[Chemistry Class Notes] on Ozone Layer Depletion Pdf for Exam

We have been hearing a lot of things about environmental pollution and the different factors causing this. Ozone layer depletion is considered as one of the most predominant matters that have to be taken into consideration. It is slowly taking its’ shape to emerge into a bigger challenge than humankind is ever going to face. A source of protection is getting destroyed due to human activities. We definitely know something about it. But now let’s take a look at the ozone layer depletion in a broader way. 

What is the Ozone Layer?

The ozone layer is referred to as a specific region in the Earth’s stratosphere that acts as a shield against the incoming ultraviolet rays of the sun. The ozone layer absorbs around 97-99% of the medium-frequency ultraviolet light emitted by the sun. The ozone layer is composed of 3 atoms of oxygen and is represented as O3. It forms a 20-30km layer above the surface of the earth. The stratosphere contains large amounts of ozone. Curiously enough, the ultraviolet radiation itself forms the ozone layer. Ozone forms when a radiation or electrical discharge causes the oxygen (O2) molecule to split into two different atoms so that they can individually join with other atoms and form ozone (O3). 

What is the Ozone Layer Depletion?

The ozone layer depletion came to the public eye after the creation of a chemical compound known as chlorofluorocarbons or CFCs (formerly used in refrigerators, aerosols, and air conditioners). It was discovered in the 1970s. These were used as refrigerants, aerosol spray propellants etc. CFCs are light and can move up in the air and reach the stratosphere. Here the chlorofluorocarbons react with the ozone layer in the presence of ultraviolet radiation and cause it to break down into oxygen molecules. The result is the depletion of the Ozone Layer. After an International Treaty was signed in 1973, the use of CFCs was lowered and subsequently banned. In the 1980s, it was observed that the ozone layer in an area of the Antarctic stratosphere had hit low levels coming at around as low as 33 percent of pre-1975 levels. This area became known as the Ozone hole.  

How is the Ozone Layer Getting Depleted?

One of the major causes of ozone layer depletion is the chemical chlorofluorocarbons or CFCs. CFCs are generally composed of carbon, fluorine and chlorine. They are quite durable and can sustain harsh conditions. CFCs generally don’t react but only react with sunlight when it breaks down to release chlorine. 

CF2Cl2 + UV light →CF2Cl + Cl

This chlorine reacts with the Ozone layer and forms oxygen and chlorine monoxide. The ozone depletion reactions are:

Cl + O3 → ClO + O2

When chlorine monoxide reacts with another molecule of oxygen, it breaks up again and releases chlorine which can again react with ozone and cause further depletion.

ClO + O → Cl + O2

However, it is not just CFCs that can deplete the ozone layer. Many climatic and natural circumstances can also result in ozone depletion. For example, when the Antarctic hole was spotted, scientists discovered a number of reasons why the ozone hole formed in Antarctica. In summers, methane and nitrogen dioxide react with chlorine atoms and chlorine monoxide. Thus, there is a shrinking of chlorine which sinks down and thus, prevents ozone depletion. 

ClO (g) + NO2 (g) → ClONO2 (g)(chlorine nitrate)

Cl (g) + CH4 (g) → CH3 (g) + HCl (g)

However, when winter comes, special clouds form over the Antarctic region. These are polar stratospheric clouds and they provide a nice surface for chlorine nitrates to get hydrolysed. After hydrolysis, it forms hypochlorous acid, which reacts with HCl and forms molecular chlorine. 

ClONO2(g) + H2O(g) → HOCl (g) + HNO3 (g)

ClONO2(g) + HCl (g) → Cl2 (g) + HNO3 (g)

When in spring, the sun emerges in Antarctica, the sun breaks down these clouds causing the release of chlorine and thus initiating the ozone depletion process. 

HOCl (g) →OH (g)  +  Cl(g)Cl2 (g) →2Cl(g)

One chlorine molecule from the CFC can be the cause of the destruction of up to 10,0000 ozone molecules. So imagine the rate at which our ozone layer is disappearing on earth! This will increase the speed at which depletion takes place.  

What are the Effects of Ozone Layer Depletion?

Ozone Layer depletion can result in many negative effects on human beings, plants and animal life and the ecology as a whole. It not only hammers the ecosystem, but also creates a shortage of production and an economic crisis. Think, if many plants and animals either die or get infectious due to the depletion, then there will be a food shortage. With increasing population and decreasing food shortage, production demand will rise. This will lead to poverty and will ultimately lead to death. These are certain extremities we have to think upon. 

Some of the negative impacts of ozone depletion are:

If the ozone layer gets depleted, more UV rays enter the atmosphere. When these UV rays come in contact with the human skin, it can cause malignant skin cancers. There are two types of cancers. Melanoma and non-melanoma. Melanoma is serious while non-melanoma is commonly seen. It can also cause cataracts, as the eye lens gets damaged by oxidative agents. In our body, Vitamin D is synthesized when it reacts with UV rays. Excess vitamin D can also raise blood calcium levels, increasing mortality rates. It also causes sunburn. UV radiations are also identified to play a role in breast cancer and leukaemia. It also affects the immune system.  

High UV rays have shown that there has been epidermal damage in whales due to the thinning of the ozone layer. More sun damage has been noticed in many aquatic animals due to ozone layer depletion. Diseases on the non-pigmented parts of sheep, cattle and squamous cell carcinoma are likely to occur if the depletion continues. Another UV-B related sickness found in dogs is Uber Reiter’s Syndrome. Keratoconjunctivitis (New Forest eye) is another severe condition found in cattle. 

Increased UV rays can affect plant life by damaging them under extreme exposure to UV rays. Plant growth will be affected as well. It affects the total vegetation of an area, reducing the life span of many plants. The ozone enters opening pores present in the epidermis of plants called the stomata. This stomata functions as a medium of gas exchange and photosynthesis. Damage in stomata causes a threat to the survival of plants. Ozone also negatively affects the moisture content of the soil, insects etc.