CBSE Class 11 Chemistry Notes For Chapter 14 Environmental Chemistry

CBSE Class 11 Chemistry Notes For Chapter 14 Environmental Chemistry Introduction

The word ‘environment’ derived from the French word ‘environment can be defined at an assembly of physical, chemical and biological factors, which act upon an organism or an ecological community to determine its form and mode of survival.

Environment mainly consists of three major components:

1. Biotic or living: 
   • For example – All living creatures)
2. Abiotic or non-living:
   • For example –  Lithosphere, hydrosphere and atmosphere) and
3. Energy components: 
   • For example – Solar energy, thermochemical energy, nuclear energy etc.)

Environmental Chemistry

Environmental Chemistry Definition

The branch of science, which deals with the sources of the chemical components of the environment, the chemical reaction occurring among them, the products formed in the reaction and their impact on the living world is called environmental chemistry.

The study of environmental chemistry is important because

• It makes us aware of the adverse effects of various chemical constituents on the environment.
• It gives us an idea about the toxic effects of various chemical substances and their by-products which are extensively used to fulfill our requirements.
• It gives us an idea about the sources of various toxic chemicals, their adverse effects and the antidotes to combat their toxicity.

1. Some terms used in environmental chemistry

1. Pollutant:

A pollutant is a solid, liquid or gaseous substance (produced either by natural sources or by human activity) which is present in the environment to such an extent that it causes harmful or detrimental effects on living organisms (plants, animals and human beings) or nonliving components.

Example:

Air contains trace amounts of CO (0.1 ppm). If, for any reason, the amount of CO increases to 40 ppm or more, then it is regarded as pollutant

Pollutants are of two kinds :

1. Primary pollutant:  The pollutants which are emitted from any source, directly escape into the environment without sustaining any change are called primary pollutants.
2. Example: S, NO, NO₂, CO, CO₂, hydrocarbons etc.
3. Secondary pollutant: There are some pollutants which do not appear in the environment directly from their source. They are produced as harmful substances by Interaction with pollutants), already present in the environment. Tills type of pollutants are called secondary pollutants.
4. Example: Peroxyacyl nitrate (PAN), dimethyl mercury [(CH₃)₂Hg]

2. Biodegradable pollutants

The pollutants which are decomposed by bacteria or germs are known as biodegradable pollutants.

Examples: Household garbage, cow dung and other biomass etc.

3. Non-biodegradable pollutants:

The pollutants which are not decomposed by bacteria or germs or decomposed very slowly are known as non-biode gradable pollutants.

Example:

Mercury, DDT, Gammaxene etc. The presence of these substances even in trace amounts is injurious to human beings and other animals.

4. Contaminant:

A contaminant is a substance which does not occur in nature under normal conditions but is introduced into the environment either accidentally or through indiscriminate human use.It may or may not be harmful to the living organisms or non-living components. The contaminant is considered as a pollutant when it has some harmful effect

Examples:

• In Delhi pyrosulphuric acid (H₂S₂O₇) leakage from a defective tank killed many persons and caused skin and breathing problems to many others. As pyrosulphuric acid does not occur in the atmosphere, therefore it is a contaminant. Again, because of its dangerous effect, it is also regarded as a pollutant.
•  In Kerala, in 1953, 108 people died after consuming wheat flour contaminated with parathion (an agricultural pesticide).

5. Source:

The source ofany contaminantis a chemical substance or the place from whereitis produced.

Example:

A source of the pollutants like CO, NO etc., is the gas 2 emitted from petrol or diesel automobile engines

6. Sink:

If any medium continuously reacts with a pollutant for a long period and causes destruction to it, then it is said to be the sink of that particular pollutant.

Examples:

1.  Sea water acts as a sink of CO₂ present in the atmosphere

⇒ H₂O (sink) +CO₂ (Pollutant ) H₂CO₃

2. An automobile wall or memorial acts as a sink of sulfuric acid, present as an atmospheric pollutant.

⇒ (Sink )CaCO₃+ (polluntant)H₂SO₄→CaSO₂↓ + H₂O + C₂O

7. Receptor or target:

If any plant or animal body or any biotic component is adversely affected by a pollutant, then that particular body or component is called a receptor or the target of that pollutant.

Examples:

•  Smoke or smog causes a burning sensation in our respirator tract and eyes.In this case, man is the receptor or target of smoke or smog.
• The aquatic animals are the receptors of the oil layer, floating on the surface of seawater.

8. Pathways Of pollutants:

The mechanism by which any pollutant is liberated from its source, spreads in the environment and ultimately gets destroyed, is called the pathway of that pollutant.

Example:

Nitric oxide, a pollutant, liberated from petrol or diesel engine reacts with oxygen present in the air to form nitrogen dioxide. Nitrogen dioxide reacts with rainwater and falls on the surface of the earth as nitric acid.

2NO + O₂ → 2NO₂; 2HNO₂+O₂→2HNO₃

2NO₂ + H₂O→HNO₃ + HNO₂

9. Threshold Limit Value (TLV):

Anypollutantin in the environment is considered to be harmful to living organisms if its concentration exceeds a particular limit. This particular limit (of concentration) is called ‘threshold limit value’ (TLV) ofthat particular pollutant.  Atmosphere

Examples: Threshold limit values for CO and CO₂ are. 40 ppm and 5000 ppm respectively. Ho However, the TLV for phosgene (COCl₂) is only 0.1 ppm.

Threshold limit value for factory workers: It is the maximum limit of a pollutant present in the environment maximum limit of a pollutant present in the environment hours per day without suffering from health hazards Examples: Threshold limit values for SO₂ and CO₂ are 5 mg^-3. m^-3 and 5000 mg m^-3 respectively

Environmental Components Of Earth

Earth’s environment is composed of the following four components—

1. Atmosphere,
2. Hydrosphere,
3. Lithosphere and
4. Biosphere.

Among these, the first three components are abiotic while the fourth one is biotic.

1. Atmosphere: The invisible gaseous layer that surrounds and protects the Earth is called the atmosphere.
2. Hydrosphere:  It includes all sources of water such as seas, oceans, rivers, fountains, lakes, polar regions, glaciers, groundwater etc.
3.  Lithosphere:  It comprises the solid crust of the earth, made of rocks, forming the outer mineral cover.It includes soil, minerals, organic matter etc., and extends up to a depth of about 30 km from the earth’s surface.
4.  Biosphere: It is the part of the earth where living organisms exist and interact with each other and also with the non-living components. The biosphere consists of all three zones,
   • Example: Soil, water, air etc., where living beings exist.

Atmosphere

The invisible blanket of gaseous layer that surrounds the earth is called atmosphere. It extends upwards to about 1600km. It is the gravitational attraction of the earth that holds this gaseous layer closely in space around the earth’s surface. The total mass of gaseous substances in the atmosphere is nearly 5.5×1015 ton. On the basis of temperature gradients and altitude,

The atmosphere has been divided into four distinct zones.

