Ozone

Ozone gas is a form of oxygen found naturally in the stratosphere or upper atmosphere that shields Earth from the Sun's harmful ultraviolet radiation. Ozone is also found in the lower atmosphere as a man-made pollutant best known as smog. In recent years, it was discovered that certain synthetic gases are destroying Earth's stratospheric ozone shield.

Ozone is a natural component of Earth's upper atmosphere and is essential to the continuance of life on this planet. It is located between 6 and 28 miles (9.7 and 45.1 kilometers) above Earth. As a high-energy form of oxygen, it is formed naturally in the air both by the electrical charges given off during lightning and by high intensity, short wavelength light. Without this ozone layer absorbing the Sun's ultraviolet radiation, living things on Earth's surface could not survive. Many scientists believe that life on Earth could not have evolved without this protective ozone shield because direct penetration of the atmosphere by solar radiation would make Earth unlivable. Besides suffering life-threatening radiation burns and cancer, all aboveground organisms would eventually experience severe genetic damage. Although beneficial in the atmosphere, at ground level ozone is a bad-smelling, colorless gas that is poisonous and usually formed by pollutants people put into the air. Ozone at this level does not shield life from harmful radiation and is instead a major component of urban smog and a threat to all living things. Ground-level ozone, therefore, has only bad effects. Because smog can cause shortness of breath, chest pain, coughing nausea, and respiratory congestion in some people, many large cities have established "Ozone Action Day" campaigns to educate citizens how to reduce ground-level ozone and to warn them when to avoid strenuous outdoor activities.

OZONE DEPLETION

What might be considered good ozone is the gas found naturally high up in the atmosphere around Earth. It was this layer of ozone that was discovered in the late 1970s to be getting thinner. Further research revealed a severely thin spot, which some called a hole, in the ozone layer over Antarctica. By the mid-1980s, monitoring by satellites and high-altitude planes revealed that ozone levels over Antarctica had declined about 50 percent. Scientists argue that such a dramatic decrease could be related to increases in human skin cancers as well as cataracts and a weakening of the body's immune system. The decline in the ozone layer might also contribute to crop failures and a reduction in phytoplankton, which forms the basis of a major food web (the connected network of producers, consumers, and decomposers).

Many scientists argue that there is extensive evidence that a group of synthetic chemicals called chlorofluorocarbons (CFCs) are the primary cause of ozone reduction. While certain natural events like volcanoes and surges in solar activity certainly affect the ozone layer, the argument against CFCs as the primary human-influenced cause is very strong. CFCs are compounds of chlorine, fluorine, and carbon. They are odorless, invisible, and otherwise harmless gases that were put to so many uses that they were thought to be miracle compounds. CFCs were widely used as propellants in aerosol spray cans, although they soon came to be used regularly as coolants in refrigerators and air conditioners. Finally, CFCs proved useful in making Styrofoam cups and packaging materials. With all this use of CFCs, molecules of the gases were released, spreading upwards into the stratosphere where they took part in an unusual and ultimately dangerous set of chemical reactions.

When a molecule of CFC absorbs ultraviolet radiation, it releases a chlorine atom. This atom then reacts with the ozone (O₃) to form an oxygen molecule (O₂) and a chlorine monoxide molecule. At this stage, a molecule of ozone has already been converted into oxygen. The cycle then continues, as the chlorine monoxide reacts with a free oxygen atom and releases yet another atom, which in turn attacks another molecule of ozone. It has been estimated that each chlorine atom released from a CFC reaction can convert as many as 10,000 molecules of ozone to oxygen. Since ozone reduction is greatest when the atmosphere is at its coldest, the extreme atmospheric conditions of the Antarctic region provide excellent conditions for these reactions to occur, thus accounting for the "ozone hole" found there.

During the 1990s, ozone depletion was shown to be happening seasonally over populated areas. This led to a decision by the United States government and several European governments to begin phasing out the production and use of CFCs by the year 2000. There is some evidence that the buildup of CFCs is declining since then, but unfortunately, the amount of highly stable CFCs already pumped into the atmosphere will remain for a century doing their destructive work. However, some comfort can be taken in the knowledge that not only are humans no longer contributing to the problem, but that an amazingly small amount of ozone in the stratosphere still can screen out more than 95 percent of the Sun's ultraviolet radiation.

[See alsoPollution ]