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ChemWiki: The Dynamic Chemistry E-textbook> Physical Chemistry> Kinetics> Case Studies: Kinetics> Depletion of the Ozone Layer

Depletion of the Ozone Layer

The earth's stratospheric ozone layer plays a critical roll in absorbing ultra violetradiation emitted by our sun. In the last thirty years, it has been discovered th

stratospheric ozone is depleting as a result of anthropogenic pollutants. There are a number of chemical reactions that can deplete stratospheric ozone; howeve

some of the most significant depletion comes from the catalytic destruction of ozone by freed halogen radicals like chlorine and bromine.

IntroductionThe atmosphere of the Earth is divided into 5 layers. From closest and thickest to farthest and thinnest the layers are: troposphere, stratosphere, mesospher

thermosphere and exosphere. The majority of the atmospheres ozone resides in the stratosphere, which extends from 6 miles above the Earths surface to 3miles. Humans rely heavily on the absorption of ultraviolet B rays by the ozone layer because UV-B radiation causes skin cancer and can lead to gene

damage. The ozone layer has historically protected the Earth from the harmful UV rays, although in recent decades this protection has diminished due

stratospheric ozone depletion.

Figure 1. These images from the Total Ozone Mapping Spectrometer (TOMS) show the progressive depletion of ozone over Antarctica from

1979 to 1999. This "ozone hole" has extended to cover an area as large as 10.5 million square miles in September 1998. The previous record o

10.0 million square miles was set in 1996. Figure courtesy of NASA.

History of Ozone DepletionOzone depletion is largely a result of man-made substances. Humans have introduced gases and chemicals into the atmosphere that have rapidly depleted th

ozone layer in the last century. This depletion makes humans more vulnerable to the UV-B rays which are known to cause skin canceras well as other gene

deformities. The possibility of ozone depletion was first introduced by scientists in the late 1960's as dreams of super sonic transport began to become a realit

Scientists had long been aware that nitric oxide (NO) can catalytically react with ozone (O 3) to produce O2 molecules; however, NO molecules produced
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ground level have a half life far too short to make it into the stratosphere. It was not until the advent of commercial super sonic jets (which fly in t

stratosphere and at an altitude much higher then conventional jets) that the potential for NO to react with stratospheric ozone became a possibility. The threat

ozone depletion from commercial super sonic transport was so great that it is often cited as the main reason why the US federal government pulled support f

its development in 1971. Fear of ozone depletion was abated until 1974 when Sherwood Rowland and Mario Molina discovered that chlorofluorocarbo

could be photolyzed by high energy photons in the stratosphere. They discovered that this process could releasing chlorine radicals that would catalytically rea

with O3and destroy the molecule. This process is called the Rowland-Molina theory of O 3depletion.

The Chapman CycleThe stratosphere is in a constant cycle with oxygen molecules and their interaction with ultraviolet rays. This process is deemed a cycle because of its consta

conversion between different molecules of oxygen. The ozone layer is created when ultraviolet rays react with oxygen molecules ( ) to create ozone (

and atomic oxygen ( ). This process is called theChapman Cycle.

1. An oxygen molecules is photolyzed by solar radiation, creating two oxygen radicals:

2. Oxygen radicals then react with molecular oxygen to produce ozone:

3. Ozone then reacts with an additional oxygen radical to form molecular oxygen:

4. Ozone can also be recycled into molecular oxygen by reacting with a photon:

It is important to keep in mind that ozone is constantly being created and destroyed by the Chapman Cycle and that these reaction are natural processes, wh

have been taking place for millions of years. Because of this, the thickness the ozone layer at any particular time can vary greatly. It is also important to kno

that is constantly being introduced into the atmosphere through photosynthesis, so the ozone layer has the capability of regenerating itself.

Chemistry of Ozone DepletionCFC 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 in

the stratosphere (most molecules are not around long enough to cross into the stratosphere from the troposphere). This prolonged life in the atmosphere allow

them to reach great altitudes when photons are more energetic. When the CFCs come into contact with these high energy photons their individual componen

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 crea

two oxygen molecules. After each reaction, chlorine is able to begin the destructive cycle again with another ozone molecule. One chlorine atom can there

destroy thousands of ozone molecules. Because ozone molecules are being broken down they are unable to absorb any ultraviolet light so we experience mo

intense UV radiation at the earths surface.

O

2

O

O

+ 2O

2

O

.

+ O

2

O

.

O

3

+ 2

O

3

O

.

O

2

+ +

O

3

O

2

O

.

O

2
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Figure 2. Much like sunscreen for the Earth, the ozone layer shields the Earth from the suns damaging UV-B radiation, which can adversely affect human

health and ecosystems. Figure courtesy of NOAA.

{C}

The Ozone HoleFrom 1985 to 1988, researchers studying atmospheric properties over the south pole kept noticing significantly reduced concentrations of ozone directly ov

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 198

researchers finally realized their error and concluded that an enormous hole in the ozone layer had indeed developed over Antarctica. Examination of NAS

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 ozo

destruction.

Because Antarctica is surrounded by water, winds over the continent blow in a unique clockwise direction creating a so called "polar vortex" th

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 stratosphe

clouds (PSC's) which are a conglomeration of frozen H2O and HNO3. Due to their extremely cold temperatures, PSC's form an electrostatic attracti

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.

the antarctic summer, high energy photons are able to photolyze the halogenated compounds, freeing halogen radicals that then catalytically destroy O

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

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 polvortex, 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

Ozone Depleting SubstancesThe 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 Pot ential. T aken directly from the Clean Air Act, as of June 2010.

