Stratospheric Ozone (DP IB Environmental Systems & Societies (ESS))

Stratospheric Ozone

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

  • It is mainly found in the Earth's stratosphere

  • This is a layer of the atmosphere located approximately 10 to 50 kilometres above the Earth's surface

• Ozone plays a very important role in protecting life on Earth

  • This is because it absorbs a significant portion of the Sun's harmful UV radiation

  • This significantly reduces the amount of UV radiation that reaches the Earth's surface

• Types of UV radiation:

  • UVA:

    • Longest wavelength

    • Least harmful but can cause skin aging and contribute to skin cancer

  • UVB:

    • Medium wavelength

    • Can cause skin burns and direct DNA damage

    • Mostly absorbed by stratospheric ozone, but some reaches the Earth's surface

  • UVC:

    • Shortest wavelength

    • Most harmful

    • Completely absorbed by stratospheric ozone

Ozone Equilibrium

• The amount of ozone in the stratosphere remains relatively constant over long periods

  • This is due to a steady state of equilibrium

• Equilibrium is maintained between the processes of ozone formation and destruction

• When UV radiation from the Sun interacts with ozone molecules, some of the ozone absorbs the energy and breaks apart

• This results in the formation of an oxygen molecule (O₂) and a free oxygen atom (O)

  • This process of ozone destruction occurs naturally in the stratosphere

  • Under normal conditions, the free oxygen atom (O) can combine with another oxygen molecule (O₂) to form ozone (O₃) again

  • This ozone destruction and reformation creates a dynamic equilibrium in the stratosphere

    • There is a continuous cycle of ozone molecules being broken apart and reformed

  • This dynamic equilibrium ensures that the concentration of ozone in the stratosphere remains relatively stable over time

    • The rate of the forward reaction equals the rate of the backward reaction in the system, so the concentrations of the reactants and products remain relatively constant 

Ozone Depletion

Ozone-depleting substances

• Ozone-depleting substances (ODSs) are chemicals that cause stratospheric ozone depletion

  • These substances cause the destruction of ozone molecules

  • In other words, they enhance the natural ozone breakdown process (beyond natural levels)

• ODSs are commonly used in various human activities and products:

Sources of Ozone Depleting Substances

Imbalance in equilibrium

• When ozone formation and destruction rates are unequal, equilibrium is disrupted

  • This leads to increased ozone depletion

  • Increased UVB radiation reaches the Earth's surface

  • Affects ecosystems and human health

  • Causes increased rates of skin cancer and cataracts

  • Reduces terrestrial and marine productivity

Ozone holes

• Ozone depletion affects the entire Earth's stratosphere

  • However, ozone holes are most prominent at the poles

  • Ozone holes are areas of low stratospheric ozone

  • These holes appear every spring due to ODSs and seasonal weather patterns