Depletion of Ozone Layer

Ozone or Trioxygen is an allotrope of oxygen that is much less stable than the diatomic allotrope (O2),  paramagnetic compared to the diamagnetic O2; and is present in low concentration in atmosphere.

At ground level, it has harmful effects on the respiratory systems of animals. However, in upper atmosphere, it creates ozonosphere, which prevents potentially damaging ultraviolet light from reaching the Earth’s surface. Ozonosphere is located 10-18 kilometres above Earth’s surface.

Factors Affecting Thickness of Ozone Layer

The thickness of the ozone layer-that is, the total amount of ozone in a column overhead-varies greatly worldwide, being thin at equator and thickest at poles. It also varies with season, being in general thicker during the spring and thinner during the autumn in the northern hemisphere. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity.

Creation of Ozone Hole

Ozone depletion has been seen everywhere beyond tropics and there is a severe depletion in the Polar Regions due to some reasons. The polar regions get a much larger variation in sunlight than anywhere else and during the 3 months of winter spend most of time in the dark without solar radiation. Due to this, the temperatures in Polar Regions go very much down.

The extremely low temperatures in Polar Regions in winter cause formation of so called Polar Stratospheric Clouds (PSCs) in the otherwise dry stratosphere there. These clouds are made of ice crystals which provide surface of many of chemical reactions. A complex interplay of chemistry, dynamics, and radiation lead to conditions conducive to significant ozone loss in the Polar Regions.

The culprit compounds are Chlorofluorocarbons as we all know. Upon reaching the stratosphere. They CFCs are subject to higher levels of ultraviolet radiation that decompose them and release atomic chlorine. Atomic Chlorine reacts with Ozone and gives out Oxygen as follows:


In the above reaction, atomic Chlorine (Cl) gets removed once it has converted an ozone molecule to Oxygen molecule but then it is regenerated through reaction of Chlorine Monoxide (CIO) with oxygen atom (O) as follows:

ClO + O→Cl + O2

The net reaction in above two sets is as follows:


The net result of the two reactions is the depletion of ozone.

Role of Chlorine Compounds

Atomic Chlorine gets regenerated again and again in the above reaction and thus plays a catalytic role. This role was not discovered until 1973. Once discovered, the efforts to bring down / ban use of Chlorofluoro Carbons was started.

Antarctic Ozone Hole

Antarctic ozone hole is an area of the Antarctic stratosphere in which the recent (since about 1975) ozone levels have dropped to as low as 33% of their pre- 1975 values. This hole occurs during Antarctic Spring (September to Early December) as the strong westerly winds start to circulate around the continent and create an atmospheric container.  In this container over 50% of the lower stratospheric ozone is destroyed.

Arctic Ozone Hole

Every March to April during the Northern Hemisphere springtime similar, but less pronounced ozone hole forms above the Arctic. There are several reasons as to why the arctic ozone hole is less prominent in comparison to the antarctic hole. Firstly, Polar Vortex, a natural circulation of wind that isolates Antarctica from rest of the world during winter is less developed over arctic. Secondly, stratospheric temperatures at Arctic, are not as low as in the Antarctic. This is a lucky proposition because formation of even a moderate ozone hole above the Arctic region can give cause for considerable concern due to the greater populations in the higher latitudes of the Northern Hemisphere. Thirdly, Earth’s magnetic field directs more positively charged solar wind particles to Earth’s south pole. These are largely hydrogen, hydrogen oxidizes to water vapor, and water vapor both destroys ozone, and blocks one path of ozone production (not really important when UV-C is not available to make ozone anyway).

Dobson units in context with Ozone Depletion

OPlease note that ozone in the atmosphere can be measured in the PPT but that is not the units of Ozone Depletion. The Ozone hole is measured in terms of reduction in the total column ozone, above a point on the Earth’s surface, expressed in “Dobson units“.

One DU is 2.69×1016 ozone molecules per square centimetre, or 2.69×1020 per square meter or 0.4462 milli moles of ozone per square meter. The base unit for an ozone hole was fixed 220 DU because total ozone values of less than 220 Dobson Units were not found in the historic observations over Antarctica prior to 1979.

Vienna Convention

Vienna convention was the first multilateral Environmental Agreement in context with the Ozone depletion. It was agreed upon at the Vienna Conference of 1985 and entered into force in 1988, thus paving the way for a legally binding treaty as its protocol called Montreal protocol. Vienna Convention itself has not placed legally binding reduction goals for the use of CFCs. Its protocol called Montreal Protocol is legally binding to all signatories.

Montreal Protocol 1989

“Montreal Protocol on Substances That Deplete the Ozone Layer” or simply Montreal Protocol is the protocol to the Vienna Convention for the Protection of the Ozone Layer. This international treaty was designed to protect the ozone layer by phasing out the production of substances believed to be responsible for ozone depletion.

  • Opened for signature on September 16, 1987
  • Ratified by 197 Countries
  • Entered into force on January 1, 1989.

The Montreal Protocol opened for signature on September 16, 1987. This date is observed as International Ozone Day every year.

Montreal Protocol stipulates that the production and consumption of compounds that deplete ozone in the stratosphere-chlorofluorocarbons (CFCs), halons, carbon tetrachloride, and methyl chloroform-are to be phased out by 2000 (2005 for methyl chloroform). These compounds significantly deplete the stratospheric ozone layer that shields the planet from damaging UV-B radiation. To date, 197 countries have signed the Protocol. As per the latest amendment, the treaty calls for complete phase out of HCFC by 2030.

It is believed that if the international community adheres to this treaty, the ozone layer will be recovered by 2050. Thus, this protocol is hailed as most successful international agreement to date.

HCFC versus HFC

Montreal Protocol currently calls for a complete phase-out of HCFCs (Hydrochlorofluorocarbons) by 2030, but does not place any restriction on HFCs (Hydrofluorocarbons). The difference between these two is of Chlorine. Hydrofluorocarbons contain only one or a few fluorine atoms are the more common type of organofluorine compounds used as refrigerants, their atmospheric concentrations are rapidly increasing, causing international concern about their rising contribution to anthropogenic radiative forcing emissions. All the HCFCs, HFCs & CHCs are now considered to be the Global Warming Potential.

India and Montreal Protocol

India became a party to the Montreal Protocol in 1992 and has been sharing the global concern for phasing out Ozone Depleting Substances. India has emerged as a global leader in promoting smooth transition for phasing out Ozone Depleting Substances (ODS).

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