Background: For many years, on-road vehicle emissions have been estimated using the EMFAC (in California) and MOBILE (in the rest of the U.S.) modeling tools. The U.S. Environmental Protection Agency is developing a new generation emission model, called MOVES, that changes the way vehicle emission estimates are generated. Planning agencies need to understand the implications of using the new model compared to traditional models. Methods: This study investigated the differences between EMFAC and MOVES. We modeled on-road emissions of the greenhouse gas pollutants carbon dioxide (CO2) and methane (CH4) using Los Angeles County, California as a case study. We compared year 2002 and 2030 emissions generated by EMFAC2007 and MOVES-HVI Demo the latest version of the MOVES model available as of mid-2008 and analyzed how underlying activity data and emission factors contributed to observed differences. Results: MOVES produced emission estimates substantially different from those generated by EMFAC. MOVES and EMFAC produced similar CO2 emissions for 2002; MOVES produced 40% higher CO2 emissions by 2030. For 2002, MOVES generated only 42% of the CH4 emissions estimated by EMFAC; however, for the year 2030, MOVES CH4 emissions were nearly double the estimates provided by EMFAC. Important contributing factors are that MOVES embeds travel activity data that differs substantially from EMFAC data MOVES assumes a younger vehicle fleet, and by 2030 includes more vehicle miles traveled (VMT), especially for light-duty trucks. MOVES and EMFAC have similar CO2 fleet-average g/mi emission factors; however, MOVES has higher fleet-average CO2 start emission factors than does EMFAC, and MOVES emission factors differ from EMFAC for some vehicle types. For 2030, MOVES estimated higher CH4 start emissions. MOVES also adjusted emissions differently than EMFAC to account for speed and deterioration. 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.
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