LEARNING OBJECTIVES
Ozone depletion in the stratosphere is recognized as a major environmental problem with serious effects. After reading this chapter, you should understand:
• What ozone is and how ozone is naturally formed and destroyed in the stratosphere.
• What the so-called ozone shield is, and why it is important.
• How chemical and physical processes and reactions link emissions of chlorofluorocarbons (CFCs) to stratospheric ozone depletion.
• What role polar stratospheric clouds play in ozone depletion.
• Why ozone depletion is a long-term problem.
• What the environmental effects of ozone depletion are, and what options are available to minimize ozone depletion.
• Why international cooperation, including significant economic aid from wealthy to less wealthy nations, is necessary to encourage future reduction or elimination of emissions of ozone-depleting chemicals into the atmosphere.
Summary
• Concentration of atmospheric ozone has been measured for more than 70 years. In the last decade, measurements have been taken from instruments mounted on satellites. Evaluation of the available data shows a clear trend: Ozone concentrations in the stratosphere have been decreasing since the mid-1970s.
• In 1974, Mario Molina and F. Sherwood Rowland advanced the hypothesis that stratospheric ozone might be depleted as a result of emissions of chlorofluorocarbons (CFCs) into the lower atmosphere. Major features of the hypothesis are: CFCs are extremely stable and have a long residence time in the atmosphere; eventually, the CFCs reach the stratosphere, where they may be destroyed by highly energetic solar ultraviolet radiation, releasing chlorine; the chlorine may then enter into a catalytic chain reaction that depletes ozone in the stratosphere. An environmentally significant result of the depletion is that more ultraviolet radiation reaches the lower atmosphere, where it can damage living cells.
• The Antarctic ozone hole was first reported in 1985 and since has captured the imagination of people around the world. Of particular importance to understanding the ozone hole are the complex reactions that occur in the polar vortex and the development of polar stratospheric clouds. Reactions in the clouds tend to denitrify the air mass in the vortex; and during the polar spring, chlorine is released to react in the catalytic ozone depletion cycle. The reactions can be very rapid, producing the observed 70% reduction in stratospheric ozone in only a few weeks.
• Tropical and midlatitude ozone depletion is also hypothesized. Polar stratospheric clouds may be present above the tropics; in addition, belts of sulfur dioxide–rich aerosol clouds may result from volcanic eruptions. These clouds may be related to processes that denitrify the atmosphere and facilitate ozone depletion.
• Millions of tons of chemicals with the potential to deplete stratospheric ozone are now in the lower atmosphere and working their way to the stratosphere. As a result, if all production, use, and emissions of these chemicals were stopped today, the problem would continue. The good news is that the concentrations of CFCs in the atmosphere have apparently peaked and are now static or in slow decline.
• Potential environmental effects related to ozone depletion include damage to Earth’s food chain, both on land and in the ocean, and human health effects, including increases in skin cancers, cataracts, and suppression of the immune system.
• Many nations around the world have agreed to the Montreal Protocol, which will reduce global emissions of CFCs to 50% of the 1986 levels. The agreement called for elimination of production of the chemicals by 1996 for industrialized nations and by 2006 for developing nations. A serious hurdle to compliance for some nations relates to the economic fact that most chemical replacements for CFCs are more expensive than CFCs. Therefore, it appears that financial aid will be required if the less wealthy nations are to eliminate CFC use.
• Potential management strategies for the ozone depletion problem, along with eliminating production of CFCs include: (1) collecting and reusing CFCs, and (2) using substitutes for CFCs.
• Given the lifetimes of the ozone-depleting chemicals, people will have to continue to live with higher levels of exposure to ultraviolet radiation over the next few decades. Banning chemicals that can deplete stratospheric ozone is a step in the right direction and will ultimately result in the reduction of atmospheric ozone depletion.