CBSE · Class 10 · Science · Our EnvironmentWhat is the ozone layer? Describe its formation in the atmosphere and explain its crucial role in protecting life on Earth. Also, discuss the causes of ozone layer depletion and the measures taken globally to check it.

Introduction to Ozone Layer\nOzone ($O_3$) is a molecule formed by three atoms of oxygen, unlike normal oxygen ($O_2$) which contains two atoms. The ozone layer is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet (UV) radiation. It is predominantly found at an altitude of 15 to 30 kilometers above the Earth's surface.

Formation of Ozone in the Atmosphere\nThe formation of ozone in the stratosphere is a photochemical process involving solar ultraviolet radiation:

1. High-energy ultraviolet (UV) radiations from the sun split molecular oxygen ($O_2$) into free oxygen ($O$) atoms: $$O_2 \xrightarrow{UV} O + O$$
2. These highly reactive free oxygen atoms combine with molecular oxygen ($O_2$) in the presence of a catalyst to form ozone ($O_3$): $$O + O_2 \rightleftharpoons O_3$$

Role in Protecting Life on Earth

• The ozone layer acts as an invisible shield for the biosphere. It absorbs harmful ultraviolet radiation—specifically UV-B and UV-C rays—coming from the sun.
• By blocking these high-energy rays, ozone prevents severe biological damage such as skin cancer, cataracts in eyes, mutations, suppression of the human immune system, and destruction of phytoplankton which forms the base of aquatic food chains.

Causes of Ozone Depletion

• Chlorofluorocarbons (CFCs): The primary cause of ozone depletion is the release of synthetic chemical compounds known as chlorofluorocarbons, which are widely used as refrigerants, in aerosol spray cans, and fire extinguishers.
• Mechanism of Depletion: When CFCs reach the stratosphere, UV rays break them down to release reactive chlorine atoms ($Cl$). A single chlorine atom acts as a catalyst and can destroy thousands of ozone molecules by converting them back into molecular oxygen: $$Cl + O_3 \rightarrow ClO + O_2$$ $$ClO + O \rightarrow Cl + O_2$$

Global Measures to Protect the Ozone Layer

• Montreal Protocol: To prevent the depletion of the ozone layer, the United Nations Environment Programme (UNEP) forged an agreement in 1987 known as the Montreal Protocol.
• Key Provisions: It successfully mandated the worldwide phasing out and reduction of the production and consumption of ozone-depleting substances (ODS) such as CFCs, halons, and carbon tetrachloride, replacing them with eco-friendly alternatives like hydrofluorocarbons (HFCs).