The air we breathe is a source of joy and satisfaction, as well as an essential part of our life support system. Yet our cities are bathed in a complex mixture of gases--from automobiles, factories, power plants, and trees--that becomes highly reactive under the stimulation of sunlight. So it is that on hot, stagnant summer days, many cities are plagued with air that is both unpleasant and unhealthy to breathe. This mixture of gases and haze particles is commonly referred to as photochemical smog, one of whose components is ozone, the same gas that in the upper atmosphere shields the earth's surface from potentially harmful ultraviolet radiation from the sun.
Ozone, a molecule consisting of three oxygen atoms, is unstable. It not only absorbs the energy of ultraviolet radiation, but also reacts readily with living tissue. As a result, high concentrations of ozone at the ground level can have detrimental effects on humans and other living organisms. In any given human population about 20 percent experience acute reduction in lung function when exposed to the high concentrations of ozone that can occur in urban smog. While there is some evidence to suggest that repeated exposure to high ozone concentrations causes permanent lung damage (e.g., emphysema, lung cancer), this evidence is not conclusive, and thus the long-term consequences of ozone exposure are not yet certain.
Although a handful of cities--including Athens, Beijing, Los Angeles, and Mexico City--have gained international notoriety for the severity of their photochemical smog, most other urban-industrial centers around the world suffer from a similar deterioration in air quality. It is estimated that about 70 million people living in the United States are exposed to unhealthy ozone concentrations each year. Moreover, while generally not as severe as in urban concentrations, ozone pollution in rural areas is estimated to reduce U.S. agricultural productivity by about 10 percent, at a cost of about $5 billion per year. Ozone pollution is also believed to cause significant losses in the production of forest products, such as pulp and paper.
In the United States, photochemical smog remains a serious environmental problem despite substantial air cleanup efforts since the 1970s, and nearly 100 cities still experience repeated occurrence of ozone in excess of the National Ambient Air Quality Standard (NAAQS). New research that deciphers the complex processes in the formation of urban ozone clarifies why the ozone problem persists and points the way to solving it.
Monitoring global ozone levels
Recent studies monitoring ozone and the key ozone-precursor gases in four types of
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