Atmospheric Pollutants And The Reactions - Practice Questions & MCQ

The branch of chemistry which deals with chemical changes in the environment is known as Environmental chemistry. Environment Includes our surroundings such as water, soil, forests, sunlight, etc  An average human being requires nearly 12-15 times more air than the food. So, even small amounts of pollutants in the air become significant compared to similar levels present in the food.

It is a protective blanket of gases that surrounds the earth. The atmosphere extends to thousands of kilometres above the earth's surface. It has no well defined upper limits and gradually merges with outer space. The atmosphere is held to the earth by the force of gravity of the total mass of the atmosphere, aobut 99% wi within a height of 30 km from the earth's surface. The total mass of the atmosphere is about 5 x 10¹⁵ metric tonnes.
The atmosphere is divided into four major regions depending on the height discussed below:

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.

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.

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

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:

NO reacts instantly with oxygen to give NO₂

Rate of production of NO₂ is faster when nitric oxide reacts with ozone in the stratosphere.

The irritant red haze in the traffic and congested places is due to oxides of nitrogen. Higher concentrations of NO₂ 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 amount 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 into 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 (CO₂) 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 CO₂ in the air is removed by green plants and this maintains an appropriate level of CO₂ in the atmosphere. Green plants require CO₂ for photosynthesis and they, in turn, emit oxygen, thus maintaining the delicate balance. As you know, deforestation and the burning of fossil fuel increases the CO₂ level and disturb the balance in the atmosphere. The increased amount of CO₂ in the air is mainly responsible for global warming.

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 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 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 radiations. 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 rain water has a pH of 5.6 due to the presence of H+ ions formed by the reaction of rain water with carbon dioxide present in the atmosphere.

When the pH of the rain water drops below 5.6, it is called acid rain​​​​​​.
Acid rain refers to the ways in which acid from the atmosphere is deposited on the earth’s surface. Oxides of nitrogen and sulphur which are acidic in nature 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, 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 produce 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 catalyse the oxidation.

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 rain drops 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 for 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 to reach 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. It 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, CaCO₃ of Taj Mahal causing damage to this wonderful monument that has attracted people from around the world.
  • Reaction: 
  • 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 less vehicles driven by fossil fuels, 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 NO_x, when pass through the converter at 573 K, are converted into CO₂ and N₂.
• We can also reduce the acidity of the soil by adding powdered limestone to neutralise the acidity of the soil.

Particulate pollutants: These are dust, mist, fumes, smoke, smog etc.

Particulated 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 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 solid or mixture of solid and liquid particles formed during 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 are 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 emission 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 the 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 as 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 as 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 (NO₂). This NO₂ in turn absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom.

Oxygen atoms are very reactive and combine with the O₂ in air to produce ozone.

The ozone formed in the above reaction (ii) reacts rapidly with the NO(g) formed in the reaction (i) to regenerate NO₂ . NO₂ is a brown gas and at sufficiently high levels can contribute to haze.

Ozone is a toxic gas and both NO₂ and O₃ 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.

The upper stratosphere consists of 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.

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 in the atmosphere, they mix with the normal atmospheric gases and eventually reach the stratosphere. In stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radical.

The chlorine radical then react with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen.

Reaction of chlorine monoxide radical with atomic oxygen produces more chlorine radicals.

The chlorine radicals are continuously regenerated and cause the breakdown of ozone. Thus, CFCs are transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer.

The Ozone Hole
In 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 summer season: In summer season, NO₂ and CH₄ reacts with  and chlorine radicals respectively forming chlorine sinks and thus prevent the ozone depletion. The reaction occurs as follows:

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

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

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 skin, cataract, 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 radiations 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. Increase in UV radiations damage paints and fibres, causing them to fade faster.

In summer season: In summer season,  and  reacts with  and chlorine radicals respectively forming chlorine sinks and thus prevent the ozone depletion. The reaction occurs as follows:

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

In spring season: Sunlight returns and the clouds are broken and  and  are photolysed and Chlorine radical thus formed initiates the ozone depletion process