Ozone layer depletion

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Ozone layer depletion


  • Ozone layer and its depletion
  • Chemicals involved in the production and depletion of ozone
  • Main Effects of Ozone Depletion
  • Mitigation measures to reduce ozone depletion.


produced naturally in the stratosphere (extending from about 6 to 30 miles above the Earth’s surface),

Stratospheric ozone is a layer in the Earth’s atmosphere that acts as a natural shield protecting life on Earth. This ozone absorbs between 97 and 99% of the Sun’s harmful ultra-violet (UV) rays and thus protects life on Earth. However, this “good” ozone is slowly being depleted by chemicals referred to as ozone-depleting substances (ODS), including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide, carbon tetrachloride, and methyl chloroform. These man-made chemicals are ultimately responsible for more cases of skin cancer, cataracts and other health problems.

Ozone is a naturally occurring gas found in very small amounts in the Earth’s atmosphere. Ozone is a special gas that is present both in the upper atmosphere and at ground level of the Earth. There are two different types of ozone; Ground-level ozone is present in the troposphere and stratospheric ozone.

Ground-level ozone (GOL) is located in the troposphere, close to the Earth’s surface. The troposphere is the lowest layer of the atmosphere, which extends up to an altitude of about 10 – 17 km above the ground. The temperature in the troposphere decreases with height. That is, as you go up in altitude, the temperature decreases. The troposphere is a region of intense vertical mixing in addition to horizontal winds. Ozone is the main component of urban smog that originates in emissions from industrial activities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents. Ground level ozone is formed by chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Ground-level ozone is a harmful pollutant to humans. Breathing ozone can trigger a variety of health problems, including chest pain, cough, throat irritation, and congestion.

Or it can worsen bronchitis, emphysema and asthma. Another aspect of GLO is its harmful effect on the ecosystem as it can damage crops, trees and other vegetation.

stratospheric ozone

Ozone is mainly present in the stratosphere. The stratosphere is the layer above the troposphere. It extends from the troposphere up to an altitude of about 45 – 55 km. Different



In the troposphere, the stratosphere, the temperature increases with height. The stratosphere is a region of high horizontal winds, but no vertical mixing, so it is “stratified” or layered horizontally.

Ozone also plays a very important role in protecting organisms on the Earth’s surface by blocking 95% of harmful ultraviolet (UV-B) radiation from reaching the Earth’s surface. Changes in stratospheric ozone levels can thus affect human and ecosystem health as well as the chemistry of the troposphere. From the above discussion we can see that ozone protects us from UV light and it is a greenhouse gas in itself.

stratospheric ozone abundance

Ozone is in a layer centered on the arrow

and reaches a peak abundance of about 10 parts per million, at an altitude of 30 km. Even at the peak of the ozone layer, however, it still has a lot of trace components.

6.3 Ozone production

Ozone is a deep blue, explosive and toxic gas. It is formed in the atmosphere by the action of sunlight on molecular oxygen. In the stratosphere, UV light is available which can split ordinary molecular oxygen into two atomic oxygen atoms.

O2 + UV photon –> O + O

Now, atomic oxygen is a very reactive species. It immediately connects with something else. In the stratosphere, atomic oxygen can rapidly combine with molecular oxygen (in the presence of a third body) to produce ozone or O3.

O + O2 + 3rd body –> O3 + 3rd body

Molecular oxygen is converted into ozone by a combination of the above two reactions in the presence of sunlight. In this way, ozone is continuously being formed in the stratosphere.




  ozone depletion

The role of ozone in protecting mankind from the harmful effects of ultra violet radiations is well understood. Measurements by scientists show that there is a seasonal decrease or thinning of ozone concentrations in the stratosphere over Antarctica and the Arctic. Ozone depletion in the stratosphere is a serious concern


There is a long-term threat to humans, animals and the sunlight-driven primary producers (mostly plants) that support Earth’s food chains and food webs.

Ozone is lost through the following pair of reactions:

O3 + UV photon –> O2 + O

O + O3 –> 2O2

Of these two reactions, the first reaction serves to regenerate atomic oxygen for the second reaction which converts ozone back to molecular oxygen. This second reaction is very slow. However, it can be greatly accelerated by catalytic reactions (see below). In the absence of such catalytic reactions, ozone can survive in the stratosphere for 1–10 years.

Chlorofluorocarbons (CFC) / Ozone Depletion Theory

CFCs are building up in the troposphere and slowly moving into the stratosphere

Chlorine is released by the breakdown of CFCs by sunlight in the stratosphere

Chlorine converts ozone to molecular oxygen

Depletion of ozone will lead to an increase in ultraviolet radiation (“UV-B”).

 Increased UV-B can lead to: o Increase in skin cancer

o Cataract

o damage to the immune system

o Potential crop and marine life damage

Catalytic destruction of ozone by chlorine from CFCs

Catalysis refers to the acceleration of a particular chemical reaction by a catalyst, a substance that is not destroyed in the reaction, enabling it to carry out the same accelerated effect over and over again.