These are

1. Troposphere,
2. Atmosphere
3. Mesosphere and
4. Thermosphere

Different zones of atmosphere

Again according to the proportion of different gases from the surface of the earth towards the vacuum of interstellar space, and

Atmosphere can be divided into two categories:

1. Homosphere
2.  Heterosphere

The homosphere extends from the surface of the earth up to about 100 km height. In this layer, the proportions of different gases are more or less identical. Thus, this layer is called the homosphere.

The layer next to it is known as the heterosphere because the proportion ofthe gases in the different parts of this layer are found to be dissimilar. Gravity holds most of the air molecules close to the earth’s surface and hence the troposphere is much more denser than the other layers.

50% of the total mass of the atmosphere exists within a height, of5.5 km from the earth’s surface and 99% exists within a height of 30 km from the earth’s surface

Average gaseous composition in homosphere

Functions of gases present in the atmosphere

Oxygen:

• The most significant gaseous constituent of the atmosphere is oxygen. Oxygen is indispensable for any kind of combustion.
• Oxygen is also used for the oxidation
  of food taken by plants and animals to produce heat and energy.
• Oxygen is a necessary component of life as all living beings (except some microorganisms) and plants take oxygen from the atmosphere for respiration.
• Plants give up oxygen to the atmosphere during the process of photosynthesis. As a result, the balance of oxygen is maintained in the atmosphere.

Nitrogen:

The major constituent of the atmosphere is nitrogen. Proteins and nucleic acids present in living bodies are nitrogenous compounds.

• But most of animals including human beings and even plants cannot utilise atmospheric nitrogen direedy for the production of proteins and amino acids.
• However, some nitrogen-fixing bacteria can take nitrogen directly from air and produce nitrate salts in the soil. These are used by plants in the synthesis of amino acids and nucleic acids.
• Herbivorous animals meet their protein demand by eating those plants. Similarly, carnivorous animals get proteins from herbivorous animals.
• After the death of plants and animals, nitrogenous compounds present in their bodies are decomposed by some bacteria releasing nitrogen gas that returns to the atmosphere.

Carbon dioxide (CO₂):

Combustion of fossil fuels and carbonaceous compounds, and respiration of plants and animals increase carbon dioxide content in the atmosphere.

• Again plants, during photosynthesis, absorb carbon dioxide from the atmosphere for the preparation of food.
• As a result, the balance of carbon dioxide is maintained in the atmosphere.
• But due to excessive … combustion of carbonaceous fuels and indiscriminate deforestation, the quantity of carbon dioxide in the atmosphere is increasing constantly leading to a constant increase in the average temperature of the earth (See greenhouse effect).

Ozone:

The quantity of ozone gas present in the atmosphere is negligible.

• Almost the entire amount of ozone (=90%) is present in the stratosphere which is about 15-35 km above the earth’s surface.
• The presence of ozone gas close to the earth’s surface hurts mankind and other animals.
• But the presence of ozone in the upper layer of tire atmosphere is beneficial since it absorbs the harmful ultraviolet rays ofthe sun

Air Pollution

When one or more undesirable chemical substances produced by natural phenomena or uncontrolled human activity get mixed with the air to bring about health hazards to human or any other living being and affect their life processes, then air pollution is said to have occurred. The substances which cause air pollution are called air pollutants

Examples:

Sulphur dioxide (SO₂), carbon monoxide (CO), carbon dioxide (CO₂), nitric oxide (NO), nitrogen dioxide (NO₂), hydrogen sulphide (H₂S), hydrocarbons (C_xH_y), ammonia (NH₃), fly ash, dust particles, smoke, fumes etc., are some air pollutants

Causes of air pollution

Natural causes of air pollution:

• Emission of SO₂, CO, H₂S gases etc., due to volcanic eruption.
• Gases are liberated due to the decomposition of living bodies.
• Dust storms, forest fires and the fall of a meteorite.
• Spreading of viruses, bacteria, fungi, pollen-grain of flowers in air etc

Causes of air pollution due to human activities:

• Sulphur dioxide gas, carbon monoxide gas and fly ash are produced by thermal power plants.
• Gases such as sulphur dioxide, sulphur trioxide, oxides of nitrogen, hydrogen chloride, chlorine etc., from acid manufacturing factories.
• Ammonia gas is liberated from fertiliser factories and cold storage.
• Sulphur dioxide, carbon monoxide and different metallic oxides were obtained during the extraction of metals.
• Fine and bulky solid particles are produced in cement and asbestos factories.
• Carbon monoxide, nitrogen dioxide, sulphur dioxide and other such gaseous substances evolved from petroleum refineries.
• Dust and sand in the region of coal mines.
• CO, SO₂ and oxides of nitrogen are released from motor vehicles.
• Extensive deforestation affects the balance of oxygen and carbon dioxide and increases the quantity of carbon dioxide.
• Highly poisonous gas evolved from destructive weapons used in warfare.0 Emission of radioactive rays due to accidents in nuclear reactors, nuclear power plants etc.
• The extensive use of fossil fuel results in the evolution of gaseous pollutants.

Major Air Pollutants

Major air pollutants are divided into two classes:

1. Inorganic and organic gaseous substances and
2. Particulates.

Inorganic and organic gaseous substances

1. Carbon monoxide (CO):

Natural sources:

• During volcanic eruptions,
• Due to decomposition of dead plants and animals in marshy land,
• During forest fires,
• During the extraction of petroleum and natural gases,
• During lightning, carbon monoxide is produced.

Man-made sources:

• Incomplete combustion of fuel in automobile engines,
• Incomplete combustion of carbonaceous matter in industrial furnaces,
• Incomplete combustion of agriculture and slash materials,
• Heating of carbon dioxide at a high temperature in the presence of coke in the blast furnace produces carbon monoxide. Sulphur dioxide (SO₂)

Harmful effects:

This colourless, odourless gas is very harmful

Natural sources:

For human beings and animals. It has a greater affinity towards haemoglobin than of oxygen. So, it readily combines with haemoglobin to give a stable compound, carboxyhaemoglobin

(HbO₂ + CO ⇌ HbCO + O₂ ).

• Consequently, oxygen carrying capacity of haemoglobin decreases which ultimately results in death.
• The presence of CO even in small amounts may result in nausea, headache, dizziness etc.
• Cigarette smoke contains a considerable amount of CO which ultimately increases CO content in the smoker’s blood.
• Thus, chain smokers often suffer from mental imbalance, headaches, heart attacks etc. Acute oxygen starvation in the body due to poisoning by CO is called anoxia or asphyxiation.

Sink of carbon monoxide:

• Some bacteria present in the soil absorb CO and convert it to CO₂.
• Inorganic radicals like HO and HOO, atomic oxygen and ozone oxidise CO to CO₂

Control of CO pollution: 

1. One of the main sources of CO pollution is the use of internal combustion engines in automobiles. These engines emit a mixture of CO, NO_x, hydrocarbons and particulates.
   • To control the CO emission suitable modifications have to be done in internal combustion engines
   • The carburettor is to be adjusted to give a proper air-fuel ratio
   • Catalytic converters can be fitted into the exhaust pipe which may bring about oxidation of CO to CO₂.
   • Internal combustion engines should be modified and improved. Some automobile makers have improved automobile engines by the use of an extra combustion chamber so that fuel undergoes complete combustion and the exhaust gas does not contain CO.
2. CNG, LNG, LPG and LHG (liquefied hydrogen gas) should be used instead of gasoline.
3. Alternative power sources such as fuel cells, solar energy etc., be used.

