Ozone layer

The ozone layer is a layer in Earth's stratosphere that absorbs most of the Sun's UV radiation. It contains relatively high concentrations of ozone (O₃), although it is still very small with regard to ordinary oxygen, and is less than ten parts per million, the average ozone concentration in Earth's atmosphere being only about 0.6 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.

The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. 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 columnar density 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 on Earth.

Ozone-oxygen cycle in the ozone layer.

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 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 also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O₂ and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle. Chemically, this can be described as:

O₂ + ℎν_uv → 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 130,000 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 millimeters thick.

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 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 would be 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 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) 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.

Ozone is transparent to most UV-A, so most of this type of longer wavelength 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, indirect genetic damage, and skin cancer (see ultraviolet for more about UV-A).

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 in the northern hemisphere. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity.

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. 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 in the extratropics, 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 by the overhead sun which photolyzes oxygen molecules. As this slow circulation bends 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.

The Brewer-Dobson circulation moves very slowly. The time needed to lift an air parcel from the tropical tropopause near 16 to 20 kilometres (9.9 to 12 mi) is about 4–5 months (about 30 feet (9.1 m) per day). 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).

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. Again, wind transport of ozone is principally responsible for the seasonal evolution of these higher latitude ozone patterns.

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).

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 have increased markedly in recent years due to the release of large quantities of man-made organohalogen compounds, especially chlorofluorocarbons (CFCs) and bromofluorocarbons. 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. The breakdown of ozone in the stratosphere results in a reduction of the absorption of ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet radiation is able to reach the Earth’s surface. Ozone levels over the northern hemisphere have been dropping by 4% per decade. Over 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.

In 2009, nitrous oxide (N₂O) was the largest ozone-depleting substance (ODS) emitted through human activities.

Regulation

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 sharply limited beginning in 1987 and phased out completely by 1996.^{[citation needed]} Since that time, the treaty has been amended to ban CFC production after 1995 in the developed countries, and later in developing countries. 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.

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. Three satellites and three ground stations confirmed that the upper atmosphere ozone depletion rate has slowed down significantly during the past decade. The study was organized by the American Geophysical Union. 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.

Compounds containing C–H bonds (such as hydrochlorofluorocarbons, or HCFCs) have been designed to replace the function of CFCs. 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.

See also

• OzonAction Programme

Further reading

Science

• Mario Molina, and F. Sherwood Rowland. "Stratospheric Sink for Chlorofluoromethanes: Chlorine Atomic Catalyzed Destruction of Ozone." Nature 249(28 June 1974): 810-12
• Sei John H.; Pandis, Spyros N. (1998). Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. John Wiley and Sons, Inc. ISBN 0-471-17816-0.
• WMO (World Meteorological Organization) et al., Scientific Assessment of Stratospheric Ozone: 2010 (Geneva: WMO, 2011).
• WMO et al. 2007. Scientific Assessment of Ozone Depletion: 2006. Geneva: WMO, 2007.
• WMO et al. 1991. Scientific Assessment of Stratospheric Ozone: 1991. Geneva: WMO, 1991.
• UNEP (United Nations Environment Programme). 2010. Environmental Effects of Ozone Depletion and its Interactions with Climate Change: 2010 Assessment. Nairobi: UNEP.
• Velders, Guus J.M., Stephen O. Andersen, John S. Daniel, David W. Fahey and Mack McFarland. 2007. “The Importance of the Montreal Protocol in Protecting Climate.” Proceedings of the National Academy of Sciences of the United States of America, 104(12):4814-4819.

Policy

• Andersen, S. O. and K. M. Sarma. (2002). Protecting the Ozone Layer: the United Nations History, Earthscan Press. London.
• Anderson, S., K. Madhavea Sarma, and K. Taddonio. 2007. Technology Transfer for the Ozone

Layer: Lessons for Climate Change. London: Earthscan, 2007.

• Benedick, Richard Elliot; World Wildlife Fund (U.S.); Institute for the Study of Diplomacy. Georgetown University. (1998). Ozone Diplomacy: New Directions in Safeguarding the Planet (2nd ed.). Harvard University Press. ISBN 978-0-674-65003-9.  (Ambassador Benedick was the Chief U.S. Negotiator at the meetings that resulted in the Montreal Protocol.)
• Chasek, Pam, David L. Downie, and J.W. Brown (2013). Global Environmental Politics, 6th Edition, Boulder: Westview Press.
• Dotto, Lydia; Schiff, Harold (1978). The Ozone War. New York: Doubleday. ISBN 0385129270. 
• David L. Downie (2013) "Stratospheric Ozone Depletion". The Routledge Handbook of Global Environmental Politics. New York: Routledge.
• Downie, David L. (2011). "Ch. 16: The Vienna Convention Montreal Protocol, and Global Policy to Protect Stratospheric Ozone". In Wexler, Philip. Chemicals, Environment, Health: A Global Management Perspective. CRC Press. pp. 243–260. ISBN 978-1-4200-8469-6. 
• Downie, David L. (1999). "The Power to Destroy: Understanding Stratospheric Ozone Politics as a Common Pool Resource Problem". In Barkin, J. Samuel; Shambaugh, IV, George E. Anarchy and the Environment: The International Relations of Common Pool Resources. SUNY Press. pp. 97–121. ISBN 978-0-7914-4183-1. 
• David L. Downie, “Road Map or False Trail: Evaluating the Precedence of the Ozone Regime as Model and Strategy for Global Climate Change.” International Environmental Affairs, 7(4):321-345 (Fall 1995).
• David L. Downie,“UNEP and the Montreal Protocol: New Roles for International Organizations in Regime Creation and Change.” In Robert Bartlett, Priya Kurian and Madhu Malik, eds., International Organizations and Environmental Policy. Westport, CT: Greenwood Press, 1995.
• Downie, David L. (December 1993). "Comparative Public Policy of Ozone Layer Protection". Political Science 45 (2): 186–197. doi:10.1177/003231879304500203. ISSN 0032-3187. 
• Dray, Philip; Cagin, Seth (1993). Between earth and sky: how CFCs changed our world and endangered the ozone layer. New York: Pantheon Books. ISBN 0-679-42052-5. 
• Grundmann, Reiner (2001). Transnational Environmental Policy: Reconstructing Ozone. Psychology Press. ISBN 978-0-415-22423-9. 
• Litfin, Karen (1994). Ozone discourses: Science and politics in global environmental cooperation. New York: Columbia University Press. ISBN 0-231-08137-5. 
• Morissette P. M. (Summer 1989). "The evolution of policy responses to stratospheric ozone depletion" (PDF). Natural Resources Journal 29 (3): 793–820. 
• Parson, Edward (2003). Protecting the Ozone Layer: Science and Strategy. Oxford: Oxford University Press.

External links

• Stratospheric ozone: an electronic textbook
• http://www.unep.org/ozone/Public_Information/4Aii_PublicInfo_Facts_OzoneLayer.asp
• NASA. Studying Earth's Environment From Space. June 2000. (accessed November 3, 2010) http://www.ccpo.odu.edu/~lizsmith/SEES/index.html.
• The MACC stratospheric ozone service delivers maps, datasets and validation reports about the past and current state of the ozone layer.