Ozone
Ozone is a gaseous form of oxygen with three atoms (O₃), differing from the more common diatomic oxygen (O₂). It is primarily found in the stratosphere, where it forms a crucial layer that protects the Earth from harmful ultraviolet radiation. While ozone plays a protective role in the upper atmosphere, it can also have detrimental effects at ground level, where it is considered a pollutant created through chemical reactions involving nitrogen oxides and volatile organic compounds. Human activities, particularly the release of substances like chlorofluorocarbons (CFCs), have led to a thinning of the ozone layer, notably over polar regions, leading to what is often referred to as the "ozone hole." International agreements, such as the Montreal Protocol, have been implemented to curb the use of ozone-depleting substances, showing some positive effects, as recent studies indicate a gradual reduction in the size of the ozone hole. Understanding the dual nature of ozone—as both beneficial in the stratosphere and harmful at ground level—highlights the importance of environmental awareness and the impact of human activities on atmospheric chemistry.
Subject Terms
Ozone
Ozone is a gaseous form of the element oxygen. It exists naturally in Earth's atmosphere, particularly the stratosphere, where it provides the planet with an important filter from ultraviolet radiation. Some human activities cause a decrease in the amount of ozone present, an effect that has been described as a hole in (or, more correctly, a "thinning" of) the ozone layer. This has been linked to climate change and other harmful impacts, and international efforts have been made to address the issue.
Overview
Ozone is a highly reactive form of oxygen. It is composed of three oxygen atoms in a molecule (O3) rather than the more usual two atoms (O2). Ozone is formed from diatomic oxygen where high energy is present. Near the Earth, ozone forms in high-temperature combustion processes, such as in automobile engines and in electrical sparks. In the stratosphere it forms because of high-energy ultraviolet radiation.
Once formed, ozone is quick to react with other molecules. Near the Earth there are many molecules with which to react, and the ozone concentration remains low. In the stratosphere, however, there are few molecules present, so the ozone concentration builds up and forms what is termed the ozone layer. Ozone also disappears naturally by decomposing to ordinary oxygen, so there is a natural limit to the concentration that accumulates, and a steady state occurs. The ozone layer is actually quite diffuse, and the ozone concentration is never very high.
Since the mid-1950s, measurements of ozone concentrations in the atmosphere have been made regularly. In the early 1970s, analysis of the measurements suggested that something was causing a reduction in the concentration of ozone in the stratosphere, particularly in the region over the South Pole. Continued measurements confirmed a similar lowering over the North Pole area and a spreading of the effect over a larger area. Laboratory experiments showed that molecular fragments containing unpaired electrons are effective in speeding the decomposition of ozone. This catalytic effect is particularly strong in the presence of small ice crystals, which are present in the stratosphere in the polar regions in winter. Much of what is known about the way the ozone layer forms and decomposes comes from the work of Paul J. Crutzen, Mario J. Molina, and F. Sherwood Rowland, who received the 1995 Nobel Prize in Chemistry for their work on this subject.
Researchers determined that various human-generated substances contribute significantly to ozone depletion. These notably include chlorofluorocarbons (CFCs), a class of chemicals that found wide use as propellants in aerosol cans, cleaning solvents for electronic circuit boards, and working fluids in air-conditioning and refrigeration. The stability of these molecules is a prime factor in their utility, but this property also allows the molecules to drift into the stratosphere when they are released. Production of CFCs was severely limited by international agreement in the 1987 Montreal Protocol and later revisions.
In 2016, researchers released a report indicating that the size of the Antarctic hole in the ozone layer was significantly smaller in September 2015 than in September 2000, and on average had decreased over that time. This suggested that, despite occasional yearly spikes, the general trend was toward a reduction in the hole's size. The "healing" was attributed to the impact of long-term removal of CFCs and other ozone-damaging chemicals from use.
“Good” vs. “Bad” Ozone
Ozone in the stratosphere protects Earth’s surface from ultraviolet solar radiation. High-energy, ultraviolet rays are absorbed by oxygen and ozone for atmospheric reactions. Oxygen molecules, which are of greater stability than ozone, require energy corresponding to a wavelength of 242 nanometers in the electromagnetic radiation spectrum to split their atoms. Triatomic ozone, which is less stable than is diatomic molecular oxygen, requires energy corresponding to a wavelength of 200–320 nanometers.
“Good” ozone can be depleted by the ozone-depleting substances, which do not undergo chemical reaction in the troposphere, but can be destroyed by ultraviolet radiation in the stratosphere. Ozone-depleting substances derive from Earth’s surface, including many created by human activity. They degrade slowly and remain in the troposphere for years, until they reach the stratosphere, where they are broken down by intense solar radiation and release chlorine and bromine molecules, which destroy the good ozone. One chlorine atom can destroy 100,000 ozone molecules.
“Bad” ozone is not emitted directly into the air, but produced in the troposphere through chemical reactions of nitrogen oxide and volatile organic compounds in the presence of sunlight. These compounds are primary air pollutants, emitted from sources that include industrial facilities, electric utilities, motor vehicles exhaust, gasoline vapors and chemical solvents, coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. In the lower atmosphere, elevated ozone levels can be dangerous to plants and animals, including humans.
Key Concepts
- greenhouse gases (GHGs): gases that trap heat in the atmosphere
- ozone hole: an area of diminished ozone in the ozone layer
- ozone layer: an atmospheric layer that contains a high (over 91 percent) concentration of ozone
- stratosphere: the layer of Earth’s atmosphere above the troposphere, situated between 10 and 50 kilometers above the planet’s surface
- troposphere: an atmospheric layer closer to Earth, where weather phenomena occur
- volatile organic compounds: substances with carbon backbones that evaporate or vaporize easily at room temperature
Bibliography
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Douglass, Anne, et al. New Results From Inside the Ozone Hole. NASA's Goddard Space Flight Center, 2015. www.nasa.gov/sites/default/files/files/Slides‗AGUbriefing‗FINAL‗to‗print.pdf. Accessed 19 Jan. 2023.
"Ozone." American Lung Association, www.lung.org/clean-air/outdoors/what-makes-air-unhealthy/ozone. Accessed 3 Dec. 2024.
Seinfeld, J. H., and S. N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. 2nd ed. Wiley, 2006.
Walker, Kira. “What Happened to the World's Ozone Hole?” BBC Future, 22 Mar. 2022, www.bbc.com/future/article/20220321-what-happened-to-the-worlds-ozone-hole. Accessed 19 Jan. 2023.
"What Is Ozone?" United States Environmental Protection Agency, 20 June 2024, www.epa.gov/ozone-pollution-and-your-patients-health/what-ozone. Accessed 3 Dec. 2024.
“WMO Ozone Bulletins." World Meteorological Organization, community.wmo.int/stratospheric-ozone-and-uv-radiation. Accessed 19 Jan. 2023.