Previous Year Question

The correct answer is Ozone (O₃).

In the Stratosphere, Ozone is naturally formed through photochemical reactions between solar Ultraviolet (UV) radiation and molecular Oxygen (O₂). Therefore, Ozone is the correct substance produced by the UV-driven reaction referred to in the question.

In the first step of this process, sufficiently energetic UV radiation breaks an Oxygen molecule (O₂) into two separate Oxygen atoms:

O₂ + UV radiation → O + O

The highly reactive Oxygen atom then combines with another O₂ molecule to form Ozone (O₃):

O + O₂ + M → O₃ + M

Here, M is generally a third atmospheric molecule, such as N₂ or O₂, which carries away excess energy released in the reaction and helps stabilize the O₃ molecule.

About 90% of Earth's atmospheric Ozone is present in the Stratosphere. This Ozone-rich region is commonly called the Ozone Layer. Its major importance lies in absorbing harmful Ultraviolet radiation coming from the Sun.

The Ozone Layer plays an important role in preventing a large proportion of UV-B and UV-C radiation from reaching Earth's surface. In contrast, a much larger proportion of UV-A radiation passes through the atmosphere and reaches the surface.

It is also useful to distinguish the other options. CO is mainly formed by incomplete combustion of carbon-containing fuels, while SO₂ is commonly released from combustion of sulfur-containing fuels and volcanic processes. They are not products of the UV-driven photochemical reaction in which O₂ forms Ozone.