SubstanceOzone- depletion -

potential

chlorofluorocarbon-11 (CFC11) 1.0









chlorofluorocarbon-212 (CFC212)

1.0






halon-1211 3.0

halon-1301 10.0
http://en.wikipedia.org/wiki/Clean_Air_Act


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halon-2402 6.0

carbon tetrachloride 1.1

methyl chloroform 0.1

hydrochlorofluorocarbon-22 (HCFC22)

0.05

hydrochlorofluorocarbon-123

(HCFC123)

0.02

hydrochlorofluorocarbon-124(HCFC124)

0.02

hydrochlorofluorocarbon-141(b)(HCFC141(b))

0.1

hydrochlorofluorocarbon-142(b)(HCFC142(b))

0.06

Outside linksAnimated Basics of Chemistry Behind CFC and Ozone Depletion: A must see!

Harvey, Erica; Sweeney, Robert. "Modeling Stratospheric Ozone Kinetics, Part II: Addition of Hydrogen, Nitrogen and Chlorin

OzoneModelingPartII.mcd." J. Chem. Educ. 1999, 76, 1310.

Koubek, Edward; Glanville, James O. "The absorption of UV light by ozone (TD)." J. Chem. Educ. 1989, 66, 338.

NASA Observation of Ozone Hole in Polar Regions: Cool little animation dating the Ozone problem in the polar caps sin

1979.xts/UCD_Chem_2C/Chem_2C_Main_Group_Chemistry/Group_13%3a_The_Boron_Family#

http://www.youtube.com/watch?v=dE1Kip70Um4&feature=related:watch the ozone deteriorate before your eyes in this colorfulanimation!!!

http://www.epa.gov/ozone/strathome.html

http://www.eoearth.org/article/Ozone

http://www.law.cornell.edu/uscode/html/uscode42/usc_sec_42_00007671---a000-.html

References1. Dessler, Andrew. The Chemistry and Physics of Stratospheric Ozone. San Diego, Ca: Academic Press, 2000

2. Hoffman, Matthew J. Ozone Depletion and Climate Change. Albany, NY: State University of New York Press, 2005

3. Parson, Edward A. Protecting the Ozone Layer: Science and Strategy. New York: Oxford University Press, 2003.

4. Petrucci, Ralph H., William S. Harwood, and Geoff E. Herring. General Chemistry : Principles and Modern Applications. 9th ed. Upper Saddle River:

Prentice Hall, 2006.Dynamic Equilibrium#

5. Varotsos, Costas, Kirill Ya. Kondratyev. Atmospheric Ozone Variability: Implications for Climate Change, Human Health and Ecosystems. Chichester,

UK: Praxis Publishing Ltd, 2000

6. Godish, Thad. Air Quality. 4th ed. Florida: CRC Press LLC, 2004.

7. United States Clean Air Act: as of June 3rd, 2010.

General QuestionsWhat are several causes that result in the depletion of our ozone layer?

the release of free radicals, the use of CFCs, the excessive burning of fossil fuels

What is the chemical reaction that displays how ozone is created?

UV + O2 -> 2O + heat, O2 + O -> O3, O3 + O -> 2O2

What reactions demonstrate the destruction of the ozone layer?

Cl + O3--- ---> ClO + O2 and ClO + O ------> Cl + O

How do CFCs destroy our ozone layer?

the atomic chlorine freed from CFC reacts in a catalytic manner with ozone and atomic oxygen to make more oxygen molecules

Why should regulations be enforced NOW in regards to pollution and harmful chemicals?

without regulation, the production and use of chemicals will run out of hand and do irreversible damage to our stratosphere

What type of atom in the CFC molecule is most destructive to the ozone?
http://www.law.cornell.edu/uscode/html/uscode42/usc_sec_42_00007671---a000-.htmlhttp://www.eoearth.org/article/Ozonehttp://www.epa.gov/ozone/strathome.htmlhttp://www.youtube.com/watch?v=dE1Kip70Um4&feature=relatedhttp://chemwiki.ucdavis.edu/Z_Wikitexts/UCD_Chem_2C/Chem_2C_Main_Group_Chemistry/Group_13:_The_Boron_Family#http://www.youtube.com/watch?v=taTzqRHNIEchttp://search.jce.divched.org/JCEIndex/FMPro?-db=jceindex.fp5&-lay=wwwform&combo=the%20absorption%20of%20uv%20light&-find=&-format=detail.html&-skip=0&-max=1&-token.2=0&-token.3=10http://www.jce.divched.org/Journal/Issues/1999/Sep/abs1310_1.htmlhttp://www.youtube.com/watch?v=I1wrEvc2URE&feature=related


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Powered by MindTou

chlorine

What layer of the atmosphere does the ozone layer reside in?

the stratosphere, the second closest to the Earth's surface

What cycle is responsible for ozone in the stratosphere?

the Chapman Cycle

What factor is responsible for breaking up stable molecules?

ultraviolet rays from the sun

ContributorsCaroline Tran, Dan Chong , Anne Keith, Jordan Shively

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depletion of the ozone layer - chemwiki

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