Ozone (chemically, a molecule of three oxygen atoms) is found mainly in the upper atmosphere, an area called stratosphere, between 10 and 50 km from the earth’s surface.

Ozone is present in the atmosphere in low concentrations and at places where this layer is thickest, there are not more than a few molecules of ozone for every million air molecules.

Why it is important?

The ozone layer protects life on Earth from the sun’s harmful ultraviolet (UV) rays.

The ozone molecules eliminate a big threat to life forms on Earth. UV rays can cause skin cancer and other diseases and deformities, in plants and animals

Body

You may incorporate some of the following points in the body of your answer:

• During experiments in Antarctica in the early 1980s, scientists noticed that during September-November, the concentration of ozone fell considerably lower than what was recorded in the 1950s.
• The destruction of the ozone layer became a major international issue in 1985 when the “ozone hole” was discovered over Antarctica.
• The ‘ozone hole’ is not really a hole. It is a region in the stratosphere, directly above Antarctica, where the concentration of ozone has been measured to become extremely low in certain months.

Causes

• The depletion of the ozone layer is not limited to that area and has happened in other regions of the stratosphere as well, but a set of special meteorological and chemical conditions that arise over Antarctica in the months of September, October and November.
• The 1987 Montreal Protocol on Substances That Deplete the Ozone Layer (the only universally ratified treaty in United Nations’ history), began the phase-out of CFCs in 1993.
• By 2005 the consumption of ozone-depleting chemicals controlled by the agreement had fallen by 90–95 percent in the 197 countries that were parties to the protocol.
• There are 96 chemicals are presently controlled by the Montreal Protocol, including:
• Halo-carbons (Chlorofluorocarbons (CFCs) and Halons): These were considered wonder gases because they are long-lived, non-toxic, non-corrosive, and non-flammable.
• CFC-11 remains in the atmosphere for 50 years, CFC-12 for 102 years, and CFC-115 for 1,700.
• Halon 1301 is used primarily in fire extinguishers and has an atmospheric lifetime of 65 years.
• Carbon tetrachloride: It is used as a solvent and takes about 42 years to break down in the atmosphere.
• Hydrochlorofluorocarbons (HCFCs): These were developed as the first major replacement for CFCs. While much less destructive than CFCs, HCFCs also contribute to ozone depletion. They have an atmospheric lifetime of about 1.4 to 19.5 years.
• There are various other chemicals that can be mentioned like Methyl chloroform, Hydrobromofluorocarbons (HBFCs), Methyl bromide, Bromochloromethane (BCM), etc.