Air pollution is viewed as a serious problem in most of the Indian cites. Air pollution is the presence of contaminants such as dust, fumes, gases, particulates, smoke etc., which are injurious to humans, plants, animals and property. Studies indicate that 70-80% of the air pollution load in Indian cities is on account of motor vehicles. Bulk of the motor vehicle emissions are due to fuel combustion and fuel evaporation.
Hundreds of chemicals are released during the combustion of fuel in motor vehicles. A total of 445 compounds have been identified as constituents of diesel engine emissions in a recent study. The major pollutants from motor vehicles are carbon dioxide, carbon monoxide, water vapour, particulate matter, oxides of nitrogen, sulphur dioxide, hydro carbons and polycyclic aromatic hydro carbons.
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In terms of absolute tonnage in pollution load, carbon monoxide dominates all other pollutants and in Indian cities, about 90% of CO in the ambient air is contributed by vehicles. But the toxicity of CO is much lower on a weight basis than those of other pollutants. Hence a correlation between pollution load in tons and health effects cannot be correctly established.
Available data indicate that the pollutants with the most damaging health impacts are fine particulate matter (causing serious respiratory illnesses and premature death) in Indian and other cities. Hence if outdoor air pollution is found to be serious, the relative source contribution of fine particulates is important.
Recent studies by Air Borne Particulates Expert Group in U.K. has concluded that “the road traffic nationally contributed 25% of primary PM10 emissions, but the relative importance of road traffic emissions increased with decreasing particle size and road transport accounted for an estimated 60 % of PM0.1“.
Different pollutants should be ranked according to their toxicity and ambient concentrations. Two sources of particles are specially visible in Indian cities- heavy duty diesel vehicles, diesel cars and diesel multi utility vehicles, and two stroke engined two and three wheelers.
In these vehicles, age and poor maintenance may result in further increase in ambient particulate matter. There is a common perception that gasoline emissions do not contain significant amounts of particulates and as such are not dangerous from health point of view.
However a recent study in Colorado, U.S.A has shown that “gasoline and not diesel vehicles were the primary contributors to particles in exhaust, especially in cold starts and when they are not maintained well”. Studies indicate that the vehicles are poorly maintained in India resulting in excess particulates.
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Adulteration of gasoline and diesel with lower-priced materials is common in India. This can lead to increased tail pipe emissions of harmful pollutants. Fuel adulteration can increase PM, CO, HC and NOx emissions. Adulterating gasoline with benzene, naphtha, used solvents etc., commonly practised in India, increase polyaromatic hydro carbon emissions especially benzene which is a known carcinogen.
1. Particulate Matter:
Based on the data collected to date, the pollutant of concern in Indian cities is particulate matter. Its concentrations have been shown to far exceed the national ambient air quality standards in city after city. Maximum suspended particulate matter concentrations have been recorded in Delhi, Kanpur and Kolkata. Low SPM concentrations have been recorded in South Indian cities of Chennai, Bangalore and Hyderabad.
Air borne particulate matter is a mixture of many sub classes of pollutants — in solid and liquid forms, with each sub class containing many different chemical species. Particulate matter may be classified as primary or secondary. Primary particulates are emitted directly by emission sources, whereas secondary particles are formed through the atmospheric reaction of gases, such as the reaction of ammonia and oxides of sulphur or nitrogen, that leads to the formation of particulate matter.
Particulate matter is typically classified according to size. The particle size can vary from approximately 0.005 microns (0.005 x 10-6 metres) to about 500 microns. About 100 microns in diameter is the thickness of human hair. All the ambient PM irrespective of size is referred to as suspended particulate matter (SPM) or total suspended particulates (TSP) when a gravimetric procedure has been used for measuring mass.
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Particulate matter less than 10 microns in diameter is referred to as respirable suspended particulate matter (RSPM) or PM10, and PM less than 2.5 microns is referred to as PM2.5. The term fine PM has come to be associated with PM2.5 and ultra-fine with PM less than 0.1 microns in diameter (PM). Of late, the focus of PM measurement in ambient air has shifted from TSP or SPM to PM10 PM2.5 and PM0.1.
