High in the atmosphere, some oxygen (O2) molecules absorbed energy from the Sun's ultraviolet (UV) rays and split to form single oxygen atoms. These atoms combined with molecular oxygen (O2) to form ozone (O3) molecules, which are very effective at absorbing UV rays. The thin layer of ozone that surrounds Earth acts as a shield, protecting the planet from irradiation by UV light.
The amount of ozone required to shield Earth from biologically lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence 600 million years ago. At this time, the oxygen level was approximately 10% of its present atmospheric concentration. Prior to this period, life was restricted to the ocean. The presence of ozone enabled organisms to develop and live on the land.
A little closer to home, you can find the ozone overhead as measured by satellite if you know your latitude and longitude. For the U at Albany Uptown Campus, the latitude is 42.7 degrees and the longitude is -73.8 degrees.
The minus sign is needed for West longitudes. If you forget the minus sign, you'll find the overhead ozone for Kara-Balta, KYRGYZSTAN which is about 200 miles south of Lake Balkhash where my wife used to vacation with her family as a child.
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September 16th is the "International Day for Preservation of the Ozone Layer" as designated by the United Nations
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United Nations Environment Programme Slide Show.
A rather detailed and well documented four-part description in the format of frequently asked questions posted in 1997.
FAQ 2010 update (Strongly recommended if you missed something in class)
The U.S. Environmental Protection Agency (EPA) has a nice site with lots of interesting links. (For the surfer at heart)
Electronic Textbook on Stratospheric Ozone is a thorough and technical resource
Scientific Assessment of Ozone Depletion: 2010
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Because sometimes it's easy to get tangled up in the web and because I'm paranoid (some internet sites are here today and down, gone, changed, or moved tomorrow), I have copied some basic points here to assist you.
Ozone is constantly being produced and destroyed in a natural cycle. However, the overall amount of ozone is essentially stable. This balance can be thought of as a tub with a tap and a drain. As long as the tap adds water as fast as the drain removes water, the water level in the tub remains constant. Similarly, while ozone production and destruction are balanced, ozone levels remain stable. This was the situation until the past several decades.
Large increases in stratospheric chlorine and bromine have upset that balance. In effect, they have added another drain to the tub, removing ozone faster than natural ozone is created. Therefore, ozone levels are beginning to fall towards a lower level until a new balance can be achieved; analogous to the tub in which water drains out more slowly as the water level goes down and eventually the tap again adds water at the same rate that water drains out but now the water level is lower.
The ozone depletion process begins when CFC's and other ozone-depleting substances (ODS) leak or are released from equipment. Winds efficiently mix the troposphere and evenly distribute the gases. CFC's are extremely stable, and they do not dissolve in rain. After a period of a few years, ODS molecules reach the stratosphere. Strong UV light breaks apart the ODS molecule. CFC's release chlorine atoms and halons release bromine atoms. It is these atoms that actually destroy ozone, not the intact ODS molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before finally being removed from the stratosphere.
Chlorofluorocarbons (CFC's) are a class of compounds that have been used as refrigerants (the fluid used in compressors for air conditioners and refrigerators), aerosol propellants (for spray cans), foam blowing agents (for manufacture of styrofoam and insulation), and as solvents (for cleaning in the electronics industry). They are chemically very unreactive, and hence safe to work with. The CFC's have lifetimes of 50-200+ years in the atmosphere and their major "sink" is photolysis by UV radiation. CFC's were invented in 1928, but only came into large-scale production after 1950.
The most important CFC's for ozone depletion are:
Trichlorofluoromethane, CFCl3 (usually called CFC-11 or R-11)
Dichlorodifluoromethane, CF2Cl2 (CFC-12 or R-12) and
1,1,2 Trichlorotrifluoroethane, CF2ClCFCl2 (CFC-113 or R-113)
"R" stands for "refrigerant". Sometimes CFC-12, for example, is called "F-12"; the"F" stands for "Freon", DuPont's trade name for these compounds. CFC's as well as related compounds such as HCFC's (Hydrochlorofluorocarbons) and Halons (Chlorofluorobromocarbons) are identified by a particular numbering scheme. More info on uses and alternatives.
NOx and the SST
In 1969, Paul Crutzen discovered that oxides of nitrogen NOx (NO and NO2) could be an efficient catalyst for the destruction of stratospheric ozone:
NO + O3 -> NO2 +
NO2 + O -> NO + O2
O3 + O -> 2 O2 (net result)
Harold S. Johnston made the connection to Supersonic transport (SST) emissions. Until then it had been thought that the radicals H, OH, and HO2 (referred to collectively as "HOx") were the principal catalysts for ozone loss; thus, investigations of the impact of aircraft exhaust on stratospheric ozone had focussed on emissions of water vapor, a possible source for these radicals. It had been argued - correctly, as it turns out - that water vapor injection was unimportant in affecting the ozone balance. The discovery of the NOx cycle again threw open the question of SST's and the ozone layer.
The natural source of stratospheric NOx is from nitrous oxide N2O, popularly known as laughing gas which is very unreactive - it has an atmospheric lifetime of more than 150 years - so it reaches the stratosphere, where most of it is converted to nitrogen and oxygen by UV photolysis. However, a small fraction reacts instead with oxygen atoms and this is the major natural source of NOx in the stratosphere. This natural source would have been matched by 500 of the SST's, designed by Boeing in the 1960's, each spending 5 hours per day in the stratosphere. (Boeing was intending to sell 800 of these aircraft.) The Concorde, a slower plane, produces less than half as much NOx and flies at a lower altitude; since the present Concorde fleet is small, its contribution to stratospheric NOx is not significant. In the meantime, there has been a great deal of progress in developing jet engines that will produce much less NOx - up to a factor of 10 - than the old Boeing SST. The most recent model calculations indicate that a fleet of the new "high-speed civil transports" would deplete the ozone layer by 0.3 to 1.8%.
One sometimes hears that the US government killed the SST project in 1971 because of concerns raised by H. S. Johnston's work on NOx. This is not true. The US House of Representatives had already voted to cut off Federal funding for the SST when Johnston began his calculations. The House debate had centered around economics and the effects of noise, especially sonic booms, although there were some vague concerns about "pollution" and one physicist had testified about the possible effects of water vapor on ozone. About 6 weeks after both houses had voted to cancel the SST, its supporters succeeded in reviving the project in the House. In the meantime, Johnston had sent a preliminary report to several professional colleagues and submitted a paper to Science. A preprint of Johnston's report leaked to a small California newspaper which published a highly sensationalized account. The story hit the press a few days before the Senate voted, 58-37, not to revive the SST. (The previous Senate vote had been 51-46 to cancel the project. The reason for the larger majority in the second vote was probably the statement by Boeing's chairman that at least $500 million more would be needed to revive the program.)
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