Contents
Environmental chemistry is a crucial area within Chemistry Topics that investigates the impact of chemicals on the environment and ecosystems.
Air Pollution – Introduction, Sources and Effects of Air Pollution
The presence of unusually high concentrations of harmful or poisonous substances in the environment (air, water, etc.) is called pollution. Pollution contaminates the air and water with poisonous substances and makes them impure to such an extent that they become harmful to the human beings, other animals, plants as well as to the non-living things.
An unwanted and harmful substance that contaminates the environment (such as air and water) is called a pollutant. In most simple words, a substance that causes pollution is called a pollutant. The air and water which contain pollutants at levels harmful to humans, other animals, plants and non-living things are said to be polluted. Air and water are both essential for our survival. For example, air is necessary for breathing. We cannot live without air even for a few minutes. Similarly, we cannot live without water for more than a few days (though we can live much longer without food).
Air is a mixture of gases (which is present all around us). The two main gases present in air are nitrogen and oxygen. Nitrogen makes up about 78 per cent of air whereas oxygen makes up about 21 per cent of air by volume. Air also contains small amounts of carbon dioxide, argon and water vapour, etc.
The mixture of gases containing nitrogen, oxygen, carbon dioxide, argon and water vapour gives us pure air (or clean air) which is good for us. In addition to the normal constituents, the polluted air may also contain harmful substances such as carbon monoxide, sulphur dioxide, nitrogen oxides, smoke and dust, etc. When air around us is polluted, we are forced to breathe it to remain alive (though it is harmful to us).
Many respiratory problems are caused by breathing in polluted air. In fact, the respiratory problems among children are increasing day by day due to increasing air pollution. Water is the most common liquid which is present around us. Fresh water (usable water) is present in rivers, lakes and ponds.
Some fresh water is also present under the ground. Due to increasing water pollution, water-borne diseases are also increasing day by day. In this Chapter we will discuss how air and water get polluted; what are the harmful effects of air and water pollution; and how air and water pollution can be controlled (or minimised).
Air Pollution
The air over large cities is heavily contaminated with harmful gases like sulphur dioxide, nitrogen oxides, carbon monoxide, smoke and dust, etc. The contamination of air with harmful gases (like sulphur dioxide, nitrogen oxides, carbon monoxide), smoke and dust, etc., is called air pollution. The substance whose presence in air makes it impure or contaminated is called an air pollutant.
A substance becomes an air pollutant when it is present in air in such concentration which is high enough to have a harmful effect on the living or non-living things. The major pollutants which cause air pollution are : Sulphur dioxide, Nitrogen oxides, Carbon monoxide, excess of Carbon dioxide, Chlorofluorocarbons, and Suspended particulate matter (such as Dust, Smoke and Fly ash).
Sources of Air Pollution
Most of the air pollution is caused by the burning of fuels such as wood, cow-dung cakes, coal, kerosene, petrol and diesel in homes, motor vehicles (automobiles), factories and thermal power plants, etc. The various sources of air pollution are given below :
- Smoke emitted from homes by the burning of fuels like wood, cow-dung cakes, kerosene and coal causes air pollution.
- Exhaust gases emitted by motor vehicles (automobiles) due to burning of petrol and diesel cause air pollution (see Figure). Motor vehicles are the major cause of air pollution in big cities.
- Smoke emitted by factories and thermal power plants due to burning of coal causes air pollution.
- Oil refineries and industries engaged in the production of metals and manufacture of chemicals cause air pollution.
- Stone crushers, cement factories, asbestos factories and lead processing units cause air pollution.
- Use of chlorofluorocarbons in refrigeration, air conditioning and aerosol sprays causes air pollution.
- Smoking causes air pollution.
All the above sources of air pollution are man-made sources of air pollution in which pollutants are added to air by various human activities. Forest fires and volcanic eruptions are the two natural sources of air pollution which put smoke and dust into the air.
Harmful Effects of Air Pollution
Air pollution produces a large number of bad effects on living and non-living things. Air pollution can cause health problems in human beings. It can kill animals and plants. It can also damage the environment and property (buildings, etc.) We will now give the names, sources and harmful effects of various air pollutants.
(i) SULPHUR DIOXIDE.
Sulphur dioxide is produced by the burning of coal in factories and thermal power plants. Sulphur dioxide is also produced by the burning of petrol and diesel in motor vehicles. Actually, the fuels such as coal, petrol and diesel contain some sulphur as impurity which burns to produce sulphur dioxide gas. Oil refineries also emit sulphur dioxide gas into air.
Sulphur dioxide gas in the polluted air causes respiratory problems. It may even cause permanent lung damage. Sulphur dioxide gas in polluted air produces acid rain. This acid rain damages trees, plants, soil, aquatic animals (like fish), statues, buildings and historical monuments. Sulphur dioxide also contributes to the formation of a deadly air pollutant called smog.
(ii) NITROGEN OXIDES.
