Spectroscopy Applied to On-Road Mobile Source Emissions

Not to be confused with emission spectroscopy, the detection of mobile source emissions by means of a remote sensing device (RSD) is almost entirely an application of absorption spectroscopy. In this article we discuss the application to measurement of individual vehicle emissions remotely as the vehicle drives by in its normal driving mode. Most studies and commercial implementation have involved automobiles and light duty trucks. However, the same technology has been applied to heavy duty trucks, locomotives, snowmobiles and airplanes in taxi and takeoff modes. The challenge in all these applications is to make precisely collocated absorption measurements at 100 Hz in both the UV and IR regions of the spectrum with low noise. The ability to measure 1/1000 absorbance units is necessary. The only compensating advantage is that an individual vehicle measurement cycle is over in one second or less, such that slow instrument drift becomes unimportant. The results are all measured as a ratio to the measured emission of CO2 and calibrated by means of a certified cylinder with known ratios. Using these ratios, the end result for each vehicle can be expressed as mass emissions per kg or per L of fuel as well as the emissions % which would be measured were the vehicle equipped with a tailpipe probe and emissions measurement system (corrected for any excess air which might be in the exhaust manifold).