(From Grolier Multimedia Encyclopedia)

Ozone Layer

The ozone layer is a layer of the upper atmosphere lying about 20 to 25 km (12 to 15 mi) above the Earth's surface. It is so named because the unstable form of oxygen called ozone< is concentrated in this layer. The ozone layer strongly absorbs ultraviolet radiation from the Sun. If this radiation reached the Earth's surface at unprotected levels, it would be deleterious to all forms of life. For example, it would raise the incidence of human skin cancers and cataracts, as well as reducing food production in general.

Dissociation and Recombination of Gases

The natural forces at work in this layer maintain a mixture of gases that includes atomic oxygen (O), ordinary oxygen (O2), and ozone (O3). The quantities of each form fluctuate diurnally and seasonally in response to solar radiation, temperature changes, and other catalytic influences. The atoms of ordinary oxygen molecules become dissociated (O2 + energy → O + O) and recombine with other molecules of ordinary oxygen to form ozone (O + O2 + energy → O3). This process consumes energy, which is supplied in nature by absorption of shortwave solar radiation and by discharges of lightning. Decomposition (2O3 → 3O2) is hastened by a rise in temperature or interaction with a variety of substances.

Robert C. Fite


Effect of Free Radicals

Ozone in the atmosphere is constantly being created by the action of ultraviolet (UV) radiation from the Sun on ordinary molecular oxygen and is destroyed by various natural processes. The balance between the creation and destruction processes always leaves some ozone present in the atmosphere, with 90 percent of it located in the stratosphere. Through the 1960s the average ozone molecule survived in the atmosphere for about 11 minutes. Human activities, however, began affecting this balance by introducing into the stratosphere chemicals that speeded up the processes that destroy ozone, shortening its average survival time. The average amount of ozone over the United States and Europe in 2000 is about 10 percent less in the winter and spring and 5 percent less in the summer and autumn than it was in the 1960s.

The natural processes that have always removed ozone from the stratosphere are chiefly chemical reactions initiated by free radicals. Free radicals are atoms and molecules that contain an odd number of electrons, as in HO2 (17 electrons) and NO (15 electrons). The final unpaired electron is responsible for the chemical reactivity. Human ingenuity has created two new classes of chemically inert compounds: the chlorofluorocarbons, usually known as CFCs, contain chlorine, and the bromofluorocarbons, or Halons, contain bromine. CFCs are used as refrigerants in automobile air conditioners and home refrigerators, as propellant gases in aerosol spray cans, as cleansers for electronic parts, and in the manufacture of foamed plastics such as those used in food packaging. Halons are used primarily in fire extinguishers.

Free radicals released from the Earth's surface react very quickly and do not reach the stratosphere. However, the only atmospheric removal process (termed as a "sink") that acts on CFCs or Halons is decomposition by solar UV radiation. Moreover, the wavelengths of the Sun's radiation that affect CFCs also react with ozone in the stratosphere and never reach the lower atmosphere. Consequently, CFCs wander randomly about the lower atmosphere for decades before drifting above most of the stratosphere ozone, where they are decomposed by UV radiation. Only about 1 percent of the CFC molecules used in automobile air conditioners are broken up each year, and cleaning them out of the atmosphere will stretch out over several centuries. This stratospheric decomposition of CFCs releases a free radical, atomic chlorine (17 electrons). Halons behave similarly, with the eventual release of atomic bromine (35 electrons).

The reaction of a free radical with a chemical having an even number of electrons to form two other compounds must create one odd-electron species and one even-electron species to account for all of the electrons. When atomic chlorine (17 electrons) reacts with ozone (24 electrons), the product chlorine oxide has an odd number of electrons (25), and the other, O2, has an even number (16). But this new free radical, chlorine oxide, is also very reactive and quickly reacts with atomic oxygen (8 electrons) to produce atomic chlorine (17 electrons) and ordinary O2 (16 electrons). This sequence of two reactions is called a free-radical catalytic chain reaction, because the atom of chlorine that initiated the first reaction is itself a product of the second reaction, and is ready to react once again with another molecule of ozone. The chlorine has acted as a catalyst<, a substance that causes chemical reactions to occur, but which itself is neither permanently created nor destroyed in the process. The number of successive times a catalyst reacts in the chain reaction before some other process intervenes is called the chain length, and the average chain length of the chlorine atom attack on stratospheric ozone is about 100,000—that is, each chlorine atom released in the stratosphere destroys about 100,000 molecules of ozone before it is converted into a permanently inactive form.

During the 1970s and 1980s about 1 million tons of CFCs were released into the atmosphere each year, and because of the chain length of each chlorine atom, a major global environmental problem was created. The most massive ozone losses occur in September, October, and November over Antarctica, when the unique atmospheric conditions allow chlorine and bromine to remove about 70 percent of the ozone over that continent, leaving an Antarctic "ozone hole" with only about one-third as much ozone as was found overhead about 30 years earlier. Ozone losses have not been as significant in the northern hemisphere, because the atmospheric conditions are different from those in the south; northern mountain ranges such as the Rocky Mountains in the United States and the Ural Mountains in Russia disrupt the flow of air above them. The primary environmental consequence from the loss of stratospheric ozone is a sharp increase in the amount of UV radiation from the Sun reaching the Earth's surface. Such UV radiation is the primary cause of human skin cancer and also has important effects on the eyes and the immune system.

Concern over the loss of stratospheric ozone led in 1987 to the United Nations agreement known as the Montreal Protocol on Substances that Deplete the Ozone Layer. Originally, this agreement called for an eventual reduction in CFC production by 50 percent. However, its 1992 modification banned CFC production by January 1996. Current measurements of CFC and Halon concentrations in the atmosphere have shown that the Montreal Protocol has effectively stopped further increases in these compounds. The 1995 Nobel Prize in chemistry was awarded to three scientists for their work on atmospheric ozone: Paul Crutzen, Mario J. Molina, and F. Sherwood Rowland.

F. Sherwood Rowland


Bibliography
Dessler, Andrew E., The Chemistry and Physics of Stratospheric Ozone (2000).

Le Prestre, Philippe G., et al., Protecting the Ozone Layer: Lessons, Models, and Projects (1998).

Makhijani, Arjun, and Gurney, Kevin, Mending the Ozone Hole (1995).

Somerville, Richard C. J., The Forgiving Air (1996).