Ozone's Janus Face: Ground-Level Ozone Presents One of the Most Complex Air-Quality Issues Facing the Nation Today

Ozone is the Dr. Jekyll and Mr. Hyde of air pollution. Under one guise, it is beneficial; in another, it is harmful. Most of the in the atmosphere-about 90 percent-is found in a layer high above the Earth's surface in a part of the atmosphere called the stratosphere. The layer begins about 10 kilometers (six miles) above the Earth's surface and extends to about 50 kilometers. Ozone is produced naturally in the upper atmosphere through the action of high-energy solar rays on molecular oxygen. Stratospheric the ozone, absorbs most of the sun's biologically damaging ultraviolet sunlight, protecting humans, plants, and animals by allowing only a fraction to reach the planet's surface. Near the Earth's surface, where comes into direct contact with people, plants, and materials, it displays its more sinister side. Numerous studies have documented the harmful effects of on human health, crop production, forest growth, and industrial materials--for instance, the deterioration of paints, rubber, and plastics exposed to the weather. (1) Because of its destructive effects, ground-level is often referred to as bad ozone. Ozone pollution forms at ground level on warm sunny days when oxides of nitrogen ([NO.sub.x]) emitted by motor vehicles and power plants mix with volatile organic compounds (VOCs) emitted by forests, motor vehicles, and industrial plants. The sunlight acts on this soup, initiating a chemical transformation that ultimately produces or smog, as the mix is sometimes called. Weather conditions that limit atmospheric mixing and transport, such as strong inversions and low winds, exacerbate the situation by allowing to accumulate. Worst of Both Worlds Unfortunately, in the case of humanity has accomplished the seemingly impossible. We have managed to decrease the beneficial in the stratosphere while increasing the harmful at the Earth's surface. Emissions of human-produced halocarbons--chemicals containing various combinations of chlorine, fluorine, bromine, and carbon atoms--are responsible for ozone holes over the southern and northern polar regions. Meanwhile, emissions of [NO.sub.x] and VOCs result in increased exposure in urban and rural regions throughout the developed world. We have, in short, created the worst of both worlds. There is encouraging news, however, about stratospheric ozone. In 1987, the international community crafted an agreement--the Montreal Protocol on Substances that Deplete the Ozone Layer--to phase out production of ozone-depleting halocarbons, such as the family of coolants known as freons, and develop more ozone-friendly substitutes. The emission reductions called for in this agreement are already improving global halocarbon levels. Measurable improvements in levels are expected in the next 10 to 15 years, with a complete reversal of the adverse impacts anticipated over the next 50 years. This agreement marked a milestone in environmental policy and was a great success story for international cooperation. The scientific community identified a problem, determined the cause, and worked with industry and policymakers to devise a solution. To date, the Montreal Protocol represents the best example of international cooperation on global environmental issues. Lowdown on Low-level Ozone The news is not as good in the case of ground-level ozone. Despite more than three decades of effort and hundreds of billions of dollars invested in mitigation programs, is still one of the most serious air pollution problems in the United States. Ozone is one of six criteria pollutants regulated under the Clean Air Act. (2) The U.S. Environmental Protection Agency is responsible for establishing and enforcing National Ambient Air Quality Standards for these pollutants. To protect public health, the current standard sets a limit on the concentration of in the air at 120 parts per billion averaged over one hour. …