On-road Evaluation of an Automobile Emission Test Program

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Document Type


Organizational Units

Chemistry and Biochemistry


Atmospheric chemistry, Emissions, Pollutants, Inspection and Maintenance


In January 1995, the State of Colorado superseded an annual decentralized idle test program with an EPA recom mended biennial centralized IM240 test program in the six-county metro Denver area. Odd model year 1982−1994 vehicles and any vehicles that entered the counties or changed ownership required testing in 1995. Even model year vehicles were to be tested in 1996. Five days of on-road remote sensing exhaust measurements in winter 1995−1996 provided 26 255 identified, unique, gasoline-fueled vehicle emission measurements to independently evaluate the marginal benefit of the 1-year-old inspection and maintenance program. Eligible, untested even model years form the control group to compare to tested odd model years. Benefits were significant for CO (between 4% ± 2% and 7% ± 2%). Benefits for HC and NO were not detectable. Error bars are obtained using each day as an independent analysis. An effect not observable by other methods shows odd model year cars outside the program jurisdiction with significantly higher average CO emissions than their even model year peers, possibly indicating out migration of registration for failing odd model year vehicles, thus avoiding the new emission test.

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Copyright held by the American Chemical Society.

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