Critical Review of Vehicle Emission Estimates of Uncontrolled Components of Particulate Matter
Research Idea Scope
There are many air pollution emission components associated with motor vehicles, which historically have been subjected to stricter and stricter environmental controls, including running exhaust, crankcase running exhaust, cold start, and several evaporative emission processes. A number of largely uncontrolled components of particulate matter (PM) emissions remain, however, including brake wear, tire wear, and re-entrained road dust. Since cleaner fuels and technology have become more effective in reducing overall emissions from vehicles, the significance of tailpipe emissions while vehicles are operating on highways has diminished relative to the proportion attributable to the uncontrolled emission components. As a result, when predicting the contributions of vehicle emissions on local PM concentrations from a set of proposed transportation project alternatives, the uncontrolled components can account for the largest fraction. Estimates of uncontrolled components of vehicle PM emissions are less refined than their counterparts. There are large discrepancies in comparable emission estimates of brake wear and tire wear produced by the MOVES and EMFAC regulatory emission models. Differences in PM emission estimates of the uncontrolled components among project alternatives are highly correlated with only the differences in the vehicle-miles of travel attributable to each alternative. The proposed research consists of two phases. The scope of Phase 1 is to perform a critical review of the methodologies for estimating the uncontrolled PM emission components attributable to a transportation project – brake wear, tire wear, and re-entrained road dust. The review will examine the legacy databases used to develop the regulatory emission models (MOVES, EMFAC, and AP-42); distinguish differences in the models, as appropriate; investigate current research and data; identify any data gaps and needs; and determine additional parameters that may affect estimates of these emissions, e.g., facility type; vehicle type; vehicle speed; VMT ceiling; improved brake, tire, and pavement technologies; etc. The research will provide recommendations for improving estimates of the uncontrolled vehicle PM emissions components attributable to a transportation project, such as the unlimited silt loading assumption for high volume roadways, and suggestions for mitigating the contributions of these components on local PM concentrations. The scope of Phase 2 is to investigate the local air quality contributions of PM emission components while vehicles are operating on highways – running exhaust; crankcase running exhaust; brake wear; tire wear; and re-entrained road dust. A sampling of highway configurations will be examined, including freeway mainline; freeway interchanges; arterial mainline; and signalized arterial intersections for a range of traffic volumes, receptor locations, and roadway grades. The investigation will address short-term PM-10 and short-term and long term PM2.5. The contributions will be estimated for analysis years representing the first year of operation and design year of a highway project. Emission estimates will be based on the MOVES, EMFAC, and AP-42 regulatory models. Concentration predictions will be made using the AERMOD area/line, AERMOD volume, and CAL3 line source air quality models. The research will differentiate the local contributions due to each PM emission component across the range of elements considered – highway configurations, PM species, analysis years, emission models, and air quality models.
Urgency and Payoff
The assessment of proposed transportation project alternatives on local PM concentrations can often hinge on differences in the uncontrolled vehicle emission components attributable to each alternative, which largely reflect only differences in VMT among alternatives. The payoff is 2-fold: 1) refining methodologies of emissions models toward improving the accuracy, reliability, and representativeness of model estimates and 2) establishing the likely contributions of the non-running exhaust vehicle emission components to local air quality considering the variability in assessment factors of highway configurations, PM species, analysis years, emission models, and air quality models.
Michael Claggett U.S. Dept. of Transportation, Federal Highway Administration (505) 820-2047