Ozone is a gas that is naturally present in our atmosphere. Most ozone (about 90%) is found in the stratosphere while most of the remaining ozone, about 10% is located in the troposphere layer.
Ozone gas is perceptibly blue in colour and has a characteristic pungent smell. Ozone in the stratosphere is considered good for humans and other life forms because it absorbs Ultraviolet-B (UV-B) radiation from the sun. UV-B radiation is harmful to a variety of life forms. In humans, increased exposure to UV-B radiation increases the risks of skin cancer, cataracts and a suppressed immune system. Excessive UV-B exposure can also damage terrestrial plant life and aquatic ecosystems.
Ozone Layer :
The stratospheric region with the highest ozone concentration is commonly known as the ‘Ozone Layer’. The highest concentrations of ozone occur at altitudes from 26 to 28 km in the tropics and from 12 to 20 km towards the poles. The thickness of the ozone layer is about 3 to 5 mm but it pretty much fluctuates depending upon the season and geography. The peak concentration of ozone in the stratosphere can be around 40 parts per billion (ppb).
Ozone can also be present in the troposphere near Earth’s surface in excess of natural amounts. It is considered as bad ozone. It is formed by reactions involving human-made pollutant gases. High ozone exposure caused by air pollution reduces crop yields and forest growth. In humans, exposure to high levels of ozone can reduce lung capacity, and cause chest pains, throat irritation and coughing. In addition, an increase in tropospheric ozone leads to a warming of the Earth’s surface because ozone is a Green House Gas (GHG).
Ozone Depletion:
Ozone layer depletion is a reduction in the amount of ozone in the stratosphere. Massive ozone destruction by up to 65% in the stratosphere over Antarctica and the Arctic in late winter and early spring is also referred to as Ozone Hole.
Ozone hole is created due to some ozone-depleting gases. These gases are released on the earth’s surface by human activities. Then they rise to the stratosphere. These gases then undergo photochemical reactions with the stratospheric ozone. This leads to the depletion of the ozone layer. Chlorofluorocarbons, halons, methyl chloroform, carbon tetrachloride and hydrofluorocarbons are some examples of ozone-depleting gases.
In 1985, an ozone hole was discovered in Antarctica. In 2006, a 2.5 million km2 ozone hole was detected in Tibet. Again in 2011, an ozone hole appeared over Tibet and Hindkush.
Why has the “ozone hole” appeared over Antarctica when ozone-depleting gases are present throughout the stratosphere?
It is because very low temperatures of the Antarctic stratosphere create ice clouds called polar stratospheric clouds (PSCs). Special reactions occur on PSCs. These produce ozone holes in the Antarctic. Winters are more affected by ozone layer depletion.
Process of ozone Layer Depletion: -
Ozone is a poisonous gas made up of molecules consisting of three oxygen atoms. This gas is extremely rare in the atmosphere, representing just three out of every 10 million molecules.90% of ozone exists in the upper atmosphere or stratosphere.
The ozone layer in the atmosphere absorbs most of the harmful ultraviolet-B (UV-B) radiation from the sun. It also completely screens out the deadly UV-B radiation. The ozone shield is thus essential to protect life.
Depleting the ozone layer allows more UV-B to reach the earth. The result would be an increase in skin cancers, eye cataracts, weakened immune systems, reduced plant yields, damage to ocean ecosystems and reduced fishing yields, and adverse effects on animals.
In the 1970s, scientists discovered that CFCs, used as refrigerants and aerosol propellants, finally break apart in the atmosphere and release chlorine atoms. This causes ozone depletion. Bromine atoms released by halons (used in fire extinguishers) have the same effect. CFCs and halons are examples of Ozone Depleting Substances (ODS).
When measurements started in the early 1980s, scientists noticed that the ozone layer over the Antarctic was weakening. The land area under the ozone-depleted atmosphere increased steadily to more than 20 million sq km in the early 1990s and has varied between 20 and 29 million sq km since then. In 2000, the area of the ozone hole reached a record 29 million sq km.
Ozone depletion in the stratosphere occurs through a series of steps. The process of ozone depletion is discussed as under-
- Firstly, there will be the emission of halogen source gases from compounds like chlorofluorocarbons (CFCs) and Halons. These halogen source gases are often called Ozone Depleting Substances (ODS). The Ozone Depleting Substances tend to accumulate in the lower atmosphere, i.e.troposphere.The accumulation occurs in the troposphere because most source gases are highly unreactive in the lower atmosphere.
- Natural air Motion transport the Ozone Depleting Substances (ODSs) to the stratosphere.
- In the stratosphere, halogen sources gases are chemically converted to reactive halogen gases like Chlorine Monoxide and Bromine Monoxide by UV radiation from the sun (Conversion occurs in the stratosphere instead of the troposphere because solar UV radiation is more intense in the stratosphere).
- Reactive halogen gases will cause the depletion of stratospheric ozone over the globe through multiple chemical reactions.
What is the Connection between Climate Change and Ozone Depletion?
Stratospheric ozone depletion has an influence on climate change since both ozone and the compounds responsible for its depletion are active greenhouse gases. In turn, climate change will affect ozone depletion through changes in atmospheric conditions that alter the chemical production and loss of stratospheric ozone.
Consequences of Ozone Depletion
The most obvious cause for concern about ozone loss is the fact the ozone layer acts as a filter for the Sun’s ultraviolet radiation.
UV-B is known to have serious effects on humans, animals, plants and materials. Most of these effects are damaging. Only a few are sufficiently well-understood at present for the impact of enhanced UV-B to be quantified.
