The stratospheric ozone layer shields Earth from harmful UV radiation. It forms through a delicate balance of reactions called the Chapman cycle, where oxygen molecules split and recombine. This protective shield is crucial for life on our planet.

Ozone depletion, caused by human-made chemicals like CFCs, threatens this vital layer. These substances release chlorine in the stratosphere, catalyzing ozone destruction. The effects are far-reaching, impacting ecosystems, agriculture, and human health worldwide.

Stratospheric Ozone Chemistry

Ozone formation in stratosphere

• Chapman cycle describes ozone formation and destruction through series of reactions
• Formation of ozone occurs in two steps:
  1. Photolysis of molecular oxygen splits O₂ into atomic oxygen
     • $O_2 + h\nu \rightarrow O + O$
  2. Atomic oxygen reacts with molecular oxygen forming ozone
     • $O + O_2 + M \rightarrow O_3 + M$ (M is collision partner like N₂)
• Destruction of ozone happens via:
  1. Photolysis of ozone breaks O₃ into O₂ and O
     • $O_3 + h\nu \rightarrow O_2 + O$
  2. Atomic oxygen reacts with ozone producing two O₂ molecules
     • $O + O_3 \rightarrow 2O_2$
• Net reaction results in null cycle maintaining steady-state ozone concentration
• Solar radiation drives cycle with UV light initiating photolysis reactions
• Ozone layer forms protective shield absorbing harmful UV radiation (UVB, UVC)

CFCs and ozone depletion

• CFCs synthetic compounds containing chlorine, fluorine, and carbon used in refrigerants and aerosols
• CFCs transport to stratosphere due to long atmospheric lifetimes (50-100 years)
• UV radiation in stratosphere photolyzes CFCs releasing chlorine atoms
• Chlorine catalyzes ozone destruction through cycle:
  1. $Cl + O_3 \rightarrow ClO + O_2$
  2. $ClO + O \rightarrow Cl + O_2$
• One chlorine atom destroys thousands of ozone molecules before removal
• Other ozone-depleting substances include halons (fire extinguishers), carbon tetrachloride (solvents), methyl chloroform (industrial cleaning)
• Bromine compounds (halons) 40-100 times more destructive than chlorine

Effects of ozone depletion

• Increased UV radiation reaching Earth's surface causes numerous impacts
• Environmental effects include:
  • Damage to phytoplankton disrupts marine food chains
  • Reduced crop yields decrease agricultural productivity (soybeans, wheat)
  • Accelerated degradation of materials like plastics and paints
• Human health impacts encompass:
  • Increased risk of skin cancer (melanoma, basal cell carcinoma)
  • Cataracts and other eye problems leading to vision impairment
  • Suppression of immune system making people more susceptible to diseases
• Polar ozone holes form seasonally:
  • Antarctic ozone hole appears during spring (September-November)
  • Arctic experiences less severe but significant ozone depletion

Effectiveness of ozone agreements

• Montreal Protocol (1987) aimed to phase out production of ozone-depleting substances
• 198 countries ratified agreement demonstrating global commitment
• Amendments strengthened protocol:
  1. London Amendment (1990) added more substances and financial mechanism
  2. Copenhagen Amendment (1992) accelerated phase-out schedules
  3. Montreal Amendment (1997) established licensing system for trade
  4. Beijing Amendment (1999) included bromochloromethane
• Effectiveness shown through:
  • 98% reduction in CFC production and consumption since 1986
  • Decline in atmospheric concentrations of ozone-depleting substances
  • Projected recovery of ozone layer by mid-21st century (2050-2070)
• Challenges remain:
  • Addressing HCFCs and HFCs as replacements with high global warming potential
  • Combating illegal production and trade of banned substances (CFC-11)
• Montreal Protocol serves as model for addressing other global environmental issues (climate change)