6.4B Ozone depletion Notes

Dynamic balance of ozone formation and destruction

1. Stratospheric ozone concentrations have remained relatively constant for thousands of years because ozone formation and ozone destruction occur at the same rate.
2. UV radiation naturally splits oxygen molecules (O₂) into individual oxygen atoms (O), which then recombine with O₂ to form ozone (O₃).
3. When an O₃ molecule absorbs UV radiation, it breaks apart into O₂ + O, meaning the destruction process is also natural.
4. The steady state equilibrium exists because ozone destroyed = ozone produced, maintaining a stable ozone concentration under natural conditions.
5. Although the ozone layer fluctuates seasonally and geographically, long-term concentrations remain stable under natural, pre-industrial conditions.

Ozone Formation and Destruction

Formation of Ozone

1. Ozone (O₃) is formed when ultraviolet (UV-C) radiation from the Sun breaks apart molecular oxygen (O₂) in the stratosphere.
2. The freed oxygen atoms (O) then react with other O₂ molecules to form ozone (O₃).

Destruction of Ozone

1. Ozone is naturally destroyed when it absorbs UV radiation, splitting back into an oxygen molecule (O₂) and a free oxygen atom (O).
2. The free oxygen atom can then recombine with another ozone molecule, breaking it down into more oxygen molecules.
3. This cycle continuously occurs, maintaining an equilibrium concentration of ozone in the stratosphere.

Ozone-Depleting Substances (ODSs) and Their Impact on the Ozone Layer

1. Ozone-depleting substances (ODSs) accelerate the destruction of ozone molecules, disrupting the natural equilibrium between ozone formation and breakdown in the stratosphere.
2. This imbalance leads to ozone depletion, especially over polar regions like Antarctica.
3. They include chlorofluorocarbons (CFCs), halons, hydrochlorofluorocarbons (HCFCs), carbon tetrachloride and methyl bromide.
4. These substances were extensively used because they were non-toxic, non-reactive and excellent refrigerants, which made them popular in:
   1. Aerosols
   2. Refrigeration and air-conditioning
   3. Gas-blown insulating foams
   4. Fire-extinguishing systems
   5. Agricultural fumigants

Key ODSs and Their Effects

1. Chlorofluorocarbons (CFCs): Used in refrigeration, air conditioning, and aerosol propellants.
2. Halons: Found in fire extinguishers.
3. Hydrochlorofluorocarbons (HCFCs): Transitional substitutes for CFCs but still have ozone-depleting potential.
4. Methyl bromide: Used as a pesticide but phased out due to its ozone-depleting effects.

How ODSs disrupt ozone equilibrium

1. ODS molecules slowly migrate to the stratosphere because they are chemically stable in the troposphere.
2. In the stratosphere, exposure to strong UV radiation breaks ODS molecules apart.
3. This releases halogen radicals (especially chlorine and bromine) that trigger chain reactions destroying ozone molecules faster than they are formed.
4. One chlorine atom can destroy up to 100,000 ozone molecules before being removed, which massively disrupts equilibrium.