Ozone Layer Depletion | Is Ozone depletion specific to Antarctica?


 There’s been a rise in the global temperature in recent times and depletion of the ozone is one of the major reasons behind it. The earth’s atmosphere is divided into several layers that appear to appear in a series as follows: Troposphere, Stratosphere, Mesosphere, and Thermosphere which is further divided into Hydrosphere, Ionosphere, and Exosphere. The ozone layer is present in the stratosphere and plays a significant role because it protects the life on earth from the harmful Ultraviolet rays due to excessive exposure to the sun that can lead to an increased risk of skin cancer and impact the immune system. The UV rays are of three types, UV-A, UV-B, UV-C. As the sunlight passes through the atmosphere, all UV-C  and about 90% of UV-B are absorbed by the ozone and at the same time allow UV-A to pass through it completely. 

UV-A: It is also called black light invisible to human eyes. It causes tanning and helps in Vitamin D synthesis. 

UV-B: It causes Erythema (skin redness), cataracts, and snow blindness. The sunscreen’ SPF (sun protection factor) in actuality is the ability to protect from UV-B. 

UV-C: It is the most dangerous as it damages the DNA and is used in the inactivation of microorganisms by destroying their nucleic acid, thus, known to be germicidal and is widely used in wastewater treatment.  Thus, the Ozone layer holds great importance.

What is Ozone Depletion? 

Ozone depletion is the progressive thinning of the earth’s ozone layer in the stratosphere caused by the chemical compounds containing chlorine or bromine released into the atmosphere from industry and other anthropogenic activities. The diminishing of ozone is prominent in the polar areas, particularly over Antarctica. The ozone layer is formed when an oxygen molecule splits to form single atoms after absorbing energy from the sun. These single atoms react with other oxygen molecules to form ozone. The global average thickness of the ozone layer is 300 DOBSON or 3mm (1 DOBSON UNIT = 0.01mm).

O2 + Sunlight (wavelength< 242nm) = O + O

O + O2 = O3

Factors responsible for Ozone Depletion

In the 1970s, scientists discovered that an ozone layer has slowly been disappearing, and evidence showed that man-made chemicals were responsible for it. Chemicals such as CFCs and nitrogen oxides emanating from a range of industrial and consumer appliances, especially halocarbon refrigerants, propellants, and foam blowing agents, release chlorofluorocarbons, hydrochlorofluorocarbons, and halons that are considered to be ozone-depleting substances (ODS). The Ozone-Depleting potential (ODP) is highest for the halons > CFCs > HCFCs > FCs. These compounds also have a long lifespan making them more detrimental to the ozone.

OZONE-DEPLETING SUBSTANCES

LIFETIME (YEARS)

   Perfluorocarbons (FCs)

10000- 50000

2    Chlorofluorocarbons (CFCs)

50- 102

3    Halons

20 - 65

4    Hydrofluorocarbons (HFCs)

14.6- 18.3

5     Hydrochlorofluorocarbons (HCFCs)

2.1- 12.1


Compounds are emitted from the surface and enter the stratosphere through turbulent mixing, which mixes molecules much faster than they can settle. Through photodissociation, they release halogen atoms (chlorine, bromine), which speeds up the breakdown of the Ozone into oxygen. Halocarbon emissions have been observed to deplete the ozone layer. 

Thomas Midgley Jr., an American chemist in 1928, synthesized a group of non-flammable non-toxic chemicals called chlorofluorocarbons, called CFCs, that were safe to use as coolants in refrigerators, replacing toxic ammonia and methyl chloride that were earlier used as coolants. CFCs are mainly released from solvents or sprays. It is a highly inert compound due to the presence of chlorine and bromine. Thus, it moves through the troposphere and reaches the stratosphere where the presence of high energy breaks the chlorine bond and releases the atom. It was widespread by the 1970s in refrigerators and air conditioners, of course, but also in deodorants, hair sprays, bug sprays, and paints, Styrofoam packaging, and fire extinguishers. This lead to a release of about one million metric tons of CFCs every year, worth about $1 billion in annual sales. 

