Atmospheric Pollutants And The Reactions

Step out on a sunny day, and instead of being blinded by brightness, you are surrounded by a thick haze. There is no sun to be seen, and your eyes sting from the air. It is not a scene out of a horror movie but a real-life situation in most urban regions of the world. The other major environmental issue across the world is atmospheric pollution, which presents immense threats to human health, ecosystems, and climate. Everything around, including the air we breathe, is getting more and more polluted with hazardous by-products that can originate from various sources.

This Story also Contains

1. Tropospheric pollution
2. Atmospheric Pollution: Gaseous Air Pollutants
3. Oxides of Carbon :
4. Atmospheric Pollution: Global Warming and Acid Rain
5. Global Warming
6. Acid Rain
7. Atmospheric Pollution: Particulate Pollutants
8. Particulate pollutants:
9. smog
10. Formation of photochemical smog
11. Stratospheric Pollution
12. Polar Stratospheric Clouds
13. The Ozone Hole
14. Effects of Depletion of the Ozone Layer
15. Some Solved Examples
16. Conclusion

They include industrial emissions, vehicle exhaust, and agricultural procedures. Their impact is not only in degrading the air quality but also carries effects that seriously damage the environment, like global warming and acid rain. In this article, various atmospheric pollutants have been taken into account with respect to sources, chemical reactions, and consequences affecting our homes. We will lead off with gaseous air pollutants, the constituents of carbon monoxide and nitrogen oxides, and how they combine to form secondary pollutants like ozone. We'll explore two of the most important environmental topics: global warming and acid rain, with some detail on their main driving chemical processes.

The troposphere is a turbulent, dusty zone containing air, much water vapour and clouds. This is the region of strong air movement and cloud formation. The stratosphere, on the other hand, contains dinitrogen, dioxygen, ozone and little water vapour.

Atmospheric pollution is generally studied as tropospheric and stratospheric pollution. The presence of ozone in the stratosphere prevents about 99.5 per cent of the sun’s harmful ultraviolet (UV) radiations from reaching the earth’s surface and thereby protecting humans and other animals from its effect.

Note: The gas leaked from a storage tank of the Union Carbide plant in Bhopal gas tragedy was Methyl Isocyanate.

Tropospheric pollution

The tropospheric pollution is caused by two types of particles, viz,

1. Gaseous pollutants:

These are those kinds of pollutants that exist in gaseous form. Common examples are oxides of sulphur, nitrogen, carbon, hydrogen sulphide, etc.

2. Particulate pollutants:

These are those kinds of pollutants that exist as particles. Some examples include dust, mist, fumes, smoke, smog, etc.

Atmospheric Pollution: Gaseous Air Pollutants

Gaseous air pollutants are gaseous substances causing contamination of the atmosphere. They pose a potential danger to human beings' health and the environment. These air pollutants are basically caused by industrialization processes, transportation, and agricultural practices that are all linked with human activities. Some common gaseous air pollutants include carbon monoxide, nitrogen oxides, sulfur dioxide, and volatile organic compounds. Most of the gases that are emitted react with other atmospheric constituents to form secondary pollutants, including ground-level ozone and photochemical smog. The behaviour and interaction of these gaseous air pollutants in the atmosphere are very vital pieces of information in devising strategies to mitigate their adverse effects on air quality and human well-being.

Gaseous air pollutants:

These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.

Oxides of Sulphur:

Oxides of sulphur are produced when sulphur-containing fossil fuel is burnt. The most common species. sulphur dioxide is a gas that is poisonous to both animals and plants. It has been reported that even a low concentration of sulphur dioxide causes respiratory diseases e.g., asthma, bronchitis, and emphysema in human beings. Sulphur dioxide causes irritation to the eyes, resulting in tears and redness. A high concentration of SO₂ leads to stiffness of flower buds which eventually fall off from plants. Uncatalysed oxidation of sulphur dioxide is slow. However, the presence of particulate matter in polluted air catalyses the oxidation of sulphur dioxide to sulphur trioxide.