2. Oxides of sulphur (SO_x):

Two oxides of sulphur, sulphur dioxide (SO₂) and sulphur trioxide (SO₃) are represented by the general formula SO_x. Both of them are colourless gases having suffocating odour.

Sulphur Dioxide (SO₂)

Sulphur Dioxide (SO₂)

Natural sources of  SO₂:

SO₂ gas is liberated in the atmosphere due to volcanic eruptions. 67% of the total SO₂ content of the atmosphere is contributed from this source.

Sources created by human activities:

1. Combustion of coal in thermal plants produces SO₂ gas in abundant quantities.
2. SO₂ gas is evolved during roasting of sulphide ores
3. For example:  FeS, CuFeS₂, Cu₂S, ZnS, PbS etc.) in the extraction of metals.
4. Smoke emitted from oil refineries and automobile engines contains a large quantity of SO₂

Harmful effects of SO₂ :

SO₂ is a gas having extremely pungent and suffocating odour. It adversely affects the respiratory system and causes a burning sensation to eyes.

• If inhaled, it may lead to breathing problems, asthma and bronchitis.
• The presence of excess SO₂ in the atmosphere may result in lung cancer.
• SO₂ also causes allergies.
• In the presence of metallic oxide or other catalysts,  SO₂ reacts with oxygen present in the air and water vapour to form H₂SO₄.
• This sulphuric acid is much more harmful since it corrodes building materials such as marble or limestone, and metals like iron, steel, aluminium etc.

The presence of SO₂ hampers the production of chlorophyll and the leaves turn colourless. This is known as chlorosis.

Control of SO₂ pollution:

• Removal of sulphur from fuel i.e., desulphurisation of fuel by using chemical scrubbers
• Use of fuel containing less amount of sulphur
• For example: Natural gas.
• Removal of SO₂ from gaseous fuels.
• Production of electricity in nuclear power plants instead of thermal powerplants

Sulphur Trioxide (SO₃)

1. Sources of SO₃

A significant amount of SO₂, emitted from natural sources or man-made processes is oxidised to sulphur trioxide (SO₃)

1. SO₃ molecules get activated by absorbing radiations having a wavelength of 300-400 run and react with aerial oxygen to give SO₃

2. SO₂ is also oxidised to SO₃ by ozone, hydrogen peroxide and oxides of nitrogen (NO_x) present in the atmosphere

⇒ \(\mathrm{SO}_2+\mathrm{O}_3 \longrightarrow \mathrm{SO}_3+\mathrm{O}_2 ; \mathrm{SO}_2+\mathrm{H}_2 \mathrm{O}_2 \rightarrow \mathrm{H}_2 \mathrm{SO}_4\)

 Harmful effects of SO₃:

SO₃ reacts with water vapour present in the atmosphere to yield sulphuric acid (H₂SO₄). Produced H₂SO₄ combines with water droplets present in air to form an aerosol and ultimately come down as acid rain.

Historical buildings For example:

Tajmahal, Victoria Memorial, monuments, sculptures etc., made up of limestone, marble etc., are adversely affected by

Acid rain*: CaCO₃+ H₂SO₄→ CaSO₄+H₂O + CO₂

Apart from these, metallic (aluminium and iron) structures, bridges, etc., are severely damaged by acid rain. Acid rain increases the acidity(i.e., decreases the pH level of ponds, lakes etc., and in consequence, aquatic plants and animals get dangerously affected.

Acid rain also decreases the stability and glossiness of paint and varnishes.

Sink of SO_x:

Edifices, historical memorials, monuments, and sculptures made of marble, limestone and natural sources of ammonia act as a sink of SO_x.

2. Oxides of nitrogen (NO_x)

Nitrogen forms five oxides—N₂O, NO, N₂O₃, NO₂ and N₂O₅. Out of these, NO and NO₂ act as chief air pollutants. NO and NO₂ are designated

Natural sources of NO_x:

1. During lightning discharge nitrogen and oxygen combine together to form nitric oxide (NO) and this nitric oxide on reaction with excess oxygen, produces nitrogen dioxide (NO₂ ).

N₂ + O₂ → 2NO ; 2NO + O₂ →2NO₂

2. Decomposition of ammonium salts in the soil by some bacteria leads to the formation of oxides of nitrogen (NOx), mainly NO.

Sources of NO_x created by human activities:

In thermal power plants and different industries, atmospheric N₂ and O₂ combine to produce large amounts of nitric oxide (NO). This NO reacts with aerial oxygen to give NO₂.

• The high temperature produced during the burning of fuels in petrol and diesel engines also favours the formation of NO by mutual interaction of atmospheric N₂ and O₂
• NO thus formed is subsequently converted to NO₂ by aerial O₂
• An abundant quantity of NO_x gas escapes into the atmosphere from industries producing organic acid.
• Atomic explosions also add to NO_x in the air.

Harmful effects of NO_x:

1. NO₂ gas is relatively toxic but its adverse effect depends on its concentration in the air and the extent of the reaction.

• Higher concentration of NO₂ in the air produces diseases like inflammation of the lungs, bronchitis, pneumonia etc.
• If NO₂ of higher concentration is inhaled, pulmonary oedema and haemorrhage of the lungs may occur.

2. The most harmful effect of NO_x in the atmosphere is the formation of photochemical smog.

• In the presence of sunlight, hydrogen, nitrogen dioxide and oxygen in the atmosphere, on reaction, gives a mixture of ozone, peroxyacyl nitrate (PAN) and aldehyde.
• All these substances unitedly form photochemical smog. Photochemical smog irritates the eyes and nose and induces sneezing, cough and difficulty in breathing (dyspnoea). If photochemical smog persists for a long period, it may cause death.

3. NO_x drastically reduces the rate of the photosynthesis of plants. Leaves and fruits start shedding due to the presence of NO_x in the air.

4. NO and NO₂ present in the atmosphere react with ozone to form NO₂ and NO₃ respectively. From the latter (z’.e., NO₂ and NO₃), N₂O₅ is obtained. N₂O₅ thus produced, on reaction with rainwater, forms nitric acid

NO +O₃→NO₂+ O₂ ; NO₂+ O₃→NO₃ + O₂

NO₂ + NO₃→N₂O₅; N₂O₅+ H₂O→2HNO

Sink of NOx:

In the atmosphere, the maximum stability of NO and NO₂ are of four days and three days respectively. After that, they are converted to nitric acid (HNO₃). This transformation may occur in two ways following path 1 and path 2 . Ozone plays a major role in the transformation that occurred by Path 2

Path 1: 

4NO₂+ 2H₂O(water vapour or water) + O₂→ 4HNO₃

Path 2:

NO₂ + O₃ → NO₃ + O₂

NO₂ + NO₃ → N₂O₅ ; N₂O₅+ H₂O → 2HNO₃

Nitric acid thus produced comes down during the rain. A part of it falls on waterbodies

For example: Ponds, rivers, lakes etc

The remaining part, on reaction with different basic Compounds belonging to hydrocarbon series (organic substances {e.g., ammonia, lime etc.) present in the soil, Is pollutants): The main air-polluting hydrocarbons are converted into nitrate salts. Here, water and the different basic materials in the soil work as the sink of NO_x.

