1. Reduction in Benzene Content :
Benzene (C6H6) is a cancer causing chemical (carcinogen) and is also known to harm the antibody mechanism of the body. About 80- 85% of the benzene in the atmosphere is contributed by the motor vehicle emission. Although benzene content in petrol used in American and other European countries has been reduced to 1% which is still as high as 5% in India which is planned to be brought down to 3% immediately.
Inorganic particulates comprise mostly oxides of metals present in the atmosphere. Such metals, oxides are produced on burning of fuels containing metals. Amongst important inorganic particulates are lead oxides which are obtained from combustion of leaded gasoline.
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3. Organic Particulates :
Organic particulates occur in the atmosphere in a variety of compounds. Amongst them polycyclic aromatic hydrocarbons are the most important component. These particulate are carcinogenic in nature which occur mostly in urban atmosphere.
Particulate matters are termed as suspended particulate matter (SPM) and total suspended particulates (TSP) depending on variations in size of the particles. Various particulates have been classified as smoke, fumes, mist, fog and aerosol. The composition of particulates is dependent upon their origin. Particles from soils and minerals primarily contain calcium, aluminium and silicon compounds.
The decline in domestic coal use in many countries has led to reduction in the emission of black carbon particles. In urban areas of developed countries it is estimated that up to 70% of back emissions are from diesel vehicles.
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4. Hydrocarbons :
Hydrocarbons are the primary pollutants released in to the atmosphere in production of petroleum and incomplete combustion of fuels like gasoline, diesel etc. The gaseous and volatile liquid hydrocarbons are the major air pollutants in the atmosphere. Their presence in the air alone is not harmful. They undergo chemical reactions in the presence of sunlight and nitrogen oxides forming photochemical oxidants.
Formation of photochemical oxidants in the atmosphere leads to the phenomenon of photochemical smog which is characterised by brown huzy fumes which irritates the eyes and lungs and damages plant life. The most important photochemical oxidant is ozone which on reaction with hydrocarbons eventually forms peroxacyl nitrate (PAN).
Both petrol and diesel fuels consist of mixture of a large number of hydrocarbons. Over 200 different hydrocarbons have been identified in ambient air samples. It has been estimated that anthropogenic sources contribute about 15 per cent of the hydrocarbons emitted in the atmosphere each year.
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5. Trace Metals :
Toxic trace metals such as arsenic, lead, cadmium and mercury are hazardous. Lead is the major urban pollutant present in the particulate fraction. The principal use of lead is an antiknock petrol additive. It is also used as a paint pigment and in the manufacture of batteries.
Lead mixed air, if inhaled, may produce injurious effects on the kidney, blood and liver. It can get mixed up with water and food and can create cumulative poisoning. The effects are far reaching in case of growing children. The increase in the level of lead in children may cause lowering of intelligence. Thus, it forms important parameters of air pollution. The widespread phase down of lead in petrol and the introduction of unleaded petrol in many developed countries have resulted in rapidly declining emission in recent years.