Previous Year Question

Ozone (O₃) occurs at different altitudes in Earth's atmosphere, but approximately 90% of atmospheric ozone is located in the Stratosphere. The Stratosphere is an atmospheric layer extending roughly between 10 and 50 km altitude.

Within the Stratosphere, the region containing relatively higher concentrations of ozone is known as the Ozone Layer. Its major importance lies in absorbing a large part of the harmful Ultraviolet (UV) radiation coming from the Sun.

Solar ultraviolet radiation is generally divided into three ranges:

UV-A → 315–400 nm
UV-B → 280–315 nm
UV-C → 100–280 nm

Stratospheric ozone together with atmospheric oxygen blocks extremely harmful UV-C almost completely, while ozone absorbs a large proportion of UV-B. A substantial part of UV-A can reach the Earth's surface.

During ozone formation, high-energy UV radiation first breaks an O₂ molecule into oxygen atoms. One of these oxygen atoms then combines with another O₂ molecule to form O₃ (ozone).

In simplified form:

O₂ + UV → O + O

Then:

O + O₂ → O₃

When Ozone absorbs UV radiation, it can again break into O₂ and atomic oxygen. Thus, an ozone-oxygen cycle continuously operates in the Stratosphere.

It is also important that not all Ozone is beneficial. Ozone present in the Stratosphere is commonly called “good ozone” because it protects against harmful UV radiation, whereas ground-level Tropospheric ozone can act as an air pollutant.

Among the other options:

Troposphere → the lowest atmospheric layer, where most weather phenomena occur. Ozone may be present here, but the protective ozone layer is not located here.

Mesosphere → lies above the Stratosphere; it is not the principal location of the ozone layer.

Thermosphere → lies at much greater altitude above the Mesosphere; the protective ozone layer is not located here.

Remember:

Troposphere → Weather
Stratosphere → Ozone Layer
Mesosphere → Meteors burn
Thermosphere → Ionospheric phenomena/Aurora