Control of pollution caused by NO_x:

When the gas released from automobile engines is passed through a catalytic converter in the presence of Pt-catalyst, oxides of nitrogen (NO_x) are reduced to produce mainly N₂ and a small amount of NO₃.

Production of NO_x may be reduced by conducting the combustion at a lower temperature in the presence of excess air.

Before releasing the gas mixture containing NO_x, produced in the factories, it is passed over the metallic oxide catalyst

For example:  Cr₂O₃, ZnO, CuO etc.)

Heated to 500°C, thus nitric oxide (NO) is reduced to N₂ and O₂: 2NO ⇌ N₂ + O₂.

In the chemical absorption process, NO_x can be removed from the waste gas mixture. In this case,

1. Acidic For example H₂SO₄
2. Basic substances For example: Ca(OH)₂ or Mg(OH)₂ are used

3. Hydrogen sulphide (H₂S)

Natural sources:

• H₂S gas is liberated from volcanoes.
• Due to the decomposition of proteinaceous compounds containing sulphur, H₂S has evolved. Thus, rotten fish or eggs smell like H₂S gas.

Sources created by human activities:

• In oil refineries, during the production of paper and paints (containing sulphur) appreciable amount of H₂S is produced. Harmful effects of H₂S .’H₂S is a poisonous gas.
• It has severe harmful effects on man. It causes headache, nausea, irritation to the eyes, throat and nose, loss of appetite and diarrhoea.
• When taken in higher doses, it may lead to respiratory problems like bronchial pneumonia or even death.
• The reaction of H₂S with essential proteins is the primary cause of its toxic effects on human bodies. H₂S binds with iron in the mitochondrial cytochrome enzymes, thus preventing cellular respiration.

Sink of H₂S : H₂S undergoes slow oxidation to SO₂ in the atmosphere

2H₂ S + 3O₂→  2SO₂ + 2H₂O

H₂S + O₃ → SO₂ + H₂O

It also combines with various metallic salts to form insoluble metallic sulphides

4. Compounds belonging to the hydrocarbon series (organic pollutants)

The main air-polluting hydrocarbons are methane, benzene etc. Beiides thiete,
acetylene, ethylene, propylene, 1,3 butadiene etc., act as air pollutants. Among the gaseous air pollutants, the one which Is present In maximum quantity In air Is methane

Natural sources:

• In paddy fields and other muddy marshy lands, bacterial decomposition of plants, almost In the absence of air, produces plenty of methane.
• Putrefaction of animal excreta produces an abundant quantity of methane.
• The anaerobic decomposition of animal bodies produces methane.
• Forest fires and the evaporation of hydrocarbons of the terpenoid class from plants In forests are also important sources of hydrocarbon.

 Sources created by human activities:

• Hydrocarbons are produced due to the incomplete combustion or evaporation of the liquid fuels used in vehicles.
• Hydrocarbons also escape into the atmosphere as a result of incomplete combustion of fuels used for different purposes

For example: Cooking, lighting etc

• Liquid substances such as benzene, toluene etc., used as solvents in different chemical industrial units get easily evaporated and pollute the atmosphere
• Gaseous hydrocarbon, 1,3-butadiene used in the preparation of rubber and other polymers causes air pollution.
• During destructive distillation of coal, some air pollutants [e.g., benzopyrene) escape into the atmosphere.

Harmful effects of hydrocarbons: 

• Methane is a greenhouse gas.
• Due to photochemical reactions with oxygen and oxides of nitrogen, hydrocarbons form photochemical oxidants and photochemical smog, which are responsible for irritating eyes, nose and lungs and also cause breathing problems.
• Polycyclic aromatic hydrocarbons (PAH)
  • For example: Benzopyrene are carcinogenic and its presence in the atmosphere in large amounts may cause cancer.

Sink of hydrocarbon:

Hydrocarbons are sufficiently stable. However, they undergo slow oxidation or photochemical reactions involving several steps and ultimately form products like CO₂ or other water-soluble compounds which are then washed away by rain.

Control of pollution caused by hydrocarbons:

Since automobiles are the main sources of hydrocarbon pollution, so such pollution can be controlled by following those steps which are taken to control CO pollution.

Particulates

Generally, finely divided solid and liquid particles, suspended in air are referred to as particulates. The diameter of the particulates varies from 0.0002μ to 500μ (1 micron or 1 μ= 10^-4cm).

The particles of this dimension, being dispersed in air, form aerosol. Depending on the diameter, density of the particles and the intensity of air current, these particles exist in the atmosphere for a period ranging from a few seconds to a few months.

Particulates can be of two types:

• Viable (living microorganisms such as bacteria, viruses, fungi etc.) and
• Non-viable (non-living matters such as mist, smoke, dust etc.)

1. Suspended particulate matter

Smoke, soot, dust particles, metallic oxides and chlorides, fly ash, asbestos dust, acid mist(H₂SO₄, HNO₃)etc.

Natural sources of particulates:

Small particles on the surface of the earth are scattered into the atmosphere by air currents, cyclones, volcanic eruptions etc.

Sources created human activities:

• Soot:  These are fine carbon particles formed by incomplete combustion of carbonaceous fossil fuels.
• Metal particles: These are generated in the metal extraction involving processes like grinding, calcination, smelting of ores etc.
• Particles of metal oxides: During the combustion of fuels containing metal oxides, fine particles of metal oxides are generated.
• For example: Coke containing FeS₂, on combustion, produces fine particles of Fe₃O₄.

3FeS₂ + 8 O₂ → Fe₃O₄+ 6SO₂

• PbCI₂ and PbBr₂:  Compounds like tetraethyl lead (TEL), dichloroethane, dibromoethane etc., are used in petrol as anti-knocking agents. The Pb-compound during combustion of petrol produces PbO which later turns into volatile PbCl₂ and PbBr₂. These further escape into the air and exist as small particles.
• Inorganic silicates: Fine particles of silicates are scattered into the atmosphere from cement industries.
• Asbestos particles: Asbestos is a fibre-like silicate mineral. During the formation of the asbestos sheet, fine particles of asbestos arÿÿatteredinto the atmosphere.
• Sulphuric and nitric acid mist: The SO_x and NO_x  vapours present in the atmosphere react with water vapour and form their corresponding acid mists
• Particles of metal oxides: During the combustion of fuels containing metal oxides, fine particles of metal oxides are generated.
  • For example: Coke containing FeS₂, on combustion, produces fine particles of Fe₃O₄
• Organic particulates: These are mainly the particles of alkane, alkene and aromatic hydrocarbons
  • For example: PAH.
• The formation of such particles is associated with the combustion of petrol and petroleum refining. PAH particles get attached to the soots floating in the air easily and create severe health hazards.

