Ozone+Depletion

Yian Chiang
 * __Ozone Depletion__**




 * __Introduction__**:

Ozone was discovered in 1840. It was first detected in the upper layer of the atmosphere (the stratosphere) in the 1889. It is produced by the photochemical reaction between oxygen molecules and oxygen atoms in the atmosphere.

oxygen atom + oxygen → ozone

O(g) + O2(g) → O3(g)

Ozone is an allotrope of oxygen. Unlike oxygen, which has two atoms in each molecule, ozone has three atoms of oxygen per molecule. Therefore, its molecular formula is O3. Ozone is a pale blue, almost colorless gas with a characteristic odor. In small concentrations, it is non-toxic, but in concentrations above 100ppm (parts per molecule), it is toxic. Breathing in air that contains high concentrations of ozone can be dangerous, especially people with asthma

There is a thin layer of ozone in the stratosphere about 20~50km above the Earth. When ozone is formed at low altitudes, it can cause severe pollution problems. However, in the stratosphere, ozone acts as a shield, filtering out some of the harmful UV radiation from the sun. If this radiation reaches the surface of the Earth, there could be a drastic increase in the number of cases of skin cancer, genetic mutations and eye damage (e.g. cataracts being formed). UV may also be harmful to marine life.

Since 1976, there has been an alarming decrease in the amount of ozone in the stratosphere over the South Pole. In recent years, a similar phenomenon has been occurring over the North Pole.


 * __Ozone Depletion__**

Natural depletion O 3 --> O 2 + O• O 3 + O• -> 2O 2

Scientists have discovered that the depletion in thw ozone layer is caused by chlorofluorocarbons. Chlorofluorocarbons, commonly known as CFCs, are man-made compounds containing the element carbon, fluorine, and chlorine. CFCs, along with other chlorine- and bromine-containing compounds, have been implicated in the accelerated depletion of ozone in the Earth's stratosphere. CFCs were developed in the early 1930s and are used in a variety of industrial, commercial, and household applications. These substances are non-toxic, non-flammable, and non-reactive with other chemical compounds. These desirable safety characteristics, along with their stable thermodynamic properties make them ideal for many applications--as coolants for commercial and home refrigeration units, aerosol propellants, electronic cleaning solvents, and blowing agents. They were also used in the manufacture of packing foam.

Within the last few decades, large amounts of CFCs have been released into the atmosphere. CFCs are very stable and can remain in the atmosphere for a very long time. Over the years, they slowly diffuse through the air and react with ozone, destroying the ozone layer



The picture above shows how the ozone layer is slowly destroyed by CFCs.

It was not until 1973 were chlorine radicals found out to act as a catalyst in the mechanism for the destruction of ozone. One atom of chlorine from a CFC molecule can damage up to 100,000 molecules of O3. The reaction mechanism of ozone depletion is shown below:

CCl2F2 -> CClF2 + Cl• Cl• + O3 -> ClO• + O2 ClO• + O• -> Cl• + O2 ClO• + O3 -> Cl• + 2O2 So therefore, the overall equation = O3 + O• -> 2O2

In the above reaction mechanism the chlorine radical is reproduced and so acts as a catalyst. The ClO radical is the reaction intermediate as it is produced and used up in the reaction. The overall effect is the destruction of a molecule of ozone by the addition of an oxygen radical, to produce two molecules of oxygen. This destroys the ozone layer and harmful UV radiation reaches earth through the holes in the ozone layer.


 * __Ozone Depleting Pollutants (ODT)__**

Unfortunately, CFCs are not the only ODTs. Other ODTs include the methylhalides, carbon tetrachloride (CCl4), carbon tetrafluoride (CF4), and the halons which contain bromine instead of chlorine. Such compounds are called halocarbons.

