Air pollution is the addition of foreign particles, gases and other pollutants into the air which has an adverse effect on human beings, animals, vegetation, etc.
It is a type of environmental pollution that affects the atmosphere and is the result of burning fossil fuels either in industries, power generation plants or in the internal combustion engines.
Causes of air pollution
Following are the various causes of air pollution:
1. The combustion of natural gas, petroleum, coal and wood in industries, automobiles, aircrafts, railways, thermal plants, agricultural burning, kitchens etc (e.g., soot, fly ash, carbon dioxide, carbon monoxide, nitrogen oxide, sulphur oxides, nitrogen oxides).
2. Metallurgical processing causes pollution ( e.g.,mineral dust, fumes containing fluorides sulphides and metallic pollutants such as lead, chromium, nickel, vanadium, cadmium, zinc, mercury).
3. Chemical Industries (pesticides, fertilizers, weedicides, fungicides)
4.Processing industries such as cotton textile, wheat flour mills.
5.Welding, stone crushing, gem grinding also add to the air pollution.
6.The natural air pollutants include pollen, spores, marsh gas, volcanic gases and synthesis of harmful chemicals by electric storms and solar flares. The major cause of pollution in the urban areas is automobiles which inefficiently burn petroleum, release 75% of noise and 80% of air pollutants.
Primary air pollutants and their effects
Following are the primary air pollutants.
Carbon monoxide :
Carbon monoxide occurs in the air due to incomplete combustion, metallurgical operations and naturally by plants as well as animals. It is poisonous on account of having an affinity for hemoglobin approximately 250 times greater than that of oxygen. Upon combining with hemoglobin, carbon monoxide forms stable compound and prevents oxygen, combining with the hemoglobin, resulting in giddiness, headache, cardiovascular malfunctioning and asphyxia. Cigarette smoking increases the carbon monoxide concentration of the blood.
Carbon dioxide :
Respiration of organisms and burning of fossil fuels releases carbon dioxide in the atmosphere. The CO2 produced as a result of respiration is taken up by plants during photosynthesis and it therefore does not accumulate in the atmosphere. The carbon dioxide produced in the burning of fossil fuels causes rise in atmospheric carbon dioxide concentration.
Industrial units, thermal power plants, trains and vehicles release tons of carbon dioxide in the atmosphere. This rise in carbon dioxide concentration prevents more of sun’s heat escaping from the earth, much in the way the glass in a greenhouse does. It results in rise in earth’s temperature and is described as greenhouse effect.
Fossil fuels are the major source of sulphur dioxide pollution. Smelting of metallic ores (e.g .,iron, copper, lead, zinc, nickel) and burning of petroleum and coal in industries, thermal plants, and motor vehicles produces SO2 in large quantities.
Sulphur dioxide causes irritation of the respiratory system and damages the epithelial lining of the lungs. It can also irritate the conjuctiva of the eye. Sulphur dioxide is injurious to higher plants such as cotton, cereals, grapes, apple, etc. It also causes erosion of building materials such as marble, limestones etc.
In the air sulphur dioxide combines with water to form sulphuric acid. It is the cause of acid rain.
It consists of soot, fly-ash, dusts of various types, fur, hair, spores, pollen grains etc. Particulate matter is differentiated into settleable (larger than 10 micrometre, remaining in air for less than one day) and suspended (less than 10 micrometre remaining in air for more than one day to several weeks).
Suspended particulate matter is differentiated into aerosol, dust and mist. Smoke is similar to dust but consists of visible suspension of carbon and other particles given off by burning or smouldering organic matter.
Soot is a result of incomplete burning of carbohydrates, smoke and fly ash are the fine particulate matter passed out along with gases during burning of coal. Soot, smoke, fly-ash and dust deteriorate the quality of articles, clog stomata, produce allergic reaction, especially bronchial asthma and chronic bronchitis.
Particulate matter from the processing industries (e.g., cotton dust, iron mill dust, mine dust, flour mill dust, gem grinding, cement industries, asbestos industries) cause pulmonary problems.
