12. Layer of Atmosphere possessing ozone is-
    (1) Stratosphere (2) Mesosphere
    (3) Ionosphere (4) Troposphere

The ozone layer is a crucial component of Earth’s atmosphere, playing a vital role in protecting life from the sun’s harmful ultraviolet (UV) radiation. Its location is not arbitrary—the ozone layer is specifically concentrated in the stratosphere, a region above the troposphere and below the mesosphere. This article explores the reasons behind the ozone layer’s location in the stratosphere, its functions, and why other atmospheric layers do not serve the same protective role.

The Structure of Earth’s Atmosphere

Earth’s atmosphere is divided into several layers, each with distinct characteristics and roles:

• Troposphere: The lowest layer, extending up to about 8–15 km (5–9 miles) above the surface, where weather occurs and most of the atmosphere’s mass is found.

• Stratosphere: The next layer, situated from about 10–50 km (6–31 miles) above the surface, containing the ozone layer.

• Mesosphere: Above the stratosphere, extending from about 50–85 km (31–53 miles), where temperatures decrease with altitude.

• Ionosphere: Not a distinct layer, but a region within the upper mesosphere and thermosphere where ions and free electrons are abundant, important for radio communications.

• Thermosphere: The layer above the mesosphere, extending to the edge of space.

The Ozone Layer and Its Location

The ozone layer is a region of the stratosphere with a relatively high concentration of ozone (O₃) molecules. Approximately 90% of atmospheric ozone is found in the stratosphere, specifically between about 15–35 km (9–22 miles) above Earth’s surface, with peak concentrations typically between 20–25 km (12–16 miles). This layer is sometimes called the “ozone shield” because it absorbs most of the sun’s harmful UV radiation before it reaches the surface.

Why Is the Ozone Layer in the Stratosphere?

The stratosphere provides the ideal conditions for ozone formation and stability:

• Ozone Formation: In the upper stratosphere, oxygen molecules (O₂) absorb short-wavelength UV radiation and split into highly reactive oxygen atoms. These atoms then combine with other oxygen molecules to form ozone (O₃).

• Ozone Stability: Ozone is continuously formed and destroyed in the stratosphere. The balance between these processes results in a stable concentration of ozone in this layer.

• Absorption of UV Radiation: Ozone molecules absorb longer-wavelength UV radiation (especially UVB, 280–320 nm), preventing it from reaching the Earth’s surface and protecting living organisms from its harmful effects.

• Temperature Profile: The stratosphere’s temperature increases with altitude due to the absorption of UV radiation by ozone, which also helps maintain the stability of the ozone layer.

The Role of the Ozone Layer

The primary function of the ozone layer is to absorb and block most of the sun’s harmful UVB radiation. Without this protective shield, UVB would reach the Earth’s surface in much higher amounts, causing:

• Increased skin cancer and cataracts in humans and animals

• Damage to crops and marine ecosystems

• Disruption of DNA in living organisms, leading to mutations and other biological effects

The ozone layer is therefore essential for the survival and health of most life forms on Earth.

Why Not Other Layers?

Let’s examine why the ozone layer is not found in the other atmospheric layers:

• Troposphere: Ozone is present in the troposphere but in much lower concentrations. Here, ozone is considered a pollutant and can cause respiratory problems in humans and damage to plants. The troposphere lacks the conditions for the stable accumulation of high ozone concentrations necessary for UV protection.

• Mesosphere: The mesosphere is above the stratosphere and has much lower air density. Ozone formation is minimal here, and the conditions are not conducive to maintaining a protective ozone layer.

• Ionosphere: The ionosphere is not a true atmospheric layer but a region within the upper mesosphere and thermosphere characterized by ionized particles. It does not contain significant amounts of ozone and does not provide UV protection.

• Thermosphere: This layer is even farther from the surface and is characterized by extremely low air density. Ozone does not form or accumulate here in significant amounts.

The Importance of the Ozone Layer’s Location

The location of the ozone layer in the stratosphere is critical for several reasons:

• Optimal UV Absorption: The stratosphere’s altitude and air density allow ozone to absorb UV radiation efficiently before it reaches the surface.

• Protection of Life: By filtering out harmful UVB, the ozone layer enables life to thrive on land and in the oceans.

• Atmospheric Stability: The presence of ozone in the stratosphere contributes to the temperature inversion that defines this layer, maintaining atmospheric structure and weather patterns.

Threats to the Ozone Layer

Human activities, particularly the release of ozone-depleting substances such as chlorofluorocarbons (CFCs), have caused significant thinning of the ozone layer, especially over Antarctica (“ozone hole”). International agreements like the Montreal Protocol have been instrumental in reducing the production and use of these substances, allowing the ozone layer to begin recovering.

Key Takeaways

• The ozone layer is located in the stratosphere, between about 15–35 km above Earth’s surface.

• It contains a high concentration of ozone molecules that absorb harmful UVB radiation.

• The stratosphere provides the ideal conditions for ozone formation and stability.

• Other atmospheric layers do not have the necessary conditions to sustain a protective ozone layer.

• The ozone layer is essential for protecting life on Earth from the harmful effects of UV radiation.

Conclusion

The ozone layer is a vital shield located in the stratosphere, where it absorbs most of the sun’s harmful ultraviolet radiation and protects life on Earth. Its unique position in the stratosphere allows for the formation and stability of ozone molecules, making this layer the most effective barrier against UVB. Understanding the location and importance of the ozone layer is essential for appreciating its role in maintaining the health and stability of our planet.

In summary, the correct answer is:
(1) Stratosphere.
The ozone layer is found in the stratosphere, not in the troposphere, mesosphere, or ionosphere. Protecting this layer remains a global priority for environmental and human health.