Ozone Depletion

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OZONE DEPLETION


CHEMISTRY 1022 LAB TA: JUHEE

With the observed decrease of the ozone layer in most parts of the world we are also observing an increase in Ultraviolet-B (UV-B) radiation penetrating to the earth’s surface. The stratospheric ozone absorbs UV-B and turns it into an oxygen molecule (O2) and an oxygen atom thereby preventing most of the harmful UV-B rays from reaching earth. It has been difficult to do exact measurements on the increasing levels of UV-B rays that reach the earth due to variations in different regions of the world like cloud cover, pollutants, and sun angles. However, in Antarctica increasing levels of UV-B rays have been easily observed. At Palmer Station, Antarctica, scientists have noticed levels reach or exceed maximum summer values in San Diego, CA.6 The ozone layer is located at an altitude of twenty to thirty kilometers and takes up about one-third the volume of the stratosphere layer, whose altitude is between twenty to fifty kilometers. The process of ozone absorbing ultraviolet radiation, which results in exclusive high temperature in the upper stratosphere, prevents harmful ultraviolet rays from reaching life on earth.1 This process is known as the "Natural Occurring Mechanism," which is illustrated by the following reaction where the ozone (O3(g)) molecule is created by the interaction of the O2 molecule and another Oxygen molecule: O3(g) Image:ozone_2.gif.gif O2(g)+ O 

O + O3(g) Image:ozone_6.gif.gif  2O2(g)

O3(g) + O + O3(g) Image:ozone_2.gif.gif O2(g)+ O + 2O2(g)

Net Reaction: 2O3(g)Image:ozone_2.gif.gif 3O2(g)

During the natural occurring mechanism process, a very small amount of ozone (O3(g)) is lost to the ultraviolet radiation. Inversely, ozone depletion is the loss of the ozone layer that surrounds the earth which allows high energy ultraviolet light to travel through the earth’s atmosphere and damage living cells. UV-B radiation plays a role in malignant melanoma, the most serious form of skin cancer. UV-B rays cause skin cancer by doing direct damage to skin cell DNA. This leads to gene mutations that affect cell division and repair, and in turn leads to DNA error. Finally, over exposure to UV-B seems to lead to the body having a more difficult time fighting off disease. This could explain why the body is more susceptible to cancer and cataracts diseases. UV-B rays may also cause reduction in the growth of aquatic life forms.2The euphotic zone of the ocean (the surface to the depth of the ocean where the light intensity falls to one percent8) is exposed to a lot of sunlight and therefore, a lot of UV-B rays. Phytoplanktons live primarily in this zone and the light is crucial to their survival. But, the increase in UV-B rays has affected their orientation mechanisms and mobility, resulting in a decrease of their survival rates.7 Plant exposure to extreme levels of UV-B could have an effect on the chemistry of the plant. This could lead to insect consumption and changes in disease rates by affecting how their nutrients are stored and how they develop. The effects of UV-B exposure can also alter the life cycles and metabolisms of plants and insects. One positive point is that plants can usually adapt to changes in their environments, but if the changes are extreme, it could result in changes in vegetation in different parts of the world.
The greatest depletion of the ozone layer comes from a substance called chlorofluorocarbon (CFC), which have the greatest lifetime in interacting with the O3(g) molecule. This data was categorized by the Global Warming Protections in the Scientific Assessment of Ozone Depletion in 2002. The most common CFC in this group is trichlorofluoromethane (CCl3F) with a lifetime of 45 years. This compound was used heavily until 1995 to keep low temperature efficiency in refrigerators and car/household air conditioners. It was also used in foam for manufacturing furniture and mattresses. The second most common chemical is sulfur dioxide (SO2) which contributes 88 % of the 19.9 million metric tons of SO2 produced annually to the air through fuel combustion. The other 15 % of ozone depletion is from human-made solid and liquid "particulate" materials such as aerosols.3 Here, we will examine how chlorine in CCl3F(g), a "homogeneous catalyst", separates from CCl3F(g) when ultraviolet ray impacts with CCl3F(g) molecule at high elevation: 

       Cl(g)
       Image:ozone_13.gif.gif         breaks apart when hit by UV           
   F Image:ozone_17.gifCImage:ozone_6.gif.gif Cl(g)   Image:ozone_14.gif.gif        Cl(g) ........     (Cl(g) now is a free-radical)
        Image:ozone_20.gif    
         Cl(g)

Cl(g) only has seven valence electrons, and tends to bond with unpaired molecule. Free-radical Cl(g) will react with O3(g). The reaction is known as Cl(g) Catalyzed Mechanism:

Cl(g) + O3(g)Image:ozone_6.gif.gif ClO(g) + O2(g)


ClO(g) + O3(g) Image:ozone_6.gif.gif ClO2(g) + O2(g)


ClO2(g) Image:ozone_6.gif.gif Cl(g) + O2(g)


Image:ozone_24.gif

Net Reaction: Cl(g) + ClO(g) +ClO2(g) + 2O3(g) Image:ozone_6.gif.gif ClO(g) + Cl(g) + ClO2(g) + 3O2(g) 

                 2O3(g) Image:ozone_6.gif.gif 3O2(g)



 In the overall reaction, free-radical Cl(g) breaks up O3(g) without the presence of ultraviolet radiation.  Free-radical Cl(g) regenerates in the reaction, reproduces a new free-radical Cl(g) and this new Cl(g) continues to react with another O3(g) molecule.  Therefore, free-radical Cl(g) turns O3(g) into O2(g) molecule much faster than ultraviolet light.






