Context:

• Before we get into the topic, let’s get one thing straight – Ozone hole does not mean there is a hole in Earth’s atmosphere. It, instead, refers to the depletion of ozone molecules in the stratosphere over Antarctica. The recurring springtime Antarctic ozone hole was first reported in a study in 1985 by British Antarctic Survey scientists, Joesph Farman, Brian Gardiner, and Jonathan Shanklin. The news sent shockwaves across the globe. It sparked debates about pollution, ultraviolet rays and our role in the depletion of ozone. It also brought together all nations to commit themselves to phase out ozone-depleting agents.

WHATis ozone?

• Earth is surrounded by an atmosphere which comprises five layers of gases held together by gravity. They include the troposphere, the stratosphere, mesosphere, thermosphere and finally the exosphere. The atmosphere consists of a mixture of 78 per cent nitrogen, 21 per cent oxygen and one per cent of other gases. Each layer has its own role in protecting Earth and supporting life on the planet. Our concern here is about the second layer – the stratosphere. It is the most stable region, where jets fly. It is also the region where ozone, a gas made of three oxygen atoms (O3), is abundant.
• The region in the stratosphere that contains relatively higher concentration of ozone (in comparison to the other parts of the atmosphere) is called the ozone layer. This layer, found about 10 to 40 kilometres above the ground, acts as a shield absorbing most of the sun’s ultraviolet radiation. It protects life on Earth from the harmful effects of UV rays. However, at the ground level, ozone, a powerful oxidant, is a respiratory hazard and a pollutant. Ozone is created in the stratosphere when high energy UV radiation causes the O2 molecule to split. The free oxygen atoms react with other O2 molecules to form O3. Interestingly, the very rays that the ozone deflects help create it. It is important to register here that the amount of ozone in the stratosphere can vary with location, season and day-to-day weather condition.

WHATis the spring-time depletion?

• The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. They also invented an instrument to measure the amount of ozone in the atmosphere. A decade before the discovery of the hole, scientists observed a steady depletion of ozone in the stratosphere. In 1985, it was discovered that the depletion is at an alarming rate over the polar regions, especially over Antarctica, during spring. This seasonal depletion is called the spring-time depletion.
• Every spring, the atmospheric ozone is rapidly destroyed by chemical processes over the Antarctic. The reduction is usually up to 70 per cent. First observed in 1985, the trend has been continuing. As you read this article, the ozone layer is likely to deplete further, as spring kicks in during September and lasts till December in the Antarctic. In the Arctic however, the greatest decline so far has been 30 per cent and the amount lost varies every year. Ozone hole refers to an area in the atmosphere where the amount of ozone present is less than 220 Dobson units.

WHATcauses ozone depletion?

• Ozone depletion occurs when chlorofluorocarbons (CFCs) and halons — gases found in aerosol spray cans and refrigerants — are released into the atmosphere. Other ozone-depleting chemicals include carbon tetrachloride and trichloroethane. Anaesthetics, fire-fighting equipment and the manufacture of materials such as styrofoam also release these substances. These chemicals can remain in the atmosphere for decades to over a century.
• When CFCs reach the atmosphere, they are first degraded by UV rays. Degradation of CFC releases free chlorine atoms. The basic cause of ozone layer depletion is that these chlorine atoms then break up ozone molecules. Ozone then disappears. The chlorine atoms are recreated in subsequent reactions. One chlorine atom can destroy 1,00,000 ozone molecules. Similarly, degradation of halons releases free bromine atoms, which destroy ozone in the same way as chlorine.

During spring

• As we saw earlier, ozone is constantly produced by the action of the sun’s ultraviolet radiation on oxygen molecules. Although ozone is created primarily in tropical latitudes, large-scale air circulation patterns in the lower stratosphere move ozone toward the poles, where its concentration builds up.
• The extreme cold conditions in the polar regions produce polar stratospheric clouds. CFCs attach to ice particles in clouds. When the sun comes out again in the polar spring, the ice particles melt, releasing the ozone-depleting molecules.
• Once released, these ozone-destroying molecules break up ozone molecules.
• Reductions of up to 70 per cent in the ozone column were observed in spring over Antarctica and first reported in 1985.

Source:TH