2. Harmful effects Of particulates

The harmful effects of particulates depend on the particle size and the nature of the substances from which these particles originate. Generally, finer particles are more harmful. This is because, the particles with a diameter greater than 5μ get trapped in the nostril but particles having a diameter less than 5μ enter the lungs through the nostril.

The magnitude of the surface area of extremely fine particles being exceedingly high, carcinogenic particulates

For example:

Polycyclic aromatic hydrocarbons, asbestos etc.) can easily find their shelter on these particles and cause cancer, asthma, tuberculosis and different lung diseases. Apart from this, finely divided suspended particles in the air, enter the body through the eyes, ears, nose etc.,

Leads to different types of diseases:

• Smoke released from automobiles contains lead particles which adversely affect the child’s brains and cause nerve diseases.
• The normal production and development of red blood corpuscles (R.B.C.) are disturbed in the presence of lead particles. If an excess amount of lead particles are inhaled regularly, haemoglobin disintegrates and is eliminated through urine
•  Accumulation of cadmium particles in the body during respiration even in trace amounts, may cause breathing trouble and heart disease.
• Beryllium compounds
  • For example: BeCl₂, BeSO, ) affect the lung and create berylliosis disease.
• Workers of coal mines and cotton mills are prone to be attacked by diseases like black lung and white lung.
•  Workers of cement factories are susceptible to five attacks of silicosis due to the intake of SiO₂ particles during respiration.
• Inhalation of asbestos particles results in asbestosis and it leads to cancer in the case of workers of asbestos factories.
• The cause of arsenicosis is due to inhalation of arsenic compounds. In this disease, the skin becomes rough and wounds appear on the skin.

The harmful effects of particulates also extend to the plants present in the atmosphere react with the Water kingdom. Accumulation of dust and other particles on the vapours and their corresponding acid mists. leaves of plants close to the stomata. As a result, transpiration as well as photosynthesis of plants get severely affected. Naturally, the growth of plants and the production of crops also get depreciated

Ozone Layer

In the lower region of the stratosphere (the region at the height of 15 km to 35 km from the ground), there exists a layer of ozone gas. This layer is known as the ozone layer or ozonosphere.

Almost 90% of total ozone gas exists in this layer. In the absence of the ozonosphere, the existence of living beings would have been at stake. It is this ozone layer in the stratosphere which absorbs the harmful ultraviolet radiations coming from the sun. Thus it prevents most of the ultraviolet rays from reaching the Earth’s surface

1. Formation of the ozone layer

Oxygen molecules (O₂) present in the stratosphere absorb ultraviolet radiations coming from the sun and decompose to form oxygen atoms (O). This atomic oxygen combines with oxygen molecules to form ozone molecule

⇒ \(\stackrel{*}{\mathrm{O}}_3+\mathrm{M} \longrightarrow \mathrm{O}_3+\stackrel{*}{\mathrm{M}}\) [*Asterisk sign indicates excited state]

M  denotes a neutral colliding species

For example:

O₂ or N₂ with which O₂  on collision, releases its excess energy] On the other hand, the ozone molecule ( O₃  ) also absorbs UVradiation and gets converted into oxygen molecule ( O₂ )

In the ozone layer of the stratosphere, these two opposite processes (i.e., the formation of ozone molecule and the decomposition of ozone molecules) occur in a cyclic order and finally attain a state of equilibrium. Due to the existence of such an equilibrium state, the quantity of ozone in the stratosphere remains fixed.

2. Role of ozone layer in the environment

The ozone sphere works as a protective layer for the fitting world. Acting as a protective umbrella, the ozone layer absorbs most of the harmful UV radiation emitted by the sun because these rays are utilised in the production and decomposition of ozone.

In the absence of the ozone layer, the earth’s surface and the adjacent air would be so heated by the UV-radiation that the existence of the plants and the animals including bacteria in land and water would have been impossible.’

Ultraviolet radiation:

• Causes cancer in human skin
• Damages the cornea of the eyes and develops premature cataracts
• Decreases immunity against diseases and fertility in living beings. Genetic diseases are also the consequence of the harmful effects of ultraviolet radiation

3. Depletion of ozone layer: Ozone hole

In 1982, the British scientist Jo Foreman first observed that the ozone layer of the stratosphere above the Antarctic (the south polar regions) was gradually getting thinner. Extensive research in the subsequent years has revealed that the ozone layer is gradually becoming thinner not only in Antarctica but almost everywhere in the stratosphere. The phenomenon of thinning of the ozone layer of the stratosphere is known as depletion of the ozone layer or the ozone hole.

Depletion of the ozone layer signifies that the equilibrium between! the two contradictory’ Processes i.e., the formation and the decomposition of ozone has been disturbed somehow. Due to the influx of several foreign substances into the atmosphere, the rate of decomposition of ozone has far exceeded than the rate of its formation. This has resulted in the thinning of the ozone layer almost every where in the stratosphere

Ozone layer before formation of hole:

Ozone layer after formation of hole:

Causes of depletion of ozone layer (formation of ozone hole):

1. Scientists believe that some chemical substances belonging to the class of chlorofluorocarbons (Freons or CFCs) are mainly responsible for the formation of ozone hole. Chlorofluorocarbons (CFCs) are the different chloro and fluoro derivatives of methane and ethane.

Some examples of CFCs are given below:

The above-mentioned chemicals were extensively used as

• Refrigerants
• Propellants in aerosols,
• Foaming agent in plastic production,
• Ingredients of fire extinguisher,
• Solvents for various purposes etc. In the troposphere, these gases are non-corrosive, non-toxic, non-inflammable and chemically inert.

As a result, these gases after being liberated from the field of their applications, gradually reach the upper stratosphere after a long period of time.

In the stratosphere, they absorb the UV radiation coming from the sun and decompose to produce highly active chlorine atoms which subsequently with O₃ of the ozone layer to liberate oxygen and chlorine monoxide free radical (CIO) which, on further reaction with ozone, forms O₂ molecules and active chlorine atoms. The chlorine atoms again combine with O₃ molecules to form O₂ molecules. In this way, a cyclic process continues, which eventually causes depletion of the ozone layer.

⇒ \(\mathrm{CFCl}_3(\mathrm{~g})+h \nu \longrightarrow \dot{\mathrm{C} F C l} 2 \text { (free radical) }+\dot{\mathrm{Cl}} \text { (active) }\)

⇒ \(\mathrm{CF}_2 \mathrm{Cl}_2(\mathrm{~g})+h \nu \longrightarrow \dot{\mathrm{C}} \mathrm{F}_2 \mathrm{Cl} \text { (free radical) }+\dot{\mathrm{Cl}} \text { (active) }\)

⇒ \(\dot{\mathrm{Cl}}+\mathrm{O}_3 \longrightarrow \mathrm{Cl} \dot{\mathrm{O}}+\mathrm{O}_2 ; \mathrm{Cl} \dot{\mathrm{O}}+\mathrm{O}_3 \longrightarrow 2 \mathrm{O}_2+\dot{\mathrm{Cl}}(active)\)

It has been experimentally found that a single Cl -atom is capable of decomposing millions of O₃ molecules.

