- The lowest region of atmosphere in which the human beings along with other organisms live is called the troposphere. It extends up to the height of ~ 10 km from sea level
- Above the troposphere, between 10 and 50 km above sea level lies stratosphere.
- 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, 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.
- Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air.
- The following are the major gaseous and particulate pollutants present in the troposphere:
1. Gaseous air pollutants: These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.
2. Particulate pollutants: These are dust, mist, fumes, smoke, smog etc.
1. Gaseous air pollutants
- (a) Oxides of Sulphur:
- Oxides of sulphur are produced when sulphur-containing fossil fuel is burnt.
- Sulphur dioxide is a gas that is poisonous to both animals and plants
- Even a low concentration of sulphur dioxide causes respiratory diseases e.g., asthma, bronchitis, emphysema in human beings.
- Sulphur dioxide causes irritation to the eyes, resulting in tears and redness
- The 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
(b) 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 the 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 )
- 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 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.
(d) 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 the organs and tissues
- It is produced as a result of 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.
Why carbon monoxide is poisonous?
- It binds to haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable than the oxygen-haemoglobin common 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 a headache, weak eyesight, nervousness and cardiovascular disorder.
- This is the reason why people are advised not to smoke.
- In pregnant women who have the habit of smoking 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 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.
- Deforestation and burning of fossil fuel increase 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 and the Greenhouse Effect
- 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, chlorofluorocarbons compounds (CFCs) and water vapour in the atmosphere.
- Thus, they add to the heating of the atmosphere. This causes global warming.
- 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.
- Carbon dioxide molecules also trap heat as they are transparent to sunlight but not to the 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 the 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 (Section 14.2.2). 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 melting of polar ice caps and flooding of low lying areas all over the earth.
- Increase in the global temperature increases the incidence of infectious diseases like dengue malaria, yellow fever, sleeping sickness, etc.
What can we do to reduce the rate of global warming?
- minimise the use of automobiles. one can use the bicycle, public transport system, or go for carpool.
- plant more trees to increase the green cover.
- Avoid burning of dry leaves, wood etc. It is illegal to smoke in public places and workplaces
- When the pH of the rainwater 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 the 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 a wet deposition.
- Acid rain is a byproduct of a variety of human activities that emit the oxides of sulphur and nitrogen in the atmosphere.
- 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
- 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 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
How We Can Help To Reduce The Formation Of Acid Rain.
- reducing the emission of sulphur dioxide and nitrogen dioxide in the atmosphere.
- useless vehicles driven by fossil fuels; use less sulphur content fossil fuels for power plants and industries.
- use natural gas which is a better fuel than coal or uses coal with less sulphur content.
- Catalytic converters must be used in cars to reduce the effect of exhaust fumes on the atmosphere.
- We can also reduce the acidity of the soil by adding powdered limestone to neutralise the acidity of the soil.
Taj Mahal and 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 a large number of industries and power plants around the area
- Use of poor quality of coal, kerosene and firewood as fuel for domestic purposes add up to this problem.
- The resulting acid rain reacts with marble, CaCO3 of Taj Mahal causing damage to this wonderful monument
- As a result, the monument is being slowly disfigured and the marble is getting discoloured and lusterless
- The Government of India announced an action plan 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.
- This plan aims at clearing the air in the ‘Taj Trapezium’– an area that includes the towns of Agra, Firozabad, Mathura and Bharatpur
- Under this plan, more than 2000 polluting industries lying inside the trapezium would switch over to the use of natural gas or liquefied petroleum gas instead of coal or oil.
- A new natural gas pipeline would bring more than half a million cubic metres of natural gas a day to this area.
- People living in the city will also be encouraged to use liquefied petroleum gas in place of coal, kerosene or firewood.
- Vehicles plying on highways in the vicinity of Taj would be encouraged to use low sulphur content diesel.
- Particulates pollutants are the minute solid particles or liquid droplets in the 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.
- Smoke particulates consist of 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.
- 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.
- Mists are produced by particles of spray liquids and by condensation of va herbicides and insecticides that miss their targets and travel through air and form mists.
- 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. Airborne 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 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.
- The word smog is derived from smoke and fog. This is the most common example of air pollution that occurs in many cities throughout the world
There are two types of smog:
(a) Classical smog occurs in the cool humid climate. 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 climate. 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 pours in air. Examples are sulphuric acid mist and 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 the free oxygen atom
(i) Oxygen atoms are very reactive and combine with the O2 in the air to produce ozone.
(ii) The ozone formed in the above reaction (ii) reacts rapidly with the NO(g) formed in the reaction to regenerate NO2. NO2 is a brown gas and at sufficiently high levels can contribute to haze.
(iii) 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 a 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
How can photochemical smog be controlled?
- control the primary precursors of photochemical smog, such as NO2 and hydrocarbons, the secondary precursors such as ozone and PAN, the photochemical smog will automatically be reduced.
- Usually, catalytic converters are used in the automobiles, which prevent the release of nitrogen oxide and hydrocarbons to the atmosphere.
- Certain plants e.g., Pinus, Juniperus, Quercus, Pyrus and Vitis can metabolise nitrogen oxide and therefore, their plantation could help in this matter.
- Formation and Breakdown of Ozone The upper stratosphere consists of the 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. O2 (g) O(g) + O(g) O(g) + O2 (g) O3 (g) Ozone is thermodynamically unstable and decomposes to molecular oxygen.
- 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 for ozone layer depletion is believed to be the release of chlorofluorocarbons 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 the stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radical.
- 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 ozone hole over the South Pole.
- In the summer season, nitrogen dioxide and methane react with chlorine monoxide) and chlorine atoms forming chlorine sinks, preventing much ozone depletion, whereas, in winter, a special type of clouds called polar stratospheric clouds are formed over Antarctica.
- These polar stratospheric clouds provide a surface on which chlorine nitrate formed gets hydrolysed to form hypochlorous acid. It also reacts with hydrogen chloride to give molecular chlorine.
- When sunlight returns to Antarctica in the spring, the sun’s warmth breaks up the clouds and HOCl and Cl2 are photolysed by sunlight,
- The chlorine radicals thus formed, initiate the chain reaction for ozone depletion
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 off much phytoplankton, damage to fish productivity etc
- plant proteins get easily affected by UV radiations which leads to the harmful mutation of cells.
- It also increases 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.