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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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Poster 9: Measurement and Modeling of Vehicle Tailpipe Emissions Based Upon On-Road Data from Portable Instruments
H. Christopher Frey, Ph.D., Associate Professor
Water Resources and Environmental Engineering Program
Department of Civil Engineering, North Carolina State University, Raleigh, NC 27695-7908
In recent years, portable emission measurement systems (PEMS) have become available that can be temporarily installed in a vehicle. For example, using PEMS, the effect of changes in traffic signal timing and coordination on selected primary arterials were evaluated. Data for CO2, CO, NO, and hydrocarbon (HC) tailpipe emissions and for vehicle activity (e.g., vehicle speed, engine parameters) were collected on a second-by-second basis. In total, over 1,200 one-way trips were made with more than 20 vehicles. A pilot study was used to identify key factors influencing on-road emissions and as input to the design of an evaluation study. In the evaluation study, data were collected intensively with a small number of vehicles on two corridors before and after signal timing and coordination changes were implemented. A 10 to 20 percent reduction in emissions of CO, NO, and HC associated with improved signalization was observed on Walnut Street. For Chapel Hill Road, a comparison of peak and off-peak time periods revealed that emissions rates were typically 40 to 60 percent less for uncongested versus congested traffic flow. Based upon second-by-second data from PEMS, as well as dynamometer data for facility-specific cycles, an empirically-based modal method for modeling vehicle emissions has been developed and demonstrated. Using hierarchical-based regression tries, vehicle specific power (VSP) was identified as the key explanatory variable for Tier 1 vehicle emissions. Fourteen VSP modes were developed. Additional explanatory variables were selected based upon HBTR. Uncertainty in mean emission rates for each mode and for predictions of trip emissions were quantified. The conceptual model was validated by comparison with two independent data sets. Recommendations were developed for emissions rate estimation in EPA´s new MOVES model.