2. Experiments head revealed that halons widely damage the ozone layer. Halons are halocarbons. Most of them contain bromine as halogen. These are mainly used as fire extinguishers

For example: Halon—1211 (CF₂BrCl, bromochlorodifluoromethane),

Halon: 1301 (CF₃Br, bromotrifluoromethane) etc.

Halons are stable in the troposphere. But in the stratosphere, they absorb UV-radiations and decompose to produce active bromine atoms. These active bromine atoms combine with O₃ of the ozone layer to liberate oxygen and bromine monoxide free radical (BrO).

The produced BrO again reacts with ozone to evolve O₂ molecules and active bromine atoms. Bromine atoms thus obtained, in reaction with O₃, give O₂ molecule. In this way, the entire process proceeds continuously in a cyclic manner which leads to the decay of the ozone layer.

3. Extensive studies have unquestionably proved that increase in quantity of die oxides of nitrogen in the stratosphere adversely affect the ozone layer. The main source of these oxides are the supersonic aeroplanes which emit plenty of NO gas while flying through the stratosphere. Like Cl and Br-atoms, NO molecule brings about catalytic decomposition of O₃ into O₂

NO + O₃→NO₂ + O₂ ; NO₂ + O → NO + O₂

The reaction of O₃ with NO yields NO₂ yields. This NO₂ combines with the oxygen atom (which ”highways produced in the stratosphere due to the decomposition of O₃ and O₂ under the influence of UV-radiation) to regenerate NO. This explains why the O₃ molecules undergo continuous decomposition although the quantity of NO is not diminished.

4. Effect of ozone hole on the environment

Effect on climate:

If the ozone layer in the stratosphere is destroyed, the UV radiation emitted by the sun, instead of being absorbed by this region, will be incident on the earth’s surface. Consequently, the temperature of the earth’s surface will increase. Owing to this rise in temperature the earth will be continuously heated and the ice in the polar regions will melt, resulting in a rise in
the water level of the sea.

Effect on mankind:

In the absence of ozone layer in the stratosphere, the UV radiation will directly reach the earth’s surface. This radiation is extremely harmful to human beings. It causes skin cancer and premature cataract in the eyes. Exposure to the UV-radiation damages the immune system which thereby increases susceptibility to viral infections. Moreover, this radiation motivates the photochemical reactions which increases the tendency of smog formation. This in turn creates severe respiratory problems such as bronchitis, tracheal irritation, lung diseases etc.

Effect on plants, animals and other living organisms:

The incidence of UV radiation on the earth’s surface will hinder the process of photosynthesis. As a result, the production of crops will decrease. UV radiation would naturally increase the earth’s average surface temperature. Water bodies will dry up and water from the soil will evaporate. Consequently, agriculture will be greatly affected and the production of crops will fall drastically.

Also, UV radiation precludes photosynthesis. Therefore plants and aquatic phytoplanktons will die. Thus, marine life which depends on phytoplankton will also perish. Therefore, UV radiation disrupts the entire ecological system and composition of the sea

Greenhouse And Greenhouse Effect

What is a greenhouse?

Greenhouse means a glass room or glass chamber. Plants of the tropical region cannot adapt themselves to the climatic conditions of the cold countries. So for sustaining plant life (particularly for plants of tropical regions in cold countries), this type of chamber made of glass is used. Sun rays enter the glass chamber through the transparent glass roof and walls and due to this the soil gets heated.

The heated soil inside the chamber radiates infrared rays of longer wavelengths which cannot pass through the glass. The glass absorbs a part of these rays and the rest are reflected to the soil inside the chamber. As a result, the temperature inside the chamber always remains higher than that of the outside temperature. Thus, proper growth of these plants becomes possible.

In fact, some gaseous substances present in the earth’s atmosphere such as CO₂ water vapour etc., together act like a glass of the greenhouse and keep the atmosphere adjacent to the earth’s surface warmer and create a favourable environment for the living world.

Greenhouse effect and its importance

Greenhouse Effect Definition:

The natural process by which CO₂  water vapour and some other gases are present in the atmosphere, prevents the return of the radiation emitted by the earth’s surface to outer space, thereby keeping the surface of the earth and the adjacent environment suitable for the effect.

‘ Some gases like CO₂, water vapour etc., present in the atmosphere allow sun rays of smaller wavelengths to be incident on the earth’s surface but prevent the rays of longer wavelength (infrared rays) emitted from the hot earth’s surface, from returning to the outer space. Those gases absorb a significant portion of the reflected radiation of longer wavelength and arc heated.

The rest of the infrared rays fall on the earth’s surface and remain in the adjacent atmosphere to keep the surface of the earth and the adjacent atmosphere warm and make it favourable for the existence of the living world. due to indiscriminate deforestation, the quantity of CO₂ absorbed by the plants is gradually decreasing.

Greenhouse gases like CO₂, water vapour etc., help to keep the atmosphere warm to a certain level of temperature (average as 5°C) which is essential for the existence of life on earth. If those gases were not present in the atmosphere then the average temperature of the earth’s surface and that of the surrounding atmosphere would have dropped to about -30°C and eventually, survival of life on earth would have been impossible

Greenhouse gases, their sources and contribution towards the greenhouse effect

The gases that absorb a significant portion of radiation of longer wavelengths (infrared rays) emitted by the hot earth’s surface and reflect the rest to the earth’s surface, to keep the adjacent environment warm, are referred to as greenhouse gases. Some greenhouse gases are—carbon dioxide (CO₂), methane (CH₄), chlorofluorocarbon (CFCs), ozone (O₃), nitrous oxide (N20), water vapour (H₂O) etc.

1. Carbon dioxide:

Out of all the greenhouse gases, CO₂ is present in the largest amount in the atmosphere. Naturally, CO₂ plays the most vital role in absorbing the radiation emitted by the earth’s surface. The contribution of CO₂ towards the greenhouse effect is approximately 50%.

During respiration, plants and animals take in O₂ and give up CO₂  while plants accept CO₂ for the preparation of their food. In this way, the CO₂  -level in the environment is maintained. But at present, due to the progressive increase in the quantity of CO₂  in air, the equilibrium of CO₂  gas in the atmosphere has been disturbed. The possible reasons for the continuous

Increase in the percentage of COz gas are as follows:

1. 1.  Due to the indiscriminate use of fossil fuels in factories, motor vehicles etc., the quantity of CO₂ released in the atmosphere is not being used up completely by different natural processes.
2. Consequently, the concentration of CO in the atmosphere keeps on increasing.
3. In industrial regions, particularly during the manufacture of cement, a large &ftU>unt of CO₂ gas is released into the atmosphere.
4. During the process of photosynthesis, plants absorb CO₂ from the air. But due to indiscriminate deforestation, the quantity of CO₂ absorbed by the plants is gradually decreasing.

