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NARSTO Workshop on Innovative Methods for Emission Inventory Development and Evaluation University of Texas, Austin October 14-17, 2003 |
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Comparison of Remote Sensing Measurements of On-Road Vehicle Emissions in Chicago and Denver with MOBILE6 predictions
Till Stoeckenius and Cuong Tran
ENVIRON International Corp., Novato, CA
Remote sensing device (RSD) measurements of vehicle exhaust plumes have been collected over a period of years in Denver (1999 – 2001) and Chicago (1997 – 2000). Each year´s measurements were made over a period of a few days with the location and time of year held constant from one year to the next. Previous analyses of these data have shown that they provide an accurate and consistent portrayal of light-duty vehicle exhaust emissions for the fleet and driving conditions observed at each monitoring location. As part of a series of analyses designed to evaluate EPA´s MOBILE6 emission factor model, ENVIRON undertook a comparison of the Denver and Chicago RSD data with corresponding fleet average vehicle exhaust emission factors predicted by MOBILE6. RSD measurements represent the ratios of hydrocarbons, carbon monoxide, and nitrogen oxide to carbon dioxide in the vehicle exhaust plume over approximately a one-half second time interval. These measurements together with a few reasonable assumptions about the combustion process can be used to determine the percent HC, CO, and NO in the exhaust plume and, from the carbon balance, mass emissions in grams per kg of fuel. MOBILE6, on the other hand, predicts tailpipe emission factors in units of grams/mile. Data used to derive MOBILE6 emission factors originate from direct measurements of HC, CO, and NOx mixing ratios in the tailpipe exhaust during vehicle operation over a prescribed dynamometer driving cycle. Expressing MOBILE6 g/mile factors in g/kg of fuel (or vice versa) requires an estimate of the instantaneous fuel economy which is not available from the RSD data used in this study. Since MOBILE6 only provides fleet average fuel economy figures and the instantaneous vehicle specific fuel economy may vary significantly from the average, comparisons were based on ratios of mass emission factors (HC/NOx and CO/NOx). Comparisons were also made of the relative changes in emissions with vehicle age and model year as predicted by MOBILE6 and as observed in the RSD data. Dependency of emissions on driving mode (as measured by the vehicle specific power) is examined and implications for the MOBILE6/RSD comparison explored.