$
2 \mathrm{SO}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) \rightarrow 2 \mathrm{SO}_3(\mathrm{~g}
$

The reaction can also be promoted by ozone and hydrogen peroxide.

$
\begin{aligned}
& \mathrm{SO}_2(\mathrm{~g})+\mathrm{O}_3(\mathrm{~g}) \rightarrow \mathrm{SO}_3(\mathrm{~g})+\mathrm{O}_2 \mathrm{~g} \\
& \mathrm{SO}_2(\mathrm{~g})+\mathrm{H}_2 \mathrm{O}_2(\mathrm{l}) \rightarrow \mathrm{H}_2 \mathrm{SO}_4(\mathrm{aq})
\end{aligned}
$

Oxides of Nitrogen:

Dinitrogen and dioxygen are the main constituents of air. These gases do not react with each other at a normal temperature. At high altitudes when lightning strikes, they combine to form oxides of nitrogen. NO₂ is oxidised to nitrate ion, NO₃ − which is washed into soil, where it serves as a fertilizer. In an automobile engine, (at high temperature) when fossil fuel is burnt, dinitrogen and dioxygen combine to yield significant quantities of nitric oxide (NO) and nitrogen dioxide ( NO₂ ) as given below:

$\mathrm{N}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) \xrightarrow{14 \mathrm{R} \mathrm{K}} 2 \mathrm{NO}(\mathrm{g}$
NO reacts instantly with oxygen to give $\mathrm{NO}_2$
$2 \mathrm{NO}(\mathrm{g})+\mathrm{O}_2(\mathrm{~g}) \rightarrow 2 \mathrm{NO}_2 \mathrm{~g}$
The rate of production of $\mathrm{NO}_2$ is faster when nitric oxide reacts with ozone in the stratosphere.

$
N O(g)+O_3(\mathrm{~g}) \rightarrow N O_2(\mathrm{~g})+O_2(\mathrm{~g}
$

The irritant red haze in the traffic and congested places is due to oxides of nitrogen. Higher concentrations of $\mathrm{NO}_2$ damage the leaves of plants and retard the rate of photosynthesis. Nitrogen dioxide is a lung irritant that can lead to acute respiratory disease in children. It is toxic to living tissues also. Nitrogen dioxide is also harmful to various textile fibres and metals.

Hydrocarbons:

Hydrocarbons are composed of hydrogen and carbon only and are formed by incomplete combustion of fuel used in automobiles. Hydrocarbons are carcinogenic, i.e., they cause cancer. They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs.

Oxides of Carbon :

(i) Carbon monoxide:

Carbon monoxide (CO) is one of the most serious air pollutants. It is a colourless and odourless gas, highly poisonous to living beings because of its ability to block the delivery of oxygen to organs and tissues. It is produced as a result of the incomplete combustion of carbon. Carbon monoxide is mainly released into the air by automobile exhaust. Other sources, which produce CO, involve incomplete combustion of coal, firewood, petrol, etc. The number of vehicles has been increasing over the years all over the world. Many vehicles are poorly maintained and several have inadequate pollution control equipment resulting in the release of greater amounts of carbon monoxide and other polluting gases. Do you know why carbon monoxide is poisonous? It binds to haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. In blood, when the concentration of carboxyhaemoglobin reaches about 3–4 per cent, the oxygen-carrying capacity of blood is greatly reduced. This oxygen deficiency results in headaches, weak eyesight, nervousness and cardiovascular disorder. This is the reason why people are advised not to smoke. In pregnant women who smoke, the increased CO level in blood may induce premature birth, spontaneous abortions and deformed babies.