Secondary air pollutants and their effects
There are usually produced photo chemically from primary pollutants and are termed as photochemical oxidants. Secondary air pollutants occur in one type of smog termed as photo chemical smog.
Smog is opaque or dark fog having condensed water vapours, dust, smoke and gases such as sulphur dioxide, hydrogen sulphide, nitrogen dioxide,etc. It causes necrosis in plants, allergies and asthma/ bronchitis in humans. Smog is of two types; classical smog and photochemical smog. Classical smog is dark brown and opaque. Condensation of water vapours with hydrogen sulphide and sulphur dioxide over dust or smoke particles forms classical smog.
Photochemical smog is a grey or yellowish brown opaque smog. Photochemical smog is formed at high temperature over cities and towns, due to still air, emission of nitrogen oxides and carbohydrates from automobile exhaust and solar energy.
Nitrogen dioxide splits into nitric acid and nascent oxygen. Nascent oxygen combines with molecular oxygen to form ozone. Ozone reacts with carbohydrates to form aldehydes and ketones.
Nitrogen oxides, oxygen and ketones all combine to form peroxyacetyl nitrate (PAN).
Photochemical oxidants or secondary air pollutants
The photochemical reactions between nitrogen oxides and unsaturated hydrocarbons produce secondary pollutants such as ozone, peroxyacetyl nitrate, aldehydes and phenols.
Ozone being strong oxidant destroys chlorenchyma, produces necrosis, corrodes surface of marble statues and heritage buildings, injures mucous membranes, dry throat, haemorrhages and eye irritation.
PAN (Peroxyacetyl Nitrate) :
They damage chloroplasts, inhibit electron transport system. In human beings they cause respiratory distress and eye irritation.
Aldehydes and Phenols:
Aldehydes cause irritation in gastrointestinal and respiratory tract. Phenols damage organs such as kidneys, liver, spleen and lungs.
Pollen and microbes:
They are normal constituents of air. Microbes directly damage the vegetation, food articles and cause diseases in plants, animals as well as human beings. Excess of pollen causes allergic reactions in several human beings. The common reactions are also collectively termed as hay fever.
Air pollution control measures
Following are some of the pollution control measures:
The control measures depends upon the type of pollutant (i.e., gas or particulate). Settling chambers are useful to separate particulate matter of larger sizes . The gaseous pollutant pass through large chambers settling down the particles of large size . Cyclones are useful in removing particulate matter ranging from 1 to 200 microns. This system works on the principle of centrifugal force. Dirty air is blasted into a conical cylinder with the hollow central tube. As the air enters the cyclone it creates violent swirls. In this process particulate matter is thrown to the periphery. These particles, after striking the wall of the cone, lose their speed due to friction and exit at the bottom. This system removes 70-75 percent particulate matter.
Several types of scrubbers are available which remove both particulate and gaseous pollutants such as sulphur dioxide.The exhaust is passed through a spray of water and lime water. Gases dissolved in water and lime water precipitate sulphur dioxide as calcium sulphate.
Thermal power plants and several large Industries use electrostatic precipitators to remove particulate matter from the exhaust gases. It works on the principle of electrical charging of dust particles and collecting them on differentially charged platform .
In automobiles catalytic converters are helpful to reduce emission of gases. Metals such as platinum, palladium and rhodium work as catalyst. These metals convert unburnt hydrocarbons into carbon dioxide and water and similarly carbon monoxide and nitric oxide into carbon dioxide and nitrogen gas respectively.
Thermal reactors are useful in vehicles which convert carbon monoxide into pure oxygen.
Use of CNG (i.e.,compressed natural gas) decreases the amount of pollutants in automobile exhausts.
Sulphur dioxide combines with atmospheric moisture and precipitates as sulphuric acid which is termed as acid rain.
Large scale emission of acidic gases into the atmosphere from thermal power plant, industries and automobiles cause acid rain. Acid rain damages plants ( e.g., chlorosis, necrosis ,defoliation, dieback). It causes leaching of essential minerals of soil. It has a corroding effect on the white marble, metals, paint, slate, stone etc. The phenomenon is termed as stone leprosy.
Acid rain kills aquatic plants and animals such as fish, molluscs, etc.