The depletion of the ozone layer is one of the most widely discussed environmental issues. Ozone is basically a gas that forms a protective covering in the Earth's upper atmosphere, which is known as stratosphere. The depletion of this protective layer was first noticed in 1970. Ozone is formed when three oxygen atoms combine together.
The process begins when an oxygen molecule absorbs ultraviolet photons, and breaks down into two oxygen atoms. This is known as photolysis or photo dissociation. The free oxygen atom then combines with one oxygen molecule to form a molecule of ozone (O3). The importance of the ozone layer lies in the fact that it absorbs the ultraviolet radiation of the sun between 200 to 310 nm wavelengths, and thus, prevents these radiations from reaching the Earth. While doing so, an ozone molecule breaks down into a molecule of oxygen and an oxygen atom.
But the free oxygen atom again combines with an oxygen molecule to form one ozone molecule. Thus, the formation and destruction of ozone is a continuous process, which helps keep its total amount at a balanced level. But certain substances can disrupt this process and cause the ozone layer to deplete. Ozone depletion and ozone hole are two related, but distinct condition. The exact mechanisms of ozone depletion and the formation of a hole in the ozone layer in the polar regions are different, but both involve the destruction of the ozone layer by substances like atomic chlorine and bromine.
Reasons Behind Ozone Depletion
The layer of ozone is depleting due to an increase in the level of atomic chlorine and bromine, as well as the rising levels of free radicals like hydroxyl radicals and nitric oxide radicals. The most important ozone-depleting substances are, chlorofluorocarbons (CFCs), bromofluorocarbons or halons, methyl chloroform, methyl bromide, and hydrofluorocarbons. Chlorofluorocarbons are the most important substances responsible for depleting the layer of ozone.
In Earth's atmosphere, i.e., in the lower atmosphere, chlorofluorocarbons are very stable and they do not break down easily. But when CFCs are transported to the stratosphere, they are broken down by the action of the ultraviolet radiation of the sun. When CFCs are broken down, they release free chlorine atoms. The free chlorine atom released thus, reacts with an ozone molecule to form chlorine monoxide. In the process, an oxygen molecule is also released.
Chlorine monoxide reacts with ozone to form a free chlorine atom, along with two oxygen molecules. The free chlorine atom again reacts with ozone, and repeats the series of chemical reactions, which leads to the depletion of the ozone layer. Similarly, the other halogen atoms, like free bromine atoms released due to the breakdown of methyl bromide and halons also deplete the ozone layer.
Chlorofluorocarbons are mainly found in industrial solvents, spray aerosols, perfumes, deodorants, and insecticides. They are also used in foam products, refrigerators, air conditioners, and freezers. The use of CFCs has been however, cut down significantly after the Montreal Protocol. Halons are primarily used in fire extinguishers, while methyl bromide is a pesticide.
Methyl chloroform on the other hand, is used as a solvent in various industrial processes. Other ozone-depleting substances like carbon tetrachloride is used as a solvent, and also in fire extinguishers. Hydrofluorocarbons are nowadays used to substitute CFCs due to the fact that they are less harmful to the ozone layer. But hydrofluorocarbons are green house gases that can cause global warming.
Effects of Ozone Depletion
Ozone when present in the lower atmosphere, is a pollutant and green house gas. Green house gases are known to contribute towards climate change and global warming. But in the stratosphere, ozone serves a very special purpose by absorbing the harmful ultraviolet radiation of the sun. These radiations can significantly increase the risk of certain types of skin cancers, like malignant melanoma, and basal and squamous cell carcinoma.
Even the incidence of cataracts and blindness can rise with the increased exposure to ultraviolet rays. If excess ultraviolet B radiation reach the Earth, it can adversely affect the reproduction, mobility, and the orientation of phytoplankton. Phytoplankton are found at the bottom of the aquatic food chain. A reduction in their population can create imbalance in the entire food chain, and affect the population of other organisms. The depletion of the ozone layer can also affect cyanobacteria, which help food crops like rice utilize atmospheric nitrogen.
It is true that a lot of damage has already been caused to the ozone layer. But it is still possible to reverse this process, if all the nations cooperate and come together to reduce the level of industrial emission of CFCs and other ozone-depleting substances.
In this regard, Montreal Protocol has been adopted in 1987. With the proper implementation of the provisions of the Montreal protocol, recovery is expected by 2050. The emission of CFCs has already been reduced significantly since the adoption of Montreal protocol. The need of the hour is to create greater awareness about the causes of ozone depletion, and take some urgent steps to mitigate this problem.