(ii) Carbon dioxide:

Carbon dioxide $\mathrm{CO}_2$ is released into the atmosphere by respiration, burning of fossil fuels for energy, and by decomposition of limestone during the manufacture of cement. It is also emitted during volcanic eruptions. Carbon dioxide gas is confined to the troposphere only. Normally it forms about 0.03 per cent by volume of the atmosphere. With the increased use of fossil fuels, a large amount of carbon dioxide gets released into the atmosphere. Excess of $\mathrm{CO}_2$ in the air is removed by green plants and this maintains an appropriate level of $\mathrm{CO}_2$ in the atmosphere. Green plants require $\mathrm{CO}_2$ for photosynthesis and they, in turn, emit oxygen, thus maintaining the delicate balance. As you know, deforestation and the burning of fossil fuel increase the $\mathrm{CO}_2$level and disturb the balance in the atmosphere. The increased amount of $\mathrm{CO}_2$ in the air is mainly responsible for global warming.

Atmospheric Pollution: Global Warming and Acid Rain

The two major consequences pertaining to atmospheric pollution are global warming and acid rain. Global warming means an increase in the global temperature and disruption of the balanced climate system on earth, which is a result of the accumulation of greenhouse gases, particularly $\mathrm{CO}_2$ and CH4. Thereafter, it melts glaciers, raises the sea level, and gives rise to extreme weather events. In another sense, acid rain forms in the atmosphere in the presence of sulfur dioxide and nitrogen oxides. Subsequently, these contaminants react with atmospheric water and form sulfuric acid and nitric acid, which fall as acidic rain to the ground. This acid rain could burn vegetation, cause pollution in bodies of water, and also cause erosion in building structures and other forms of infrastructure. The solutions to these problems demand an integrated approach that reduces emissions, manages resources sustainably, and works collaboratively with each other across borders.

Global Warming

About 75 % of the solar energy reaching the earth is absorbed by the earth’s surface, which increases its temperature. The rest of the heat radiates back to the atmosphere. Some of the heat is trapped by gases such as carbon dioxide, methane, ozone, chlorofluorocarbon compounds (CFCs) and water vapour in the atmosphere. Thus, they add to the heating of the atmosphere. This causes global warming.

We all know that in cold places flowers, vegetables and fruits are grown in glass-covered areas called greenhouses. Do you know that we humans also live in a greenhouse? Of course, we are not surrounded by glass but by a blanket of air called the atmosphere, which has kept the temperature on Earth constant for centuries. But it is now undergoing change, though slowly. Just as the glass in a greenhouse holds the sun’s warmth inside, the atmosphere traps the sun’s heat near the earth’s surface and keeps it warm. This is called the natural greenhouse effect because it maintains the temperature and makes the earth perfect for life.

• In a greenhouse, solar radiations pass through the transparent glass and heat up the soil and the plants.
• The warm soil and plants emit infrared radiation. Since glass is opaque to infrared radiations (thermal region), it partly reflects and partly absorbs these radiations. This mechanism keeps the energy of the sun trapped in the greenhouse.
• Similarly, carbon dioxide molecules also trap heat as they are transparent to sunlight but not to heat radiation. If the amount of carbon dioxide crosses the delicate proportion of 0.03 per cent, the natural greenhouse balance may get disturbed. Carbon dioxide is a major contributor to global warming.
• Besides carbon dioxide, other greenhouse gases are methane, water vapour, nitrous oxide, CFCs and ozone.
• Methane is produced naturally when vegetation is burnt, digested or rotted in the absence of oxygen. Large amounts of methane are released in paddy fields, coal mines, from rotting garbage dumps and by fossil fuels.
• Chlorofluorocarbons (CFCs) are man-made industrial chemicals used in air conditioning etc. CFCs are also damaging the ozone layer.
• Nitrous oxide occurs naturally in the environment.
• In recent years, their quantities have increased significantly due to the use of chemical fertilizers and the burning of fossil fuels. If these trends continue, the average global temperature will increase to a level which may lead to the melting of polar ice caps and the flooding of low-lying areas all over the earth.
• An increase in the global temperature increases the incidence of infectious diseases like dengue, malaria, yellow fever, sleeping sickness etc.
• We should plant more trees to increase the green cover. Avoid burning of dry leaves, wood etc.

