Chemistry of Ozone Depletion
CFC molecules are made up of chlorine,
fluorine and carbon atoms and are extremely stable. This extreme
stability allows CFC's to slowly make there way into the stratosphere
(most molecules are not around long enough to cross into the
stratosphere from the troposphere). This prolonged life in the
atmosphere allows them to reach great altitudes when photons are more
energetic. When the CFCs come into contact with these high energy
photons their individual components are freed from the whole. The
following reaction displays how Cl atoms have an ozone destroying cycle:
Cl + O3 → ClO + O2
ClO + O → Cl + O2
________________
O3 + O → 2O2 : Overall reaction
Chlorine is able to destroy so much of
the ozone because it is a catalyst. Chlorine initiates the break down of
ozone and combines with a freed oxygen to create two oxygen molecules.
After each reaction, chlorine is able to begin the destructive cycle
again with another ozone molecule. One chlorine atom can thereby destroy
thousands of ozone molecules. Because ozone molecules are being broken
down they are unable to absorb any ultraviolet light so we experience
more intense UV radiation at the earths surface.
The Ozone Hole
From 1985 to 1988, researchers studying
atmospheric properties over the south pole kept noticing significantly
reduced concentrations of ozone directly over the continent of
Antarctica. For three years it was assumed that the ozone data was
incorrect and was due to some type of instrument malfunction. In 1988,
researchers finally realized their error and concluded that an enormous
hole in the ozone layer had indeed developed over Antarctica.
Examination of NASA satellite data later showed that the hole had begun
to develop in the mid 1970's.
The ozone hole over Antarctica is formed
by a slew of unique atmospheric conditions over the continent that
combine to create an ideal environment for ozone destruction.
- Because antarctica is surrounded by water, winds over the continent blow in a unique clockwise direction creating a so called "polar vortex" that effectively contains a single static air mass over the continent. As a result, air over Antarctica does not mix with air in the rest of the earth's atmosphere.
- Antarctica has the coldest winter temperatures on earth, often reaching -110 F. These chilling temperatures result in the formation of polar stratospheric clouds (PSC's) which are a conglomeration of frozen H2O and HNO3. Due to their extremely cold temperatures, PSC's form an electrostatic attraction with CFC molecules as well as other halogenated compounds
As spring comes to Antarctica, the PSC's
melt in the stratosphere and release all of the halogenated compounds
that were previously absorbed to the cloud. In the antarctic summer,
high energy photons are able to photolyze the halogenated compounds,
freeing halogen radicals that then catalytically destroy O3.
Because Antarctica is constantly surrounded by a polar vortex, radical
halogens are not able to be diluted over the entire globe. The ozone
hole develops as result of this process.
Resent research suggests that the
strength of the polar vortex from any given year is directly correlated
to the size of the ozone hole. In years with a strong polar vortex, the
ozone hole is seen to expand in diameter, where as in years with a
weaker polar vortex, the ozone hole is noted to shrink.
For more references
For more references
http://Depletion_of_the_Ozone_Layer#Chemistry_of_Ozone_Depletion
Ozone Depleting Substances
The following substances are listed as ozone depleting substances under Title VI of the United State Clean Air Act:Table 1: Ozone Depleting Substances And Their Ozone-Depletion Potential. Taken directly from the Clean Air Act, as of June 2010.
| Substance | Ozone- depletion potential | ||||
|---|---|---|---|---|---|
| chlorofluorocarbon-11 (CFC–11) | 1.0 | ||||
| chlorofluorocarbon-12 (CFC–12) | 1.0 | ||||
| chlorofluorocarbon-13 (CFC–13) | 1.0 | ||||
| chlorofluorocarbon-111 (CFC–111) | 1.0 | ||||
| chlorofluorocarbon-112 (CFC–112) | 1.0 | ||||
| chlorofluorocarbon-113 (CFC–113) | 0.8 | ||||
| chlorofluorocarbon-114 (CFC–114) | 1.0 | ||||
| chlorofluorocarbon-115 (CFC–115) | 0.6 | ||||
| chlorofluorocarbon-211 (CFC–211) | 1.0 | ||||
| chlorofluorocarbon-212 (CFC–212) | 1.0 | ||||
| chlorofluorocarbon-213 (CFC–213) | 1.0 | ||||
| chlorofluorocarbon-214 (CFC–214) | 1.0 | ||||
| chlorofluorocarbon-215 (CFC–215) | 1.0 | ||||
| chlorofluorocarbon-216 (CFC–216) | 1.0 | ||||
| chlorofluorocarbon-217 (CFC–217) | 1.0 | ||||
| halon-1211 | 3.0 | ||||
| halon-1301 | 10.0 | ||||
| halon-2402 | 6.0 | ||||
| carbon tetrachloride | 1.1 | ||||
| methyl chloroform | 0.1 | ||||
| hydrochlorofluorocarbon-22 (HCFC–22) | 0.05 | ||||
| hydrochlorofluorocarbon-123 (HCFC–123) | 0.02 | ||||
| hydrochlorofluorocarbon-124 (HCFC–124) | 0.02 | ||||
| hydrochlorofluorocarbon-141(b) (HCFC–141(b)) | 0.1 | ||||
| hydrochlorofluorocarbon-142(b) (HCFC–142(b)) | 0.06 |
