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Ozone - Ozone Ozone Can t live with it can t live without...

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Ozone Can t live with it, can t live without it 1
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http://www.theozonehole.com/stratosphericchemistry.htm 2
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Ozone: What is it and how did it form?? Ozone (O 3 ) is a colourless gas with an unpleasant odour It is a naturally occurring gas It is a chemical cousin of molecular oxygen (O 2 ) Early plants protected from harmful solar UV radiation by evolving in water Photosynthesis created an enriched oxygen atmosphere within which an ozone layer formed 3
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Ozone: Where do we find it? Most ozone is found in a layer between 15 and 35 km above the surface (Stratosphere) The ozone layer here is beneficial to life on earth Absorbs harmful UV radiation from the sun 4
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Electromagnetic radiation 5
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What is UV radiation? tude Altit 6
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Ozone: the UV radiation connection All UV Ͳ C (highest energy Ͳ most destructive) is All UV C (highest energy absorbed by O 3 layer Destroys DNA Most UV Ͳ B is absorbed but holes are causing increased exposure and health risks Skin cancer etc. Nearly all UV Ͳ A reaches the surface Mostly benign, however... 7
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Ozone: photochemistry of creation O 3 rapidly created in a 3 Ͳ body reaction: ± ² 3 2 2 2 M O M O O O O UV O ³ o ³ ³ ³ o ³ rapidly created in a 3 body reaction: where M is a third molecule which carries off the 3 2 2 3 O O o excess energy during the 3 Ͳ way collision but remains unaffected ( M is typically oxygen or nitrogen) If M not there, too much energy to stick together If nothing else happened, all oxygen would be converted to O 3 not a great thing! 8
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Ozone chemistry 9
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Ozone: photochemistry of destruction A competing process reduces O 3 : 2 3 O O O O O O UV O ³ o ³ ³ o ³ O 3 is photodissociated by UV radiation into atomic and 2 3 2 2 3 3 2 O O o molecular oxygen Th ( t l) ti d t ti l t The (natural) creation destruction cycle repeats O 3 dissociation is sped up by the presence of catalytic agents 10
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Ozone: catalytic destruction Catalytic reactions destroy O 3 in the presence of molecule X which reacts and removes O 3 while being regenerated in the process 2 3 2 O X O XO O XO X O ³ o ³ ³ o ³ In the stratosphere X is: nitric oxide (NO): XO is NO 2 2 3 2 3 3 2 O O o hydroxyl (OH): HO 2 chlorine (Cl): ClO bromine (Br): BrO 11
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Catalytic destruction of O 3 12 http://www.nas.nasa.gov/About/Education/Ozone/chemistry.html http://www.theozonehole.com/ozonedestruction.htm
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Ozone: the CFC connection Chlorofluorocarbons (CFC s) are a family of chemically stable compounds with low toxicity that are purely artificially created compounds with low toxicity
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