for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone.

In places, chlorine can destroy ozone at a far faster rate than the gas is replenished naturally, tte ozone loss then exceeds the ozone creation, as if water were being poured into a bucket with its bottom cut out. tte Sun's powerful ultraviolet rays can then penetrate to the ground, producing widespread damage to living things.

Why doesn't chlorine keep on consuming ozone until there is none left in the stratosphere? tte cycle of destruction is eventually broken when a chlorine atom, instead of interacting with ozone, interacts with methane in the atmosphere to form hydrochloric acid, tte acid diffuses downward from the stratosphere to the troposphere, and because it's soluble in rain the chlorine is finally washed out, closing the cycle.

tte simple scenario of ozone-destroying chlorine, released from CFCs by ultraviolet sunlight, does not explain why severe ozone depletion is limited to the Antarctic region or why it is observed only in the spring. Something must be focusing and intensifying chlorine's destructive power both in space and time.

Although the CFCs were mainly released by human activities in the Northern Hemisphere, where the world's industries are concentrated, the chemicals were redistributed by global winds. Each sunless winter, steady winds blow in a circular pattern over the ocean that surrounds Antarctica, trapping a huge air mass inside, tte whirling winds concentrate the ozone-destroying chemicals within a vast, towering polar vortex, confining the most serious ozone depletion to regions above the South Pole and isolating it from the rest of the world for months at a time.

tte ozone hole also requires the presence of polar stratospheric clouds of ice crystals. Whipping around the pole, the high-speed vortex winds shut out air from the warmer equatorial regions and keep the temperatures very cold inside. During the long, dark winter, the stratospheric air over Antarctica is therefore colder than elsewhere, allowing ice crystals to form, tte crystals provide surfaces or platforms on which chlorine and ozone can alight and interact more readily than if they were free.

Who cares if chemicals are punching a few holes in the sky and letting a little more sunlight reach the ground? You care, and I care, for the solar ultraviolet rays, which are no longer absorbed in the ozone layer, can produce skin cancer and cloud up your eyes with cataracts. It also increases the vulnerability of animals to infections, and can damage or wipe out one-celled plants, called phytoplankton, which live near the surface of the sea and are the base of the ocean food chain. When the

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