Air borne PM has numerous sources, ranging from naturally occurring dust, sea salt, and pollen to products of combustion such as forest fires, domestic cooking and heating, garbage burning, power generation, vehicles, railways and ships. Combustion processes normally contribute much more to the fine and ultra-fine PM fractions, whereas non combustion processes (such as road dust, sea salts etc.,) contribute more to large size PM fractions.
Fine PM tends to be distributed uniformly over large areas, thereby making it difficult to trace it to individual sources. They have long residence time and distributes over a region. Larger particles, however have shorter atmospheric life times (minutes to hours) and do not travel long distances. As a result, they tend to be less evenly distributed, and are usually found closer to their sources.
Studies have shown that particles of different size vary in their respiratory tract deposition, movement, clearance and consequent retention time in human body. Ultra-fine particles tend to behave more like gases and hence travel to the lower regions of the lungs as compared to larger particles which tend to get deposited in the upper or middle region of the respiratory tract.
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Particles above 10 microns in diameter are deposited almost exclusively in the nose and throat, whereas particles smaller than one micron are able to reach the lower regions of the lungs. The intermediate size range gets deposited in between these two extremes of the respiratory tract.
Ultra-fine particles are highly toxic to the lungs. Hence particle size and particle numbers may be more relevant indicators than particle mass- the smaller the particle, the greater the fraction of particles deposited in air ways and lungs, and the greater the surface area available for interaction with biological systems. Hence monitoring fine particles and numbers is more relevant than monitoring SPM and particle mass.
The main components of particulate matter from vehicles are the metals and organic compounds (HC and PAHS) adsorbed on a core of carbon. Organic compounds adsorbed on the carbon core are known to lead to mutations and even cause cancer.
A recent study by the U.S Environmental Protection Agency (EPA) highlighted the likely cancer risk from diesel emissions, declaring it as a potential carcinogen. The diesel particles, many of which are smaller than one micron in diameter, have a carbonaceous core with a large surface area to which various organic compounds including polycyclic and nitro-polycyclic hydro carbons are adsorbed.
The monitoring and regulation of PM2.5 is on its way to becoming standard across U.S.A and other countries. While there is a strong case for better understanding of both the ambient concentrations and sources of fine particles, it is also important to account for the complexity of measurement techniques. These techniques are expensive and more technically challenging.
National Environmental Research Institute India has started monitoring RSPM (PM10) recently at its 30 stations located in important cities. However there is no agency in India so far, to measure PM2.5 at regular intervals. Recent studies in India indicates that “PM10 make up one- third to two-thirds of TSP. PM2.5 has been found to constitute about one-half of PM10“.
2. Carbon Monoxide (CO):
Carbon monoxide (CO) is a colorless, odorless gas and is a product of incomplete combustion of fossil fuels. In most of the Indian cities, gasoline fueled vehicles account for the bulk of CO emissions. The most important parameter influencing CO emissions is the air-fuel ratio during engine operation.
CO emissions increase with a decreasing air-fuel ratio. Gasoline engines often operate close to stoichiometric (amount of air needed for complete combustion of fuel) at part load and fuel rich at full load. As a result, CO emissions are significant from gasoline engines. Diesel engines operate well on the lean side of stoichiometric, thus, CO emissions from diesel engines are minimal.
The computations indicate that while CO emissions from a gasoline car are 24.75 gms/km, an equivalent diesel car produces only 2 gms/km. A heavy duty diesel bus produces about 4.45gm/km. A two-stroke two wheeler produces about 2.0 gms/km and a four-stroke 2 wheeler produces about 1.60 gms/km. In India 90% of the ambient CO is contributed by vehicles. In terms of tonnage of emissions, CO typically leads all other pollutants, but is less toxic compared to other pollutants from vehicles.
Improved engine design and fitment of catalytic converters are playing an important part in reducing production of carbon monoxide from vehicles. Three-way catalytic converters installed on gasoline-fueled vehicles can reduce CO emissions by about 90 per cent from uncontrolled levels. The catalytic converter oxidises CO to carbon dioxide.