Acid Rain

We are aware that normally rainwater has a pH of 5.6 due to the presence of H+ ions formed by the reaction of rainwater with carbon dioxide present in the atmosphere.

$\begin{aligned} & \mathrm{H}_2 \mathrm{O}(\mathrm{l})+\mathrm{CO}_2(\mathrm{~g}) \rightleftharpoons \mathrm{H}_2 \mathrm{CO}_3(\mathrm{aq}) \\ & \mathrm{H}_2 \mathrm{CO}_3(\mathrm{aq}) \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{HCO}_3^{-}(\mathrm{aq})\end{aligned}$

When the pH of the rainwater drops below 5.6, it is called acid rain.
Acid rain refers to how acid from the atmosphere is deposited on the earth’s surface. Oxides of nitrogen and sulphur which are acidic can be blown by wind along with solid particles in the atmosphere and finally settle down either on the ground as dry deposition or in water, fog and snow as wet deposition.

Acid rain is a byproduct of a variety of human activities that emit the oxides of sulphur and nitrogen in the atmosphere. As mentioned earlier, the burning of fossil fuels (which contain sulphur and nitrogenous matter) such as coal and oil in power stations and furnaces or petrol and diesel in motor engines produces sulphur dioxide and nitrogen oxides. SO2 and NO2 after oxidation and reaction with water are major contributors to acid rain because polluted air usually contains particulate matter that catalyses the oxidation.

$\begin{aligned} & 2 \mathrm{SO}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{H}_2 \mathrm{SO}_4(\mathrm{aq}) \\ & 4 \mathrm{NO}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow 4 \mathrm{HNO}_3(\mathrm{aq})\end{aligned}$

Ammonium salts are also formed and can be seen as an atmospheric haze (aerosol of fine particles). Aerosol particles of oxides or ammonium salts in raindrops result in wet deposition. SO2 is also absorbed directly on both solid and liquid ground surfaces and is thus deposited as dry-deposition.

• Acid rain is harmful to agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth.
• It causes respiratory ailments in human beings and animals. When acid rain falls and flows as groundwater reaches rivers, lakes etc. it affects plants and animal life in the aquatic ecosystem.
• It corrodes water pipes resulting in the leaching of heavy metals such as iron, lead and copper into the drinking water. Acid rain damages buildings and other structures made of stone or metal.
• The Taj Mahal in India has been affected by acid rain.
  • The air around the city of Agra, where the Taj Mahal is located, contains fairly high levels of sulphur and nitrogen oxides. This is mainly due to the large number of industries and power plants around the area. The use of poor-quality of coal, kerosene and firewood as fuel for domestic purposes adds up to this problem. The resulting acid rain reacts with marble, $\mathrm{CaCO}_3$ of the Taj Mahal causing damage to this wonderful monument that has attracted people from around the world.
  • Reaction: $\left(\mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{CaSO}_4+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2\right)$
  • As a result, the monument is being slowly disfigured and the marble is getting discoloured and lustreless. The Government of India announced an action plan(Taj Trapezium) in early 1995 to prevent the disfiguring of this historical monument. Mathura refinery has already taken suitable measures to check the emission of toxic gases.
• We should use fewer vehicles driven by fossil fuels, and use less sulphur content fossil fuels for power plants and industries.
• We should use natural gas which is a better fuel than coal or use coal with less sulphur content.
• Catalytic converters must be used in cars to reduce the effect of exhaust fumes on the atmosphere. The main component of the converter is a ceramic honeycomb coated with precious metals - Pd, Pt and Rh. The exhaust gases containing unburnt fuel, CO and $\mathrm{NO}_{\mathrm{x}}$ when passing through the converter at 573 K , are converted into $\mathrm{CO}_2$ and $\mathrm{N}_2$
• We can also reduce the acidity of the soil by adding powdered limestone to neutralise the acidity of the soil.

