The Combustion Process  Gasoline and diesel fuels are mixtures of hydrocarbons (made of hydrogen, oxygen and carbon atoms.) Hydrocarbons are burned by combining with oxygen. Nitrogen and sulphur atoms are also present and combine with oxygen when burned to produce gases. Automotive engines emit several types of pollutants.

Typical Engine Combustion:

Fuel + Air => Hydrocarbons + Nitrogen Oxides + Carbon Dioxide + Carbon Monoxide + water

Hydrocarbon emissions are fragments fuel molecules, only partially burned. Hydrocarbons react in the presence of nitrogen oxides and sunlight to form ground-level ozone, a major component of smog. Ozone irritates the eyes, damages the lungs, and aggravates respiratory problems. A number of exhaust hydrocarbons are also toxic, with the potential to cause cancer.

NITROGEN OXIDES (NOx)  Under high pressure and temperature conditions in an engine, nitrogen and oxygen atoms react to form nitrogen oxides, NOx. Nitrogen oxides, like hydrocarbons, are precursors to the formation of ozone and contribute to acid rain. Catalytic converters in car exhaust systems break down heavier nitrogen gases, forming nitrous oxide (NO) - 300 times more potent than carbon dioxide as a greenhouse gas. Nitrous oxide makes up about 7.2 percent of the gases cited in global warming. Vehicles with catalytic converters produced nearly half of that nitrous oxide. Nitrous oxide also comes from nitrogen-based fertilizer and manure from farm animals.

CARBON MONOXIDE  Carbon monoxide (CO) is a colorless, odorless, poisonous gas. A product of incomplete burning of hydrocarbon-based fuels. Carbon monoxide consists of a single carbon atom and a single oxygen atom linked together (CO), the product Carbon monoxide of incomplete combustion of fuel. Most CO is produced when air-to-fuel ratios are too low in the engine during vehicle starting, when cars are not tuned properly, and at higher altitudes, where thin air reduces the amount of oxygen available for combustion. Two-thirds of the carbon monoxide emissions come from transportation sources, with the largest contribution coming from highway motor vehicles. In urban areas, the passenger vehicle contribution to carbon monoxide pollution can exceed 90 percent.

See more about Carbon Monoxide

Carbon Dioxide U.S. Environmental Protection Agency (EPA) originally viewed carbon dioxide as a product of "perfect" combustion, but now views CO2 as a pollution concern. Carbon dioxide is a greenhouse gas" that traps the earth's heat and contributes to global warming. 

Evaporative Emissions  Hydrocarbon pollutants also escape into the air through fuel evaporation - evaporation causes significant hydrocarbon pollution from cars on hot days when ozone levels are highest. Evaporative emissions occur several ways:

Running Loses: The hot engine and exhaust system can vaporize gasoline when the car is running.

Sitting Evaporation: The engine remains hot for a period of time after the car is turned off, and gasoline evaporation continues when the car is parked.

Adding Fuel: Gasoline vapors are always present in fuel tanks. These vapors are forced out when the tank is filled with liquid fuel.

Adapted from Cars and Pollution EPA Fact Sheet OMS-5

Long term solutions require reduced combustion of all kind. While vehicles with new energy sources such ethanol, biofuels, propane and natural gas can contribute to reduced air pollution, their benefit is limited if vehicle use continues at current intensity.

If you pay more money to buy a hybrid car, but drive it more, you have contributed little or nothing to solving air pollution problems. If you buy a gas guzzling clunker and only drive it 20 km a week, you have contributed more to the solution.

The problem with all alternative fuels is that the manufacture of all fuels requires energy, distribution, and a manufacturing infrastructure that produces affordable fuel without negative effects on the environment. No alternative fuel is ideal.

See Switch to Biofuels

Hydrogen  Ultimately cars might burn hydrogen in fuel cells, but despite working prototypes, a hydrogen economy is a distant fantasy. The biggest problem is that producing hydrogen requires a large amount of energy. In Canada, there are opportunities to dam rivers and produce electricity with falling water, a non polluting, renewable energy resource. A more problematic energy source would be be nuclear reactors that "burn" uranium or plutonium.

Even if new non-polluting energy sources are developed, hydrogen storage and distribution requires a new, very expensive infrastructure that could replace gasoline and diesel fuels. With rich countries such as the USA on the verge of bankruptcy and facing the repair of already aging, derelict infrastructures, adding a new, unprecedented development cost would seem unlikely.