3.3 Ozone Depletion and Recovery

Ozone depletion threatens Earth's protective shield against harmful UV radiation. This atmospheric crisis, caused by human-made chemicals like CFCs, has far-reaching consequences for human health, ecosystems, and climate patterns.

The Montreal Protocol's global effort to phase out ozone-depleting substances shows promising results. While the ozone layer slowly recovers, ongoing challenges include managing existing chemical banks and regulating replacement compounds with high global warming potential.

Formation and Importance of Ozone Layer

Ozone Formation Process

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• Stratospheric ozone layer forms through photochemical reactions involving oxygen molecules and ultraviolet radiation from the sun
• Ozone (O₃) creation occurs when oxygen molecules (O₂) split by UV radiation, resulting oxygen atoms combine with other oxygen molecules
• Chemical equation for ozone formation: $O_2 + UV \rightarrow O + O$ $O + O_2 + M \rightarrow O_3 + M$ (where M represents a third molecule, typically N₂ or O₂, that absorbs excess energy)
• Ozone layer primarily located in stratosphere between 15 and 35 kilometers above Earth's surface
• Highest ozone concentration occurs around 25 kilometers altitude
• Ozone concentration typically measured in Dobson Units (DU)

Protective Functions of Ozone

• Stratospheric ozone plays crucial role protecting life on Earth by absorbing harmful ultraviolet-B (UV-B) radiation from sun
• UV-B absorption by ozone prevents DNA damage (skin cells, plant cells)
• Ozone layer reduces risk of skin cancer (melanoma, basal cell carcinoma)
• Protects plant life and ecosystems from harmful UV effects (reduced crop yields, marine phytoplankton damage)
• Influences atmospheric temperature distribution and global climate patterns by absorbing and re-emitting infrared radiation
• Helps maintain stratospheric temperature inversion important for atmospheric stability

Ozone-Depleting Substances and Sources

Major Anthropogenic Ozone Depleters

• Chlorofluorocarbons (CFCs) serve as primary ozone-depleting substances
  • Historically used in refrigerants (R-12, R-11)
  • Employed as aerosol propellants (hairsprays, deodorants)
  • Utilized in foam blowing agents (insulation materials)
• Hydrochlorofluorocarbons (HCFCs) developed as CFC replacements
  • Lower ozone-depleting potential than CFCs
  • Examples include HCFC-22 (air conditioning), HCFC-141b (foam blowing)
• Halons containing bromine serve as potent ozone-depleting substances
  • Used in fire extinguishers and fire suppression systems
  • Examples include Halon 1211 (portable extinguishers) and Halon 1301 (total flooding systems)
• Methyl bromide used as soil fumigant and pesticide in agriculture
  • Particularly effective against soil-borne pests and diseases
  • Phased out in developed countries, limited critical use exemptions remain

Additional Ozone-Depleting Compounds

• Carbon tetrachloride used as solvent and cleaning agent
  • Applications include dry cleaning and industrial degreasing
  • Ozone depletion potential approximately 0.82 relative to CFC-11
• Methyl chloroform (1,1,1-trichloroethane) employed in various industries
  • Used as solvent for adhesives, coatings, and electronic components
  • Ozone depletion potential approximately 0.1 relative to CFC-11
• Natural sources of ozone-depleting substances contribute minimally
  • Volcanic eruptions release chlorine and bromine compounds
  • Oceanic emissions produce small amounts of methyl bromide and methyl chloride
  • Biomass burning generates methyl chloride and methyl bromide

Ozone Depletion Chemistry

Catalytic Ozone Destruction Cycle

• Ozone depletion process initiates when chlorine or bromine atoms release from ozone-depleting substances through photolysis in stratosphere
• Free chlorine or bromine atoms catalyze destruction of ozone molecules through chain reactions
• Key reaction converts ozone (O₃) to oxygen (O₂) by chlorine or bromine atoms
• Catalytic cycle regenerates chlorine or bromine atoms to continue process
• General catalytic cycle for chlorine-mediated ozone destruction: $Cl + O_3 \rightarrow ClO + O_2$ $ClO + O \rightarrow Cl + O_2$ Net: $O_3 + O \rightarrow 2O_2$
• Single chlorine or bromine atom destroys thousands of ozone molecules before removal from stratosphere
• Bromine atoms approximately 60 times more effective at destroying ozone than chlorine atoms

Polar Stratospheric Chemistry

• Polar stratospheric clouds (PSCs) form in extremely cold conditions, particularly over Antarctica
• PSCs enhance ozone depletion by providing surfaces for heterogeneous reactions
• Heterogeneous reactions convert inactive chlorine reservoirs (HCl, ClONO₂) to active forms (Cl₂, HOCl)
• Example heterogeneous reaction on PSC surface: $HCl + ClONO_2 \rightarrow Cl_2 + HNO_3$
• Antarctic ozone hole forms due to unique combination of factors:
  • Extremely low temperatures (below -78°C) allowing PSC formation
  • Isolation of air masses within polar vortex
  • Return of sunlight in spring triggering rapid ozone destruction
• Arctic ozone depletion occurs to lesser extent due to warmer temperatures and less stable polar vortex

Montreal Protocol Effectiveness

Implementation and Global Cooperation

• Montreal Protocol signed in 1987 as international treaty to phase out production and consumption of ozone-depleting substances
• All 197 UN member states ratified agreement and subsequent amendments
• Protocol established gradual reduction and elimination schedule for ozone-depleting substances
  • Different timelines set for developed and developing countries
  • Example: CFC production phased out by 1996 in developed countries, 2010 in developing countries
• Kigali Amendment (2016) added hydrofluorocarbons (HFCs) to controlled substances list
  • Aims to reduce HFC consumption by 80-85% by late 2040s
  • Addresses climate change concerns as HFCs potent greenhouse gases

Observed Results and Ongoing Challenges

• Atmospheric concentrations of many ozone-depleting substances significantly decreased since peak in late 1990s
  • CFC-11 levels declined by 14% from peak values
  • CFC-12 concentrations decreased by 7% from maximum
• Ozone layer shows signs of recovery
  • Antarctic ozone hole gradually decreasing in size (maximum area reduced by approximately 20% since 2000)
  • Ozone levels increasing in upper stratosphere (3-4% per decade since 2000)
• Protocol success yielded co-benefits for climate change mitigation
  • Many ozone-depleting substances potent greenhouse gases (CFCs have global warming potentials thousands of times higher than CO₂)
  • Estimated 135 billion tonnes of CO₂-equivalent emissions avoided between 1990 and 2010
• Challenges remain in ozone protection efforts:
  • Management of banks of ozone-depleting substances in existing equipment (refrigerators, air conditioners)
  • Regulation of replacement compounds with high global warming potential (some HFCs)
  • Addressing unexpected emissions (e.g., recent increase in CFC-11 emissions traced to eastern China)
  • Ensuring continued compliance and funding for developing countries