Atmospheric Pollution: Particulate Pollutants

Particulate pollutants, also known as particulate matter (PM), are small portions of solid or liquid suspended in the atmosphere. They differ by their size, composition, and source and are classified based on their aerodynamic diameter. Fine particulate matter (PM2.5) and coarse particulate matter (PM10) are of special concern since they have the capability of reaching deep parts of the respiratory system, posing potential damage to health. Particulate pollutants can be primary or directly emitted into the atmosphere, or they may be secondary and formed by some chemical reactions of gaseous pollutants. Combustion processes, industry, construction, and natural sources of particulate pollutants include dust storms and volcanic eruptions. The impact of particulate matter exposure has been associated with various respiratory diseases, cardiovascular problems, and even higher rates of mortality. The mitigation of particulate pollutants is effected through the introduction of technologies in emission control, clean energy sources, and raising awareness among the public about the importance of air quality.

Particulate pollutants:

These are dust, mist, fumes, smoke, smog etc.

Particulate pollutants are the minute solid particles or liquid droplets in air. These are present in vehicle emissions, smoke particles from fires, dust particles and ash from industries. Particulates in the atmosphere may be viable or non-viable. The viable particulates e.g., bacteria, fungi, moulds, algae etc., are minute living organisms that are dispersed in the atmosphere. Human beings are allergic to some of the fungi found in the air. They can also cause plant diseases.

Non-viable particulates may be classified according to their nature and size as follows:

(a) Smoke particulates consist of a solid or mixture of solid and liquid particles formed during the combustion of organic matter. Examples are cigarette smoke, smoke from the burning of fossil fuel, garbage and dry leaves, oil smoke etc.

(b) Dust is composed of fine solid particles (over 1µm in diameter), produced during the crushing, grinding and attribution of solid materials. Sand from sandblasting, sawdust from woodworks, pulverized coal, cement and fly ash from factories, dust storms etc., are some typical examples of this type of particulate emission.

(c) Mists are produced by particles of spray liquids and by condensation of vapours in air. Examples are sulphuric acid mist and herbicides and insecticides that miss their targets and travel through air and form mists.

(d) Fumes are generally obtained by the condensation of vapours during sublimation, distillation, boiling and several other chemical reactions. Generally, organic solvents, metals and metallic oxides form fume particles

The effect of particulate pollutants is largely dependent on the particle size. Air-borne particles such as dust, fumes, mist etc., are dangerous for human health. Particulate pollutants bigger than 5 microns are likely to lodge in the nasal passage, whereas particles of about 10 microns enter into the lungs easily. Lead used to be a major air pollutant emitted by vehicles. Leaded petrol used to be the primary source of air-borne lead emissions in Indian cities. This problem has now been overcome by using unleaded petrol in most of the cities in India. Lead interferes with the development and maturation of red blood cells.

smog

The word smog is derived from smoke and fog. There are two types of smog:
(a) Classical smog occurs in cool humid climates. It is a mixture of smoke, fog and sulphur dioxide. Chemically it is a reducing mixture and so it is also called reducing smog.
(b) Photochemical smog occurs in a warm, dry and sunny climates. The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories. Photochemical smog has a high concentration of oxidising agents and is, therefore, called oxidising smog.

Formation of photochemical smog

When fossil fuels are burnt, a variety of pollutants are emitted into the earth’s troposphere. Two of the pollutants that are emitted are hydrocarbons (unburnt fuels) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which NO is converted into nitrogen dioxide $\left(\mathrm{NO}_2\right)$. This $\mathrm{NO}_2$ in turn absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atoms.

$
\mathrm{NO}_2(\mathrm{~g}) \xrightarrow{\mathrm{hv}} \mathrm{NO}(\mathrm{g})+\mathrm{O}(\mathrm{g})
$

Oxygen atoms are very reactive and combine with the $\mathrm{O}_2$ in air to produce ozone.