3. Hydrocarbons (HC):
Hydro carbon compounds are defined as compounds consisting of carbon and hydrogen. Hydrocarbon emissions from motor vehicles occur from unburned or partial combustion of fuels. Engine oil leakage into the engine cylinders also results in HC emissions. Emissions of HC from two-stroke gasoline engines are very high.
The exhaust from a two-strike engine contains both unreacted fuel that did not burn at all and organic compounds formed from partial burning of the fuel. Engine oil added to the fuel or air stream to lubricate the piston in a two stroke gasoline engine may not burn completely and hence comes out as HC emissions.
In gasoline-fueled vehicles, with no catalytic converters, about 55 per cent of HC emissions are from the exhaust system, 20 to 30 per cent from evaporation of fuel in the fuel lines, fuel tank and carburetor and 13 to 25 per cent come from the crank case blow- by. Catalytic converters convert HC into CO2 and water and considerably reduces HC emissions. Modern gasoline fueled cars with 3-way catalysts produce considerably less HC emissions.
Diesel fueled vehicles normally emit low levels of hydrocarbons. Hydrocarbons from diesel fueled vehicles are mainly due to incomplete combustion of the fuel or the engine oil. HC emissions from diesel vehicles can be significantly controlled by reducing the engine oil entering the engine cylinder. In most of the 4-stroke gasoline and diesel engines, the main source of HC emissions is due to fuel-air mixture in the crevices.
Crevice is a narrow region in the combustion chamber into which a flame cannot propagate. Volume between the piston and the cylinder liner above the top piston ring is a typical crevice. Design changes have been made in modern engines through high top ring location to reduce the crevice volume. However this needs higher strength materials for pistons. Reducing crevice volumes in gasoline and diesel engines had significantly improved fuel economy and reduced HC and PM emissions significantly.
4. Polycyclic Aromatic Hydrocarbons (PAHs):
Toxic emissions from vehicles include polycyclic aromatic hydrocarbons – benzene, 1-3 butadiene, aldehydes etc., Aldehydes includes acetaldehyde and formaldehyde. Polycyclic aromatic hydrocarbons also include toluene, xylene, ethyl benzene etc.
Polycyclic aromatic hydrocarbons are a group of chemical compounds with two or more fused aromatic rings. Their major source is the incomplete combustion of organic material. They are emitted in vapour phase and gets adsorbed on the carbon particles. PAHS are emitted at a higher rate in diesel fueled vehicles than gasoline fueled vehicles. Vehicles are responsible for more than 30% of PAHS in the ambient air.
Benzene constitutes 63 to 85 per cent of the toxic emissions in modern catalyst fitted gasoline vehicles. About 85-90% of benzene emissions in gasoline engines come from exhaust gases and the balance through gasoline evaporation.
The benzene in exhaust originates both from partial combustion of other aromatic hydrocarbons in gasoline (such as xylene, toluene, etc.) and from benzene in the gasoline fuel. Benzene is added in gasoline to improve octane value, especially in unleaded gasoline. Gasoline is also adulterated in India through addition of benzene.
Benzene is considered to be a human carcinogen, hence exposure to it is a cause of concern. In many countries, benzene and aromatics content in gasoline are regulated. The maximum benzene content in gasoline was fixed at 5% by volume in 1996 in India.
Subsequently it was reduced to 3% by volume for the tour major metros in 2000 A.D. It is further reduced to 1% in Nov. 2000 for the National Capital Territory of Delhi and Mumbai. It is fixed at 1% by volume in Europe, USA and many other developed countries. Though Europe and U.S.A have specified the total aromatics content in gasoline, it is not specified in India so far.
1, 3 – butadiene is not present in the fuels (both gasoline and diesel) but is present in their exhaust emissions. 1, 3-butadiene is a product of partial hydrocarbon combustion. Vehicles are responsible for more than 95% of the 1, 3 butadiene in the ambient air. Catalytic converters are efficient at removing 1, 3-butadiene. The exhaust from a conventional gasoline vehicle (not fitted with catalytic converter) contains significantly more 1, 3-butadiene than a conventional diesel vehicle. 1, 3-butadiene is considered to be a human carcinogen.