2.  Methane:

The role of methane gas in preventing the outflow of the heat emitted from the earth’s surface is worth mentioning. Due to the bacterial decomposition plants in paddy fields and other marshy lands, putrefaction of dung and other excreta and anaerobic decomposition of dead animals, methane gas is produced. Besides these, different waste organic compounds, oil mines etc., are the other sources of methane gas.

The capacity of each methane molecule to prevent the outflow of heat is 25 times a much as that of a molecule of CO₂  gas. But the quantity of methane gas in the atmosphere being less than that of CO₂ gas, its contribution to the greenhouse effect is about 16-20%.

3. Chlorofluorocarbons (CFCs):

CFCs are widely used as refrigerants, propellants in aerosol sprays, fire extinguishing agents, solvents for cleaning electronic types of equipment and foaming agents. These compounds destroy the ozone layer in the stratosphere and act as greenhouse gases in the troposphere.

The capacity of the chlorofluorocarbon molecules to prevent the release of heat emitted from the earth’s surface is 15000-20000 times greater than that of CO₂ molecules. These compounds are extremely stable. So, they can exist in the atmosphere for a long time. The contribution of these compounds towards the greenhouse effectis found to be 13-18%.

4. Tropospheric ozone:

Ozone gas present in the troposphere acts as a greenhouse gas. The combustion of fossil fuels in automobile engines, thermal power plants and different chemical industries gives rise to a profuse quantity of oxides of nitrogen (NO_x). The combination of ; different hydrocarbons and oxygen present in the atmosphere with these oxides results in the formation of ozone gas. The contribution of ozone gas to the greenhouse effect is about 7-8%.

5. Nitrous oxide:

Extensive combustion of fossil fuels, motor vehicles, and bacterial decomposition of nitrogenous chemical fertilisers in agricultural lands generates nitrous oxide. Again, its heat retention capacity per molecule is 200 times greater than that of CO₂ per molecule. The contribution of this gas towards the greenhouse effect is about 4-5%.

Global warming

For tire last few centuries, the average temperature of Tire Earth has been gradually increasing.

For example: During the period from 1800-1900 AD, the average temperature of the earth has increased by nearly 0.4°G. Again, in the following century i.e., 1900-2000 AD., this increase in temperature has been almost of 1°C. So it cannot be denied that the natural environment is gradually becoming warmer.

The phenomenon of this progressive rise in temperature all over the world is called global warming. The reason for this global warming can be attributed to the increased concentration of greenhouse gases in the atmosphere, caused by various human activities

Harmful effects of global warming: 

1. Because of global warming 2100 AD, the polar ice caps will melt, thereby releasing an enormous amount of water. Then the situation will be almost similar to what it was 1,30,000 years ago, when the surface water level of the sea was 6 years ago, when the surface water level of the sea was 6 coastal regions like Holland, America, New Orleans, Florida, Bangladesh etc., will be inundated and will go under water forever.

2. Global warming is a great threat to human health. Respiratory problems occur frequently due to human health.

Respiratory problems occur frequently due great concern. Global warning would initiate a favourable temperature for the breeding of microorganisms resulting in the epidemic spread of dreadful diseases such as dengue, malaria, encephalitis etc. If the CO₂ content in the air becomes twice that of the present value, then many species will become extinct from the earth

Consequences of the greenhouse effect

Scientists have predicted about the effect of increased concentration of greenhouse gases such as

• The temperature of the earth’s surface and the troposphere will go to increase day by day and by the middle of this century, the temperature of the earth will be increased by at least of 2°-4°C.
• Due to the increase in earth’s temperature, the polar caps, accumulated in tyre polar regions (Greenland and Antarctica) will melt and this will cause an increase in the water level of the sea. As a consequence, vast coastal regions like India, Bangladesh, Myanmar, Maldives etc., will sink, causing colossal devastation. If the populated area be inundated in this way, the resettlement of the affected people will pose a great problem to many countries
• Owing to the increased greenhouse effect, droughts will be more frequent during summer in the countries of the mid¬ latitudes in the northern hemisphere. As a result, crop production in the fertile lands of North America and the previous Soviet Russia will be reduced.
• More devastating cyclones, supercyclones, tornados or hurricanes will occur with an increase in temperature.
• Increase in temperature may lead to the destruction of forests due to forest fires.
• Due to the inability to sustain at high temperatures, living beings will die. Consequently, the ecosystem will be severely affected. In the marshy lands, due to increased decomposition of plants, methane will be liberated.

In a word, it can be said that by the middle of this century, man will have to face severe natural calamities.

Measures to check global warming

Global warming cannot be eradicated or reduced overnight. A concerted effort is necessary to attain this goal. In overnight. A concerted effort is necessary to attain this goal.

In measures are mentioned below:

• The addition of CO₂ to the atmosphere should be minimised by reducing the use of fossil fuels such as wood, coal, petroleum etc.
• Unlawful cutting of trees should be stopped and the forests should be saved from destruction.
• Afforestation ought to be encouraged so that plants absorb more CO₂ (for the preparation of their food). Koiget !r ’Boron
• Use of unconventional forms of energy
• For example: Solar energy, wind energy, tidal energy etc.) is to be increased.
• Use of CFCs is to be prohibited

Smog Or Classical Smog

In December 1952, the city of London was covered with a dense layer of fog continuously for five days. The people, irrespective of age and sex, fell ill and 4000 people ultimately lost their lives.

Subsequently, it was found that the fog contained poisonous gases emitted from automobile engines and factories and the constituent which was present in the largest quantity, was found to be sulphur dioxide gas (SO₂). A British physician named it smog (smog = smoke + fog). As the horrible effect of such smog was first observed in London, it was called London smog

Several accidents caused by such smog (of course of less alarming proportions), occurred in different cities. Smog is frequently observed in big cities

For example:

Delhi, Mumbai Kolkata) of India, during the winter season. Mixture of particulates with gaseous oxides of Mixture of particulates with gaseous oxides of the presence of SO₂ and carbon (soot) particles, classical smog possesses a reducing character. Thus it is also called reducing smog

1. Formation of smog

During winter, particularly after evening or early in the During winter, particularly after evening or early in the earth’s surface becomes heavier If it is suddenly cooled down earth’s surface becomes heavier ifit is suddenly cooled down cannot go upwards and remains confined in that layer. Impure cannot go upwards and remains confined in that layer. Impure finely divided particles liberated from local factories motor vehicles, mix with that confined air to create finely divided particles liberated from local factories motor vehicles, mix with that confined air to create and and

Harmful effects of smog:

• Smog irritates the nose, eyes and throat, resulting in sneezing and coughing.
• It affects the respiratory system, causing bronchitis, asthma, heart disease etc.
• It lowers visibility level, posing great problems while driving cars. So, accidents are likely to happen.
• It also has adverse effects on electronic systems and plants

2. Photochemical smog or Los Angeles smog

This type of smog was first observed in the city of Los Angeles in America, in the year 1950. So, it is called Los Angeles smog. Highly poisonous substances like nitrogen dioxide(NO₂) and ozone (O₃). peroxyacyl nitrate (PAN), smog was formed due to chemical reactions in the presence of bright sunlight, it is commonly known as photochemical smog. Generally, during the mid-days of the summer when the sun shines brightly, (i.e., the intensity of solar radiation is very high) this kind of smog is observed

In big cities, where there is considerable vehicular traffic on the roads throughout the whole day and night, the atmosphere contains the oxides of nitrogen particularly, nitrogen dioxide (NO₂) in the largest proportions.