$
\mathrm{O}(\mathrm{g})+\mathrm{O}_2(\mathrm{~g}) \rightleftharpoons \mathrm{O}_3(\mathrm{~g})
$

The ozone formed in the above reaction (ii) reacts rapidly with the $\mathrm{NO}(\mathrm{g})$ formed in the reaction (i) to regenerate $\mathrm{NO}_2 \cdot \mathrm{NO}_2$ is a brown gas and at sufficiently high levels can contribute to haze.

$\mathrm{NO}(\mathrm{g})+\mathrm{O}_3(\mathrm{~g}) \rightarrow \mathrm{NO}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g})$

Ozone is a toxic gas and both $\mathrm{NO}_2$ and $\mathrm{O}_3$are strong oxidising agents and can react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN).

Effects of photochemical smog

The common components of photochemical smog are ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN). Photochemical smog causes serious health problems. Both ozone and PAN act as powerful eye irritants. Ozone and nitric oxide irritate the nose and throat and their high concentration causes headache, chest pain, dryness of the throat, cough and difficulty in breathing. Photochemical smog leads to cracking of rubber and extensive damage to plant life. It also causes corrosion of metals, stones, building materials, rubber and painted surfaces.

Stratospheric Pollution

As most of the discussion about atmospheric pollution relates to the troposphere—that is, the lowest layer of the atmosphere—the problem of stratospheric pollution is equally important. Above the troposphere lies the stratosphere, which contains the ozone layer. The latter plays a relevant role in the absorption of harmful UV radiation from the Sun. However, some man-made chemicals like CFCs and other halogenated compounds are able to split the ozone molecules and eventually lead to the formation of the ozone hole mostly above the Antarctic region. The widespread effects of stratospheric pollution include an increase in the amount of UV radiation finally reaching the Earth's surface, resulting in increased cases of skin cancers and eye damages in humans, and disruption of marine ecosystems. These efforts on stratospheric pollution resulted in the Montreal Protocol, a global treaty targeting the phase-out of ozone-depleting substances.

Polar Stratospheric Clouds

Polar stratospheric clouds, more commonly known as nacreous clouds, are exceptionally special kinds of atmospheric phenomena that occur in the stratosphere over the polar regions during winter. These clouds form at very low temperatures—less than -78°C (-108°F)—and are known to play an essential role in the depletion of the ozone layer. PSCs provide surfaces for chemical reactions that turn rather inert chlorine compounds into active, ozone-destroying forms. In the spring, when the sun rises again, these compounds of active chlorine undergo very fast reactions that break up the molecules of ozone to form the ozone hole. The understanding of PSCs formation and behavior underpins the forecast and monitoring of the state of the ozone layer and the formulation of strategies for mitigating the effect of stratospheric pollution on the environment and human health.

