Stratospheric ozone shields Earth from harmful UV radiation. This protective layer absorbs most UV-B and blocks UV-C, preventing DNA damage, skin cancer, and ecological harm. Understanding its chemistry is crucial for addressing ozone depletion.

The Chapman Cycle explains ozone formation and destruction in the stratosphere. Key reactions involve oxygen molecules, atomic oxygen, and ozone, establishing a dynamic equilibrium. Various factors influence ozone levels, including solar radiation, temperature, and human-made substances.

Stratospheric Ozone and UV Protection

Protection from UV radiation

• Stratospheric ozone layer forms protective shield 15-35 km above Earth's surface consisting of O₃ molecules
• UV radiation types filtered include UV-A (315-400 nm), UV-B (280-315 nm), UV-C (100-280 nm)
• Ozone's protective function absorbs 97-99% of UV-B radiation and completely blocks UV-C radiation
• Consequences of UV exposure lead to DNA damage causing skin cancer (melanoma), cataracts, immune system suppression
• Ecological impacts reduce plant growth and damage marine ecosystems (coral bleaching)

Chapman Cycle and Ozone Chemistry

Chapman Cycle reactions

• Ozone formation reactions:
  1. $O_2 + hv → O + O$ (λ < 242 nm) splits oxygen molecule into atoms
  2. $O + O_2 + M → O_3 + M$ (M is third body, usually N₂ or O₂) forms ozone
• Ozone destruction reactions:
  1. $O_3 + hv → O_2 + O$ (λ < 1180 nm) breaks down ozone
  2. $O + O_3 → 2O_2$ recombines oxygen
• Net reaction $3O_2 ⇌ 2O_3$ establishes dynamic equilibrium

Key species in ozone layer

• Oxygen (O₂) serves as source of atomic oxygen and participates in ozone formation
• Atomic oxygen (O) forms by photolysis of O₂ and reacts with O₂ to create ozone
• Ozone (O₃) acts as primary UV absorber, formed and destroyed in Chapman Cycle
• Nitrogen oxides (NOₓ) catalyze ozone depletion (NO, NO₂)
• Chlorine (Cl) and bromine (Br) act as anthropogenic ozone depleting substances (CFCs, halons)
• Hydroxyl radical (OH) serves as natural ozone depleting substance

Ozone production vs destruction

• Solar radiation intensity varies with latitude, season, time of day affecting photolysis rates
• Temperature influences reaction rates, stratospheric cooling increases ozone depletion
• Atmospheric circulation (Brewer-Dobson) transports ozone and precursors globally
• Natural variability includes solar cycles (11-year) and volcanic eruptions (sulfur aerosols)
• Anthropogenic factors encompass CFCs and climate change impacts on stratospheric conditions
• Altitude affects ozone concentration, peak ~25 km in stratosphere
• Catalytic cycles (NOₓ, ClOₓ, HOₓ) accelerate ozone destruction through chain reactions