Sunday, September 16, 2012

What is Ozone Hole?

The United Nations Environment Program (UNEP) celebrates 16th September as “World Ozone Day” since 1995.  The day commemorates the signing of the “Montreal Protocol” – an international treaty to phase out substances that deplete the ozone. The day creates awareness about the ozone layer which protects the earth from possible catastrophic events. It also reminds us about the harm, human activities can cause to the environment of the Earth. 

But what is ozone and who discovered it? The word “ozone” comes from a Greek word ozein which means ”to smell”. It is a naturally occurring gas in the atmosphere. A molecule of ozone consists of three atoms of oxygen and hence its chemical formula is O3. It was discovered in 1840 by a German-Swiss chemist Christian Friedrich Schoenbein.

Where is it present in our atmosphere? Ozone is present in two regions of our atmosphere. These two regions form 99.99 % of our atmosphere. Let us consider the earth’s atmosphere to be a (say) 140 storied building. Let each storey be one km tall. The region occupied by the first 16 floors is then called troposphere. The region from 16th floor to 50th floor is called stratosphere. Approximately 10% of the total ozone resides in the troposphere. The remaining 90% of the ozone lies in the stratosphere. Since most of the ozone resides in the stratosphere, this ozone is often referred as ozone layer.

How is ozone produced and destroyed in the atmosphere? The sources of ozone in the troposphere are burning of fossil fuels and other reactions involving natural gases and atmospheric pollutants. It is destroyed by natural reactions and by human-produced chemicals. In the stratosphere, ozone is produced naturally in two steps. In the first step oxygen molecule (O2) is broken by sunlight into two oxygen atoms (O + O).  In step two, an oxygen atom (O) reacts with another oxygen molecule (O2) to form ozone (O3). This production is balanced by destruction. It could happen naturally when O3 is broken by sunlight or it reacts with other naturally occurring or human produced chemicals.

Is ozone harmful? An increased amount of ozone in the troposphere can reduce crop yield and forest growth. An inhalation of ozone can cause multiple respiratory diseases, reduce lung capacity, coughing, throat irritation and can lead to death owing to its poisonous nature. It could also increase the temperature of Earth’s surface as it is also a greenhouse gas. Thus it is an atmospheric pollutant whose presence is troposphere is harmful and hence could be termed bad ozone.

On other hand, the stratospheric ozone absorbs the harmful solar rays – in particular the ultra-violet (UV) rays –and prevents them from reaching the Earth. The UV rays can interfere with skin immune system and cause skin cancer, mutate chromosomes, damage cornea, kill single celled organisms, damage macromolecules such as proteins and nucleic acid which are the characteristic of a living cell. They can also be harmful to the surface cells of plants. This could result in stunted growth of plants and decrease the crop yield. The UV rays could also destroy the phytoplankton thus harming the base of food chain. Since the stratospheric ozone prevents these events, it could be termed as good ozone.

Is the concentration of ozone same in the atmosphere? NO! It changes with location and time. The concentration is lowest over the equator and maximum at poles.  

But what is ozone hole and who discovered it? “Ozone hole” is the dramatic decrease in the concentration of ozone over the Antarctic stratosphere. The event occurs only in spring when the Sun returns to the Antarctic. The formation of “Ozone Hole” over the Antarctic was first reported in an article in the international journal “Nature” by J. C. Farman, B. G. Gardiner, and J. D. Shanklin. These scientists working for the British Antarctic Survey at their research station at the Halley Bay (76oS) were keeping a close watch on it since late 1970s and finally reported in “Nature”.
How does ozone hole occur? The first step in the formation of ozone hole is emission of ozone depleting gases (Chlorofluorocarbons). These un-reactive gases travel with the atmosphere. The atmosphere moves in loops from equator to poles like an elongated giant wheel. There is one giant wheel of atmosphere for the northern hemisphere and another for the southern hemisphere.  The giant wheels rise in the tropics, travels towards poles and then descends over the poles. After coming down to poles, it moves back to the tropics over the surface.

As the CFCs rise in the stratosphere they are converted into reactive gases by UV radiation and travel towards the poles through the atmosphere. The poles have night for six months and day for 6 months. The winter rules during the night period while summer rules during the day period. During the winter period these gases freeze on clouds of  ice crystals formed due to extremely low temperatures over the Antarctic. As the Sun returns to the Antarctic in the spring the ice crystals melt and these gases now participate in catalytic reactions which starts destroying ozone in large amount- thus causing “ozone hole”. In the recent past similar events have been found to occur over the Arctic stratosphere as well. “Ozone Hole” leads to the flooding of the harmful UV radiations from the Sun to the Earth. It is very similar to a big hole in umbrella, making it useless either for summer or rainy season. Hence it is called “Ozone Hole”.

The ozone hole raised the hackles for it challenged the habitation on this planet. A flurry of scientific research revealed that chlorofluorocarbons (CFCs) were the main culprits causing ozone depletion over the Antarctic. The fluorinated hydrocarbons were developed 1930 by General Motors Research Laboratories in a search for non-toxic, non-flammable refrigerant to replace chemicals that were in use. Freon (Du Pont, US) and Arcton (ICI, UK) are trade names for CFCs. These were termed as revolutionary invention because of they were cheap to produce, easy to store, non-flammable, non-explosive, non-reactive with other gas molecules – in short perfect business product for minting money. CFCs were primarily used as propellants in spray cans, blowers for making soft foams, refrigerants and coolants. Since the atmosphere moves from tropics to poles concerns were raised about their increased usability, especially in the developed countries of the world and immense potential to cause ozone destruction. The scientists proposed that the CFC molecules would be broken in the stratosphere by the high energy sun radiations and the favorable meteorological conditions in the Antarctic (and sometimes the Arctic) during winter and then in spring would lead to a series of catalytic reactions leading to depletion of ozone thus forming an ozone hole. The fears were confirmed with the observation of large ozone depletion in the Antarctic in the 1980s.

Fortunately the international community realized the seriousness of threat due to ozone depletion to the planet Earth. Consequently an agreement was reached in 1987 by the international community in what is known as Montreal Protocol. It came into force in 1989. It called for a systematic phasing out of the halocarbons with large ozone depleting potential in developed as well as developing countries. It is believed that if the international agreement is adhered to, the ozone layer would recover by 2050. Due to its widespread adoption and implementation it has been hailed as an example of exceptional international co-operation with Kofi Annan – former Secretary General of the United Nations quoted as saying that “perhaps the single most successful international agreement to date has been the Montreal Protocol”. India acceded to the Vienna Convention for the Protection of the Ozone Layer in 1991 and ratified the Montreal Protocol in 1992.
The treaty has set milestone years for phasing out of the chemicals causing ozone depletion. For Example - The year 2010 was the final dead line year all the countries to phase out the CFCs in their respective lands. The CFCs have been replaced by other chemicals which do have ozone depleting potential albeit less than CFCs. The protocol calls for phasing out of the new chemicals as well without being discriminatory to the developing world by 2030. Hopefully the Kyoto Protocol (to limit green house has emissions) will see the light of the day and become as successful as Montreal Protocol. Amen!!

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