The upper stratosphere consists of a considerable amount of ozone (O₃), which protects us from the harmful ultraviolet (UV) radiations (λ 255 nm) coming from the sun. These radiations cause skin cancer (melanoma) in humans. Therefore, it is important to maintain the ozone shield.
Ozone in the stratosphere is a product of UV radiations acting on dioxygen (O₂) molecules. The UV radiations split apart molecular oxygen into free oxygen (O) atoms. These oxygen atoms combine with the molecular oxygen to form ozone.
$\begin{aligned} & \mathrm{O}_2(\mathrm{~g}) \xrightarrow{\mathrm{UV}} \mathrm{O}(\mathrm{g})+\mathrm{O}(\mathrm{g}) \\ & \mathrm{O}(\mathrm{g})+\mathrm{O}_2(\mathrm{~g}) \rightleftharpoons \mathrm{O}_3(\mathrm{~g})\end{aligned}$
Ozone is thermodynamically unstable and decomposes to molecular oxygen. Thus, a dynamic equilibrium exists between the production and decomposition of ozone molecules. In recent years, there have been reports of the depletion of this protective ozone layer because of the presence of certain chemicals in the stratosphere. The main reason of ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs), also known as freons. These compounds are nonreactive, non-flammable, non-toxic organic molecules and therefore used in refrigerators, air conditioners, in the production of plastic foam and by the electronic industry for cleaning computer parts etc. Once CFCs are released into the atmosphere, they mix with the normal atmospheric gases and eventually reach the stratosphere. In the stratosphere, they get broken down by powerful UV radiations, releasing chlorine-free radicals.
$\mathrm{CF}_2 \mathrm{Cl}_2(\mathrm{~g}) \quad \xrightarrow{\mathrm{Cl}}(\mathrm{g})+\dot{\mathrm{C}} \mathrm{F}_2 \mathrm{Cl}(\mathrm{g})$
The chlorine radical then reacts with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen.
$\mathrm{C} 1(\mathrm{~g})+\mathrm{O}_3(\mathrm{~g}) \rightarrow \mathrm{ClO}(\mathrm{g})+\mathrm{O}_2(\mathrm{~g})$
The reaction of chlorine monoxide radical with atomic oxygen produces more chlorine radicals.
$\mathrm{ClO}(\mathrm{g})+\mathrm{O}(\mathrm{g}) \rightarrow \dot{\mathrm{C}} 1(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g})$
The chlorine radicals are continuously regenerated and cause the breakdown of ozone. Thus, CFCs are transporting agents for continuously generate chlorine radicals into the stratosphere and damaging the ozone layer.

The Ozone Hole

In the 1980s atmospheric scientists working in Antarctica reported about the depletion of the ozone layer commonly known as the ozone hole over the South Pole. It was found that a unique set of conditions was responsible for the ozone hole.

In the summer season:

In the summer season, $\mathrm{NO}_2$ and $\mathrm{CH}_4$ react with ClO and chlorine radicals respectively forming chlorine sinks and thus preventing ozone depletion. The reaction occurs as follows:

$\begin{aligned} & \mathrm{ClO}+\mathrm{NO}_2 \rightarrow \mathrm{ClONO}_2 \\ & \mathrm{CH}_4+\dot{\mathrm{Cl}} \rightarrow \dot{\mathrm{C}} \mathrm{H}_3+\mathrm{HCl}\end{aligned}$

In winter season:

In this season, a special type of clouds called Polar Stratospheric Clouds are formed over Antarctica. The reaction occurs as follows:

$\begin{aligned} & \mathrm{ClONO}_2(\mathrm{~g})+\mathrm{H}_2 \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{HOC}+\mathrm{HNO}_3 \\ & \mathrm{ClONO}_2(\mathrm{~g})+\mathrm{HCl}(\mathrm{g}) \rightarrow \mathrm{Cl}_2(\mathrm{~g})+\mathrm{HNO}_2\end{aligned}$

In the spring season:

Sunlight returns the clouds are broken HOCl and Cl₂ are photolysed and the Chlorine radical thus formed initiates the ozone depletion process

$\begin{aligned} & \mathrm{HOCl}(\mathrm{g}) \xrightarrow{\mathrm{W}} \mathrm{OH}(\mathrm{g})+\mathrm{Cl}(\mathrm{g} \\ & \mathrm{Cl}_2(\mathrm{~g}) \xrightarrow{\mathrm{W}} \underset{\sim}{2 \mathrm{Cl}(\mathrm{g}}\end{aligned}$

Effects of Depletion of the Ozone Layer

With the depletion of the ozone layer, more UV radiation filters into the troposphere. UV radiations lead to ageing of the skin, cataracts, sunburn, skin cancer, killing of many phytoplanktons, damage to fish productivity etc. It has also been reported that plant proteins get easily affected by UV radiation which leads to the harmful mutation of cells. It also increases the evaporation of surface water through the stomata of the leaves and decreases the moisture content of the soil. An increase in UV radiation damages paints and fibres, causing them to fade faster.