Aldehydes are not present in the fuel (gasoline, diesel, ethanol or methanol), but are present in their exhaust emissions. The major types of aldehydes in vehicle emissions are acetaldehyde and formaldehyde. Diesel engines emit significantly more aldehydes than comparable gasoline engines.
Aldehydes emission is largely responsible for “diesel odour” and cause irritation of eyes and nasal passages. Both formaldehyde and acetaldehyde are irritants and suspected human carcinogens. PAH emissions are higher in the exhaust of diesel-fueled vehicles than gasoline fueled vehicles.
5. Oxides of Nitrogen (NOx):
Nitrogen oxides are formed during combustion as nitrogen in the air reacts at high temperatures with oxygen. High flame temperatures during combustion breaks down molecular oxygen and nitrogen of the inducted air inside the cylinder. Then they recombine to form nitric oxide (NO). Nitrogen dioxide (NO2) is formed by oxidation of NO.
Reactions forming NO are temperature dependent and hence NOx emissions are more in diesel engines which operate at higher temperature compared to gasoline engines. Diesel cars produce about 116.9 gms/km of nitrogen oxides against about 3.3 gms/km produced by an equivalent gasoline car. A heavy duty diesel bus produces about 354.3 grams/km and a diesel truck about 405.3 gms/km of nitrogen oxide emissions.
Nitrogen oxide emissions from vehicles are controlled by reducing the temperature in the engine cylinder through retarding combustion, exhaust gas recirculation (EGR), water injection, inter cooled turbo charging etc. It is also controlled by after treatment devices like three-way catalytic converters in gasoline vehicles and de NOx converters in diesel vehicles.
Motor vehicles are the main contributors to nitrogen oxide in the ambient air. About 60-70 per cent of the nitrogen oxides in the ambient air are due to vehicles. The studies indicate that compared to 1995, nitrogen oxide emissions in the ambient air have come down in most of the Indian cities in 2000, basically due to the improvements in diesel engine designs. NOx contributes to acid rain and soil acidification. NOx also contributes to secondary particulate formation and is a precursor of ground-level ozone.
6. Sulphur Dioxide (SO2):
The sulphur in the fuel is oxidized during combustion process to produce sulphur dioxide. A fraction of SO2, is oxidized to sulphur trioxide (SO3), which combines with water to form sulphuric acid. Gasoline fuel has a negligible amount of sulphur compared to diesel fuel and therefore SO2, emissions from gasoline engines are insignificant.
Diesel fuel in India contains between 0.05 to 0.5% of sulphur. Hence bulk of SO2, emissions from vehicles is contributed by diesel vehicles. Vehicles are responsible for 2 to 6 per cent of the sulphur dioxide in the ambient air. Sulphur dioxide contributes to acid rain and acidification of soils. Sulphur dioxide reacts with metals to form metallic sulphides, which form part of particulate matter emissions.
Sulphur dioxide emissions are estimated as 10.3 grams/km for passenger cars with diesel engines, while for buses and trucks using high sulphur diesel, SO2 emissions are estimated as 25.4 gms/km. In metropolitan Delhi, where low sulphur diesel is used, SO2 emissions are estimated as 2.5 grams/km for diesel cars.
Sulphur in the fuel and SO2 emissions coming out of the engine cylinder affects the performance of catalytic converters significantly. Hence the sulphur in the fuel is getting reduced throughout the world to enable fitment of emission control equipment and to improve ambient air quality.
Recent studies by the Central Pollution Control Board in India have revealed that there is significant reduction in SO2 levels in the ambient air in 2000 compared to 1995 in all the major cities in India. The reduction in SO2 levels may be due to the use of upgraded fuels in power generation and low sulphur gasoline and diesel in vehicles.
7. Lead:
Motor vehicles fueled with leaded gasoline are the main source of lead in ambient air. Tetra ethyl lead is added to gasoline to increase the fuel’s octane value which improves the and-knock characteristics of gasoline engines.