Nitrogen oxides are produced by the burning of fuels like petrol and diesel in motor vehicles. They are also produced by the burning of coal in factories and thermal power plants. Actually, the high temperature produced by the burning of fuels like petrol, diesel and coal makes some of the nitrogen and oxygen of air to combine to form nitrogen oxides.
Oil refineries also produce and emit nitrogen oxides into the air. Nitrogen oxides attack breathing system and lead to lung congestion. They also attack skin. Just like sulphur dioxide, nitrogen oxides present in polluted air produce acid rain. Nitrogen oxides also contribute to the formation of smog.
Smog is a deadly air pollutant which is formed by the combination of smoke and fog (The minute water particles suspended in air near the surface of earth during cold weather in winter, is called fog). Smoke contains tiny carbon particles, and harmful gases such as nitrogen oxides and sulphur dioxide, etc. The carbon particles, nitrogen oxides and sulphur dioxide, etc., of smoke combine with the condensed water vapour called fog to form ‘smog’. Smog causes cough and aggravates (makes worse) asthma and other lung diseases, especially in children.
(iii) CARBON MONOXIDE.
Carbon monoxide is produced by the incomplete combustion of fuels like wood, coal, kerosene, petrol and diesel in homes, factories and motor vehicles. The exhaust gases of motor vehicles (cars, buses and trucks, etc.) contain carbon monoxide which they emit into air (Incomplete combustion of fuels which produces carbon monoxide, takes place in insufficient supply of air). Cigarette smoke also contains carbon monoxide.
Carbon monoxide is a very poisonous gas. When inhaled, carbon monoxide combines with the haemoglobin of our blood and reduces the oxygen-carrying capacity of blood. Due to this, blood is not able to carry sufficient oxygen to our body parts. This lack of oxygen causes respiratory problems (breathing problems). It causes suffocation. If too much carbon monoxide is inhaled, it may even cause death.
(iv) CARBON DIOXIDE.
Carbon dioxide is produced in excessive amounts by the burning of large quantities of fuels such as wood, coal, kerosene, petrol, diesel, LPG, and CNG in homes, factories and motor vehicles. Though carbon dioxide is a normal constituent of air but excess of carbon dioxide in air is considered a pollutant (because it produces undesirable changes in the environment). Carbon dioxide is the main greenhouse gas which traps sun’s heat in the earth’s atmosphere by producing greenhouse effect which leads to global warming. We will study this in detail after a while.
(v) CHLORO-FLUORO-CARBONS (CFCs).
Chlorofluorocarbons are the chemical compounds made of chlorine, fluorine and carbon elements. They are commonly known as CFCs. Chlorofluorocarbons are used in refrigeration, air conditioning and aerosol sprays. So, all these sources release chlorofluorocarbons into the air. Chlorofluorocarbons are industrially useful gases but they also behave as air pollutants because of their damaging effect on ozone layer (which exists high up in the atmosphere or upper atmosphere).
Chlorofluorocarbons are depleting the useful ozone layer of the upper atmosphere. This happens as follows : Chlorofluorocarbons released into the air go up and ultimately reach high into the atmosphere where the protective ozone layer exists. The chlorofluorocarbons react with the ozone gas of ozone layer and destroy it gradually. Ozone layer prevents the harmful ultraviolet radiations of the sun from reaching the earth. So, ozone layer protects us from the harmful effects of the ultraviolet radiations of the sun.
The destruction of ozone layer by CFCs will allow the extremely harmful ultraviolet radiations of the sun to reach the earth. These ultraviolet radiations can cause skin cancer, cataract, and destruction of plants, including crops. In fact, a big hole has already been made by the destruction of ozone gas in the ozone layer over the South Pole of the earth. It is called “ozone hole”. The good news is that less harmful chemicals are now being used in place of CFCs.
(vi) SUSPENDED PARTICULATE MATTER (SPM).
The finely divided solid or liquid particles suspended in air are called suspended particulate matter. Some of the examples of suspended particulate matter are : Dust, Smoke and Fly ash. They remain suspended in air for long periods.
(a) Dust consists of tiny particles of earth. Dust is produced by blowing wind, heavy traffic on roads, stone crushers and construction activities. Dust in air spoils our clothes and reduces visibility. Dust produces allergic reactions in human body and aggravates diseases like bronchitis. Dust covers the leaves of plants and trees and prevents photosynthesis.
(b) Smoke is mainly tiny particles of carbon in air. Smoke is produced by the burning of fuels like wood, cow-dung cakes, coal, kerosene, petrol, and diesel in homes, factories, thermal power plants and motor vehicles. Smoke present in air spoils our clothes and blackens the buildings. Smoke attacks our lungs and causes respiratory diseases.
(c) The minute ash particles formed by the burning of coal and carried into air by the gases produced during burning, is called fly ash. Fly ash is emitted by the chimneys of coal based thermal power plants. Fly ash particles present in air cause irritation to the eyes, skin, nose, throat and respiratory tract. Continued breathing in air containing fly ash causes diseases like bronchitis and lung cancer.
The two extremely harmful effects of air pollution on the environment are acid rain and greenhouse effect (or global warming). So, we will now discuss acid rain and greenhouse effect produced by polluted air in detail, one by one. Let us start with acid rain.
