Ozone layer - Definition


Ozone layer - Definition

The ozone layer is that part of the Earth's stratosphere which contains ozone. The total quantity of ozone in the ozone layer is not very large; if just the ozone were compressed to the pressure of the air at sea level, it would be only a few millimeters thick.

Ozone is notable for its ability to absorb certain wavelengths of ultraviolet radiation:

"The stratospheric ozone layer is important because, among other things, it reduces the amount of solar ultraviolet-B radiation reaching the Earth's surface. UV-B is the main cause of basal and squamous cell skin cancers, but not of malignant melanoma; the latter is primarily caused by UV-A-- which is not absorbed by ozone." [1] (http://www.sepp.org/ozone/natsci_ozone.html)

Ozone in the earth's atmosphere is generally 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 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, thus creating an ozone layer 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 as one moves towards the poles. It also varies with season, being in general thicker during the spring and thinner during the fall. The reasons for this latitude and seasonal dependence are complicated, involving stratospheric circulation patterns as well as solar intensity.

The ozone layer can be destroyed by the presence of free radical catalysts, including nitric oxide (NO), hydroxyl (OH), and atomic chlorine and bromine, leading to ozone depletion. While there are natural sources for all of these species, the concentrations of Cl and Br have increased markedly in recent years due to the release of large quantities of manmade organohalogen compounds, especially chlorofluorocarbons (CFCs). These highly stable compounds find their way to the stratosphere, where Cl and Br atoms are liberated by the action of ultraviolet light on them. Each chlorine atom is capable of breaking down approximately one hundred thousand ozone molecules during the time that it resides in the stratosphere, and bromine is even more efficient. Ozone levels, over the northern hemisphere, have been dropping by ~4% per year. Over approximately ~4.6% of the Earth's surface much larger declines have been seen; these are the ozone holes.

On January 23, 1978 Sweden became the first nation to ban aerosol sprays that are thought to damage the ozone layer.

On August 2, 2003, scientists announced that the depletion of the ozone layer may be slowing down due to an international ban on chlorofluorocarbons. [2] (http://news.independent.co.uk/world/environment/story.jsp?story=429802) 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 CFCs used by nations which have not banned them, and due to gases which are already in the stratosphere.

External links

http://www.unep.org/ozone/Public_Information/4Aii_PublicInfo_Facts_OzoneLayer.asp

External links

See also