Human Health: The link between UV-B and the incidence of skin cancer is particularly emotive. Melanoma - the particular form of skin cancer as reported in many areas is actually much rarer than other types, but it is the most serious type with a substantially high mortality rate. The incidence of the more common non-melanoma skin cancer is loosely correlated with long-term UV-B exposure. Cataracts in the eyes, reduction in reproductive power and loss of immunity are some of the other harmful effects.
Terrestrial Plants: Typically, sensitive plants show reduced growth and smaller leaves. Unable to photosynthesize as efficiently as other plants, they yield smaller amounts of seeds or fruit. In some cases, these plants also show changes in their chemical composition, which can affect food quality.
Aquatic Ecosystems: Enhanced UV-B certainly damages a range of small aquatic organisms - zooplanktons, larval crabs, shrimps and juvenile fishes.
Climate: There would be no stratosphere without the ozone layer. Thus any depletion of stratospheric ozone is predicted to cool this region and hence change the temperature regime of the atmosphere to a significant extent.
The International Initiative against the Depletion of the Ozone Layer-
Intergovernmental negotiations for an international agreement to phase out ozone-depleting substances started in 1981 and concluded with the adoption of the Vienna Convention for the protection of the Ozone layer in March 1985.
Vienna Convention for the Protection of the Ozone Layer (1985)
The treaty was signed by 20 nations that agreed to take appropriate measures to protect the ozone layer from human activities.
The Vienna Convention encouraged intergovernmental cooperation on research, systematic observation of the ozone layer, monitoring of CFC production, and the exchange of information. The Convention committed the signatories to take general measures to protect human health and the environment against human activities that modify the ozone layer.
The Vienna Convention was a framework agreement and did not contain legally binding controls or targets. However, it set an important precedent. For the first time, nations agreed in principle to tackle a global environmental problem before its effects were felt, or even scientifically proven.
In May 1985, British scientists published their discovery of severe ozone depletion over the Antarctic. Their findings were confirmed by American satellite observations and offered the first proof of severe ozone depletion. The discovery of the ozone ‘hole’ shocked the world. It is regarded as one of the major environmental disasters of the twentieth century.
The governments now recognized the need for stronger measures to reduce the production and consumption of a number of CFCs and several halons. As a result, the Montreal Protocol on Substances that Deplete the Ozone Layer was adopted in September 1987.
Montreal Protocol on Substances that Deplete the Ozone Layer
It was signed in 1987. It came into force in 1989.
The objective of the protocol is the protection of the ozone layer through control of global production and consumption of Ozone Depleting Substances (ODSs)
Important Features of the Montreal Protocol:
Montreal protocol provided for the use of Hydrofluorocarbons (HFCs) as substitute compounds for Chloro Fluoro Carbons (CFCs), and other ozone Depleting Substances. However, Hydrofluorocarbons and all ODSs are greenhouse gases and contribute to human-induced climate change.
Ninety-six chemicals are presently controlled by the Montreal Protocol and phase-out schedules. The Protocol was designed so that these schedules could be revised on the basis of periodic scientific and technological assessments.
In 2009, the Montreal Protocol became the first UN Treaty to achieve universal ratification (that is, by all the member countries), demonstrating the world’s commitment to ozone protection, and more broadly, to global environmental conservation.
Implementation of the Montreal Protocol progressed well in developed and developing countries. All phase-out schedules were adhered to in most cases, some even ahead of schedule. Its implementation led to the phase-out of around 98% of ozone-depleting chemicals.
The Impact of the Montreal Protocol
Montreal Protocol came into effect on 16th September 1987. It is celebrated every year as the International Day for the Preservation of the Ozone Layer or World Ozone Day.
The Montreal Protocol has been an outstanding success.
It is estimated that without the Protocol, by the year 2050, the amount of Ozone Depleting Substance (ODS) in the atmosphere would have been five times greater. Ozone depletion would have risen to at least 50% in the northern hemisphere’s mid-latitudes and 70% in the southern mid-latitudes, about 10 times worse than current levels.
The Montreal Protocol has resulted in the phasing out of Ozone Depleting Substances (ODSs). As most of these substances are also potent greenhouse gases, the Protocol has delivered substantial climate benefits too.
The success of ozone protection has been possible because science and industry were able to develop and commercialize alternatives to ozone-depleting chemicals. Developed countries ended the use of CFCs faster and with less cost than was originally anticipated.
Frequently Asked Questions (FAQs)
Question: What is the ozone layer?
Answer - The stratospheric region with the highest ozone concentration is commonly known as the ‘Ozone Layer’. The highest concentrations of ozone occur at altitudes from 26 to 28 km in the tropics and from 12 to 20 km towards the poles.
Question: What is the ozone layer made of?
Answer - Ozone, a triatomic molecule of oxygen is formed from oxygen naturally in the upper levels of the earth’s atmosphere by high energy Ultraviolet (UV) radiation from the Sun. UV radiation breaks down oxygen molecules, releasing free atoms, some of which bond with other oxygen molecules to form ozone. 90% of ozone exists in the upper atmosphere or stratosphere.
Question: In which year ozone layer is destroyed?
Answer- In 1985, an ozone hole was discovered in Antarctica. In 2006, a 2.5 million sq. km ozone hole was detected over Tibet. Again in 2011, an ozone hole appeared over Tibet and Hindkush.
Question: What destroys the ozone Layer?
Answer - An ozone hole is created due to some ozone-depleting gases. Chlorofluorocarbons(CFCs), Halons, Methylchloroform, carbon tetrachloride and Hydrofluorocarbons are some examples of ozone-depleting gases.
Question: How important is the ozone layer?
Answer - The formation of stratospheric ozone is initiated by ultraviolet (UV) radiation coming from the sun ozone in the stratosphere is considered good for humans and other life forms because it absorbs Ultraviolet-B (UV-B) radiation from the sun.
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