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    Atmosphere - Stratospheric Ozone
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Introduction

Ozone occurs naturally in the atmosphere; 10% of it occurs in the troposphere -the lowest part of the atmosphere- and the remaining 90% in the stratosphere, at an elevation of approximately 10 to 50 kilometers. The highest ozone concentration occurs between 25 and 35 km, where it forms the so-called stratospheric ozone layer. Although this layer constitutes a very small part of the atmosphere, life on the planet would not be possible without it, as it absorbs most of the incoming ultraviolet (UV) radiation from the sun, thus protecting living organisms from its harmful effects. Separately, absorbed UV radiation produces heat in the stratosphere, thus contributing to shaping the thermal structure of the atmosphere. Therefore, any disruption in the thickness of the ozone layer has potentially serious consequences for life on Earth (WMO et al., 2007).

The thickness of the stratospheric ozone layer is commonly determined by measuring the amount of ozone in a vertical column of air. This thickness is expressed in Dobson units (DU); 100 UD are equivalent to one millimeter in thickness of the ozone layer at sea level and 0 °C (WMO et al., 2007). Ozone levels fluctuate naturally from 200 to 500 DU, depending on latitude. The highest concentrations occur in middle and high latitudes; the lowest, in the tropics (between 250 and 300 DU; WMO et al., 2007).

In 1970, it was discovered that certain man-made organic chemicals containing chlorine, fluorine and bromine were destroying the ozone layer (Molina, MJ and FS Rowland, 1974; UNEP, 2002; WMO et al., 2007); a single molecule of chlorine or bromine can destroy one hundred thousand ozone molecules. These substances, known as Ozone Depleting Substances (ODS), are used in cooling appliances, air conditioning, rigid polyurethane foam, solvents, insecticides, aerosols and fire extinguishers. ODS participate in a complex series of reactions leading to ozone destruction. Examples of these substances are chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide (MBR), carbon tetrachloride (TET) and methyl chloroform (MCF) (UNEP, 2002; WMO et al., 2007).

Although ODS emissions occur in virtually all worldwide, weather conditions in the South Pole (polar clouds and wind) favor reactions that transform ODS into reactive gases that destroy ozone (UNEP, 2002; WMO et al., 2007; NASA, 2009). The thinning of the ozone layer over Antarctica has produced what is known as the "ozone hole" (less than 220 DU; WMO et al., 2007). This was first observed in the early eighties with its peak size recorded in 2000, covering about 29.4 million square kilometers; in 2008 it spanned 27 million square kilometers -an area slightly larger than North America, which covers about 25 million square kilometers (NASA, 2008). The degradation of the ozone layer increases the exposure to UV radiation, causing negative effects for human health, such as skin cancer, cataracts and immune system depression, as well as alterations in ecosystem composition and functioning (damage to crops and early stages of important species such as fish, shrimp, crabs and amphibians; alteration of biogeochemical cycles; changes in trophic structure; potential decline in marine productivity, etc.) (WMO et al., 2007).

The concern of the scientific community and governments of various countries led to the adoption of the Vienna Convention on the Protection of the Ozone Layer (1985) and the Montreal Protocol on Substances that degrade the Ozone Layer (1987), in which commitments were made to reduce ODS consumption and production (UNEP, 2009; WMO et al., 2007). Mexico signed the treaties in 1985 and 1987, respectively, ratified the Montreal Protocol in 1988 and adopted the amendments of London (1991), Copenhagen (1994) and, more recently, Montreal (2006) and Beijing (2007; UNEP, 2009). Compliance with the commitments to the Montreal Protocol involves economic costs; hence, the Multilateral Fund for the Implementation of the Montreal Protocol was created, with contributions from industrialized countries, to support developing countries (Multilateral Fund for the Implementation of the Montreal Protocol, 2009).

 

 

References

NASA. Ozone Hole Watch. 2009. Disponible en:
http://ozonewatch.gsfc.nasa.gov/index.html Fecha de consulta: 31-08-2009.

NASA. The ozone hole. 2008. Disponible en:
http://www.theozonehole.com/nasa0ct2008.htm Fecha de consulta: 31-08-2009.

Molina, M. J. y F. S. Rowland. Stratospheric sink for chlorofluoromethanes: chlorine atom-catalysed destruction of ozone.  Nature 249, 5460: 810-812. 1974.

Multilateral Fund for the Implementation of the Montreal Protocol. 2009. Disponible en http://www.multilateralfund.org/ Fecha de consulta: agosto de 2009.

UNEP. Ozone secretariat. Evolution of the Montreal Protocol. Status of Ratification. 2009. Disponible en: http://ozone.unep.org/Ratification_status/ Fecha de consulta: agosto de 2009.

UNEP. Protecting the Ozone Layer. The United Nations History. United Kingdom. 2002.

WMO, NOAA, NASA, UNEP y European Commission. Scientific Assessment of Ozone Depletion: 2006. Global Ozone Research and Monitoring Project—Report No. 50. Switzerland. 2007.