However, CFCs started to create an unrecognized problem high in the stratosphere. CFCs are harmless in the lower atmosphere,  but have a long lifespan of 40 to 150 years, as suggested by researchers Frank Sherwood Rowland and Mario Molina in 1974. 

Over time, the CFC molecules would drift up into the stratosphere, where ultraviolet light would break them down into free chlorine atoms. Upon contact with ozone, these atoms would trigger a chain reaction, causing the ozone layer to thin. The theory quickly gained scientific support. Countries including the US, Canada, Norway, and Sweden banned non-essential aerosols which contained CFCs, but the production of CFCs continued for refrigerants and other uses. However, chemical companies such as du Pont, the largest manufacturer of CFC, denied the link with ozone depletion, saying conclusive evidence had yet to be presented. 

In 1985, British scientists who had measured total over the interactive since 1957 made an extremely concerning discovery. Compared to 10 years earlier, there was 40% less ozone above the southern hemisphere in September and October. NASA satellite data soon confirmed that the ozone layer was indeed thinning, with a growing hole over Antarctica.



Why is Antarctica more prone to ozone depletion as compared to other regions? 

The pollutants are released in the troposphere. As the sunlight falls on the earth, the atmosphere heats up and so is the air in the troposphere making it warm. This warm air starts to move upward are reach the stratosphere. These pollutants, specifically the ozone-depleting substances are all over the atmosphere, but they have a significant impact in the Antarctica region. 

The ozone hole is prominent in Antarctica reason due to  three factors as follows: 

Temperature –

Antarctica lies in the south pole which is even colder than the north pole or the arctic region. The natural temperature in Antarctica can vary from – 10 degrees on the coast, to -55 degrees on the inland. It is the coldest place on earth. 

Sunlight – 

Normally,  the sunlight reacts with the ozone layer and the ozone molecules dissociate into oxygen molecules and release heat. Thus, sunlight participates in limited ozone destruction.    O3 + sunlight release heat. But as the layer reduces, the reaction between the sun and the ozone would decrease which would, in turn, bring down the amount of heat released. This is another reason for reduced temperature levels in Antarctica. 

Polar vortex – 

Antarctica has one of the strongest clockwise winds on earth, which begin blowing in May and June. An enormous ring of moving air forms over the Antarctica continent during winters as these howling stratospheric winds create a cold polar vortex. During winters, the stratosphere over the continent becomes colder than anywhere else on earth. 

Due to the spherical shape of the Earth, the speed at which the earth rotates at its axis, is highest at the equator as compared to that at the poles, simply because the wind has to cover a larger distance at equators than the poles in 24 hours. Therefore, the wind from the equator follows a curved path to reach the north and the south pole to form a polar vortex. Thus, the polar vortex is formed at extremely low temperatures and low air pressure. It occurs in the Northern hemisphere and the southern hemisphere in the Artic and the Antarctica region respectively. It whirls in an anti-clockwise direction in artic and clockwise in the Antarctica region. The temperature inside the vortex drops below -76 degrees Celsius, causing other molecules to condense into icy particles that form the polar stratospheric clouds. 

Polar stratospheric clouds - 

At about 60 degrees Southern latitude, the polar stratosphere clouds originate at a height between 10 – 25 km only during the winters and the early springs. 

Based on their particle size and formation temperature, clouds are classified into type 1 and type 2. Type 2 clouds are also called nacreous or mother-of-pearl clouds because they typically range between 10 - 200 km in length and thickness, and are comprised of ice crystals that form when temperatures are below the ice frost point, which is normally below minus 83 degrees Celsius. These clouds appear colorful as the ice crystals behave as a prism and split the light into colorful components. 

