On-Road Remote Sensing of Automobile Emissions in the Denver Area: Year 4, January 2003

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Publication Date


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Final Report prepared for CRC




The University of Denver has completed the fourth year of a multi-year remote sensing study in the Denver area. The remote sensor used in this study is capable of measuring the ratios of CO, HC and NO to CO2 in motor vehicle exhaust. From these ratios, we calculate the percent concentrations of CO, CO2, HC and NO in the exhaust that would be observed by a tailpipe probe, corrected for water and excess oxygen not involved in combustion. Mass emissions per mass or volume of fuel can also be determined. The system used in this study was configured to determine the speed and acceleration of the vehicle, and was accompanied by a video system to record license plates.

Measurements were conducted on December 31, 2002 and January 7, 8, and 31, 2003 in Denver. The measurement site was the interchange ramp from northbound I-25 to westbound 6th Avenue in central Denver. A database was compiled containing 21,321 records for which the State of Colorado provided make and model year information. These 21,321 records contained valid measurements for CO and CO2. A total of 21,235 records contained valid measurements for CO, HC, and NO. The database, along with earlier databases and reports, can be found at www.feat.biochem.du.edu.

The mean CO, HC and NO emissions respectively for the fleet measured in this study were 0.35%, 88 ppm (offset adjusted – see below) and 456 ppm. These values are somewhat lower than the mean emissions for fleets measured by the University of Denver at the same site in the winters of 1996 and 1997, in separate studies, and in 1999, 2000 and 2001 as part of the current CRC study. The measured lower emissions can be attributed to the current fleet consisting of more modern vehicles with advanced emissions control systems.

As expected, the fleet emissions observed at the site in Denver exhibited a skewed distribution, with most of the total emissions contributed by a relatively small percentage of the measurements. For example, the cleanest 91% of the measurements are responsible for only 35% of CO emissions. This skewed distribution was also seen in previous studies at the site.

Using vehicle specific power, it was possible to adjust the emissions of the vehicle fleet measured in 2003 to match the vehicle driving patterns of the fleet measured in 1999, 2000, and 2001. After doing so, it was seen that the CO, HC, and NO emissions of the current year are lower than the previous years.

A model year adjustment was applied to a fleet of specific model year vehicles to track deterioration. The analysis indicated continued deterioration of model years measured in 1999. Tracking of model year fleets through five measurements in six years (including three years of this study) showed that the rate of emissions deterioration increases significantly after the vehicle has aged several years. An analysis of high emitting vehicles showed that there is considerable overlap of CO and HC high emitters, for instance 4.7% of the measurements contributed 40% of the total CO and 37% of the total HC. The noise in the CO and NO measurements was determined to be minimal, while noise in the HC channel was somewhat significant.