Carbon tetrachloride (CCl4), despite its toxicity, was first used in the early 1900s as a fire extinguisher, and more recently as an industrial solvent, an agricultural fumigant, and in many other industrial processes including petrochemical refining, and pesticide and pharmaceuticals production. Recently it has also been used in the production of CFC-11 and CFC-12. It has accounted for less than 8% of total ozone depletion. The use of carbon tetrachloride in developed countries however, has been prohibited since the beginning of 1996 under the Montreal Protocol

Methyl chloroform, also known as 1,1,1 trichloroethane is a versatile, all-purpose industrial solvent used primarily to clean metal and electronic parts. It was introduced in the 1950s as a substitute for carbon tetrachloride. Methyl chloroform has accounted for roughly 5% of total ozone depletion. The use of methyl chloroform in developed countries has been prohibited since the beginning of 1996 under the Montreal Protocol

Halons, unlike CFCs, contain bromine, which also destroys ozone in the stratosphere. Halons are used primarily in fire extinguishers. Halon-1301 has an ozone depleting potential 10 times that of CFC-11. Although the use of halons in developed countries has been phased out since 1996, the atmospheric concentration of these potent, ozone destroyers is still rising because of their long atmospheric lifetimes. To date halons have accounted for about 5% of global ozone depletion.

Methyl bromide, another bromine-containing halocarbon, has been used as a pesticide since the 1960s. Today, scientists estimate that human sources of methyl bromide have been responsible for approximately 5 to 10% of global ozone depletion.


 * __Alternatives to CFCs__**

Many countries have now agreed to ban CFCs. In 1992, an international agreement was reached for a complete ban on the release of CFCs by 1996. Until now, most of the countries in the world have completely banned the use of CFCs. However, even if the use of CFCs is totally banned, the depletion of the ozone layer will still continue perennially due to the CFCs already present in the atmosphere.

To replace CFC, the new substance will have to be as stable, non-toxic, non-flammable, and non-reactive with other chemical compounds as CFCs are as these are the desirable traits CFCs have that made them so practical in the first palce. The alternative will also need to have a lower percentile of, or do not contain chlorine as chlorine is what causes ozone depletion.

__**//Bibliography//**__

//Environmental Chemistry handout//

//Information on CFCs, 2006, http://www.portaec.net/library/pollution/air_pollution/information_on_chlorofluorocarbo.html//

//Ozone layer protection, 2010, http://www.epa.gov/ozone/science/ods/classone.html//

//Ozone depleting substance, 20009, http://www.environment.gov.au/atmosphere/ozone/ods/index.html//

//The second image, http://www.google.com.hk/imglanding?q=ozone+depletion&um=1&hl=zh-CN&newwindow=1&safe=strict&client=firefox&sa=N&rls=org.mozilla:zh-TW:official&source=og&tbs=isch:1&tbnid=_PDj2HNaZWUIFM:&imgrefurl=http:climatechange.thinkaboutit.eu/think4/post/ozone_layer_depletion/&imgurl=http://climatechange.thinkaboutit.eu/scripts/tinymce/jscripts/tiny_mce/plugins/imagemanager/files/ozone_depletion.jpg&zoom=1&w=480&h=285&ei=m8G6TI3-GcPQcYy26PUM&iact=rc&oei=k8G6TJ-aIsidcY-f3a0M&esq=2&page=2&tbnh=97&tbnw=163&start=21&ndsp=23&ved=1t:429,r:1,s:21&biw=1280&bih=560

The first image, http://www.google.com.hk/imglanding?q=ozone+depletion+dog&um=1&hl=zh-CN&newwindow=1&safe=strict&client=firefox&rls=org.mozilla:zh-TW:official&source=og&biw=1280&bih=560&tbs=isch:1&tbnid=6u7SXOTscRCsTM:&imgrefurl=http:www.cartoonstock.com/directory/o/ozone_depletion.asp&imgurl=http://www.cartoonstock.com/lowres/vsh0735l.jpg&zoom=1&w=400&h=311//