Stratospheric Ozone Depletion and Reducing Ozone Depletion

1. Learning Objectives

  • Objective STB-4.A: Explain the importance of stratospheric ozone to life on Earth.

    • STB-4.A.1: The stratospheric ozone layer is important for the evolution of life and the continued health and survival of life on Earth.
    • STB-4.A.2: Stratospheric ozone depletion is caused by:
    • Anthropogenic factors: Such as chlorofluorocarbons (CFCs).
    • Natural factors: Such as the melting of ice crystals in the atmosphere at the beginning of the Antarctic spring.
    • STB-4.A.3: A decrease in stratospheric ozone increases the UV rays that reach the Earth's surface, leading to health issues such as skin cancer and cataracts in humans.

  • Suggested Skill 1.A: Describe environmental concepts and processes.

2. Importance of Stratospheric Ozone

  • Ozone in the stratosphere absorbs UV-C and much of the UV-B radiation.

  • Without the ozone layer, life on land would not be possible due to significant tissue damage and DNA mutations caused by UV-B and UV-C radiation.

  • Human Health Benefits:

    • Prevention of Skin Cancer: UV-B and UV-C radiation can cause DNA mutations, leading to skin cancer.
    • Prevention of Cataracts: UV radiation causes oxidative stress in the eyes, potentially leading to cataracts.

  • Understanding Tropospheric Ozone: In contrast to stratospheric ozone, tropospheric ozone is a respiratory irritant, damaging to plant tissue, and is a precursor to photochemical smog.

3. Mechanism of Ozone Absorption

  • How Ozone Absorbs UV-B and UV-C:
    • UV-C breaks O$_2$ into two free oxygen atoms (2 O).
    • A free oxygen atom from this reaction combines with an O$2$ molecule to form ozone (O$3$).
    • UV-C can also reverse this reaction by breaking ozone (O$3$) into O$2$ and O, which can bond with another free O to reform O$_2$.
    • The continued formation and breakdown of O$_3$ in the stratosphere absorbs all UV-C and much UV-B radiation, thus protecting organisms on Earth.

4. Anthropogenic Ozone Depletion

  • Chlorofluorocarbons (CFCs):
    • CFCs are a primary anthropogenic cause of O$_3$ breakdown.
    • Used as refrigerants and propellants in aerosol containers (e.g., hair spray, Febreeze).
    • UV radiation causes a free chlorine atom to separate from CFCs.
    • The highly electronegative chlorine atom bonds to one of the oxygen atoms in ozone (O$3$), converting it to oxygen (O$2$).
    • The free oxygen atom then bonds with chlorine monoxide to form O$2$ and results in the free chlorine atom breaking down more O$3$.
    • A single chlorine atom can persist in the atmosphere for 50-100 years and can destroy up to 100,000 ozone molecules.

5. Natural Ozone Depletion

  • During the Antarctic spring, the melting of ice forms polar stratospheric clouds (PSC).
    • PSCs are composed of water and nitric acid (HNO$_3$) and can only form in the extreme cold temperatures (around -1000°F) found above Antarctica.
  • In the presence of PSCs, chlorine nitrate (ClONO$2$) and hydrochloric acid (HCl) react to produce Cl$2$.
    • Chlorine gas (Cl$_2$) is then photolyzed (broken down by sunlight) into two free chlorine atoms.
  • Free chlorine atoms act similarly to those derived from CFCs, breaking down O$3$ into O$2$ repeatedly.

6. Learning Objectives for Alternatives

  • Objective STB-4.B: Describe chemicals used to substitute for chlorofluorocarbons (CFCs).

    • STB-4.B.1: Ozone depletion can be mitigated by replacing ozone-depleting chemicals with substitutes that do not deplete the ozone layer. Hydrofluorocarbons (HFCs) are examples of such replacements but some strong greenhouse gases.

  • Suggested Skill 7.B: Describe potential responses or approaches to environmental problems.

7. Reducing Ozone Depletion

  • The main method to reduce anthropogenic O$_3$ depletion involves phasing out and replacing CFCs.
    • Montreal Protocol (1987): A global agreement aimed at phasing out the production of CFCs in refrigerators, aerosols, and other applications.
    • CFCs are being replaced with Hydrochlorofluorocarbons (HCFCs)—CFCs that have hydrogen added.
  • Replacement for HCFCs: Hydrofluorocarbons (HFCs) are considered, although they still serve as greenhouse gases; however, they are not ozone-depleting since they do not contain chlorine.
    • HCFCs continue to deplete O$_3$ but to a lesser degree than CFCs.
    • The replacements are not permanent solutions but rather transition options to phase out in developed nations after 2020 and developing nations by 2030.
    • Replacements for HFCs: Hydrofluoroolefins (HFOs), which are basically HFCs with shorter atmospheric lifetimes and lower global warming potential (GWP).