Ozone Depletion – Vienna Convention, Montreal Protocol – Why does Ozone deplete mainly at the poles ?

Ozone (O3) is a naturally occurring gas found in the stratosphere – in the range of 15 to 35 kilometres above earth’s surface, called the ozone layer or the ozone shield. The ozone layer absorbs around 98 per cent of medium frequency ultraviolet rays coming from the sun, which otherwise can potentially damage life forms on the earth’s surface. Ozone depletion, or thinning of the ozone layer due to conversion of O3 to O2 (oxygen), is a major threat to environment because it increases the harmful UV rays coming to earth’s surface, thereby increasing chances of skin cancer, eye cataracts, and even genetic damage.

Ozone depleting substances are mainly carbon compounds containing Chlorine or Bromine, such as Carbon tetrachloride, hydro bromo fluoro carbons (HBFCs), Hydro Chloro Fluoro Carbons (HCFCs). These substances are banned under the Montreal Protocol on Substances that Deplete the Ozone Layer, 1987.

Vienna Convention for the Protection of the Ozone Layer 1985

The Vienna Convention is a framework convention for international efforts to protect the ozone layer. It does not contain legally binding goals, protocols to the convention can be created for that purpose. Legally binding terms are laid in the accompanying Montreal Protocol.

Montreal Protocol to the Vienna Convention 1987

  • Legally binding treaty. It is counted among the most successful treaties of all time for its effectiveness in reducing ozone depletion.
  • Its effectiveness has been a result of commitment of the nations and also the Multilateral Fund, a financing instrument to help countries implement the treaty.

As a result of the effective implementation of the Montreal Protocol, the once growing Ozone hole over antarctica has recovered significantly and continues to recover.

Kigali agreement 2016 brought HFCs (Hydro Fluoro Carbons) into the ambit of the Montreal Protocol, even though they are not ozone depleting. HFCs have ozone depletion potential of zero but they are very potent greenhouse gases. HFCs had already been targeted for reduction by the Kyoto Protocol under UNFCCC framework (to reduce greenhouse gas emissions). Their further coverage under Montreal Protocol would effectively curb their release into the environment.

Why does ozone layer deplete primarily at the poles ?

The chemistry of polar vortex has created severe ozone depletion. The nitric acid in polar stratospheric clouds reacts with chlorofluorocarbons to form chlorine, which catalyses the photochemical destruction of ozone. Chlorine concentrations build up during winter (as polar vortex is stronger in winter), and subsequently maximum depletion occurs when the sun shines in the spring.

Polar Stratospheric Clouds (Nacreous clouds) act as catalyst for reactions, turning chlorine from other less reactive forms into free radicals, thus greatly speeding the destruction of ozone.

Thus, ozone depletion is more severe wherever the ozone depleting substances can accumulate. This also explains why ozone depletion at the north pole is much less severe than at the south pole, because there is greater air exchange between the Arctic and the mid-latitudes, which disperses the ozone depleting substances to wider areas.

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