Chlorofluorocarbons (CFCs) are chemically inert, nonflammable, nontoxic compounds that are superb solvents and have been used in many industrial processes; they are excellent coolants for air conditioners and refrigerators. Two CFCs that have been widely used are Freon-11 (CCl3F) and Freon-12 (CCl2F2) (Freon is a DuPont trade name).
The compound known as HCFC-134, a fluorocarbon currently used in home and automobile air conditioners.
The CFCs are so unreactive that they do not readily decompose, that is, break down into simpler compounds, when they are released into the atmosphere. Over time the CFCs are carried into the stratosphere by air currents, where they are exposed to large amounts of ultraviolet radiation.
In 1974, Mario Molina and Sherwood Rowland of the University of California–Irvine demonstrated in their laboratory that when CFCs are exposed to ultraviolet radiation they break down to form chlorine radicals (Cl•).
Molina and Rowland predicted that these very reactive radicals could cause problems by catalyzing the destruction of ozone in the stratosphere.
Each spring since 1979, researchers have observed a thinning of the ozone layer over Antarctica. Each spring (autumn in the Northern Hemisphere) beginning in 1983, satellite images have shown a “hole” in the ozone layer over the South Pole. During August and September 1987, a NASA research team flew a plane equipped with sophisticated analytical instruments into the ozone hole 25 times. Their measurements demonstrated that as the concentration of the chlorine oxide radicals, Cl-O increased, the concentration of ozone decreased.
By September 1992, this ozone hole was nearly three times the area of the United States. In December 1994, three years of data from NASA’s Upper Atmosphere Research Satellite (UARS) provided conclusive evidence that CFCs are primarily responsible for this destruction of the ozone layer. Considerable thinning of the ozone layer in the Northern Hemisphere has also been observed.
The following is a simplified representation of the chain reaction that is now believed to account for most of the ozone destruction in the stratosphere
Cl• + O3 -> Cl−O• + O2 (step 1)
Cl−O• + O -> Cl• + O2 (step 2)
O3 (g) + O -> 2O2 (g) (net reaction)
A sufficient supply of oxygen atoms, O, is available in the upper atmosphere for the second step to occur. The net reaction results in the destruction of a molecule of ozone. The chlorine radical that initiates the first step of this reaction sequence is regenerated in the second step, however, and so a single chlorine radical can act as a catalyst to destroy many thousands of O3 molecules. Other well-known reactions also destroy ozone in the stratosphere, but the evidence shows conclusively that the CFCs are the principal culprits.
Since January 1978, the use of CFCs in aerosol cans in the United States has been banned; increasingly strict laws prohibit the release into the atmosphere of CFCs from sources such as automobile air conditioners and discarded refrigerators. The Montreal Protocol, signed by 24 countries in 1989, called for reductions in production and use of many CFCs. International agreements have since called for a complete ban on CFC production. Efforts to develop suitable replacement substances and controls for existing CFCs continue. The good news is that scientists expect the ozone hole to decrease and possibly disappear during the twenty-first century if current international treaties remain in effect and if they are implemented throughout the world. These are two very large ifs.
Additional information on stratospheric ozone can be found at the EPA Web site, http://www.epa.gov/docs/ozone. The Web site for this textbook will direct you to additional information about ozone.
A computer-generated image of part of the Southern Hemisphere on October 17, 1994, reveals the ozone “hole” (black and purple areas) over Antarctica and the tip of South America. Relatively low ozone levels (blue and green areas) extend into much of South America as well as Central America. Normal ozone levels are shown in yellow, orange, and red. The ozone hole is not stationary but moves about as a result of air currents. (Courtesy NASA)