Environmental Chemistry

Ozone Layer

The ozone layer or ozone shield is a part of Earth’s stratosphere that absorbs most of the ultraviolet radiation emitted by the Sun. It contains high concentrations of ozone \({O_3}\) than other parts of the atmosphere, but it is also very minimal compared to other gases in the stratosphere.

The ozone layer comprises less than 10 parts per million of ozone, although the average concentration of ozone in the Earth’s atmosphere as a whole is around 0.3 parts per million. The ozone layer is dominantly available in the lower stratosphere, as in 15 to 35 kilometres (9.3 to 21.7 mi) above Earth but, the thickness of the ozone layer varies seasonally and geographically.

Ozone Layer

                                                                                              Ozone Layer

What is an Ozone Layer?

The ozone layer is a layer in the Earth’s stratosphere that comprises higher ozone concentrations and protects the Earth from excessive ultraviolet radiation produced by the Sun. The ozone layer absorbs over 97 to 99 per cent of the Sun’s medium-frequency ultraviolet radiation (from around 200 nm to 315 nm wavelength), which would otherwise potentially harm exposed life near the surface.

The famous French physicists Charles Fabry and Henri Buisson in 1913 discovered the ozone layer. Sun’s measurements have shown that the radiation emitted from its surface and hitting Earth is usually consistent with the spectrum of a black body with a temperature range of 5500–6,000 K except that there was no radiation below a wavelength of about 310nm range at the ultraviolet end of the spectrum. In conclusion, they observed that something in the atmosphere was absorbing the missing radiation.

Eventually, only one known compound, ozone, matched the spectrum’s missing radiation. Its properties have been explored in detail by British meteorologist G. M. B. Dobson, who invented a basic spectrophotometer, which is useful in measuring the stratospheric ozone from the ground. Dobson developed a global network of ozone monitoring stations between 1928 and 1958, which continues to monitor ozone to date. In his honour, the “Dobson unit,” a convenient measure of the amount of ozone overhead, was named.

Source of Ozone Layer

A British physicist Sydney Chapman discovered the photochemical pathways that give rise to the ozone layer in the year 1930. Ozone in the Earth’s stratosphere forms by ultraviolet radiation, which strikes ordinary oxygen molecules containing two oxygen atoms \({O_2}\) and splits them into separate oxygen atoms (atomic oxygen); atomic oxygen is then combined with unbroken \({O_2}\) to produce ozone, \({O_3}\).

The ozone molecule is unstable (though long-lived in the stratosphere), but as ultraviolet light reaches ozone, it breaks into an O2 molecule and an individual oxygen atom, a constant process called the ozone-oxygen cycle. Chemical representation of this is:

\({O_2} + {hv_{uv}} \longrightarrow 2O\\\\\)

\({O + {O_2} \leftrightarrow {O_3}}\)

About 90% of the ozone in the atmosphere is in the stratosphere. Ozone concentrations are highest between about 20 and 40 kilometres and range from about 2 to 8 parts per million. If you compress all of the ozone to the air pressure at sea level, it would be just 3 millimetres \({\frac{1}{8}inch}\) thick.

Distribution in the Stratosphere

The ozone layers thickness differs across the world and is usually thin near the equators and thick near the poles. Thickness refers to the amount of ozone in a column over a given region which differs from season to season. The reasons for these changes are due to patterns in atmospheric circulation and solar intensity.

Most of the ozone is formed over the tropics and distributed by stratospheric wind patterns to the poles. In the northern hemisphere, these cycles, known as the Brewer-Dobson circulation, make the ozone layer thinner in the fall season and thicker in the spring.

When solar UV radiation in the tropics generate ozone, it happens so when the circulation of ozone-poor air out of the troposphere and into the stratosphere, where the Sun photolyzes oxygen molecules and transforms them into ozone. The ozone-rich air is then transported to higher latitudes and falls to lower layers of the atmosphere.

The United States research has shown that ozone levels are the highest in the spring, April, and May months and the lowest in October. Although the average volume of ozone rises from the tropics to higher latitudes, the amounts are higher in high northern latitudes than in high southern latitudes due to the ozone-hole phenomenon.

Depletion of Ozone Layer

In 1976, the atmosphere research studies revealed that the ozone layer depletion has started because of the chemicals released by industries, primarily chlorofluorocarbons (CFCs). Many concerns that increased UV radiation due to ozone depletion endangered life on Earth, including increased human skin cancer and other environmental issues, lead to chemical bans and, the latest evidence suggests that ozone depletion has reduced or stopped.

Depletion of the ozone layer is the thinning of the ozone layer present in the stratosphere. It happens as the chlorine and bromine molecules in the atmosphere come in contact with ozone molecules in the stratosphere and destroy the ozone molecules. One chlorine will damage 100,000 ozone molecules. The ozone molecules deplete at rates faster than they form.

Some compounds release chlorine and bromine when exposed to extreme ultraviolet light, which contributes to the degradation of the ozone layer. These compounds are ozone-depleting substances (ODS). Ozone-depleting compounds containing chlorine include chlorofluorocarbons, carbon tetrachloride, hydrochlorofluorocarbons and methyl chloroform. The ozone-depleting compounds that contain bromine are halons, methyl bromide and hydro Bromo-fluorocarbons. Chlorofluorocarbons are the most common ozone-depleting compounds. It is when the chlorine atom interacts with some other molecule that it does not react with ozone.

The Montreal Protocol was proposed in the year 1987 to avoid the usage, production and import of ozone-depleting compounds and, to reduce their accumulation in the atmosphere to protect the Earth’s ozone layer.

Causes of Ozone Layer Depletion

Depletion of the ozone layer is a significant problem and is due to a variety of causes. The major causes of the ozone layer depletion are:

Chlorofluorocarbons

Chlorofluorocarbons or CFCs are the primary sources of depletion of the ozone layer. Solvents, spray aerosols, refrigerators, air conditioners, etc., emit CFCs.

Nitrogen compounds

Nitrogen substances such as NO2, NO, N2O are responsible for the ozone layer depletion.

Natural Causes

Any natural phenomena, such as sunspots and stratospheric waves, also degrade the ozone layer. However, it does not affect more than 1-2 per cent depletion of the ozone layer. Volcanic eruptions are also the source of the depletion of the ozone layer.

FAQs about Ozone Layer

Q.1.  Which other planet is known to have an ozone layer?

Answer. Venus is another planet known to have an ozone layer. It has a thin layer of ozone at an altitude of 100 kilometres above the planet’s surface.

Q.2. What are the Ozone-Depleting Substances?

Answer. Ozone-Depleting Substances are substances that are responsible for ozone layer depletion, like chlorofluorocarbons, halons, carbon tetrachloride, hydrofluorocarbons, etc.

Q.3. What are the effects of ozone layer depletion?

Answer. The ozone layer has several harmful effects on the environment, such as;

Effects on animals

In animals, direct exposure to ultraviolet radiation causes skin and eye cancer.

Effects on Human Health

Humans directly exposed to harmful ultraviolet radiation from the Sun due to the depletion of the ozone layer. That leads to severe human health conditions, such as skin diseases, cancer, sunburn, cataract, rapid ageing, and a compromised immune system.

Effects on the Environment

High concentration ultraviolet rays can result in limited growth, flowering and photosynthesis in plants. Forests must also withstand the adverse effects of ultraviolet rays.

Effects on Marine Life

Planktons bear great harm from exposure to harmful ultraviolet rays. They are higher on the aquatic food chain. If planktons are gone, the species present in the lower food chain may also be affected.

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