Ozone Layer Notes with PDF

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1. The Ozone Layer: Basics & Location

• Primary Location: The stratosphere.
  • Approximately 90% of atmospheric ozone is located here.
• Altitude: Found in the lower part of the stratosphere, approximately 15 to 30 km above the Earth’s surface.
  • Its exact thickness varies by season and geographical location.
• Measurement:
  • Thickness is measured in Dobson Units (DU).
  • 1 Dobson Unit is defined as a 0.01 mm thick layer of pure ozone at 0°C and 1 atmosphere of pressure.
• Natural Regulation: Naturally regulated by Nitrogen Dioxide (NO₂).
• Variability: Ozone levels are not constant; they vary seasonally (e.g., more in spring, less in rainy seasons).
• Atmospheric Layers (for reference): The layers above the troposphere, in order, are: Stratosphere, Mesosphere, Thermosphere, and Exosphere.

2. Formation, Properties, and Function of Ozone

• Chemical Composition: A molecule composed of three oxygen atoms (O₃).
• Formation Process: Created in the upper atmosphere when sunlight (ultraviolet radiation) reacts with oxygen molecules (O₂).
• Toxicity: While protective in the stratosphere, ozone gas (O₃) at ground level is toxic to living organisms.
• Primary Function: Acts as a protective shield for the biosphere by absorbing the sun’s harmful Ultraviolet (UV) radiation.
  • It absorbs 93-99% of the sun’s harmful UV light.
• Consequence of UV Exposure:
  • Ultraviolet radiation can cause skin cancer by inducing genetic mutations.
  • The types of UV rays that reach the Earth and are harmful are UV-A and UV-B.

3. Ozone Depletion & the Ozone Hole

• Primary Cause of Depletion: Chlorofluorocarbons (CFCs) are the main cause of the ozone hole.
  • CFCs contain chlorine, which destroys ozone molecules.
  • CFCs are very stable and can remain in the atmosphere for 80 to 100 years.
• Other Ozone-Depleting Substances (ODS):
  • Halons (e.g., HALON-1211 used in firefighting)
  • Carbon Tetrachloride
  • Methyl Chloroform (a solvent)
  • Nitrous Oxide (N₂O)
• Mechanism of Depletion (The Ozone Hole over Antarctica):
  • The depletion is maximum over Antarctica, forming the “ozone hole.”
  • This is due to a combination of factors:
    1. The presence of a prominent polar front.
    2. The formation of Polar Stratospheric Clouds (PSCs).
    3. The inflow of Chlorofluorocarbons (CFCs).
  • Nitric acid in the PSCs reacts with CFCs, releasing chlorine atoms that destroy ozone molecules. The clouds also help convert chlorine into its active, ozone-destroying form.
• Discovery of Ozone Holes:
  • The Antarctic ozone hole was first discovered in 1985 by a British team using a Total Ozone Mapping Spectrometer.
  • An ozone halo over the Tibetan Plateau was discovered in 2005 by a research team led by G.W. Kent Moore.

4. Chlorofluorocarbons (CFCs): Common Uses (Now largely phased out)

• As pressurizing agents in aerosol cans.
• In the production of plastic foams.
• As solvents for cleaning electronic components (e.g., Trichlorofluoromethane).
• As refrigerants in cooling systems.
• (Note: CFCs are not used in the production of tubeless tires.)

5. Protection and International Efforts

• Key International Agreement: The Montreal Protocol.
  • It is an international treaty for the protection of the ozone layer.
  • It aims to phase out the production of ozone-depleting substances, primarily CFCs.
  • It was signed in 1987 and came into force on January 1, 1989.
• International Ozone Layer Preservation Day: Celebrated annually on September 16 to commemorate the signing of the Montreal Protocol.
• Refrigerants: Modern refrigerators often use a type of halonic hydrocarbon (historically referred to as Mafron). While ammonia is used in large industrial plants, it was not the primary replacement for CFCs in domestic units.

6. Related Facts and Common Misconceptions

• Greenhouse Effect:
  • CFCs are responsible for contributing to the greenhouse effect.
  • Other major greenhouse gases include water vapor, carbon dioxide (CO₂) (a primary contributor), methane, and nitrous oxide.
• Incorrect Associations:
  • Acid Rain: Primarily caused by Sulfur Dioxide (SO₂) and Nitrogen Oxides (NOx), not Nitric Acid alone.
  • Rocket Fuel: Often a highly refined form of Kerosene Oil, not directly related to ozone-depleting substances.

Know More About Ozone Layer:

• Ozone Layer – Old Year Questions
• Ozone Layer One Liner Questions & Answers