Lead emissions in ambient air also originate from coal combustion and various lead-based industries such as lead smelters and lead battery plants. Although lead in gasoline accounts for less than 10 per cent of all refined lead production, about 80 to 90 per cent of lead in ambient air originates from combustion of leaded gasoline.
After identifying its adverse health effects, lead is being replaced by other chemicals (benzene, aromatics, oxygenates etc.) to improve octane value of the gasoline fuel. Unleaded gasoline is being manufactured and marketed by most of the countries including India.
8. Ozone:
Ozone is a secondary pollutant formed by reactions between oxides of nitrogen and hydro carbons in the presence of sun light. Ozone is a colour less gas that occurs in two separate layers of the atmosphere. Ozone in the outer (stratosphere) layer of the atmosphere is generated by photolysis of oxygen and protects the earth from ultra violet rays. In the lower (troposphere) layer, ground level ozone is formed by the reaction of non-methane HCS and NOx with ambient oxygen in the presence of sun light and high temperatures.
Motor vehicles are the main anthropogenic emission source of ozone’s precursors (HCS and NOx). Ground level ozone concentrations depend on the absolute and relative concentrations of its precursors and the intensity of solar radiation. Peak concentrations of ozone occur on high temperature days. Ozone is responsible for photochemical smog and has been associated with transient effects on the human respiratory system.
9. Carbon Dioxide (CO2):
All the carbon in the fuel should be oxidized to carbon dioxide (CO2) in an ideal condition. However, part of the carbon in the fuel does not get completely oxidised and is emitted as carbon, CO and hydrocarbons. CO2 emissions increase with fuel consumption, hence heavy duty vehicles have much higher CO2 emissions.
CO2 emissions from 2-stroke two wheelers and 4-stroke two wheelers are estimated as 26.6 and 28.3 grams/km respectively. Estimates for gasoline and diesel cars are 223.6 and 208.3 grams per km. respectively. CO2 emissions from heavy duty buses and trucks are about 515.1 gms/km., which are more than double the emissions from cars. No norms are available in India for CO2 emissions from vehicles.
Carbon dioxide is a greenhouse gas. Greenhouse gases are gases that absorb some of the heat radiated from the earth’s surface, which would otherwise escape into the space. This process raises the temperature of the atmosphere. It is estimated that in India, vehicles are responsible for about 16% of the total CO2 produced by different sources.
10. Non Exhaust Emissions from Vehicles:
The air conditioning system, tyres, brakes, and other vehicle components produce pollutant emissions. Chlorofluro carbons (CFCs) and particles are the major non exhaust emission sources in vehicles. The sources of CFC emissions from motor vehicles is the freon gases used in air conditioners of vehicles. CFCS destroy ozone layer in the stratosphere, which results in excessive ultra violet radiation. CFCS are also greenhouse gases. The contribution of motor vehicles to global CFC emissions is estimated at about 28 per cent.
The other factors affecting the non-exhaust particulate emissions include:
1. Tyres and their interactions with road surface.
2. Wear of brake linings on vehicles.
3. Type of roads (paved versus unpaved)
The U.S.A, EPA estimates emission factors of 0.001 and 0.008grams/km of PM10 due to tyre wear and wear of brake lining on cars respectively. A recent emissions inventory study in the United Kingdom indicated that wear of tyres and brake lining contributed to 1 and 7.5 per cent respectively of total PM10 emissions from vehicles. The Swedish National Chemicals Inspectorate estimates that tyre wear releases approximately 40,000 Tons of particulate matter across Europe each year.
The health impact of these non-exhaust particles is expected to be serious because they are typically very small, with an aerodynamic diameter of just 1 micron. Tyres use a blend of natural and synthetic rubber. There is growing evidence of a relationship between the incidence of asthma and the concentrations of particles in the atmosphere resulting from the abrasion of tyres and roads.
The brake linings are manufactured in most of the developing countries including India from asbestos. Asbestos particles are known carcinogens. Unpaved roads create different problems – the significant resuspension of road dust.
It is difficult to quantify the impact of non-exhaust particles on over all ambient air concentrations. In particular, it is difficult to estimate the impact of dust resuspension by vehicles on roads.