Ozone Depletion

Learning Outcomes:

1. Understand the dual nature of ozone, as both beneficial and harmful, depending on its atmospheric location.
2. Identify key factors and substances contributing to ozone depletion.
3. Explore the mechanisms of ozone depletion and the role of CFCs, nitrogen oxides, and other substances.
4. Comprehend the environmental impacts of ozone layer thinning on living organisms and ecosystems.
5. Recognize monitoring efforts and mitigation strategies for ozone depletion.

Ozone, an allotrope of oxygen with the chemical symbol O₃, consists of three oxygen atoms bonded in a non-linear fashion. It plays a dual role in the atmosphere: bad in the troposphere, causing air pollution and smog, and good in the stratosphere, shielding the Earth from harmful ultraviolet (UV) radiation. This chapter delves into the processes and effects of ozone depletion, the sources of destructive substances, and the implications on life and the environment.

Ozone Depletion

Change in Equilibrium

Ozone depletion refers to the significant decrease in the concentration of ozone in the atmosphere, disrupting the balance between its formation and destruction:

1. Disturbance: Several atmospheric substances react with ozone, accelerating its destruction at a rate much faster than its formation.
2. Depletion: This results in a substantial decline in ozone concentration, notably over the Antarctic, where only about 50% of the original ozone remains.
3. Discovery: The phenomenon of ozone depletion became widely recognized in 1985.

Sources of Ozone-Depleting Substances

1. Chlorofluorocarbons (CFCs):

• Composition: Made of chlorine, fluorine, and carbon.
• Usage: Widely used as refrigerants, aerosol propellants, foaming agents, fire extinguishing agents, solvents in electronic cleaning, and freezing agents.
• Applications: About two-thirds as refrigerants and one-third as blowing agents in foam products.
• Properties: Non-corrosive, non-flammable, low toxicity, and chemically stable, contributing to their widespread use.
• Longevity: Resistant to natural atmospheric removal processes; they can persist for 40 to 150 years.
• Escape: CFCs slowly evaporate from their sources, survive in the troposphere, and break down in the stratosphere when exposed to UV radiation.
• Chemical Reaction: UV exposure frees chlorine atoms, which destroy thousands of ozone molecules through a catalytic cycle.

1. Nitrogen Oxides:

• Sources: Originating from thermonuclear explosions, industrial emissions, and agricultural fertilizers.
• Destruction Mechanism: Nitric oxide (NO) catalytically breaks down ozone, forming nitrogen dioxide and oxygen.
• Nitrous Oxide (N₂O): Released from soil under specific conditions, reaches the stratosphere, decomposes into nitric oxide, and further depletes ozone.

1. Bromine Compounds:

• Compounds: Include halons and hydrobromofluorocarbons (HBFCs), used in fire extinguishers and pesticides.
• Potency: Bromine atoms are 100 times more destructive to ozone than chlorine atoms.

1. Sulfuric Acid Particles: Free chlorine from molecular reservoirs, converting reactive nitrogen into inert forms.
2. Other Substances: Includes carbon tetrachloride (a toxic solvent) and methyl chloroform (used in cleaning solvents and aerosols).

Important Note: Each molecule of chlorine or bromine released into the atmosphere has the potential to destroy thousands of ozone molecules before it becomes neutralized by other compounds.

Monitoring the Ozone Layer

Key organizations engaged in atmospheric monitoring include:

1. World Meteorological Organization (WMO).
2. World Weather Watch (WWW).
3. Integrated Global Ocean Services Systems (IGOSS).
4. Global Climate Observing System (GCOS).

Role of Polar Stratospheric Clouds (PSCs) in Ozone Depletion

Polar stratospheric clouds contribute significantly to ozone depletion:

1. Types of PSCs:

• Nacreous Clouds: Stretch 10-100 km in length, glowing with iridescence.
• Nitric Acid Clouds: Composed of nitric acid instead of water.
• Non-Iridescent Clouds: Chemically similar to nacreous clouds but form slower.

