Air Pollution | Air Pollutants

Air Pollution

The air pollution is defined as an imbalance in quantity of air which adversely affects the biotic community. In other words, the contamination of undesirable air pollutant which adversely affects environment because of lowering quality of air.

  • Air pollution comes from a wide variety of sources. Some of the most excessive sources include vehicle or manufacturing exhaust, forest fires, volcanic eruptions, dry soil erosion and other natural sources, and building construction or demolition.
  • Automobiles of all classes emit CO, SO2, unburnt hydrocarbons and nitrogen oxide. It is estimated that a car (without cleaning device) on burning 1000 litre of fuel emits 350 kg CO, 0.6 kg SO2, 0.1 kg lead.with other pollutants.
  • Depending on the concentration of air pollutants, several effects can be noticed. The effect of pollution can be seen as higher rain acidity, crop depletion, higher rates of asthma, etc.

Types of Air Pollutants

Air pollutants can be categorized into two groups:

  • Primary Air Pollutants: These are derived from the direct burning of source like coal which causes a release of pollutants. The example of primary pollutants is CO2, CO, SO2, Hydrocarbons like CH4 (Methane), ethylene; particulate matters, Nitrogen dioxide (NO2), etc.
  • Secondary Air Pollutants: They are formed because of a reaction of primary pollutants among themselves. About 10% of global air pollution is caused by secondary pollutants.
    The examples of secondary air pollutants are photochemical smog, acid rain, etc.

Composition of Primary Air Pollutants

The primary air pollutants are composed of

1. Particulate Matter

  • It comprises solid particulars or liquid droplets (aerosols) which are small enough to remain suspended in air, e.g. smoke, dust, asbestos, fibres, pesticides, some metals (including Hg, Pb, Cu, and Fe) and also biological agents like tiny dust mites, spares, and pollen.
  • The presence of particulate matter reduces sunlight and visibility.
  • The atmospheric particle having diameter 7-10 mm, generally settle out in less than a day, whereas particles with diameter 1 mm or less can remain suspended in the air for weeks.
  • This suspended particulate matter in the lower atmosphere (troposphere) causes human respiratory illness, like asthma chronic bronchitis, etc.
  • A level above 100µg/m3 (year average) results in the respiratory problem.
  • A level above 300µg/m3 (year average) results in bronchitis.
  • The annual average permissible limit is 75µg/m.
  • It also lowers the temperature at the earth’s surface by altering radiation.

2. Carbon Monoxide

  • It is the product of incomplete combustion of fossil fuels.
  • It reduces the oxygen carrying capacity of blood and linked with increased health risk for a human.
  • CO concentration of 100 ppm causes a headache, 500 ppm causes collapse and 1000 ppm may cause death.
  • Smokers inhale CO concentration of 400-450 ppm.

3. Hydrocarbons

  • These are compounds composed of hydrogen and carbon and are produced naturally during decomposition of organic matter.
  • Methane is evolved from soil microbes in flooded rice fields, whereas Formaldehyde emitted from indoor sources, such as newly manufactured carpeting, which causes indoor pollution which is carcinogenic.

4. Sulphur Dioxide (SO2)

  • It is produced when sulphur containing coal is burnt.
  • The ore smelters and oil refineries also emit a significant amount of S02.
  • It causes acid rain and severe respiratory problem.

5. Nitrogen Oxides

  • It is formed mainly from N2 and O2 during combustion of fossils at high temperature in automobiles engine.
  • Nitrogen oxides cause the reddish-brown haze (brown air) in the traffic-congested city, which contributes to heart and lung problems and may be carcinogenic.

Composition of Secondary Air Pollutants

The secondary air pollutants are composed of

1. Photochemical Smog

  • Photochemical smog is composed mainly of Ozone (O3), Peroxyacetyl Nitrate (PAN) and NOx.
  • It is also called brown air where solar radiation is intense. In areas or seasons of lesser solar radiation, smog formation is incomplete and the air is referred as grey air.
  • Automobile exhausts contain Hydrocarbon (HC) and NOx which play an important role in O3 and PAN formation in urban areas.