Apart from these, hydrocarbons (produced by evaporation or incomplete combustion of liquid fuels) and other gaseous substances

For example: SO₂> CO₂  are present.

In the presence of bright substances like ozone (O₃), peroxyacyl nitrate (RCO₃NO₂), aldehyde and ketone. These gaseous substances and the particulates mix together in the air to form photochemical smog.

It formation of smog is to be noted that, it is not real smog, because it contains particulates mix in air to form photochemical smog. The formation of smog is to be noted that, it is not a real smog, because it contains Hence it is also known as oxidising smog

Mechanism for the formation of photochemical smog:

In the presence of sunlight, nitrogen dioxide (NO₂) molecule decomposes into nitric oxide (NO) molecules and atomic oxygen

In the reaction of hydrocarbons with this atomic oxygen, at first highly reactive free radical RCO is produced and this radical again combines with oxygen molecule to give peroxyacetyl radical

Peroxyacyl radical is highly reactive. It combines with hydrocarbon,O₂ and XO₂ to form a mixture of aldehyde, ketone, ozone and peroxyacyl nitrate respectively. This peroxyacylnitrate is extremely harmful for eyes.

Harmful effects of photochemical smog:

• Presence of large amounts of ozone (O₃), peroxyacetyl nitrate (PAN), acetaldehyde, ketone etc., causes
  • Irritation of eyes, nose, and throat but it sill effect on the eyes is much more intense
  • Congestion of nostrils, sneezing and cough
  • Respiratory problems and chest pain.
• By brown colour, it reduces visibility and hence the car drivers and pilots face extreme difficulties.
• PAN and other oxidising materials damage plant cells and produce white spots on leaves.
• PAN hinders the process of photosynthesis.
• Rubber goods lose elastic properties and become brittle.

To control or suppress the formation of photochemical smog, the following methods can be adopted.

• Certain chemical compounds, which are capable of generating free radicals, are sprayed into the atmosphere. The free radicals readily combine with the free radicals responsible for the formation of photochemical smog (such as O^•, R^•, R^•O, ^•H etc.) and hence nullify their effects.
• Efficient catalytic converters are being developed for installation in automobiles so that emission of nitrogen oxides and hydrocarbons can be prevented or minimized

Certain plants such as Pinus, Juniperus, Pyrus, Vritis etc., can directly assimilate oxides of nitrogen for their metabolic activity. So their plantation could be helpful.

Comparison between ordinary smog and photochemical smog:

Acid Rain

Ordinary water is slightly acidic (pH = 5.6) because a portion of carbon dioxide gas present in the air gets dissolved in water and forms carbonic acid

CO₂ + H₂O→H₂CO₃ .

But if rainwater contains an excess amount of dissolved acid, then it is called acid rain. Acid rain is mainly a mixture of H₂SO₄, HNO₃ and HCl. The pH of such rainwater generally lies within the range of 5.6 to 3.5. The proportion of the above acids in the rainwater of different localities depends upon the quantities of sulphur dioxide (SO₂), nitrogen oxides (NO_x) and hydrogen chloride (HCl) present in the air of that particular locality

1. Origin of Acid Rain

Huge quantities of the oxides of sulphur, nitrogen and carbon (SO_x, NO_x, CO_x etc.) are released In the air due to natural processes as well as tyre human activities. These oxides combine with oxygen, ozone and water vapour present in air to give different acids. These acids float in the air in the form of fine particles as an aerosol. Moreover, HCl gas is also liberated in sufficient quantity from the factories. These acids come down to the earth through dew, snow and rainfall

H₂SO₄ has the highest contribution (60-65%) to acid rain followed by HNO₃ having 30-35% contribution.

2. Harmful effects of acid rain

Effect on soil and plants:

Acid rain increases the acidity of the soil, changes the solubility of different metals and metallic oxides present in the soil. Thus, living creatures and bacteria living inside the soil are severely affected or die. As a result, the fertility of the soil decreases and the production of crops Is drastically reduced. Due to increased acidity of the soil, leaf pigments get spoiled, the process of photosynthesis and as a consequence the growth of plants and their immunity drastically fall, i.e., agricultural productivity is reduced.

Effect on aquatic plants and animals:

Due to acid rain, the pH of different water bodies decreases significantly. As a result, the production of spawn of fish is reduced. The biological processes of fishes are affected. An increase in acidity results in the elimination of many species of algae and zooplankton, aquatic insects, fishes etc. That is, polluted water disrupts the aquatic food chains and consequently disturbs the ecosystem. In countries like America, Sweden etc., in a large number of lakes, virtually no fish exists due to acid rain.

Effect on mankind:

Acid rain dissolves different metallic substances. These dissolved substances enter human bodies through water and result in severe health hazards. Acid rain has profound ill effects on human skin, hair and cells. H₂SO₄ and HNO₃ present in acid rain enter the human body and adversely affect the nervous, respiratory and digestive systems.

 Effect on architecture and edifice:

Because of acid rain buildings, monuments of historical importance

For example:

Tajmahal, Victoria Memorial, British Parliament House), states, sculptures etc., made of marble;, limestone, dolomite, mortar and slate suffer irreparable damages

Marble, limestone etc., react with H₂SO₄ to form an insoluble layer of CaSO₄ and lose its glossiness

CaCO₃+ H₂SO₄ → CaSO₄ ↓+ CO₂↑ + H₂O

Few years ago, scars developed on the surface of Tajmahal Few years ago, scars developed on the surface ofTajmahal Few years ago, scars developed on the surface of Tajmahal

Slone cancer:

• The scars that are developed on the surface of architectural edifices, memorials, sculptures etc., are termed stone leprosy or stone cance
• Metallic surfaces,
  • For example: Aluminium, steel or iron structures, bridges etc., exposed to acid rain, suffer steady.
• Textile materials, leather and paper products are also not spared from the ill effects of acid rain

3. Measures to check acid rain

Only the drastic reduction in the quantities of SOx and NOx in the environment can eliminate the apprehension rather than the threat of acid rain.

The following measures can be adopted to check acid rain:

Use of fossil fuels is to be decreased as far as practicable. Fuels of low sulphur content should be used so that the emission of SO₂ can be controlled.

Suitable technological devices should be developed for the removal of those gases

For example: SO_x, NO_x etc.)

Released from the thermal plants, factories, furnaces for metal extraction and various other sources

Vehicles involving engines with improved technology must be launched so that the emission of NO_x can be controlled