Formation of the Ozone Layer
Ozone (O₃) is a highly reactive gas composed of three oxygen atoms. It is primarily found in the stratosphere; the atmospheric layer located between 10 km and 50 km above the Earth’s surface. This region contains a thin layer of ozone, which plays a crucial role as a natural shield, filtering out harmful ultraviolet (UV) radiation from the Sun. By doing so, the ozone layer protects life on Earth from the damaging effects of UV rays.
- Since the early 1970s, the stratospheric ozone layer has significantly thinned over specific parts of the Earth, especially over the Antarctic region. This phenomenon of reduced ozone concentration is commonly referred to as the “ozone hole”.
| Ultra violet (UV) radiation, with wavelengths shorter than visible spectrum has high energy. UV radiations can be divided into three forms: UV-A (wavelength between 320–400 nm), UV-B (wavelength lesser than 280 nm), and UV-C (wavelength lesser than 280 nm). UV-C is most damaging to biological systems. |
Causes of ozone layer depletion
Ozone (O₃) layer can be destroyed both by natural and man-made causes—
(i) Natural causes:
- A number of naturally occurring substances destroy stratospheric ozone. Most important of these compounds are:
- Hydrogen oxide (HOₓ), Methane (CH₄), Hydrogen gas (H₂), Nitrogen oxides (NOₓ), Chlorine monoxide (ClO); during volcanic eruptions, significant amount of chlorine may be released in the stratosphere. Tiny particulate matter in the stratosphere, known as stratospheric aerosols, may also lead to ozone destruction.
(ii) Human activity related causes:
- Any event, which release chlorine atoms into the atmospheric, can cause severe ozone destruction, because chlorine atoms in the stratosphere can destroy ozone very efficiently. Most damaging among such agents are human made chlorofluorocarbons (CFCs), which is widely used as refrigerants and to pressurize sprays cans. In stratosphere, chlorine atoms from CFCs react with ozone to form chlorine monoxide and oxygen molecule.
Important ozone depleting chemicals and their uses
Name of the compound | Used in |
CFCs | Refrigeration, aerosol, foam, food freezing, warming devices, cosmetics, heat detectors, solvents, cosmetics, refrigerants, firefighting |
Halon | Fire fighting |
HCFC-22 | Refrigeration, aerosol, foam, fire fighting |
Methyl chloroform | Solvent |
Carbon tetrachloride | Solvent |
Effect of Ozone (O₃) Layer Depletion
The depletion of the ozone layer, especially the formation of the ozone hole, poses a serious threat because the ozone shield protects Earth from harmful ultraviolet (UV) radiation. Without it, increased UV radiation reaches the Earth’s surface, posing risks to all forms of life.
While a small amount of UV-B radiation is essential (e.g., for vitamin D synthesis in humans), excess exposure can be harmful and even fatal to many organisms. UV radiation also serves as a germicide, but its increased levels can have widespread biological and ecological consequences.
Harmful Effects on Human Beings
- Increases susceptibility to skin cancer
- Leads to higher incidence of cataracts
- Damages DNA
- Harms the cornea
- Causes retinal diseases
- Suppresses the human immune system
Harmful Effects on Plants
- Inhibits photosynthesis
- Affects plant metabolism
- Represses growth
- Destroys plant cells
- Causes mutations
- Reduces forest productivity
Harmful Effects on Other Organisms
- Marine and freshwater organisms are highly sensitive to UV rays
- Fish larvae are especially vulnerable
- Plankton populations are severely impacted
- Affects larvae of fish, shrimp, and crab species
Harmful Effects on Non-Living Materials
- Accelerates the breakdown of paints and plastics
- Alters temperature gradients in the atmosphere
- Affects global atmospheric circulation patterns
- Contributes to climatic changes
Why is Ozone Depletion Most Severe Over Antarctica?
- Colder Stratosphere and PSC Formation: The Antarctic stratosphere remains extremely cold, allowing the formation of Polar Stratospheric Clouds (PSCs) at altitudes below 20 km. These clouds provide the necessary surfaces for chemical reactions that lead to ozone depletion.
- Ozone Cooling Effect and Vortex Formation: Ozone normally absorbs sunlight, causing a rise in temperature with altitude in the stratosphere. When ozone is depleted, this warming effect is reduced, leading to further cooling. This promotes the formation of more PSCs and helps stabilize the Antarctic vortex — a tight circulation of air that traps ozone-depleting substances within the polar region.
- Longevity of Antarctic Vorte: The Antarctic vortex remains stable for a longer duration, persisting from winter into mid-spring. In contrast, the Arctic vortex is weaker and breaks up earlier in the spring. This prolonged stability in Antarctica enhances ozone loss.
Arctic Ozone Depletion
- Rising Concern in the Arctic:Ozone depletion has also been observed over the Arctic. A significant event occurred in March 1996, when Britain experienced the most severe ozone loss recorded in the northern hemisphere.
- Role of Atmospheric Cooling:Researchers believe that sudden cooling in the upper atmosphere is responsible for this phenomenon in the Arctic, where such extreme cold conditions were once rare.
- Steady Ozone Decline in the North: Since the winter of 1992, a consistent decline in ozone levels has been observed across the northern hemisphere.
- Key Factor: Low Temperatures and PSCs: While ozone-depleting chemicals are involved, the increasing frequency of cold temperatures in the Arctic stratosphere plays a crucial role. These frigid conditions promote the formation of PSCs, which enable destructive chemical reactions that reduce ozone.