In the summer season:

In the summer season, $\mathrm{NO}_2$ and $\mathrm{CH}_4$ react with ClOand chlorine radicals respectively forming chlorine sinks and thus preventing ozone depletion. The reaction occurs as follows:

$\begin{aligned} & \mathrm{ClO}+\mathrm{NO}_2 \rightarrow \mathrm{ClONO}_2 \\ & \mathrm{CH}_4+\dot{\mathrm{Cl}} \rightarrow \dot{\mathrm{C}} \mathrm{H}_3+\mathrm{HC}^1\end{aligned}$

In winter season:

In this season, a special type of clouds called Polar Stratospheric Clouds are formed over Antarctica. The reaction occurs as follows:

$\begin{aligned} & \mathrm{ClONO}_2(\mathrm{~g})+\mathrm{H}_2 \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{HOCl}+\mathrm{HNO}_3 \\ & \mathrm{ClONO}_2(\mathrm{~g})+\mathrm{HCl}(\mathrm{g}) \rightarrow \mathrm{Cl}_2(\mathrm{~g})+\mathrm{HNO}_3\end{aligned}$

In the spring season:

Sunlight returns the clouds are broken and HOCl and $\mathrm{Cl}_2$ are photolysed and the Chlorine radical thus formed initiates the ozone depletion process

$\begin{aligned} & \mathrm{HOCl}(\mathrm{g}) \stackrel{\text { ll }}{\longrightarrow} \mathrm{OH}(\mathrm{g})+\mathrm{Cl}(\mathrm{g} \\ & \mathrm{Cl}_2(\mathrm{~g}) \xrightarrow{\text { MP }} 2 \mathrm{Cl}^{\prime}(\mathrm{g}\end{aligned}$

Recommended topic video on (Atmospheric Pollutants And The Reactions)

Some Solved Examples

Example 1
Question:
Among the gases (a) - (e), the gases that cause the greenhouse effect are:

(a)CO₂
(b) H₂O
(c) CFCs
(d) O₂
(e) O₃
1) (a), (b), (c) and (d)
2) (a), (c), (d) and (e)
3) (a) and (d)
4) (a), (b), (c) and (e)

Solution:
The gases that cause the greenhouse effect are CO_2, O_2, and H₂O. Hence, the correct answer is option (4) - (a), (b), (c) and (e).

Example 2
Question:
The statement that is not true about ozone is:

1) In the stratosphere, CFCs release chlorine free radicals C which react with O₃ to give chlorine oxide radicals.
2) In the atmosphere, it is depleted by CFCs.
3) In the stratosphere, it forms a protective shield against UV radiation.
4) It is a toxic gas and its reaction with NO gives NO₂

Solution:
The correct statement that is not true about ozone is: "In the stratosphere, it forms a protective shield against UV radiation." The correct answer is option (3).

Example 3
Question:
The gas leaked from a storage tank of the Union Carbide plant in the Bhopal gas tragedy was:

1) Phosgene
2) Methyl isocyanate
3) Methylamine
4) Ammonia

Solution:
The gas leaked in the Bhopal gas tragedy was Methyl Isocyanate. Hence, the correct answer is option (2).

Example 4
Question:
The smog is essentially caused by the presence of:

1) O₂ and O₃
2) O₂ and N₂
3) Oxides of sulphur and nitrogen
4) O₃ and N₂

Solution:
Smog is caused due to the oxides of sulphur and nitrogen. Hence, the correct answer is option (3).

Conclusion

Atmospheric pollution is an ultrabroad and wide-ranging subject that can be understood only by a detailed understanding of the kinds of pollutants, their interactions, and the effects all these bring. Ranging from gaseous air to particulate matter, each type of pollutant comes with its challenges and has a certain solution to be entitled. Its effect does not stop at just the local air quality level but transcends to global phenomena, including climate change and ozone depletion.