MOVES Versus EMFAC: Comparison of Greenhouse Gas Emissions Using Los Angeles County

The U.S. Environmental Protection Agency is developing a new generation emission model, MOVES (Motor Vehicle Emission Simulator), to replace MOBILE6. MOVES changes the basis for mobile source emissions estimation from average speed to modal activity. The paper examines differences in features, methods, and results between MOVES and EMFAC, the mobile source emissions model approved for California. Using a Los Angeles County, California application; two greenhouse gases, carbon dioxide (CO2) and methane (CH4); and two analysis years, 2002 and 2030, we analyzed how underlying activity data and emission factors contributed to observed differences between the two models. At the county level, for 2002 MOVES produced similar CO2 emissions, but only 42% of the CH4 emissions estimated by EMFAC; for 2030, MOVES produced 40% higher CO2 emissions and CH4 emissions were nearly double the estimates provided by EMFAC. Important contributing factors to these differences are the activity data and emission rates embedded in MOVES. The default vehicle activities indicated a younger fleet and higher miles traveled for light-duty trucks by 2030. The CO2 emissions differences between the two models appear to be mainly affected by the magnitude of forecasted vehicle miles traveled; CH4 emissions results tend to hinge on the base emission rates. EPA considers the underlying MOVES database for CO2 and CH4 emissions to be a draft and emissions results will likely change with upcoming model releases.