Ozone layer

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Ozone-oxygen cycle
in the ozone layer

The ozone layer or ozone shield is a region of

parts per million of ozone, while the average ozone concentration in Earth's atmosphere as a whole is about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 15 to 35 kilometers (9 to 22 mi) above Earth, although its thickness varies seasonally and geographically.[1]

The ozone layer was discovered in 1913 by French physicists

Dobsonmeter) that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958, Dobson established a worldwide network of ozone monitoring stations, which continue to operate to this day. The "Dobson unit", a convenient measure of the amount
of ozone overhead, is named in his honor.

The ozone layer absorbs 97 to 99 percent of the Sun's medium-frequency ultraviolet light (from about 200 

nm to 315 nm wavelength), which otherwise would potentially damage exposed life forms near the surface.[3]

In 1985, atmospheric research revealed that the ozone layer was being depleted by chemicals released by industry, mainly

chlorofluorocarbons (CFCs). Concerns that increased UV radiation due to ozone depletion threatened life on Earth, including increased skin cancer in humans and other ecological problems,[4] led to bans on the chemicals, and the latest evidence is that ozone depletion has slowed or stopped. The United Nations General Assembly has designated September 16 as the International Day for the Preservation of the Ozone Layer
.

Venus also has a thin ozone layer at an altitude of 100 kilometers above the planet's surface.[5]

Sources

The

ozone-oxygen cycle
. Chemically, this can be described as:

About 90 percent of the ozone in the atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 kilometres (66,000 and 131,000 ft), where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only 3 millimetres (18 inch) thick.[6]

Ultraviolet light

UV-B energy levels at several altitudes. Blue line shows DNA sensitivity. Red line shows surface energy level with 10 percent decrease in ozone
UV-A
(315–400 nm), is hardly affected by ozone, and most of it reaches the ground. UV-A does not primarily cause skin reddening, but there is evidence that it causes long-term skin damage.

Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation coming from the Sun. Extremely short or vacuum UV (10–100 nm) is screened out by nitrogen. UV radiation capable of penetrating nitrogen is divided into three categories, based on its wavelength; these are referred to as UV-A (400–315 nm),

UV-C
(280–100 nm).

UV-C, which is very harmful to all living things, is entirely screened out by a combination of dioxygen (< 200 nm) and ozone (> about 200 nm) by around 35 kilometres (115,000 ft) altitude. UV-B radiation can be harmful to the skin and is the main cause of

mammals
.

Ozone is transparent to most UV-A, so most of this longer-wavelength UV radiation reaches the surface, and it constitutes most of the UV reaching the Earth. This type of UV radiation is significantly less harmful to DNA, although it may still potentially cause physical damage, premature aging of the skin, indirect genetic damage, and skin cancer.[8]

Distribution in the stratosphere

The thickness of the ozone layer varies worldwide and is generally thinner near the equator and thicker near the poles.[9] Thickness refers to how much ozone is in a column over a given area and varies from season to season. The reasons for these variations are due to atmospheric circulation patterns and solar intensity.[10]

The majority of ozone is produced over the

photolyzes oxygen molecules and turns them into ozone. Then, the ozone-rich air is carried to higher latitudes and drops into lower layers of the atmosphere.[9]

Research has found that the ozone levels in the United States are highest in the spring months of April and May and lowest in October. While the total amount of ozone increases moving from the tropics to higher latitudes, the concentrations are greater in high northern latitudes than in high southern latitudes, with spring ozone columns in high northern latitudes occasionally exceeding 600 DU and averaging 450 DU whereas 400 DU constituted a usual maximum in the Antarctic before anthropogenic ozone depletion. This difference occurred naturally because of the weaker polar vortex and stronger Brewer–Dobson circulation in the northern hemisphere owing to that hemisphere’s large mountain ranges and greater contrasts between land and ocean temperatures.

ozone hole phenomenon.[9]
The highest amounts of ozone are found over the Arctic during the spring months of March and April, but the Antarctic has the lowest amounts of ozone during the summer months of September and October,

Brewer–Dobson circulation in the ozone layer

Depletion

NASA projections of stratospheric ozone concentrations if chlorofluorocarbons had not been banned

