What is Ozone Layer Depletion?

Ozone (O₃) layer depletion refers to the process of thinning of ozone layer in the stratosphere due to the presence of Ozone Depleting Substances (ODS) emitted due to anthropogenic activities. The major ODS are chlorofluorocarbons (CFCs), halons, carbon tetrachloride, methyl bromide, and methyl chloroform. CFCs are mainly used as coolants in air conditioners and refrigerators, propellants in aerosol cans and as in manufacture of insulation and packaging materials. Halons are used in fire retardants, methyl bromide in pesticides and methyl chloroform and carbon tetrachloride in industrial solvents. All these substances release either chlorine or bromine and cause catalytic destruction of ozone in the stratosphere. The scientists (Sherwood Rowland, Mario Molina and Paul Crutzen) who explained the process of ozone layer depletion due to ODS were awarded Nobel Prize in Chemistry in 1995.

The chlorine that are released from the ODS cause the destruction of ozone molecule in the presence of sunlight in a couple of chain reactions:

• Cl (Chlorine) + O₃ (Ozone) = ClO (Chlorine monoxide) + O₂ (Oxygen)
• ClO (Chlorine monoxide) + O (Oxygen) = Cl (Chlorine) + O₂ (Oxygen)

The chlorine released in the above process (2nd step) goes on to break another molecule of ozone. Like this, one molecule of chlorine can destroy over 1,00,000 molecules of ozone over a period of 50 years.

Microsoft Word – EVS. SEM-2 _Lesson 10-15_

Maximum ozone layer depletion (thinning of ozone layer) occurs in Antarctica between September and November every year. Two important conditions favor the ozone depletion during these seasons: (i) Presence of sunlight in the Polar Regions, and (ii) Development of circumpolar vortex (mass of cold air around the southern polar region). The presence of sunlight promotes the catalytic destruction of ozone while the development of circumpolar vortex causes the water vapor in the atmosphere to cool rapidly and form ice crystals. The ice crystals thus formed adhere to each other to form ‘Polar Stratospheric Clouds’ that in turn provide a platform for the chlorine and bromine to destroy ozone molecules. When the circumpolar vortex breaks down after November, the ozone-depleted air spread to the neighbouring regions of Australia, New Zealand and South America and starts thinning the ozone layer out there.

Recognizing the disastrous consequences of using ODS, a worldwide agreement was signed in 1987 in Montreal, popularly known as ‘Montreal Protocol’ to ban the use of ODS globally, and CFCs. It was initially decided in the agreement that 50% reduction in CFC production would be achieved by 1998 and would be replaced by hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). Unlike CFCs, HFCs and HCFCs do not destroy ozone molecules. The satellite measurements revealed in 1997 that ODS had started declining in the atmosphere and ozone layer has started recovering. In the absence of further use of ODS, the ozone layer is slated to fully recover by 2050.