Ozone layer

Ozone-oxygen cycle in the ozone layer.

The ozone layer or ozone shield refers to a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet (UV) radiation. It contains high concentrations of ozone (O3) relative to other parts of the atmosphere, although still very small relative to other gases in the stratosphere. The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth's atmosphere as a whole is only about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 20 to 30 kilometres (12 to 19 mi) above Earth, though the thickness varies seasonally and geographically.[1]

The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. Measurements of the sun, showed that the radiation sent out from its surface, and which reaches the ground on Earth, is generally consistent with the spectrum of a black body with a temperature in the range of 5,500–6,000 K (5,227 to 5,727°C), except that there was no radiation below a wavelength of about 310 nm at the ultraviolet end of the spectrum. It was deduced that the missing radiation was being absorbed by something in the atmosphere. Eventually the spectrum of the missing radiation was matched to one known chemical, Ozone.[2] Its properties were explored in detail by the British meteorologist G. M. B. Dobson, who developed a simple spectrophotometer (the 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 97–99% 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]

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 from the planet's surface.[4]

Sources

The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sydney Chapman in 1930. Ozone in the Earth's stratosphere is created by ultraviolet light striking ordinary oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O2 to create ozone, O3. The ozone molecule is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an individual atom of oxygen, a continuing process called the ozone-oxygen cycle. Chemically, this can be described as:

O2 + ℎνuv → 2O
O + O2 ↔ O3

About 90% of the ozone in our 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 (1⁄8 inch) thick.[5]

Ultraviolet light

UV-B energy levels at several altitudes. Blue line shows DNA sensitivity. Red line shows surface energy level with 10% decrease in ozone
Levels of ozone at various altitudes and blocking of different bands of ultraviolet radiation. Essentially all UVC (100–280 nm) is blocked by dioxygen (from 100–200 nm) or else by ozone (200–280 nm) in the atmosphere. The shorter portion of the UV-C band and the more energetic UV above this band causes the formation of the ozone layer, when single oxygen atoms produced by UV photolysis of dioxygen (below 240 nm) react with more dioxygen. The ozone layer also blocks most, but not quite all, of the sunburn-producing UV-B (280–315 nm) band, which lies in the wavelengths longer than UV-C. The band of UV closest to visible light, 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-B (315–280 nm), and 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 sunburn; excessive exposure can also cause cataracts, immune system suppression, and genetic damage, resulting in problems such as skin cancer. The ozone layer (which absorbs from about 200 nm to 310 nm with a maximal absorption at about 250 nm)[6] is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface. Nevertheless, some UV-B, particularly at its longest wavelengths, reaches the surface, and is important for the skin's production of vitamin D.

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.[7]

Distribution in the stratosphere

The thickness of the ozone layer—that is, the total amount of ozone in a column overhead—varies by a large factor worldwide, being in general smaller near the equator and larger towards the poles. It also varies with season, being in general thicker during the spring and thinner during the autumn. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity.[8]

Since stratospheric ozone is produced by solar UV radiation, one might expect to find the highest ozone levels over the tropics and the lowest over polar regions. The same argument would lead one to expect the highest ozone levels in the summer and the lowest in the winter. The observed behavior is very different: most of the ozone is found in the mid-to-high latitudes of the northern and southern hemispheres, and the highest levels are found in the spring, not summer, and the lowest in the autumn, not winter in the northern hemisphere. During winter, the ozone layer actually increases in depth. This puzzle is explained by the prevailing stratospheric wind patterns, known as the Brewer-Dobson circulation. While most of the ozone is indeed created over the tropics, the stratospheric circulation then transports it poleward and downward to the lower stratosphere of the high latitudes.[8] However, owing to the ozone hole phenomenon, the lowest amounts of column ozone found anywhere in the world are over the Antarctic in the southern spring period of September and October and to a lesser extent over the Arctic in the northern spring period of March, April, and May.

Brewer-Dobson circulation in the ozone layer.

The ozone layer is higher in altitude in the tropics, and lower in altitude outside the tropics, especially in the polar regions. This altitude variation of ozone results from the slow circulation that lifts the ozone-poor air out of the troposphere into the stratosphere. As this air slowly rises in the tropics, ozone is produced as the sun overhead photolyzes oxygen molecules. As this slow circulation levels off and flows towards the mid-latitudes, it carries the ozone-rich air from the tropical middle stratosphere to the mid-and-high latitudes lower stratosphere. The high ozone concentrations at high latitudes are due to the accumulation of ozone at lower altitudes.[8]

The Brewer-Dobson circulation moves very slowly. The time needed to lift an air parcel by 1 km in the lower tropical stratosphere is about 2 months (18 m per day). However, horizontal poleward transport in the lower stratosphere is much faster and amounts to approximately 100 km per day in the northern hemisphere whilst it is only half as much in the southern hemisphere (~51 km per day).[9] Even though ozone in the lower tropical stratosphere is produced at a very slow rate, the lifting circulation is so slow that ozone can build up to relatively high levels by the time it reaches 26 kilometres (16 mi).[8]

Ozone amounts over the continental United States (25°N to 49°N) are highest in the northern spring (April and May). These ozone amounts fall over the course of the summer to their lowest amounts in October, and then rise again over the course of the winter.[10] Again, wind transport of ozone is principally responsible for the seasonal changes of these higher latitude ozone patterns.[8]

The total column amount of ozone generally increases as we move from the tropics to higher latitudes in both hemispheres. However, the overall column amounts are greater in the northern hemisphere high latitudes than in the southern hemisphere high latitudes. In addition, while the highest amounts of column ozone over the Arctic occur in the northern spring (March–April), the opposite is true over the Antarctic, where the lowest amounts of column ozone occur in the southern spring (September–October).[8]

Depletion

Main article: Ozone depletion
NASA projections of stratospheric ozone concentrations if chlorofluorocarbons had not been banned.