The burning of fossil fuels like coal and oil in factories, thermal power plants and oil refineries, and petrol and diesel in motor vehicles produce acidic gases like sulphur dioxide and nitrogen oxides which go into air and pollute it. Sulphur dioxide reacts with water vapour present in the atmosphere to form sulphuric acid whereas nitrogen oxides react with water vapour present in the atmosphere to form nitric acid. These acids dissolve in rainwater and fall to the earth in the form of acid rain.
Thus, acid rain is that rain which contains small amounts of acids formed from acidic gases like sulphur dioxide and nitrogen oxides present in polluted air. Acid rain contains very dilute solutions of sulphuric acid and nitric acid. Acid rain causes great damage to living and non-living things. The damage caused by acid rain is very, very slow and hence cannot be seen immediately. Acid rain has the following harmful effects :
(i) Acid Rain Destroys Forests.
Acid rain damages the forest trees by destroying their leaves [see Figure (a)]. It causes the leaves of trees to turn yellow and fall off. In the absence of leaves, the roots of trees cannot absorb water from the soil. And due to lack of water the trees die. Acid rain also damages a lot of crop plants every year and causes a big loss to the farmers. Acid rain makes the soil acidic. This acidic soil is not good for the growth of crop plants.
(ii) Acid Rain Kills Aquatic Animals Such as Fish. Acid rain causes the water in ponds, lakes and rivers to become much more acidic and unsuitable for the survival of aquatic animals and plants. Due to high acidity of water, the aquatic animals such as fish get killed [see Figure (b)].
(iii) Acid Rain Corrodes the Statues, Buildings and Historical Monuments and Damages Them Slowly. The statues, buildings and monuments are made of marble or limestone, etc. The acids present in acid rain react with the carbonates present in marble and limestone of a statue, building or monument and corrode it slowly (dissolve it slowly). In this way, acid rain makes the statues, buildings and monuments to crumble away slowly [see Figure (c)].
The Case of Taj Mahal. The Taj Mahal at Agra (near Delhi) is a beautiful historical monument made of pure, white marble (see Figure ). The experts have warned that air pollution around Taj Mahal area is discolouring its white marble and also corroding it slowly. This poses a threat to the beauty of Taj Mahal.
Actually, the Mathura Oil Refinery near Agra as well as the various industries in and around Agra are emitting gaseous pollutants such as sulphur dioxide and nitrogen oxides into the air which cause acid rain. The acids present in acid rain react with the marble (calcium carbonate) of Taj Mahal monument and corrode it slowly. The slow corrosion (or eating up) of marble of a monument by acid rain is also known as ‘Marble Cancer’.
The suspended particulate matter such as soot particles emitted in the smoke from Mathura Oil Refinery is discolouring the pure white marble of Taj Mahal by turning it yellowish. The Supreme Court of India has taken several steps to save Taj Mahal from the damage being caused by air pollution (acid rain, etc.). It has ordered all the industries in Agra area to switch over to cleaner fuels like CNG and LPG to reduce air pollution. It has also asked vehicles to be run on CNG or unleaded petrol in the Taj Mahal area.
From the above discussion we conclude that it is not only the living things (such as humans, other land animals, aquatic animals like fish, trees and crop plants, etc.) which are affected by air pollution, even the non-living things (such as soil, statues, buildings and historical monuments) get affected by air pollution.
Before we go further and describe the greenhouse effect, we should .know the meaning of ‘greenhouse’ and how it works. This is described below : The greenhouse is a structure or building made of glass walls and glass roof in which the plants that need protection from cold weather are grown (see Figure).
The glass walls and glass roof of a greenhouse allow the sun’s heat rays to by soil, plants and other things in the greenhouse) to go out. In this way, more and more of sun’s heat rays are trapped inside the greenhouse due to which the temperature in the greenhouse rises.
So, even without an internal supply of heat, the temperature inside a greenhouse becomes higher than that outside. This heat is beneficial for the growth of plants inside the greenhouse (when the outside temperature is very low during winter season). Thus, greenhouse acts as a heat trap (which traps sun’s heat energy). We will now describe why a greenhouse acts as a heat trap.
The sun is an extremely hot object due to which the heat rays emitted by the sun are of shorter wavelengths. The shorter wavelength heat rays coming from the sun can pass through the glass walls and glass roof of a greenhouse and go inside it.
On the other hand, the inside objects of glasshouse are much less hot (than the sun), so they emit heat rays of longer wavelengths. The longer wavelength heat rays emitted by the inside objects of the greenhouse cannot pass through the glass walls and glass roof of a greenhouse and go out. Thus, glass is a material which allows sun’s heat rays to come in but does not allow heat rays to go out.
This is called greenhouse effect. Due to the presence of a carbon dioxide layer around the earth, our atmosphere acts like the glass roof of an ordinary greenhouse and allows sun’s heat rays to be trapped within the earth’s atmosphere. This is why it is called greenhouse effect.