APES 9.1 Stratospheric Ozone Depletion

Overview of Stratosphere and Ozone Layer

  • The stratosphere is the layer of Earth's atmosphere directly above the troposphere.

  • Key layers of the atmosphere: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.

  • The ozone layer, located in the stratosphere, plays a crucial role in protecting the Earth from harmful UV radiation.

Importance of the Ozone Layer

  • Ozone Formation: Ozone (O3) is a molecule composed of three oxygen atoms.

  • Location: Ozone layer is situated between 9 and 18 miles above Earth's surface.

  • Protection Mechanism: The ozone layer acts as a shield against different types of UV radiation:

    • UVA: Passes through the ozone layer.

    • UVB: Mostly filtered by ozone.

    • UVC: Completely blocked by the ozone layer.

  • Health Risks of UV Radiation:

    • Sunburn

    • Eye Damage

    • Skin Cancer

Natural Ozone Dynamics

  • Normal stratospheric conditions consist of around 21% oxygen, present as:

    • Free oxygen (single atom) and molecular oxygen (O2).

  • Formation Process: UV radiation strikes molecular oxygen (O2), breaking it into two free oxygen atoms:

    • Chemical Equation: UV + O2 → 2O

  • The free oxygen atom (O) can then react with another O2 molecule to form ozone (O3) without further solar input:

    • Chemical Equation: O + O2 → O3

  • Recycling Mechanism: Ozone can also break down into molecular oxygen and free oxygen atom under UV radiation:

    • Chemical Equation: O3 + UV → O2 + O

  • This cycle of ozone formation and breakdown maintains balance under normal conditions.

Impacts of Human Activity on Ozone Layer

  • Ozone Depletion: Human-produced chemicals, primarily chlorofluorocarbons (CFCs), significantly accelerate ozone breakdown.

  • Characteristics of CFCs:

    • Non-toxic, nonflammable, used in refrigerants, air conditioners, aerosol sprays, and styrofoam.

    • Chemical structure: carbon atom bonded to chlorine and fluorine atoms.

  • Mechanism of Depletion:

    • CFCs enter the stratosphere and are broken down by UV radiation, releasing chlorine:

      • Chemical Reaction: CFC → Cl + (other products)

    • The released chlorine reacts with ozone to form chlorine monoxide (ClO) and oxygen gas (O2):

      • Chemical Reaction: O3 + Cl → ClO + O2

    • Chlorine monoxide can react with another free oxygen atom, regenerating chlorine and creating more O2:

      • Chemical Reaction: ClO + O → Cl + O2

  • A single chlorine atom can deplete 100,000 ozone molecules, resulting in significant ozone layer thinning.

Consequences of Ozone Layer Depletion

  • Increased UV radiation reaching the Earth's surface leads to:

    • Higher incidence of skin cancer and cataracts in humans.

    • Reduced photosynthesis in plants, impacting food chains and energy availability.

  • Seasonal Trends: Ozone depletion is most pronounced at the poles, especially during the Antarctic spring (August-November) due to polar stratospheric clouds (PSCs) forming under low temperatures.