Ozone Layer and Ozone Hole Explained for UPSC: Formation, Depletion, and Antarctic Ozone Hole

What is Ozone (O₃)?

• Ozone (O₃) is a molecule made of three oxygen atoms.
• It is found mainly in the stratosphere between roughly 10–50 km above the Earth’s surface.
• A small amount of ozone is also present in the troposphere, but there it behaves as a pollutant and is harmful to human health (causes lung issues, breathing problems).

The stratospheric ozone layer, however, acts like a protective shield for life on Earth.

Role of the Ozone Layer: Protection from UV Radiation

The ozone layer absorbs harmful ultraviolet (UV) radiation coming from the Sun, especially UV-B and UV-C with wavelengths less than 0.4 microns.

Without it:

• Skin cancers
• Eye damage
• Reduced crop productivity
• Damage to marine ecosystems

…would all increase.

So even though ozone is toxic at ground level, in the stratosphere it is life-saving.

How is Ozone Formed and Destroyed? (Chapman Cycle – Concept)

Ozone in the stratosphere is continuously:

• Formed
• Destroyed
• And kept in dynamic balance

This process is often explained through the Chapman Cycle (you don’t need equations for UPSC, just the idea):

1. High-energy UV rays split O₂ (oxygen) into individual O atoms
2. These O atoms combine with O₂ → form O₃ (ozone)
3. Ozone itself can again absorb UV and break down

This constant formation + breakdown maintains a natural equilibrium of ozone — until humans interfere.

Measuring Ozone: What is a Dobson Unit (DU)?

Ozone concentration in the atmosphere is measured in Dobson Units (DU).

• If all the ozone over a given area were compressed to standard pressure and temperature, it would form a thin layer.
• 1 Dobson Unit = 0.01 mm thickness of pure ozone at standard conditions.

Typical values:

• Tropical regions: around 250–280 DU
• Polar regions (Antarctica): normally higher, around 300–350 DU

When it drops sharply below this, we start talking about ozone depletion and ozone hole conditions.

Where is Ozone Produced and Where is it Concentrated?

Important nuance for UPSC:

• Most ozone is produced over the tropics (equatorial region)
  because this is where sunlight and UV radiation are strongest.
• But maximum ozone concentration is found over the polar regions, especially Antarctica.

Why?

Because of stratospheric circulation (sometimes referred to as Dobson circulation/winds):

• Winds move ozone from tropical regions → towards the poles
• Over the Antarctic, cold temperatures + higher pressure help accumulate and retain more ozone

So:

Ozone is produced mostly in the tropics, but concentrated more at the poles.

This is a favorite conceptual angle for UPSC.

Why is the Ozone Hole Strongest Over Antarctica?

We often hear about the “ozone hole” over Antarctica. Let’s clarify two things:

1. There is no actual physical hole.
   It means severe thinning of the ozone layer.
2. The ozone values fall from normal 300–350 DU to even 100 DU or less over Antarctica in spring.

Reasons:

1. Very low temperatures
   • Antarctica is extremely cold, especially in winter.
   • Cold conditions affect chemical reactions and allow the formation of polar stratospheric clouds (PSCs), which support ozone-destroying reactions.
2. Presence of ozone-depleting substances (ODS)
   • These include chlorofluorocarbons (CFCs) and other halogen compounds.
   • They are mostly emitted in tropical and mid-latitude regions,
     but transported to polar areas by stratospheric winds.
3. Lower concentration of NOx and HOx in polar regions
   • In tropics and mid-latitudes, NOx (nitrogen oxides) and HOx (hydroxyl radicals) also react with ozone and participate in its removal, but they also help regulate some chlorine chemistry.
   • In the polar stratosphere, under very cold conditions, the balance shifts and chlorine and bromine compounds (from CFCs & halons) dominate ozone destruction.
4. Stable polar vortex
   • During Antarctic winter, a strong polar vortex (circulating wind system) traps air masses, allowing ozone-depleting reactions to intensify.
   • When sunlight returns in spring, massive ozone destruction occurs — this is when the ozone hole becomes most pronounced.

So the Antarctic region becomes a “trap” for ozone-depleting substances, leading to strong, seasonal ozone depletion.

“Ozone Hole” is a Misnomer

The term “ozone hole” does not mean:

• Zero ozone
• Or an actual gap in the atmosphere

It simply means:

• Ozone concentration falls dramatically below normal levels
• The effective thickness of the ozone layer reduces
  (from around 3 mm equivalent → to less than 1 mm in extreme cases)

UPSC may test this conceptual clarity in both Prelims MCQs and Mains.

Key UPSC-Relevant Points (Quick Recap)

• Ozone formula: O₃
• Layer location: Mainly in the stratosphere (10–50 km)
• Function: Absorbs UV radiation, protects life
• Measured in: Dobson Units (DU)
• Formed mainly over: Tropics
• Concentrated mainly over: Poles (especially Antarctica)
• Ozone hole: Severe thinning, not a physical hole
• Main cause of depletion: CFCs and other ODS, carried to poles via stratospheric circulation
• Most pronounced ozone hole: Over Antarctica (Southern Hemisphere)

Check out our GS Geography Course – Click Here

FAQs

1. #OzoneLayerUPSC – What is the ozone layer and in which part of the atmosphere is it mainly found?
2. #OzoneMeasurement – What is a Dobson Unit and how is it used to measure ozone concentration in the atmosphere?
3. #OzoneHoleAntarctica – Why is the ozone hole more pronounced over Antarctica compared to other parts of the world?
4. #CFCsAndOzoneDepletion – How do chlorofluorocarbons (CFCs) and other ozone-depleting substances lead to stratospheric ozone depletion?