Ozone Depletion — Revision Notes

⚡ 30-Second Revision

Ozone Layer: — Stratosphere (10-50 km), $O_3$ molecules.
Function: — Absorbs harmful UV-B and UV-C radiation.
ODS (Ozone Depleting Substances): — CFCs ($CF_xCl_y$), Halons (bromine-containing), Methyl Bromide ($CH_3Br$), Carbon Tetrachloride ($CCl_4$).
Mechanism: — ODS release Cl/Br atoms in stratosphere $\rightarrow$ Cl/Br act as catalysts $\rightarrow$ destroy $O_3$ molecules.

- $Cl + O_3 \rightarrow ClO + O_2$ - $ClO + O \rightarrow Cl + O_2$

Ozone Hole: — Severe thinning over Antarctic, exacerbated by Polar Stratospheric Clouds (PSCs).
Consequences (UV-B): — Skin cancer, cataracts, immune suppression (humans); damage to phytoplankton, reduced photosynthesis (ecosystems).
Solution: — Montreal Protocol (1987) - phase out ODS.

2-Minute Revision

Ozone depletion is the thinning of the Earth's protective ozone layer in the stratosphere, primarily caused by human-made Ozone Depleting Substances (ODS) like Chlorofluorocarbons (CFCs) and Halons. These stable chemicals rise to the stratosphere, where intense UV radiation breaks them down, releasing highly reactive chlorine and bromine atoms.

These halogen atoms then catalytically destroy ozone ($O_3$) molecules in a chain reaction, disrupting the natural balance of ozone formation and destruction. The most significant manifestation is the 'ozone hole' over Antarctica, intensified by unique polar meteorological conditions and Polar Stratospheric Clouds (PSCs).

The primary consequence of ozone depletion is increased penetration of harmful UV-B radiation to Earth's surface. This leads to severe health impacts in humans, including a higher risk of skin cancer, cataracts, and a weakened immune system.

Ecosystems are also affected, with damage to terrestrial plants (reduced photosynthesis) and crucial marine organisms like phytoplankton. The global community responded with the highly successful Montreal Protocol in 1987, an international treaty to phase out ODS, which has led to a gradual recovery of the ozone layer, demonstrating effective environmental governance.

5-Minute Revision

The ozone layer, located in the stratosphere (10-50 km above Earth), is a vital shield of ozone ($O_3$) molecules that absorbs the majority of harmful UV-B and UV-C radiation from the Sun. This absorption protects life from DNA damage, mutations, and other adverse effects. Naturally, ozone is formed and destroyed via the Chapman cycle, maintaining a dynamic equilibrium.

However, human activities introduced Ozone Depleting Substances (ODS) such as Chlorofluorocarbons (CFCs), Halons, and Methyl Bromide. These chemicals are extremely stable in the lower atmosphere and slowly migrate to the stratosphere.

There, intense UV radiation breaks them down, releasing highly reactive chlorine ($Cl$) and bromine ($Br$) atoms. These halogen atoms act as catalysts, meaning they participate in and accelerate the destruction of ozone molecules without being consumed themselves.

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$Cl + O_3 \rightarrow ClO + O_2$
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$ClO + O \rightarrow Cl + O_2$

The most dramatic thinning, known as the 'ozone hole,' occurs annually over Antarctica during spring. This is due to extremely cold temperatures forming Polar Stratospheric Clouds (PSCs), which convert inactive chlorine compounds into reactive forms, and the subsequent return of sunlight that triggers rapid ozone destruction.

Consequences of increased UV-B radiation include: Human Health: increased incidence of skin cancer (melanoma, non-melanoma), cataracts, and suppression of the immune system. Ecosystems: damage to DNA and reduced photosynthesis in terrestrial plants, and significant harm to phytoplankton (base of marine food web), impacting aquatic productivity. Materials: degradation of plastics and other synthetic materials.

In response to this global threat, the Montreal Protocol on Substances that Deplete the Ozone Layer was signed in 1987. This landmark international treaty successfully phased out the production and consumption of ODS, leading to a projected recovery of the ozone layer by the mid-21st century. The Kigali Amendment later addressed HFCs, which are non-ODS but potent greenhouse gases used as ODS replacements, highlighting the interconnectedness of environmental issues.

Prelims Revision Notes

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Ozone ($O_3$): — Allotrope of oxygen. Found in two main atmospheric layers:

* Stratospheric Ozone (Good Ozone): 10-50 km altitude. Forms the ozone layer. Crucial for absorbing UV-B and UV-C radiation. * Tropospheric Ozone (Bad Ozone): Ground level. A pollutant, component of smog, harmful to respiratory system and plants.

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Ozone Depleting Substances (ODS):

* Examples: Chlorofluorocarbons (CFCs), Halons, Carbon Tetrachloride ($CCl_4$), Methyl Chloroform ($CH_3CCl_3$), Methyl Bromide ($CH_3Br$). * Properties: Chemically stable, non-toxic, non-flammable (initially seen as beneficial). Contain chlorine (Cl) or bromine (Br). * Mechanism: Drift to stratosphere $\rightarrow$ UV radiation breaks them down $\rightarrow$ releases reactive Cl/Br atoms $\rightarrow$ Cl/Br act as catalysts to destroy $O_3$.

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Catalytic Cycle of Ozone Destruction (by Cl):

* $Cl + O_3 \rightarrow ClO + O_2$ * $ClO + O \rightarrow Cl + O_2$ * Net reaction: $O_3 + O \rightarrow 2O_2$. Chlorine is regenerated and can destroy many more $O_3$ molecules.

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Ozone Hole:

* Location: Most pronounced over the Antarctic region during its spring (Sept-Nov). * Contributing Factors: Extremely cold temperatures (below -78°C) leading to formation of Polar Stratospheric Clouds (PSCs). PSCs provide surfaces for converting inactive chlorine reservoirs ($HCl$, $ClONO_2$) into reactive forms ($Cl_2$). Sunlight in spring then photodissociates $Cl_2$ to release active Cl atoms.

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Consequences of Ozone Depletion (Increased UV-B Radiation):

* Human Health: Increased risk of skin cancer (melanoma, non-melanoma), cataracts, suppression of the immune system. * Ecosystems: Damage to DNA and reduced photosynthesis in terrestrial plants (affecting crop yields). Damage to phytoplankton (base of marine food web), impacting aquatic productivity and $CO_2$ absorption. * Materials: Degradation of plastics, paints, and other synthetic materials.

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International Response:

* Montreal Protocol (1987): Landmark international treaty to phase out the production and consumption of ODS. Highly successful in allowing ozone layer recovery. * Kigali Amendment (2016): Amendment to Montreal Protocol to phase down Hydrofluorocarbons (HFCs), which are potent greenhouse gases used as ODS replacements (though not ODS themselves).

Vyyuha Quick Recall

Can Harm Marine Creatures, Causing Skin Cancer, Immune Damage. Montreal Protocol Saved Ozone.

CFCs, Halons, Methyl Chloroform, Carbon Tetrachloride (ODS)
Skin Cancer, Immune Damage (Human health effects)
Montreal Protocol Saved Ozone (Solution)