The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), nitrous oxide (N2O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). While there are natural sources for all of these species, the concentrations of chlorine and bromine increased markedly in recent decades due to the release of large quantities of man-made organohalogen compounds, especially chlorofluorocarbons (CFCs) and bromofluorocarbons.[11] 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.[12]

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% since the late 1970s. For approximately 5% of the Earth's surface, around the north and south poles, much larger seasonal declines have been seen, and are described as "ozone holes".[10] The discovery of the annual depletion of ozone above the Antarctic was first announced by Joe Farman, Brian Gardiner and Jonathan Shanklin, in a paper which appeared in Nature on May 16, 1985.[13]

Regulation

To support successful regulation attempts, 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". The specific metaphors used in the discussion (ozone shield, ozone hole) proved quite useful[14] and, compared to global climate change, was much more seen as an "hot issue" and imminent risk.[15] Lay people were cautious about a depletion of the ozone layer and the risks of skin cancer.

In 1978, the United States, Canada and Norway enacted bans on CFC-containing 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.[16] Since that time, the treaty was amended to ban CFC production after 1995 in the developed countries, and later in developing countries.[17] 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.[18]

On August 2, 2003, scientists announced that the global depletion of the ozone layer may be slowing down due to 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 down significantly during the previous decade. Some breakdown can be expected to continue due to ODSs used by nations which have not banned them, and due to 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.[10]

Compounds containing C–H bonds (such as hydrochlorofluorocarbons, or HCFCs) have been designed to replace CFCs in certain applications. These replacement compounds are more reactive and less likely to survive long enough in the atmosphere to reach the stratosphere where they could affect the ozone layer. While being less damaging than CFCs, HCFCs can have a negative impact on the ozone layer, so they are also being phased out.[19] These in turn are being replaced by hydrofluorocarbons (HFCs) and other compounds that do not destroy stratospheric ozone at all.

See also

References

  1. ↑ "Science: Ozone Basics". Retrieved 2007-01-29.
  2. ↑ http://www.tandfonline.com/doi/pdf/10.3137/ao.460101
  3. ↑ "Ozone layer". Retrieved 2007-09-23.
  4. ↑ SPACE.com staff (October 11, 2011). "Scientists discover Ozone Layer on Venus". SPACE.com. Purch. Retrieved October 3, 2015.
  5. ↑ "NASA Facts Archive". Retrieved 2011-06-09.
  6. ↑ Matsumi, Y.; Kawasaki, M. (2003). "Photolysis of Atmospheric Ozone in the Ultraviolet Region" (PDF). Chem. Rev. 103 (12): 4767–4781. doi:10.1021/cr0205255. PMID 14664632. Archived from the original (PDF) on June 17, 2012. Retrieved March 14, 2015.
  7. ↑ Narayanan, D.L.; Saladi, R.N.; Fox, J.L. (2010). "Review: Ultraviolet radiation and skin cancer". International Journal of Dermatology 49 (9): 978–986. doi:10.1111/j.1365-4632.2010.04474.x. PMID 20883261.
  8. 1 2 3 4 5 6 Shagoon Tabin (2008). Global Warming: The Effect Of Ozone Depletion. APH Publishing. p. 194. Retrieved 12 January 2016.
  9. ↑ Flury, T.; Wu, D.L.; Read, W.G. (2013). "Variability in the speed of the Brewer–Dobson circulation as observed by Aura/MLS". Atmos. Chem. Phys. 13 (9): 4563–4575. doi:10.5194/acp-13-4563-2013.
  10. 1 2 3 "Stratospheric Ozone and Surface Ultraviolet Radiation". Scientific Assessment of Ozone Depletion: 2010 (PDF). WMO. 2011. Retrieved March 14, 2015.
  11. ↑ "Halocarbons and Other Gases". Emissions of Greenhouse Gases in the United States 1996. Energy Information Administration. 1997. Retrieved 2008-06-24.
  12. ↑ "NOAA Study Shows Nitrous Oxide Now Top Ozone-Depleting Emission". NOAA. 2009-08-27. Retrieved 2011-11-08.
  13. ↑ Farman, J. C.; Gardiner, B. G.; Shanklin, J. D. (1985). "Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction". Nature 315 (6016): 207–210. Bibcode:1985Natur.315..207F. doi:10.1038/315207a0.
  14. ↑ Ungar, Sheldon (2000). "Knowledge, ignorance and the popular culture: climate change versus the ozone hole". Public Understanding of Science 9 (3): 297–312. doi:10.1088/0963-6625/9/3/306. Retrieved March 14, 2015.
  15. ↑ Grundmann, Reiner (2007). "Climate Change and Knowledge Politics" (PDF). Environmental Politics 16 (3): 414–432. doi:10.1080/09644010701251656. Retrieved March 14, 2015.
  16. ↑ Morrisette, Peter M. (1989). "The Evolution of Policy Responses to Stratospheric Ozone Depletion". Natural Resources Journal 29: 793–820. Retrieved 2010-04-20.
  17. ↑ "Amendments to the Montreal Protocol". EPA. 2010-08-19. Retrieved 2011-03-28.
  18. ↑ "Brief Questions and Answers on Ozone Depletion". EPA. 2006-06-28. Retrieved 2011-11-08.
  19. ↑ "Ozone Depletion Glossary". EPA. Retrieved 2008-09-03.

Further reading

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