Ozone is a gas comprised of three atoms of oxygen that is found both in the Earth’s upper atmosphere and at ground level. According to the US Environmental Protection Agency, ozone can be good or bad, depending on where it is found.
According to the federal government’s AirNow.gov website, ozone occurs naturally in the Earth's upper atmosphere – 6 to 30 miles above the Earth's surface – where it forms a protective layer that shields us from the sun's harmful ultraviolet rays.
Manmade chemicals are known to destroy this beneficial ozone. An area where the protective "ozone layer" has been significantly depleted – for example, over the North or South Pole – is sometimes called "the ozone hole.” The United States, along with over 180 other countries, recognized the threats posed by ozone depletion and, in 1987, adopted a treaty called the Montreal Protocol to phase out the production and use of ozone-depleting substances. EPA has established regulations to phase out ozone-depleting chemicals in the United States.
In the Earth's lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight. Ozone at ground level is a harmful air pollutant.
Currently, for the 2015 Ozone National Ambient Air Quality Standards (NAAQS) – the most stringent ozone standard – all of Wyoming’s counties are meeting the standard and have been designated as attainment/unclassifiable by the U.S. Environmental Protection Agency (EPA).
However, this is not the case for the less-stringent 2008 Ozone NAAQS. In July 2012, the U.S. EPA designated all of Sublette county and portions of Lincoln and Sweetwater Counties as an ozone nonattainment area, collectively known as the Upper Green River Basin Ozone Nonattainment Designation Area (UGRB). Monitored ozone in the UGRB was in attainment for the 2008 Ozone NAAQS by the attainment date of July 2015. The UGRB remains classified as a nonattainment area. The Wyoming Department of Environmental Quality is assessing a pathway for submitting a request to the EPA to redesignate the UGRB back to attainment for the 2008 Ozone NAAQS.