1. Chlorine Release: CFC breakdown releases chlorine atoms, which combine to form stable compounds like chlorine nitrate (ClONO₂) and HCl.
2. Activation: PSCs provide substrates for chemical reactions that free chlorine, facilitating its destructive interaction with ozone.

Ozone Depletion Mechanisms

1. Chlorine and Methane Reaction: Forms HCl and methylium cation.
2. Reaction with Nitrogen Dioxide: Forms chlorine nitrate.
3. Molecular Chlorine Formation: In PSC presence, reactions occur faster, freeing molecular chlorine, which decomposes to atomic chlorine under sunlight, continuing ozone depletion.
4. Dimer Formation: Chlorine monoxide dimers break down under sunlight, releasing free chlorine for further ozone destruction.

Concept: Catalytic Ozone Destruction – A single chlorine atom can destroy thousands of ozone molecules, significantly impacting the stratospheric ozone concentration.

Why Ozone Depletion is Predominant at the Antarctic

1. Cold Stratosphere: Antarctic’s low temperature enables the formation of PSCs, enhancing conditions for ozone destruction.
2. Ozone Absorption: Depletion cools the stratosphere, favoring PSC formation.
3. Longevity of Vortex: The Antarctic vortex, a ring of circulating air, traps ozone-depleting substances, stabilizing conditions for prolonged depletion.
4. Winter-Spring Sequence: During June-November, cold temperatures and PSC formation trigger catalytic cycles, peaking in October.

Environmental Effects of Ozone Depletion

Increased UV-B radiation due to ozone depletion impacts multiple aspects of the environment:

1. Human and Animal Health:

• Skin Cancer: Higher UV-B radiation increases the risk, particularly in light-skinned populations.
• Eye Diseases: UV radiation damages the cornea and lens, leading to eye disorders.
• Immune Suppression: Reduces immune response, increasing susceptibility to infections.

1. Terrestrial Plants:

• Photosynthesis: UV-B affects growth, development, and processes, varying across species.
• Agriculture: Necessitates breeding UV-B tolerant crop varieties.
• Biodiversity: Alters species composition, affecting ecosystem balance.

1. Aquatic Ecosystems:

• Phytoplankton: UV-B exposure impairs orientation, reducing survival rates.
• Marine Life: Affects early developmental stages of fish, shrimp, crabs, decreasing reproductive capacity.

1. Biogeochemical Cycles:

• Altered Cycles: Increased UV impacts terrestrial and aquatic cycles, affecting greenhouse gas emissions.

1. Air Quality:

• Trace Gases: Changes the concentration of hydroxyl radicals, affecting atmospheric lifetimes of climatically significant gases.

Important Note: UV-B radiation not only directly damages organisms but also induces changes in their environment, altering ecological balance and global climate dynamics.

Measurement of Ozone

Ozone is measured using instruments like the Dobson spectrophotometer, M83 filter ozonometer, and Total Ozone Mapping Spectrometer (TOMS). The Dobson Unit is a common measure, representing the thickness of ozone compressed at standard temperature and pressure.

Environmental Protection and Alternatives

CFC Substitutes:

1. Should be safe, cost-effective, energy-efficient, and have low ozone depletion and global warming potential.
2. HFC-134a is a widely adopted alternative, alongside R-143a and R-152a.

Concept: Choosing suitable substitutes is crucial to mitigate the adverse effects of ozone-depleting substances and curb global warming.

MCQ:
Which of the following compounds is most harmful to the ozone layer?

1. Carbon dioxide (CO₂)
2. Chlorofluorocarbon (CFC)
3. Nitrogen (N₂)
4. Hydrogen (H₂)
   Correct Answer: 2. Chlorofluorocarbon (CFC)

This content highlights the intricate balance between natural and human-made factors in ozone depletion, emphasizing the importance of understanding and mitigating its effects on global ecosystems and health.