Ozone, 1979-2008
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Ozone can refer to two forms of ozone: ground-level ozone is not released directly into the air but is a product of chemical reactions between nitrogen oxide and organic compounds in sunlight. Ozone can also refer to a gas in the Earth's stratosphere that absorbs ultraviolet (UV) radiation from the sun. Ground-level ozone is commonly known as smog, which can impair visibility and irritate lungs in high concentrations. Meanwhile, ozone in the ozone layer (a region of Earth's stratosphere) protects living organisms, including human beings, from harmful UV radiation.[1]
Background
Ozone is a colorless, inorganic gas that exists at ground level and in the Earth’s upper atmosphere. Naturally occurring ozone in the stratosphere (a layer from six to 30 miles above the Earth’s surface) shields the Earth from the sun’s ultraviolet radiation. Ground-level ozone is formed in the troposphere (the lowest layer of the Earth’s atmosphere) when nitrogen oxide and volatile organic compounds react in sunlight to produce smog, which may damage crops and vegetation. Smog can also cause health problems depending on the concentration levels of ozone, duration of exposure, the average amount of air breathed per minute, and the length of time between short-term exposures to ozone. Ground-level ozone concentrations generally peak during the afternoon, and average concentrations are typically highest during summer months.[2]
There are natural and human-caused contributors to ground-level ozone. Volatile organic compounds (VOCs) are carbon-containing reactive compounds that are emitted naturally by plants and vegetation. Natural sources of nitrogen oxides, which are contributors to ground-level ozone, include anaerobic plant respiration, lightning, and volcanic activity. Human-caused VOCs and nitrogen oxides are emitted by the burning of gasoline, coal, natural gas, and wood; they are also emitted by solvents, paints, glues, and other household products. Human-caused sources include motor vehicles, chemical and petroleum facilities, dry cleaners, electric power plants, and other industries.[2]
National ozone standards
The Clean Air Act established nationwide air quality standards for six air pollutants known as criteria pollutants: ground-level ozone, sulfur dioxide, particulate matter, nitrogen oxide, carbon monoxide, and lead. These standards, known as National Ambient Air Quality Standards (NAAQS), set ceilings for the six air pollutants, and states are required to enact implementation plans outlining enforceable, source-specific emissions limits for these pollutants. Each state plan must show that the state will meet and maintain the NAAQS.[3][4]
In setting NAAQS, the EPA sets two standards for criteria pollutants—primary and secondary standards. Primary standards limit pollution to protect human health, and secondary standards limit pollution to protect against visibility impairment and damage to animals, vegetation, and buildings. If a geographical area exceeds the NAAQS for one or more of the six criteria pollutants, the area is considered a nonattainment area by the EPA. Geographical areas with pollutant concentrations below the NAAQS are known as attainment areas.[3][5]
In 1979, the EPA set the primary and secondary ozone standards at 120 parts per billion (ppb). The standards established the maximum hourly average of ground-level ozone in a geographical area. Areas were considered in compliance with the standards if they had one day or less per calendar year of ozone concentrations greater than 120 ppb over a one-hour period. In 1997, the EPA replaced the one-hour standards with eight-hour standards and set the maximum eight-hour average of ground-level ozone at 80 ppb. The standards were lowered to 75 ppb in 2008.[6] The Supreme Court affirmed the legality of the ozone NAAQS in Whitman v. American Trucking Associations.
Ozone layer protection
Under the Clean Air Act, the federal government is required to regulate substances that deplete ozone molecules in the stratospheric ozone layer. This layer is located 12 to 19 miles above the Earth's surface and filters harmful ultraviolet (UV) rays from the sun. Similar to its authority over acid rain reduction, the EPA has sole authority over regulating ozone-depleting substances. Federal regulations classify ozone-depleting substances as either Class I or Class II depending on their potential impact on the ozone layer. The production and import of five of six Class I substances were gradually banned from 1994-1996. The last substance was banned in 2005. Class II substances have a lesser impact on the ozone layer than Class I substances; the production and use of these substances are scheduled to be banned by the year 2030.[7][8]
See also
Footnotes
- ↑ U.S. Environmental Protection Agency, "Climate change glossary, M," accessed November 26, 2014
- ↑ 2.0 2.1 World Bank Group, "Ground-Level Ozone," July 1998
- ↑ 3.0 3.1 U.S. Senate Environment and Public Works Committee, "Clean Air Act - Full Text," accessed June 11, 2017
- ↑ U.S. Environmental Protection Agency, "Regulatory Actions," accessed March 10, 2015
- ↑ Environmental Protection Agency, "Applying or Implementing Ozone Standards," accessed June 1, 2017
- ↑ Environmental Protection Agency, "Table of Historical Ozone National Ambient Air Quality Standards (NAAQS)," accessed June 11, 2017
- ↑ U.S. Environmental Protection Agency, "Ozone Layer Protection Glossary," accessed September 16, 2014
- ↑ U.S. Environmental Protection Agency, "The Phaseout of Ozone-Depleting Substances," accessed September 16, 2014
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