In 1981, British researchers discovered that the ozone hole in the Artic and Antarctic was the largest zone loss in the earth’s ozone layers4.  The explanation for this is that the extreme wind around the two regions blow CFCs and SO2(g) (another type of homogeneous catalyst) to the poles.  The year-round low temperatures (average under 100F) create ice crystals, which act as a heterogeneous catalyst to disintegrate CCl3F(g) and SO2(g).  The free-radical Cl(g) then travels to higher elevation and breaks apart the ozone molecule in the same way.
  



















Image:ozone_28.jpg













These two images were collected by NASA to illustrate the ozone loss. In the Antarctic in 2002 the ozone loss is about half the size of the ozone loss in 2001 (left to right)5.  


Due to the destructive nature of these compounds to the ozone layer, as depicted above, an environmental committee drafted an agreement in 1985 for the reduction of products containing CFCs, which they called the Vienna Convention.  The Montreal Protocol of 1987 followed these guidelines by stating that the production of products containing CFCs would be reduced to half by the late 1990s.  Another protocol was then drafted in 1991 called the "United Nations Environmental Programme," which superceded the Montreal Protocol.  This Protocol was designed to aid in the complete disuse of CFCs by 1996 due to the increased damage in the ozone.  Since this Protocol was signed, the destruction of the ozone has dropped sharply and hope for repair of the ozone has once again been achieved.


The biggest alternative that is being used instead of Chlorofluorocarbons (CFC) seem to be primarily Hydrofluorocarbons (HFC) and Hydrochlorofluorocarbons (HCFC)10. The main reason why HFC’s and HCFC’s work effectively is because they are more readily broken up in the lower atmosphere, well before the point where the Chlorine can get into the ozone layer. These then fall down as acid rain but not at any significant level. HFC’s do not have any affect on the Ozone at all and the HCFC’s only have a minor effect9. Another alternative for CFC’s in  a form of propellant is Hydrofluoroalkane11, which does not have any adverse affect on the ozone layer and has a much more efficient use for inhalers than do CFC’s.


There are other new chemicals in use today with unknown side effects to our precious ozone.  But for now we seem to have stopped the most violent damage as we continue to research and replace other damaging chemicals and compounds.


 
References:



  1.  Burt, James E., Aguado, Eward., Understanding of Weather and Cl(g)imate, 3rd Edition, Composition and Structural of the Atmosphere, Pearson Education, Inc: New Jersey, 2004; pp. 13-19.

  2.  Burt, James E., Aguado, Eward., Understanding of Weather and Cl(g)imate, 3rd Edition, Human Effects: Air Pollution and Heat Islands, Pearson Education, Inc: New Jersey, 2004; pp. 424-442.

  3.  Ozone-Depleting Substances, http://www.epa.gov/ozone/ods.html, June 21st, 2004 (accessed 09/20/2005).

  4.  National Academy of Sciences. "The Ozone-Depletion Phenomenon", http://www.beyonddiscovery.org/content/view.page.asp?I=88, 2003 (accessed 09/20/2005). 

  5.  NASA Scientific Visualization Studio. "Ozone Depletion" http://svs.gsfc.nasa.gov/stories/toms/byyear.html  (accessed 09/20/2005).

  6.  U.S. Global Change Research Information office, "Executive Summary,"http://www.gcrio.org/UNEP1998/UNEP98p2.html.  9/23/2005

  7.  "The Effects of Ozone Depletion," http://www.epa.gov/ozone/science/effects.html  9/25/2005

  8.  "Photic Zone," 

  9.  http://en.wikipedia.org/w/index.php?title=Photic_zone&printable=yes 9/21/2005 AFEAS, "Atmospheric Chlorine: CFC\’s and Alternative Fluorocarbons." 2004. AFEAS. 25 Sept. 2005. <http://www.afeas.org/atmospheric_chlorine.html>.

  10.  Nordenberg, Tama. "CFC-Free Medication for an Ailing Ozone Layer." FDA Consumer Magazine. 1997. US FDA. 25 Sept. 2005. <http://www.fda.gov/fdac/features/1997/497_cfcs.html>.

  11.  Clinical Drug Investigation. "Long-Term Safety of Flunisolide Hydrofluoroalkane Metered-Dose Inhaler in Adults and Adolescents with Asthma." Medscape. 2001. ADIS International Limited. 25 Sept. 2005. <http://www.medscape.com/viewarticle/418157>.







Researched and written by: HOANG TRINH, MIKE STANG, PAULA STURM, SHAWN SILLIVAN, THER YANG.








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