Ozone Depletion (HL) (DP IB Environmental Systems & Societies (ESS))

Ozone Depletion

• Ozone depletion refers to the gradual thinning of the ozone layer in the Earth's stratosphere

• This is due to human-made chemicals called ozone-depleting substances (ODSs)

  • These ODSs are released from products such as refrigerants, aerosols, and solvents

• One of the most well known groups of ODSs are chlorofluorocarbons (CFCs)

How CFCs cause ozone depletion

• CFCs are stable compounds and do not readily degrade in the lower atmosphere (troposphere)

• As CFCs are released into the atmosphere, they gradually drift upward into the stratosphere

• CFCs absorb UV radiation in the stratosphere

• This causes them to break down, releasing chlorine radicals

• Stage 1:

  • A chlorine radical (Cl•) reacts with an ozone (O₃) molecule to form an oxygen (O₂) molecule and a chlorate radical (ClO•)

Cl• + O₃ → ClO• + O₂

• Stage 2:

  • The chlorate radical then combines with another ozone molecule to produce another chlorine radical and two oxygen molecules

ClO• + O₃ → Cl• + 2O₂

• Overall reaction:

  • Overall, as a result of these two stages, ozone is converted to oxygen

2O₃ → 3O₂

• As long as chlorine radicals are present in the stratosphere, ozone molecules will be continuously broken down through this cycle of reactions

  • The chlorine radicals are not used up in these reactions (the act as catalysts)

  • As a result, a single chlorine radical can destroy thousands of ozone molecules before being removed from the stratosphere