The ozone layer can be depleted by

organohalogen compounds, especially chlorofluorocarbons (CFCs) and bromofluorocarbons.[12] These highly stable compounds are capable of surviving the rise to the stratosphere, where Cl and Br radicals are liberated by the action of ultraviolet light. Each radical is then free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. By 2009, nitrous oxide was the largest ozone-depleting substance (ODS) emitted through human activities.[13]

The breakdown of ozone in the stratosphere results in reduced absorption of ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet radiation is able to reach the Earth's surface at a higher intensity. Ozone levels have dropped by a worldwide average of about 4 percent since the late 1970s. For approximately 5 percent of the Earth's surface, around the north and south poles, much larger seasonal declines have been seen, and are described as "ozone holes". "Ozone holes" are actually patches in the ozone layer in which the ozone is thinner. The thinnest parts of the ozone are at the

Jonathan Shanklin, in a paper which appeared in Nature
on May 16, 1985.

Regulation attempts have included but not have been limited to the Clean Air Act implemented by the United States Environmental Protection Agency. The Clean Air Act introduced the requirement of National Ambient Air Quality Standards (NAAQS) with ozone pollutions being one of six criteria pollutants. This regulation has proven to be effective since counties, cities and tribal regions must abide by these standards and the EPA also provides assistance for each region to regulate contaminants.[15] Effective presentation of information has also proven to be important in order to educate the general population of the existence and regulation of ozone depletion and contaminants. A scientific paper was written by Sheldon Ungar in which the author explores and studies how information about the depletion of the ozone, climate change and various related topics. The ozone case was communicated to lay persons "with easy-to-understand bridging metaphors derived from the popular culture" and related to "immediate risks with everyday relevance".[16] The specific metaphors used in the discussion (ozone shield, ozone hole) proved quite useful and, compared to global climate change, the ozone case was much more seen as a "hot issue" and imminent risk. Lay people were cautious about a depletion of the ozone layer and the risks of skin cancer.

"Bad" ozone can cause adverse health risks respiratory effects (difficulty breathing) and is proven to be an aggravator of respiratory illnesses such as asthma, COPD and emphysema.[17] That is why many countries have set in place regulations to improve "good" ozone and prevent the increase of "bad" ozone in urban or residential areas. In terms of ozone protection (the preservation of "good" ozone) the European Union has strict guidelines on what products are allowed to be bought, distributed or used in specific areas.[18] With effective regulation, the ozone is expected to heal over time.[19]

Levels of atmospheric ozone measured by satellite show clear seasonal variations and appear to verify their decline over time.

In 1978, the United States, Canada and

aerosol sprays that damage the ozone layer. The European Community rejected an analogous proposal to do the same. In the U.S., chlorofluorocarbons continued to be used in other applications, such as refrigeration and industrial cleaning, until after the discovery of the Antarctic ozone hole in 1985. After negotiation of an international treaty (the Montreal Protocol), CFC production was capped at 1986 levels with commitments to long-term reductions.[20] This allowed for a ten-year phase-in for developing countries[21] (identified in Article 5 of the protocol). Since that time, the treaty was amended to ban CFC production after 1995 in the developed countries, and later in developing countries.[22] Today, all of the world's 197 countries have signed the treaty. Beginning January 1, 1996, only recycled and stockpiled CFCs were available for use in developed countries like the US. This production phaseout was possible because of efforts to ensure that there would be substitute chemicals and technologies for all ODS uses.[23]

On August 2, 2003, scientists announced that the global depletion of the ozone layer may be slowing down because of the international regulation of ozone-depleting substances. In a study organized by the American Geophysical Union, three satellites and three ground stations confirmed that the upper-atmosphere ozone-depletion rate slowed significantly during the previous decade. Some breakdown can be expected to continue because of ODSs used by nations which have not banned them, and because of gases which are already in the stratosphere. Some ODSs, including CFCs, have very long atmospheric lifetimes, ranging from 50 to over 100 years. It has been estimated that the ozone layer will recover to 1980 levels near the middle of the 21st century.[24] A gradual trend toward "healing" was reported in 2016.[25]

Compounds containing

hydrofluorocarbons
(HFCs) and other compounds that do not destroy stratospheric ozone at all.

