In other words, picking out the vehicles that are the worst polluters would be an effective target to reduce environmental effects. Even vehicles with catalytic converters emit significant amounts of carbon monoxide before they reach operating temperature. Ideally, an on-road remote monitor that can measure auto emissions accurately during real driving conditions would vastly improve the environmental monitoring of carbon monoxide. To solve the problem, a team including Jian Wang and Ronald K Hanson at Stanford University, California, have developed a highly sensitive laser diode sensor to monitor the pollution. Infrared light from the diode laser passes across the road through the exhaust gases of the vehicle.
Different gases absorb at different wavelengths of light, referred to as transitions.
When the wavelength of light is tuned to a carbon monoxide transition, some of the light is absorbed and less light reaches the sensor. As vehicle exhausts also produce carbon dioxide, a second diode laser is used to monitor this and by accounting for the carbon dioxide, precise concentrations of the carbon monoxide emission can be determined. Analyser, which is able to identify gross polluters, and laser diode sensors, operating at other wavelengths which are limited to a quarter of a percent accuracy by weak line-strengths and interference from other transitions from water vapour and carbon dioxide. The new diode lasers developed by Wang and colleagues are tuned to carbon monoxide transitions free from interference from water vapour. In addition, by using the ratio between carbon monoxide and carbon dioxide as a measurement of the amount of carbon monoxide in the undiluted exhaust, their accuracy is better than any other type of sensor.