Why in news?
- During September and October, 2019, the annual ozone hole over the Antarctic reached its peak extent of 16.4 million sq. km and then shrank to less than 10 million sq. km. This has been the smallest observed since 1982.
What is an ozone hole and how is it formed?
- It is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of the Southern Hemisphere spring (August–October).
- It is caused by chemicals called CFCs, i.e. Chlorofluorocarbons.
- These CFCs escape into the atmosphere from refrigeration and propellant devices and processes.
- In the stratosphere, ultraviolet light breaks the bond holding chlorine atoms (Cl) to the CFC molecule.
- A free chlorine atom goes on to participate in a series of chemical reactions that both destroy ozone and return the free chlorine atom to the atmosphere unchanged, where it can destroy more and more ozone molecules.
- Polar stratospheric clouds (PSCs) and ozone:
- In the long months of polar darkness over Antarctica in the winter, atmospheric conditions are unusual.
- An endlessly circling whirlpool of stratospheric winds called the polar vortex isolates the air in the center.
- Because it is completely dark, the air in the vortex gets so cold that clouds form.
- Unusual chemical reactions can occur only on the surface of polar stratospheric cloud particles, which may be water, ice, or nitric acid, depending on the temperature.
- These frozen crystals provide a surface for the reactions that free chlorine atoms in the Antarctic stratosphere.
- These reactions convert the inactive chlorine reservoir chemicals into more active forms, especially chlorine gas (Cl2).
- When the sunlight returns to the South Pole in October, UV light rapidly breaks the bond between the two chlorine atoms, releasing free chlorine into the stratosphere, thus ozone molecules get destroyed.
- In the early spring as polar vortex weakens, the ozone-destroying forms of chlorine disperse.
- The ozone layer stabilizes until the following spring.