Ozone Dangers

The potential dangers of uncontrolled climate change were joined by yet another possible human-induced problem, high surface ozone concentrations in urban areas and the reduction of stratospheric ozone concentrations. Ozone is a naturally occurring unstable gas that makes up 0.000007 percent of the lower atmosphere (or 0.07 part per million [ppm]). It is also created by photochemical reactions when nitrogen oxides produced by motor vehicle exhaust and a variety of hydrocarbons from both human and natural sources react in the presence of sunlight. Combined with other photochemical compounds, ozone composes the hazy, eye-stinging, breathing-impairing air known as smog. In thick smog, ozone concentrations may be 10 times normal levels, causing a variety of respiratory problems.


In the troposphere, ozone is a pollutant. It is harmful to

In the stratosphere, ozone is helpful. It absorbs the Sun's

plants and to the lungs of animals and people.


ultraviolet rays that can damage plants and cause skin cancers.

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(Atmosphere not to scale)

Surface ozone became the lesser of the two ozone problems. Early Atmospheric ozone may be in the decade, the discussion around ozone depletion centered on the good or bad, depending on effect of supersonic transports (SSTs) on the stratosphere. At the time, where lt isfound.

SSTs were thought to be the transportation mode of the immediate future. Flying higher and faster than normal jet aircraft, they would whisk people across continents and oceans in just a few hours. The U.S. Federal Aviation Administration estimated that by 1985 there would be 500 SSTs, each spending seven hours per day cruising through the stratosphere. The problem: Scientists estimated that the water vapor in the SSTs' contrails would reduce stratospheric ozone by 3.8 percent. Although this estimate was too high, further research showed that other exhaust chemicals would halve stratospheric ozone—ozone that screened out cancer-causing ultraviolet radiation. When the SST proved to be uneconomical and only a few were built, the concern over ozone depletion temporarily faded away.

In the early 1970s, the University of California, Irvine, scientists F. Sherwood Rowland (1927- ) and Mario Molina (1943- ) became intrigued by the behavior of chlorofluorocarbon (CFC) gases. These human-made inert (that is, nonreactive) gases were used as propellants in spray cans and as coolants in air-conditioning systems and refrigerators. Using laboratory experiments, Rowland and Molina showed that when CFCs combined with solar radiation and decomposed, they released chlorine atoms and chlorine monoxide, which destroyed large numbers of ozone molecules. In their 1974 scientific article in Nature, they hypothesized that CFCs released from millions of spray cans eventually found their way to the stratosphere and started destroying the ozone.

As more scientists studied ozone depletion, they determined that even if CFC emissions remained constant starting in 1974, ozone destruc-



a. CFCs are released and rise into the atmosphere b. UV releases Cl from CFCs c. Cl destroys the ozone, allowing more UV to enter the atmosphere d. UV causes skin cancer, making it more dangerous to stay outdoors a. CFCs are released and rise into the atmosphere b. UV releases Cl from CFCs

© Infobase Publishing c. Cl destroys the ozone, allowing more UV to enter the atmosphere d. UV causes skin cancer, making it more dangerous to stay outdoors

Chlorofluorocarbons (CFCs) released on Earth's surface enter the stratosphere, where they attack ozone.

tion would reach a maximum in 1990 and remain high for decades. Additional studies indicated that a 10 percent annual increase in the refrigerant Freon could lead to a 16 percent reduction in stratospheric ozone by 2000. As research continued, additional ozone-destroying chemicals were added to the list. Alarmist warnings that people were in imminent danger from spray cans led some to doubt that there was any problem at all.

The debate was settled, at least in scientific circles, with the publication of the National Research Council's (NRC's) report on stratospheric ozone. The NRC reported that continued release of CFCs into the atmosphere could lead to drastic climate changes (by enhancing the role of carbon dioxide in the greenhouse effect) as well as the previously discussed problem of allowing too much ultraviolet radiation to reach Earth's surface. The report recommended banning

CFCs from spray cans after January 1, 1978, and restricting their use in automobile air conditioners and industrial refrigerating units a few years later.

The United States did ban CFC propellants in 1978, but production increased outside the United States and threatened to cut the amount The ozone hole over Antarctica of stratospheric ozone in half by 2000. Convincing other nations to grew significantly between reduce CFC use was difficult. Scientists had no direct measurements of 1980 and 1991.


Ozone, October 1980 Ozone, October 1991

Ozone, October 1980 Ozone, October 1991


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1980 1985 1990 1995 2000 2005

© Infobase Publishing Year

1980 1985 1990 1995 2000 2005

stratospheric ozone—they were using laboratory results and computer models—and industries dependent upon CFCs resisted a change that could potentially cost them hundreds of millions of dollars. In the next decade, observations of ozone depletion would exert pressure on the international community to curb CFC use.

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