Introduction 

• The ozone layer is also known as ozonosphere or ozone shield.  
• It is a region of the Earth’s stratosphere that absorbs most of the Sun’s ultraviolet radiation. 
• It contains a high concentration of ozone (O3) in relation to other parts of the atmosphere.  
• Its thickness varies seasonally and geographically.  
• The ozone layer was first discovered in 1913 by the French physicists Charles Fabry and Henri Buisson.  
• Dobson Meter can be used to measure stratospheric ozone from the ground.  
• The “Dobson unit” is a measure of the amount of ozone overhead.  
• 16^th September is designated as the International Day for the Preservation of the Ozone Layer by the United Nations General Assembly.  

 Ultra  Violet (UV) Rays  

• UV rays are the electromagnetic radiation produced by high temperature surfaces such as the Sun.  
• Cause direct damage to the genetic material or DNA of animal and plant cells and also makes the human body more susceptible to diseases.  
• The ozone layer absorbs Sun’s medium-frequency ultraviolet light which otherwise would potentially damage exposed life forms near the surface. 

 Distribution of Ozone Layer in the Stratosphere 

• It is the second major layer of the Earth’s atmosphere, just above the troposphere and below the mesosphere.  
• The thickness of the ozone layer varies worldwide and is generally thinner near the equator and thicker near the poles.  
• The reasons for these variations are due to atmospheric circulation patterns and solar intensity.  
• The majority of ozone is produced over the tropics and is transported towards the poles by stratospheric wind patterns.  
• In the northern hemisphere these patterns are known as the ‘Brewer-Dobson circulation’, which make the ozone layer thickest during the spring and thinnest during the fall.  
• Ozone is produced by solar UV radiation in the tropics by lifting ozone-poor air out of the troposphere and into the stratosphere where the sun photolysis oxygen molecules and turns them into ozone.  
• Then, the ozone-rich air is carried to higher latitudes and dropped into lower layers of the atmosphere.  
• The highest amounts of ozone are found over the Arctic in the month of March-April.  
• The Antarctic has their lowest amounts of ozone during the months of SeptemberOctober. 

 Ozone Depletion  

• Ozone layer depletion is the gradual thinning of the earth’s ozone layer in the upper atmosphere.  
• It is caused due to the release of chemical compounds containing gaseous bromine or chlorine from industries or other human activities.  
• The thinning is most pronounced in the polar regions, especially over Antarctica.  
• Many research revealed that the ozone layer was being depleted mainly because of chlorofluorocarbons (CFCs) released by Industries. 

 Ozone Depleting Chemicals  

• Chlorofluorocarbons (CFCs): Used in refrigerators and air conditioners. Halon: Used in fire extinguishers. 
• Carbon tetrachloride (CCl4): Used in fire extinguishers, refrigerant and as a cleaning agent.  
• Methyl chloroform (CH3CCl3): Solvent for organic compounds; used for cleaning metal parts and circuit boards.  
• Methyl bromide (CH3Br): Used as fumigant. Fumigation is a method of pest control by suffocating pests with poison.  
• Hydrochlorofluorocarbons (HCFCs): Used as a substitute for CFCs. They do less damage to the ozone layer than CFCs. 

 International Treaties and Cooperation  

Vienna Convention for the Protection of the Ozone Layer  

• The convention was signed in 1985.  
• It established global monitoring and reporting on ozone depletion.  
• It also created a framework for the development of protocols for taking more binding action. 

 Montreal Protocol on Substances that Deplete the Ozone Layer  

• The Montreal Protocol under the Vienna Convention (the protocol) was agreed in 1987.  
• It facilitates global cooperation in reversing the rapid decline in atmospheric concentrations of ozone.  
• Under the protocol countries agreed to phase out the production and consumption of certain chemicals that deplete ozone. 
•  The Vienna Convention and its Montreal Protocol are the first and only global environmental treaties to achieve universal ratification, with 197 parties.  
• As a result of the international agreement, the ozone hole in Antarctica is slowly recovering.  

 Kigali Amendment to Montreal Protocol 

• Kigali is the capital city of Rwanda. Kigali Amendment came into effect from 1st January 2019.  
• Under this protocol, countries are expected to reduce the manufacture and use of HFCs by roughly 80-85% from their respective baselines, till 2045.  
• It is a legally binding agreement between the signatory parties with non-compliance measures. 

 Good and Bad Ozone 

• Good ozone is in the stratosphere.  
• Good ozone protects life on earth from the sun’s harmful ultraviolet radiation.  
• Bad ozone is not emitted directly into the air but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight.  
• Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapours, and chemical solvents are some of the major sources of NOx and VOC.  
• Bad ozone is the ground-level pollutant.  
• Bad ozone is harmful to breathe, damages crops, trees and other vegetation