Evaluation and Recommendation of a Modal Method for Modeling Vehicle Emissions

The Environmental Protection Agency’s new Multi-scale Motor Vehicle and Equipment Emissions System (MOVES) is intended to replace the MOBILE6 and NONROAD models in coming years. To establish a methodology for the emission rate estimator portion of MOVES, the following questions were addressed: (1) how should second-by-second data be used to estimate emission rates?; (2) what explanatory variables should be selected to refine the emission rates?; (3) what averaging time is preferred?: (4) what emission factor units should be used?; (5) what weighting approach should be used, when comparing time-, vehicle-, and trip-weighted?; (6) how should variability and uncertainty be characterized?; and (7) how should the conceptual model be validated and what are the results of validation exercises? A modal emissions estimation approach was demonstrated for hot stabilized Tier 1 vehicle emissions based upon binning second-by-second data with respect to 14 vehicle specific power categories. The use of 5 or 10 second averaging times as a basis for estimating modal emission rates did not improve prediction of total cycle or trip emissions. A time-weighted approach, based upon gram per second emission rates, offers the most flexibility for model development. Inter-vehicle variability and uncertainty in mean modal emission rates were characterized and methods for propagating uncertainty in MOVES were recommended. The conceptual approach was evaluated based upon comparison of model predictions to independent data. The predictions were found to be sensitive to a wide range of driving cycles and were accurate in most cases. Opportunities to refine and improve the approach were identified and recommended. INTRODUCTION EPA is undertaking an effort to develop a new set of modeling tools for the estimation of emissions produced by on-road and off-road mobile sources. The product of this effort will be the Multi-scale mOtor Vehicle & equipment Emission System, referred to as MOVES. The design of MOVES is guided by the following four considerations: 1) the model should encompass all pollutants (e.g., HC, CO, NOx, particulate matter, air toxics, and greenhouse gases) and all mobile sources at the levels of resolution needed for the diverse applications of the system; 2) the model should be developed according to principles of sound science; 3) the software design of the model should be efficient and flexible; and 4) the model should be implemented in a coordinated, clear, and consistent manner. A critical element of MOVES is the use of data gathered using on-board emissions measurement devices. To explore this issue, in Fall 2001 EPA issued an on-board emission analysis “shootout” contract in order to solicit several approaches for incorporating on-board emissions into moves. Three shootout contracts were issued to three organizations that worked