Environmental Science 3(3+0)

Automobile emissions

 
• The automobile, powered by piston-type internal combustion engine, is so widely used that it has become the dominant source of air pollutants in large urban cites.


• Automotive engines generally operate on "fuel rich" mixtures, which mean that there is not quite enough oxygen to completely burn the fuel. As a result there is an excess of unburnt hydrocarbons, particularly along the cylinder walls, and substantial amounts of carbon monoxide. This efficient production of carbon monoxide has made automobiles the most important source of this poisonous gas in the urban atmosphere.


• Many of the carcinogens found in the exhaust from diesel engines are polycyclic aromatic hydrocarbons (PAH) and are archetypical carcinogens. Best known of these is benzo-a-pyrene. Benzene represents a large part of the total volatile organic emissions from automobiles. Yet the compound is also recognized by many as imposing a substantial carcinogenic risk to modern society. Toluene, although by no means as carcinogenic as benzene, is also emitted in large quantities. Toluene proves a very effective compound at initiating photochemical smog and also reacts to form the eye irritant peroxybenzoyl nitrate. The highly dangerous compound dioxin can be produced in auto exhausts where chlorine is present (anti-knock agents often contain chlorine).


• Many exotic elements that are added to improve the performance of automotive fuels produce their own emissions. The best known is the anti-knock agent tetraethyl lead, which was added in such large quantities that it became the dominant source of lead particles in the air. A wide range of long-term health effects, such as lowering IQ, have been associated with exposure to lead. Although lead in urban populations is still rather high, the use of unleaded gasoline has decreased the problem somewhat. Although huge quantities of fossil fuels are burnt in power generation and a range of industrial processes, automobiles make a significant and growing contribution to carbon dioxide emissions which enhance the greenhouse effect.


• The nitrogen oxides emitted by automobiles are ultimately converted to nitric acid and these are making an increasing contribution to rainfall acidity. Diesel-powered vehicles use fuel of higher sulfur content and can contribute to the sulfur compounds in urban air.Thus while air pollution problems might well be cured by a wide range of sociological changes, a technological fix has been favoured, such as the use of catalytic converters. Although much attention is being given to lowering emissions of volatile organic compounds, it is likely that non-polluting vehicles will have to be manufactured and better a mass transmit system created.