Shankar IAS: Summary of Ozone Depletion - Famous Books for UPSC Exam (Summary & Tests)

• It is found in two different layers of the atmosphere.
• Ozone in the troposphere is ’’bad’' because it dirties the air and helps to form smog, which is not good to breathe.
• Ozone in the stratosphere is "good" because it protects life on Earth by absorbing some of the sun's harmful Ultra Violet (UV) rays decrease in the concentration of ozone in a particular region of the atmosphere of ozone hole 
• The best example of such an ozone hole is the atmosphere over the Antarctic which has only about 50 percent of the ozone that originally occurred there.

Sources
(i) Chlorofluorocarbons (CFCs):

• CFCs molecules are made up of chlorine, fluorine and carbon.
• They are used as refrigerants (66%): Propellants in aerosol sprays, foaming agents in plastic manufacturing (30%), fire extinguishing agents, solvents for cleaning electronic and metallic components, for freezing foods etc 
• CFCs has a wide and varied application due to its properties like non- corrosiveness, noninflammability, low toxicity and chemical stability, etc. the residence time of CFCs in the atmosphere estimated to be between 40 and 150 years

(ii) The chemical reaction

• CFCs + UV radiation = freeing chlorine atoms. A free chlorine atom + ozone molecule to = chlorine monoxide (00). chlorine monoxide + atom of oxygen. = (O₂) and reformation of the free chlorine atom (Cl).
• The element that destroys 03 (Le chlorine ) is being reformed at the end of cycle.
• A single chlorine atom destroy thousands of ozone molecules before encountering reactive nitrogen or hydrogen compounds that eventually return chlorine to its reservoirs.

(iii) Nitrogen Oxides:

• The chemical reaction- Nitric oxide (NO) catalytically destroys ozone.
• Nitric oxide + ozone - Nitrogen dioxide + O₂
• Nitrogen dioxide + monoxide = Nitric oxide + O₂

_{(iv) Bromine}

• _{Containing compounds called halons and HBFCs, i.e. hydrobromo fluorocarbons [both used in fire extinguishers and methyl bromide (a widely used pesticide)].}
• _{Each bromine atom destroys hundred times of more ozone molecules than what a chlorine atom does.}

_{(v) Sulphuric acid particles
These particles free chlorine from molecular reservoirs, and convert reactive nitrogen into inert forms thus preventing, the formation of chlorine reservoirs.}

Role of polar stratospheric clouds in ozone depletion

• The ice particles of the cloud provided substrates for chemical reactions which freed chlorine from its reservoirs.
• The reaction between HC1 and C10NO₂ is very slow, but this reaction occurs at a faster rate in the presence of a suitable substrate which is provided by the stratospheric clouds at the poles.
• The PSCs not only activate chlorine, but they also absorb reactive nitrogen.
• If nitrogen, oxides were present they would combine with chlorine monoxide to form a reservoir of chlorine nitrate, (C10NO₂). 
• Every spring, a hole as big as the USA develops in the ozone layer over Antarctica, in the South pole 
• A smaller hole develops each year over the Arctic, at the North Pole.

Why is the ozone hole predominant at the Antarctic?

• The Antarctic stratosphere is much colder. The low temperature enables the formation of Polar stratospheric Clouds (PSCs), below 20 km
• The vortex is a ring of rapidly circulating air that confines the ozone depletion in the Antarctic region. The longevity of the Antarctic vortex is another factor, enhancing favourable conditions for the depletion of ozone. 
• The vortex remains, in fact, throughout the polar winter, well into midspring Whereas- the vortex in the Arctic disintegrates by the time the polar spring (March-April) arrives
• The ozone measurement instruments and techniques are varied, Some of them are the Dobson spectrophotometer and the filter ozonometer called M83, and total ozone mapping pectrometer (TOMS) in the Nimbus-7 satellite
• The Umheher technique- The most common measure of total ozone abundance is the Dobson unit (named after the pioneering atmospheric physical Gordon Dobson) which is the thickness of the ozone column ( compressed at Standard Temperature and Pressure (STP)) in milli-centimeters.