5.6 Depletion of stratospheric ozone




5.6.1 Outline the overall structure and composition of the atmosphere.


external image uvrayjp.gif



Composition
Gas
percentage
Nitrogen
78.1
Oxygen
20.9
Argon
0.9
Carbon dioxide
0.04
Water
Variable
Trace Gases
>0.002

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Troposphere -
  • 0-12km thick;
  • 75% of all gases in atmosphere;
  • all weather happens here;
  • temperature falls with altitude
Stratosphere -
  • 12-50 km thick
  • temperature increases with altitude (gets warmer!)
  • horizontal winds (jet stream)
  • contains ozone layer
Mesosphere -
  • 50-80km thick
  • temperature falls with altitude
  • meteor showers happen here
Thermosphere -
  • ionosphere here (reflects radio waves for communications)
  • can be very very hot (2000 C+) !


5.6.2 Describe the role of ozone in the absorption of ultraviolet radiation.

Role of Ozone: Ultraviolet radiation is absorbed during the formation and destruction of ozone from oxygen.

  • allows UVa and UVb through, but blocks UVc waves

external image ozone2.jpg


5.6.3: Explain the interaction between ozone and halogenated organic gases.

Halogenated organic gases(e.g. CFC) are very stable under normal conditions but can liberate halogen atoms when exposed to ultraviolet radiation in the stratosphere. These atoms react with monatomic oxygen and slow the rate of ozone re‑formation. Pollutants enhance the destruction of ozone, thereby disturbing the equilibrium of
the ozone production system

How ozone is depleted by CFC’s:
  1. UV radiation breaks off a chlorine atom from a CFC molecule.
  2. The chlorine atom attacks an ozone molecule (O3), breaking it apart and destroying the ozone.
  3. The result is an ordinary oxygen molecule (O2) and a chlorine monoxide molecule (ClO).
  4. The chlorine monoxide molecule (ClO) is attacked by a free oxygen atom releasing the chlorine atom and forming an ordinary oxygen molecule (O2).
The chlorine atom is now free to attack and destroy another ozone molecule (O3). One chlorine atom can repeat this destructive cycle thousands of times.

Visual Aid (very brieft and please ignore the step about scientists and balloons.)
external image chemistry.gif


5.6.4 State the effects of ultraviolet radiation on living tissues and biological productivity.

  • Increase in mutation rates in DNA causing cancer
  • Can cause eye cataracts
  • Can damage the ability to carry out photosynthesis in plants and phytoplankton
  • Reduces primary production and therefore total productivity
external image Fig-1-Zepp-et-al-2003.png











5.6.5 Describe three methods of reducing the manufacture and release of ozone‑depleting substances.

  • Replace the old fridge with the existing "greenfreeze" technology which does not deplete the ozone
  • Recycle old CFC coolants from old fridges and air conditioners
  • Protection from excess UV e.g. sunglass and sun block


(For more info)




5.6.6 Describe and evaluate the role of national and international organizations in reducing the emissions of ozone‑depleting substances.
UNEP (United Nations Environment Programme) forges agreements on:
  • ITIHC (International Trade in Harmful Chemicals)
  • air pollution
  • contamination of international waterways
  • provide information to countries and public on disadvantages of pollution

1987 Montreal Protocol
  • international agreement on the emission of ozone-depleting substances
  • froze production and consumption of CFC’s with goal of zero production by year 2000
  • LEDC’s granted a longer time to implement the treaty
  • China and India have not met their quotas under the MP because of their rapid economic growth and high demand for refrigeration & AC’s
  • good example of a successful international cooperative effort to alter human impact on the environment