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Important Pollutants

Some of the important air pollutants are given below:
  1. Oxides of carbon:- Carbon monoxide and carbon dioxide.
  2. Oxides of nitrogen:- Nitrous oxide and nitric oxide
  3. Oxides of sulphur:- Sulphur dioxide
  4. Ozone
  5. Chloro fluro carbons (Freons)
  6. Hydrocarbons:- Methane, butane etc.
  7. Smog
  8. Metals:- Lead, mercury etc.
  9. Organic pollutants:-  Benzopyrene, and biocides etc.
  10. Dust
When freons were discovered at first, it was thought as a "boon" to industry, but very soon it turned out to be a "curse" to the environment. CFC's are synthetic compounds of chlorine, fluorine and carbon and are often referred to by the trade name "freon". CFC is commonly used as a refrigerant in air conditioning, as propellants, as solvents, and also as foaming agents. They exist as colourless and odourless gases. They are light and also cheap. Unlike other pollutants, they are stable in troposphere and can remain stable for over a century. However, if it percolates into the upper atmosphere, (stratosphere) they absorb u.v. radiation and get photolysed(decompo These chlorine atoms catalyse the decomposition of ozone resulting in the "depletion of ozone layer".

Cl* + O3 ClO* + O2

In the above reaction, ozone is converted into oxygen, thereby decreasing the density of ozone in stratosphere. This results in the formation of the so-called holes in the ozone layer. Scientists have observed these holes at the north and the south poles. They allow more of the u.v. radiation from the sun to fall on the earth, causing a number of problems like irritation of the skin, ultimately leading to skin cancer and damaging eyes and causing cataract of the eyes. So, the environmentalists all over the world met in Canada in 1987 and agreed to freeze the production and use of CFC's at 1986 level. They also resolved to decrease the production by 20% by 1994, and 30% by the end of the century. Even if this occurs, it would take at least 50-100 years for the restoration of the already damaged or depleted ozone layer to the original / normal level.

Oxides of Nitrogen

The important oxides of nitrogen which cause air pollution are Nitric oxide (NO), nitrous oxide (N2O) and nitrogen dioxide (NO2). The above oxides are produced during combustion of fossil fuels from automobiles. Plants can withstand a concentration of about 10 ppm., but when it goes beyond this limit, the plants cannot perform photosynthesis. The nitric oxide let out by the supersonic jets directly enters the stratosphere and combines with ozone resulting in the decomposition of O3 to O2, thus, decreasing its density.
NO + O3 NO2 + O2
NO2 + O3 NO + 2O2

Sulphur dioxide is released into the atmosphere during:
  1. Burning of sulphur
  2. Roasting of sulphide ores.
  3. Burning of fuels containing the sulphur in air.
S + O2 SO2
Cu2S + 2O2 2CuO + SO2
2H2S + 3O2 2H2O + 2SO2
When these pollutants mix with air, they affect the human beings, animals, plants and global temperature also. In big cities, the automobiles pollute the air.
They are responsible for 80% of air pollution and 75% of sound pollution. These pollutants are present in solid, liquid and gaseous states. Some substances give out bad odours as well. The solid pollutants include carbon, carbon compounds, heavy metals like lead, mercury etc. Let us examine how some of these pollutants mentioned above pollute the air.

After the industrial revolution, there has been a tremendous development in the mode of transportation. Eg. Cars, lorries, buses, trains, ships and aeroplanes generally use petrol and diesel as a fuel. They undergo incomplete combustion at high pressures and temperatures in the engines. The waste substances come out in the form of smoke and get mixed up with air.
This smoke mainly contains carbon monoxide.
2C + O2 2CO

When organic substances decompose or degrade methane gas is liberated. This gives carbon monoxide on oxidation.
2CH4 + 3O2 2CO + 4H2O

80% of the carbon monoxide released is form automobiles. In urban areas, at peak times the level of the CO gas increases up to 50-100 ppm. (the acceptable level being 9 ppm). The figure shows carbon monoxide levels at different times of the day and the hourly traffic (automobile) at the same time in a particular area.

These two are parallel to each other showing that the increase or decrease of CO level in urban areas is mostly due automobile exhausts.

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