A central role played by polar stratospheric clouds in the development of the ozone hole in Antarctica is that these clouds are responsible for facilitating heterogeneous chemical reactions. During these reactions, free radicals of chlorine are produced in the stratosphere, which directly destroys ozone molecules. The chlorine reservoir molecules attach to these ice particles that make up the clouds and all through the dark long winters of July and August. More chlorine binds inside the vortex and the polar stratosphere becomes increasingly large. Ozone molecules are destroyed by chlorine atoms in normal concentrations of chlorine, but eventually, the chlorine atom reacts with the molecule of ozone and gives rise to chlorine monoxide (ClO). Chlorine nitrate (ClONO2) and hydrochloric acid HCl can be formed when this ClO reacts with nitrogen dioxide or methane, respectively. They act as stable reservoirs of Cl, which do not readily lose atoms. Here, the ozone molecules constantly form and destroy, thus, the rate of formation is almost equal to the rate of destruction of the ozone layer.

Cl3 + O3 -----à ClO

ClO  + NO2 -----àClONO2

or

ClO + CH4 ----à HCl

If the process aggravates, this reaction occurs inside the PSCs where the situation such as the presence of acids like HNO3 & H2SO4 breaks the stability of the Chlorine reservoirs (ClONO2 & HCl) and forces it to break the bonds with Chlorine atoms. Some nitric acid is naturally present in the polo stratospheric clouds but some nitric acid is formed due to the reaction as follows :

ClONO2 + HCl ---à HNO3

ClONO2/HCl ---HNO3--àCl2 = Cl + Cl

Cl+ O3 ---à ClO + ClO

ClO + ClO ----à Cl2O2 + O2

Toward the end of August, the sun begins to rise, and the conditions change rapidly. This Dichlorine dioxide (Cl2O2) breaks easily in the presence of sunlight and gives rise to more Chlorine atoms that attack the ozone molecules, which causes the ozone molecule to be destroyed inside the vortex, resulting in an ozone hole. Ozone holes appear in the spring season due to the sun's radiation breaking the bond between chlorine and chlorine molecules. The resulting attack on ozone reduces the thickness of the layer. All these factors contribute to the ozone hole.

Initiatives For control and prevention of ozone depletion. 

Recognizing the deleterious effects of ozone layer depletion, in 1985, the Vienna Convention, a multilateral environmental agreement, was signed regarding the depletion of the ozone layer and provided a framework for the international reduction of CFCs as a consequence of their destructive nature on the ozone layer, resulting in higher skin cancer rates. It entered into force in 1988. 

The World leaders came together to create the landmark Montreal Protocol, signed in Montreal Canada in 1987 which became effective in 1989 on substances that deplete the ozone layer. Signatory nations to this treaty agreed to phase out the use of ozone-depleting chemicals by monitoring and reporting on their efforts to protect the ozone layer. CFC production was eventually banned in 197 countries and amendments were made to the module protocol to phase out other ozone layer-damaging chemicals. 

In 1958, du Pont agreed to phase out CFC manufacturing by the early 2000s. As a result of these efforts, the ozone hole has been steadily shrinking on average and has been relieved, which is a result of the Montreal Protocol largely working. Over the years, the level of ozone-depleting chemicals has dropped steadily and their production has ceased. This protocol has been considered to be the most successful one.  Following the CFC ban, hydrofluorocarbons were used in place of various ozone-depleting substances such as CFCs and HCFCs. While they do not deplete the ozone layer, they are potent greenhouse gases that contribute to climate change. As a result, on the 15th of October 2016, the Montreal Protocol made an amendment called the Kigali amendment that called for the phase-out of HFCs. 

Several estimates suggest that the ozone hole may disappear by 2065 if all nations continue to meet their treaty obligations 

As the world becomes more conscious about the harmful effects of CFCs and other harmful substances, the production and consumption of these substances have decreased to a significant level, which helps to heal the ozone layer and ensure it returns to its original form. However, as socially responsible citizens, we need to do our part to maintain the balance of the earth.

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