The residual effects of CFCs accumulating within the atmosphere lead to a concentration gradient between the atmosphere and the ocean. This organohalogen compound is able to dissolve into the ocean's surface waters and is able to act as a time-dependent tracer. This tracer helps scientists study ocean circulation by tracing biological, physical and chemical pathways.[27]

Implications for astronomy

As ozone in the atmosphere prevents most energetic ultraviolet radiation reaching the surface of the Earth, astronomical data in these wavelengths have to be gathered from satellites orbiting above the atmosphere and ozone layer. Most of the light from young hot stars is in the ultraviolet and so study of these wavelengths is important for studying the origins of galaxies. The Galaxy Evolution Explorer, GALEX, is an orbiting ultraviolet space telescope launched on April 28, 2003, which operated until early 2012.[28]

  • This GALEX image of the Cygnus Loop nebula could not have been taken from the surface of the Earth because the ozone layer blocks the ultra-violet radiation emitted by the nebula.
    This GALEX image of the Cygnus Loop nebula could not have been taken from the surface of the Earth because the ozone layer blocks the ultra-violet radiation emitted by the nebula.

See also

References

  1. ^ "Ozone Basics". NOAA. March 20, 2008. Archived from the original on November 21, 2017. Retrieved January 29, 2007.
  2. S2CID 128994884
    .
  3. ^ "Ozone layer". Retrieved September 23, 2007.
  4. ^ An Interview with Lee Thomas, EPA's 6th Administrator. Video, Transcript (see p13). April 19, 2012.
  5. ^ SPACE.com staff (October 11, 2011). "Scientists discover Ozone Layer on Venus". SPACE.com. Purch. Retrieved October 3, 2015.
  6. ^ "NASA Facts Archive". Retrieved June 9, 2011.
  7. PMID 14664632. Archived from the original
    (PDF) on June 17, 2012. Retrieved March 14, 2015.
  8. .
  9. ^ . Retrieved January 12, 2016.
  10. ^ "Nasa Ozone Watch: Ozone facts". ozonewatch.gsfc.nasa.gov. Retrieved September 16, 2021.
  11. .
  12. ^ "Halocarbons and Other Gases". Emissions of Greenhouse Gases in the United States 1996. Energy Information Administration. 1997. Archived from the original on June 29, 2008. Retrieved June 24, 2008.
  13. ^ "NOAA Study Shows Nitrous Oxide Now Top Ozone-Depleting Emission". NOAA. August 27, 2009. Retrieved November 8, 2011.
  14. ^ "ozone layer | National Geographic Society". education.nationalgeographic.org. Retrieved May 30, 2022.
  15. ^ US EPA, OAR (December 14, 2016). "Ozone Implementation Regulatory Actions". www.epa.gov. Retrieved May 30, 2022.
  16. S2CID 7089937
    .
  17. .
  18. ^ "Ozone Regulation". ec.europa.eu. Retrieved May 30, 2022.
  19. ^ US EPA, OAR (July 15, 2015). "International Treaties and Cooperation about the Protection of the Stratospheric Ozone Layer". www.epa.gov. Retrieved May 30, 2022.
  20. ^ Morrisette, Peter M. (1989). "The Evolution of Policy Responses to Stratospheric Ozone Depletion". Natural Resources Journal. 29: 793–820. Retrieved April 20, 2010.
  21. ^ An Interview with Lee Thomas, EPA's 6th Administrator. Video, Transcript (see p15). April 19, 2012.
  22. ^ "Amendments to the Montreal Protocol". EPA. August 19, 2010. Retrieved March 28, 2011.
  23. ^ "Brief Questions and Answers on Ozone Depletion". EPA. June 28, 2006. Retrieved November 8, 2011.
  24. ^ "Stratospheric Ozone and Surface Ultraviolet Radiation" (PDF). Scientific Assessment of Ozone Depletion: 2010. WMO. 2011. Retrieved March 14, 2015.
  25. PMID 27365314
    .
  26. ^ "Ozone Depletion Glossary". EPA. Retrieved September 3, 2008.
  27. PMID 21329203. Archived from the original
    (PDF) on February 10, 2015.
  28. ^ "ozone layer". National Geographic Society. May 9, 2011. Retrieved September 16, 2021.

Further reading

Science
Policy

External links