6.4 Stratospheric ozone

Ultraviolet (UV) Rays

UV rays are a type of electromagnetic radiation emitted by the Sun.

Types of UV Rays:

• UVA: Longest wavelength; penetrates the atmosphere and reaches Earth's surface.
• UVB: Medium wavelength; partially absorbed by the ozone layer.
• UVC: Shortest wavelength; completely absorbed by the ozone layer and atmosphere.

Role of UV rays in the Biosphere:

• Positive Impacts:
  • Vitamin D Synthesis: UV rays help humans and animals produce vitamin D.
  • Sterilization: Natural disinfectant for water and surfaces.
• Negative Impacts:
  • Health Risks: Causes skin cancer, cataracts, and immune suppression in humans.
  • Ecosystem Damage: Harms phytoplankton, the base of marine food chains, and affects plant growth.

Stratospheric Ozone

The ozone layer, located in the stratosphere (15–35 km above Earth), absorbs and blocks most harmful UVB and UVC rays, protecting life on Earth.

Steady-State Equilibrium:

Ozone is constantly created and destroyed in the stratosphere in a natural cycle:

1. Formation: UV radiation splits oxygen molecules (O₂) into single oxygen atoms, which combine with O₂ to form ozone (O₃).
2. Destruction: UV radiation also breaks down ozone into O₂ and single oxygen atoms.

This dynamic balance maintains a stable concentration of ozone in the stratosphere under normal conditions.

Ozone-Depleting Substances:

1. Chlorofluorocarbons (CFCs): Found in refrigerants, aerosol propellants, and foam production.
2. Halons: Used in fire extinguishers.
3. Methyl Bromide: Used as a pesticide.
4. Carbon Tetrachloride: Used in solvents and cleaning agents.

Ozone Depletion Process:

• ODS releases chlorine (Cl) and bromine (Br) atoms into the stratosphere.
• These atoms break down ozone molecules:
  1. CFCs are broken down by UV radiation, releasing chlorine atoms.