The rapid catalytic destruction of ozone is best explained in terms of the well-known example of CFCs (also known as freons) in the stratosphere. Chlorofluorocarbons (CFCs) were


Developed to be colorless, odorless, non-staining, chemically inert, non-toxic, non-flammable, and a few other properties that make them excellent refrigerants, solvents, propellants for aerosol cans, and foam-blowing agents . These same properties make them essentially inactive in the troposphere.

In the stratosphere, however, CFCs can be broken down into more reactive fragments under the action of UV light. When this breakdown occurs, free chlorine is released which can catalytically destroy ozone. The process takes place in two stages:

Step 1. “Photolysis” (fragmentation by sunlight) of CFCs in the stratosphere Cl2CF2 + UV light –> ClCF2 + Cl

Step 2. Catalytic destruction of ozone

Cl + O3 –> ClO + O2 ClO + O3 –> Cl + 2O2

Note that the net effect of this pair of fast reactions is to convert two ozone molecules into three normal oxygen molecules. In the second reaction (the catalyst) atomic chlorine is recovered, making it available to start. In fact, each chlorine atom can destroy hundreds of thousands of ozone molecules.

These two steps turn a very inert chemical into a devastatingly effective destroyer of ozone. whenever the stratosphere

As free chlorine atoms are present in L, ozone is quickly depleted. Other species (such as bromine and fluorine) can also act as ozone-destroying catalysts.

In looking at this chemical it is useful to consider the typical life history of CFCs in the environment:

  1. The spray starch aerosol can is emptied
  2. CFCs expand rapidly until they are evenly distributed throughout the troposphere. It takes about a year to mix in the Southern Hemisphere, depending on weather patterns.
  3. After a few years, some of the CFCs leak into the stratosphere. At sufficient altitude (~30 km), available UV light can photolyze CFCs, releasing chlorine.
  4. Each atom of chlorine participates in the catalytic destruction of thousands



Ozone molecule.

  1. Eventually the chlorine atom reacts with the methane to form a molecule of HCl, hydrochloric acid.
  2. Some of the HCl reacts with OH again to release Cl, but a small fraction of it enters the troposphere where it can dissolve in rainwater and may be released into the atmosphere via precipitation. lost.
  3. The time scale of this process is ~100 years!

6.5 Antarctic ozone hole

The famous Antarctic ozone hole was discovered by British scientists who systematically observed ozone using a simple ground-based instrument – the Dobson meter. He published this famous figure which describes the total ozone depletion over Halley Bay, Antarctica in the month of October (Australian Spring). Farman et al. These measurements provided a wake-up call to the atmospheric science community. They were quickly verified by satellite observations and several expeditions were organized to find out what was happening in the region and during this particular time of year.

Farman et al. Paper published in 1985 showed a dramatic decrease in ozone. The year-on-year decline has more or less continued to this day.

The Antarctic ozone hole is now well understood and can be summarized as follows:

The Antarctic ozone hole is confined in space and time to the time of year when the Sun first appears above the horizon after the long polar night. During the polar winter, a polar vortex forms and the polar air mass separates from other air masses in the stratosphere. The temperature continues to drop and drop, eventually causing the stratospheric air trapped in the vortex to become very cold—in fact the coldest air found in any part of Earth’s stratosphere. In this cold vortex, polar stratospheric ice crystal clouds form. The gas phase HCl dissolves in the surfaces or sticks to the surfaces of the clouds. The CFC reacts with HCl ice, converting the relatively unreactive chlorine into the more active species, Cl2, ClONO2, and HOCl. At sunrise, in October, chlorine-containing compounds are

Photolysis, releasing highly reactive Cl atoms that attack ozone. ozone density falls

rapidly, only to recover when the polar vortex breaks up, mixing and releasing the warm air


Ozone-free air to move away from the polar region. Ozone loss is felt globally.



northern hemisphere ozone

The northern hemisphere is not immune to ozone holes. In the north, the stratospheric polar vortex is not as well formed as in the south. This is due to the large difference between land and water in the northern hemisphere. The existence of the land mass breaks the symmetry of the polar vortex in the north. However, the same processes operate to the south and satellite data show the effect occurring in March (the time of spring in the Northern Hemisphere).

Sooner or later, we will see colder than normal northern polar stratospheric temperatures in early spring and heavily populated areas will be warned of abnormally low ozone levels. Since ozone depleting compounds will remain in the atmosphere for many tens of years, we have to live with these effects. Eventually the chlorine compounds will clear themselves from the stratosphere and Earth’s ozone shield will return to normal – for the sake of our grandchildren’s children.

6.7 Potential Effects of Depleting Ozone

The primary concern is the increased levels of UV radiation reaching the Earth’s surface due to depletion of stratospheric ozone. The UV spectrum can be broken down into two parts:

UV-A: 400 – 320 nm

UV-B: 320 – 290 nm

The more energetic UV-B part of the spectrum is responsible for sunburns, cataracts, potential ecological damage, and skin cancer. It can be absorbed by glasses as well as by sunscreen and hats.

Relatively little is known or understood about the consequences of increased UV-B levels. However, we do know that a 1% decrease in ozone abundance causes an increase in UV-B of about 2%. Increased UV-B exposure to the Earth’s surface could affect humans, agricultural and forest development, marine ecosystems, biogeochemical cycles and materials. Table 1 summarizes some of the potential effects of increasing UV-B.



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