tional diesel. GREET further specifies sulfur contents for low-sulfur diesel. Outside of fuel sulfur levels, vehicle emission factors are fixed based on inputs and assumptions from 1990 through 2020.

The differences between GREET and Mobile6.2 emission factors are most likely due to the variations in vehicle operation and fuel input parameters. These differences could be from cold start and warm running, fuel vapor pressure, summer or winter fuel mix, and vehicle model assumptions. While the Mobile6.2 factors tend to be larger than the GREET factors. The GREET factors are assumed to be reasonable, given the uncertainty in vehicle and fuel parameters and that they are within the bounds of Mobile6.2 estimates for the year.

EPA Mobile6 Ammonia Emissions Factors

Ammonia emissions, which ultimately contribute to particulate formation, are evaluated by APEEP but not included in the default transportation damage assessment. GREET does not evaluate ammonia emissions but Mobile6.2 does for a subset of vehicle and fuel combinations included in GREET. Table D-13 summarizes the Mobile6.2 ammonia emission factors for 2005 and 2030.

For light-duty gasoline vehicles, the ammonia factors are about 0.1 g/VMT; for light-duty diesel vehicles, they range from 0.01 to 0.03 g/VMT for both years. The heavy-duty gasoline and diesel vehicle factors are 0.05 and 0.03 g/VMT.

Applying GREET Feedstock and Fuel Production Factors to Heavy-Duty Vehicles

Feedstock and fuel production factors from GREET are used to supplement the Mobile6.2 heavy-duty-vehicle operational emissions. Because Mobile6.2 evaluates only the operational phase of heavy-duty vehicles, there is a need to supplement this component with feedstock and fuel production requirements so that results are commensurate with light-duty vehicles evaluated in GREET. To do this, the GREET feedstock and fuel production factors from reformulated gasoline and low-sulfur diesel light-duty vehicles are used. Using the energy content of gasoline or diesel consumed during vehicle operation, the corresponding GREET feedstock and fuel production factors are prorated and assessed to the heavy-duty vehicles. This procedure is done across all of the energy and emissions factors for each of the heavy-duty vehicles assessed with Mobile6.2.

Heavy-duty vehicle-manufacturing factors are not included in the assessment. Unlike feedstock and fuel production processes that are specific to a fuel (which is the same for both light- and heavy-duty vehicles), vehicle-

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