Chemistry

Topics: Ozone depletion, Ultraviolet, Ozone Pages: 6 (1739 words) Published: November 14, 2014
Chemistry
Ozone Depletion
Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon is referred to as theozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polartropospheric ozone depletion events. The details of polar ozone hole formation differ from that of mid-latitude thinning, but the most important process in both is catalytic destruction of ozone by atomic halogens.[1] The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants (CFCs, freons, halons). These compounds are transported into the stratosphere after being emitted at the surface.[2] Both types of ozone depletion were observed to increase as emissions of halo-carbons increased. CFCs and other contributory substances are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of ultraviolet light (UV light) from passing through theEarth's atmosphere, observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol that bans the production of CFCs, halons, and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in skin cancer, cataracts,[3] damage to plants, and reduction of plankton populations in the ocean'sphotic zone may result from the increased UV exposure due to ozone depletion.

Causes

Scientific evidence indicates that stratospheric ozone is being destroyed by a group of manufactured chemicals, containing chlorine and/or bromine. These chemicals are called "ozone-depleting substances" (ODS). ODS are very stable, nontoxic and environmentally safe in the lower atmosphere, which is why they became so popular in the first place. However, their very stability allows them to float up, intact, to the stratosphere. Once there, they are broken apart by the intense ultraviolet light, releasing chlorine and bromine. Chlorine and bromine demolish ozone at an alarming rate, by stripping an atom from the ozone molecule. A single molecule of chlorine can break apart thousands of molecules of ozone. What's more, ODS have a long lifetime in our atmosphere — up to several centuries. This means most of the ODS we've released over the last 80 years are still making their way to the stratosphere, where they will add to the ozone destruction. The main ODS are chlorofluorocarbons (CFCs), hydrochlorofluorcarbons (HCFCs), carbon tetrachloride and methyl chloroform. Halons (brominated fluorocarbons) also play a large role. Their application is quite limited: they're used in specialized fire extinguishers. But the problem with halons is they can destroy up to 10 times as much ozone as CFCs can. For this reason, halons are the most serious ozone-depleting group of chemicals emitted in British Columbia. Hydrofluorocarbons (HFCs) are being developed to replace CFCs and HCFCs, for uses such as vehicle air conditioning. HFCs do not deplete ozone, but they are strong greenhouse gases. CFCs are even more powerful contributors to global climate change, though, so HFCs are still the better option until even safer substitutes are discovered. The Main Ozone-Depleting Substances (ODS)

Chlorofluorocarbons (CFCs)
The most widely used ODS, accounting for over 80% of total stratospheric ozone depletion. Used as coolants in refrigerators, freezers and air conditioners in buildings and cars manufactured before 1995. Found in industrial solvents, dry-cleaning agents and hospital sterilants. Also used in foam products — such as soft-foam padding (e.g. cushions and mattresses) and rigid foam (e.g. home...
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