Auto emissions are a leading cause of air pollution and a major contributor to smog. To control air pollution, many states require regular auto emissions tests, but these tests are costly and not always reliable. Donald Stedman, a chemistry professor at the University of Denver, has developed a fast and inexpensive method of measuring auto emissions that targets those vehicles that are the worst offenders.

Emissions tests measure three main pollutants: hydrocarbons, carbon monoxide, and nitrogen oxide. Hydrocarbon emissions result when unburned fuel leaves the engine. The catalytic converter, which is part of the exhaust system, helps control these emissions by oxidizing some of the hydrocarbons.

Carbon monoxide (CO) is emitted when the air/fuel ratio is too rich-that is, there is too much fuel. Normally, when fuel burns, the carbon in the fuel combines with oxygen to form carbon dioxide (CO₂). But if oxygen levels are too low, the excess carbon is oxidized into carbon monoxide instead.

Nitrogen oxide forms when excess oxygen enters the combustion chamber. Because of the high temperature in the combustion chamber the oxygen reacts with nitrogen from air in the cylinder to form nitrogen dioxide: N₂ + O₂ 2 NO (1).

Testing stations usually measure these emissions by placing a device that generates infrared (IR) and ultraviolet (UV) radiation in or near the tailpipe. Each kind of emission absorbs IR/UV radiation at a unique frequency. A detector on the testing instrument measures amount of IR/UV radiation absorbed by the auto emissions.

The reason molecules like hydrocarbons can be detected with IR radiation is that the bonds between atoms vibrate. For example, the C-H bond(s) in a hydrocarbon vibrate at a unique frequency. If this frequency matches that of the IR radiation, the molecule absorbs the radiation. The principle is similar to a person jumping off a diving board. When the person’s meets the board right at the lowest point of the board’s cycle of motion, the energy is absorbed and the person springs higher and higher. If the person meets the board while it is on its way up, the energy is not absorbed.

Donald Stedman has adapted this instrument into a remote sensing device that can test auto emissions in real traffic. Essentially he just cut the instrument in half, with the IR/UV source on one side of the road and the detector on the other side. The radiation is directed at the exhaust plumes from passing automobiles. The detector is connected to a computer that processes the data. Also connected to the computer is a video camera that records the license plate number of the passing automobile. Motor vehicle departments can use this information to notify drivers whose vehicles do not pass the test.

One advantage of a remote sensing system is that it can test tens of thousands of vehicles in a day; in comparison, state testing stations can test only a few hundred each day. Another is cost: while testing stations in Denver cost motorists about $25.00 per test, the remote sensing system costs only about 50 cents. (2) Further, when Stedman tested his system in Denver, Los Angeles, and other sites, he found that less than 10 percent of cars cause more than 50 percent of the pollution. Identifying and repairing these vehicles could significantly reduce air pollution without inconveniencing all drivers.

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