The reaction occurring inside engine:

N2 + O2 —————> 2NO

The reaction occurring in the atmosphere:

O + O2 —————–> O3

NO + O3 —————> NO2 + O2

HC + NO + O2 —————> NO2 + PAN

  • Ozone, an effective oxidant, corrodes the heritage building surfaces and damaged marble status and other cultural assets. Ozone also aggravates lung diseases in human.
  • PAN damages chloroplasts and thus, the photosynthetic efficiency and growth of plants are reduced. In humans, PAN causes acute irritation of eyes.

2. Acid Rain

  • When pollutants like SO2 and NO2 get added in H2O of rainfall, then they cause the formation of H2SO4 and HNO3 respectively.
  • Acid rain refers to several ways in which acids from the atmosphere are deposited on the earth which are oxides of Nitrogen and Sulphur.
  • Nitrogen oxides and SO2 are produced during the combustion of coal (in an industry) and petroleum (in automobiles).
  • They rapidly oxidise to acids (Sulphuric or nitric) which quickly dissolve in water and are washed out to the ground as acid rain.

2SO2 + O2 ——————->  2SO3
SO3 + H2O ————–> H2SO4

2NO2 + H2O ————> HNO2 + HNO3

  • The word “acid rain” was first coined by Robert Angus Smith in 1872. The first time, acid rain was noticed in Sweden in 1972 but Norway and Canada have been badly affected.
  • In India, acid rain is responsible for corrosion of monument like Taj Mahal. The emission of SO2 from Mathura Oil Refinery is believed to be responsible for corrosion of Taj Mahal.

Effects of Acid Rain

It corrodes monuments.

  • It causes the death of biota of lakes (therefore known as Lake-killer) in North-west Europe and Canada.
  • It degrades fertility of the soil.

3. Smog

  • When fog is mixed with smoke, it is called smog.
  • If in smoke, SO2, NO2 are mixed, they cause the formation of acidic nature of smog and if ozone is in smog then it causes the death of people because it is poisonous.
    O3 + H2O ————> Poisonous for Biota (mainly humans)

4. Acid deposition

  • Acid deposition includes a wet and dry deposition. The wet deposition refers to acidic water received through rain, fog and snow, whereas dry deposition relates to the wind blown acidic gases and particles in the atmosphere, which settle down on the ground.

Diseases Caused by Air Pollutants

1. Silicosis: Silico-Tuberculosis or silicosis occurs due to inhalation of free silica or silicon dioxide while working in industries related to pottery, ceramics, glass making, building and construction works. The workers get a chronic cough and pain in the chest.

2. Asbestosis: It is caused by asbestos which is used in making ceilings. It is also considered as cancer causing agent.

3. Byssinosis: It is a disabling lung disease which is marked by a chronic cough and bronchitis due to inhalation of cotton fibers over a long period.

4. Pneumoconiosis: It occurs due to inhalation of coal dust from coal mining industry. The workers suffer from lung problem due to this.

Measures to Curb Air Pollution

Air Pollution can be controlled by:

  1. Absorption: In absorption technique, gaseous pollutants are absorbed in a suitable absorbent material.
  2. Combustion: In the combustion process, oxidizable gaseous pollutants are completely burnt at a high temperature.
  3. Adsorption: This technique is applied to control toxic gases, vapours and inflammable compounds that could not be efficiently removed by aforesaid techniques.

Apart from these, there are certain parameters to curb air pollution:

  • Suitable fuel selection (e.g. fuel with low-sulphur content) and its efficient utilization to reduce pollutant level in emission.
  • Modification in industrial processes or/and equipments to reduce emissions.
  • Correct selection of manufacturing site and zoning for the industrial set-up to disperse pollution sources. The most common methods of eliminating or reducing pollutants to an acceptable level include destroying the pollutant by thermal or catalytic combustion, changing the pollutant to a less toxic form.
  • The devices which can be used to remove particulate air pollutants are
  • Arresters: These are used ideally to separate particulate matters from contaminated air.
  • Scrubbers: These are used to clean air for both dusts and gases by passing it through a dry or wet packing material.
  • Filters: These are usually used to collect extremely fine particulate matters. However, electrostatic precipitation is the most effective device to remove particulate pollutant.

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