In total there are five primary pollutants which constitute about 90% of the global air pollution. These are carbon monoxide, nitrogen oxides, sulphur oxides, particulate matter and hydrocarbons.

Carbon Monoxide and Carbon Dioxide :

Carbon monoxide is a primary pollutant which is originated from incomplete combustion of fuels especially automobile exhaust. Other sources of carbon monoxide are volcanic action, natural gas emissions, electric discharge during storm, seed germination, marsh gas production, motor vehicles, forest fuels, iron and steel industries, paper and petroleum industries and agricultural burning.

Out of total content of carbon monoxide, 60 per cent emissions are attributed to anthropogenic sources. Of the rest about 80 per cent emissions of carbon monoxide is attributed to petroleum products. Accordingly the global emission of carbon monoxide has increased in proportion to increase in the number of motor vehicles.

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Carbon monoxide is highly poisonous gas and is generally considered as an asphyxiant. The toxicity of carbon monoxide is due to its affinity for haemoglobin which is the oxygen career of the blood. It has no detrimental effects on materials and surfaces.

Urban concentration of carbon monoxide varies according to the time and distance from the source peak level of carbon monoxide coincides with morning and evening rush hour traffic. A decrease in the emission of carbon monoxide has been observed as a result of introduction of catalytic convertors in the vehicles. Another approach to control carbon monoxide pollution is the substitute for gasoline. Gasoline in both compressed and liquid forms have been used as fuel.

Carbon dioxide is also produced along with carbon monoxide but it is not considered as pollutant. It is a natural component of the atmosphere and is a primary requirement for photosynthesis. Burning of forest fuels such as coal, oil and natural gas release carbon dioxide. Huge man-made emissions of carbon dioxide have been entering the atmosphere faster than the natural cycle. The temperature of surface of earth increases when the level of carbon dioxide increases in the atmosphere.

Sulphur Oxides (SOx) :

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The needs of sulphur are produced from the combustion of sulphur containing materials. Among man-made sources, coal combustion alone accounts for 74%, industries 22% and transportation 2% of the total sulphur oxide emissions.

When the fuel is burnt the sulphuric acid present as an impurity burns and produces sulphur dioxide and sulphur trioxide gas in the atmosphere. Amongst both sulphur dioxide is most important oxide. About 90% of all anthropogenic emissions of sulphur dioxide are in the northern atmosphere. Man-made emissions sulphur dioxide is partly converted into sulphur trioxide and sulphuric acid. In many industrialised countries, the level of sulphur dioxide has declined due to various control strategies.

Nitrogen Oxides (NOx) :

Oxides of nitrogen are relatively harmless in ordinary concentrations. They are released into the atmosphere mainly from vehicular smoke, electric power plants, industries, commercial institutions and residential units. The atmospheric emission contributes 50% of the total nitrogen oxides electric power plants 33% commercial institutions 14%, industries 12% and residential units 1%.

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In urban areas ambient nitric oxide follow a regular pattern depending on sunlight and traffic density. Its level increases and becomes maximum at 6-8 AM and 5-8 PM.

The use of catalytic converter provides removal of nitric oxide. Another method is the rise of chemical absorption method using sulphuric acid solutions on alkaline scrubbing solution containing calcium hydroxide and magnesium hydroxide.

Particulate Matter :

Small solid particles and the liquid droplets suspended in the air are known as particulate matter. The size and weight of the particulate matter determine their suspensions from a few seconds to months in the atmosphere. On global scales, particulate matter is one of the most prevalent atmospheric pollutants. Particulates are composed of inert or extremely reactive materials ranging from 100 µm to 0.1 pm or less.

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The particulates are present in the atmosphere in fairly large numbers and sometimes pose serious pollution problems. Several natural processes inject particulate matter into the atmosphere. The contribution from man- made activities are fly ash from power plants, smelters and mining operations and smoke from incomplete combustion processes. It has been estimated that fuel combustion from coal, fuel, oil, natural gas, industrial processes and miscellaneous sources forest fires, coal refuse, burning and agricultural burning account for about one third of the total emission of particulates.

Status of Vehicular Air Pollution in India

Air Quality, with respect to sulphur dioxide (SO2), oxides of nitrogen (NOx) and suspended particulate matter (SPM) are presented as under:

Sulphur Dioxide (SO2) :

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The locations having highest top ten values of annual mean concentration of sulphur dioxide are listed. The annual mean concentration of Sulphur dioxide exceeded the standards in the city of Jharia and Dhanbad in Jharkhand.

Oxides of Nitrogen :

The location where annual standards of industrial areas, exceeded in the city of Udaipur, Pune and Kota (one sensitive), Alwar is a satellite town and Delhi with a high concentration of industries. Due to inadequatic power supply, the industrial, commercial and residential activities often resort to captive power generation resulting in high emissions of Oxides of Nitrogen. It enlists locations having top eight highest values of annual mean concentration of Nitrogen dioxide.