feedstock production through vehicle operation. Without using GREET, individual process assessments throughout the supply chain would need to be performed and combined for each vehicle and fuel of interest.

EMISSIONS DATA AND MODELING

The GREET Model

The Argonne GREET model is used to determine emissions from light-duty autos and trucks. The GREET model is a vehicle operation and fuel production life-cycle assessment tool, which captures fuel feedstock production, fuel refining, vehicle operation, and vehicle manufacturing. Feedstock production, fuel refining, and vehicle operation are estimated with the GREET 1.8b model; vehicle manufacturing is determined with GREET 2.7a. The version designations (1.8b and 2.7a) do not imply different generations of GREET but distinguish between a version developed for the fuel cycle (1.8b) versus a version developed for the vehicle cycle (2.7). The strength of the GREET model lies in its ability to estimate a variety of fuel inputs and vehicle combinations and their associated well-to-wheel life-cycle components. GREET allows for specification of critical inputs to these components (for example, emission factors, combustion technologies, energy efficiencies, and fuel types).

GREET evaluates several life-cycle components for the feedstock production, fuel production, and vehicle-manufacturing emissions inventory. For the feedstock and fuel production cycle, GREET captures extraction and creation of raw feedstock, transport to refineries, refinery processes, and transport to fueling stations. These constitute the well-to-pump components. On the vehicle-cycle side, GREET performs a materials-based life-cycle assessment capturing raw material extraction, processing, transport, and ultimately assembly into an automobile or light-duty truck. GREET does not estimate heavy-duty vehicle life-cycle factors, so additional data sources were needed to evaluate these vehicle classes.

GREET allows for the adjustment of many feedstock, fuel, and vehicle operation input parameters; however, particular inputs were targeted for the vehicle and fuel combinations evaluated. The evaluation year was toggled for the 2005 and 2030 scenarios to capture changes in both vehicle operational performance as well as efficiency changes in other devices, such as engines and turbines. The fraction of crude oil that comes from tar sands and the amount of reformulated gasoline were adjusted on the basis of the vehicle and fuel combination. For ethanol, GREET inputs for feedstocks (corn, herbaceous, and corn stover) and milling processes (dry or wet) were changed. Another critical input parameter for the assessment is the fraction of low-sulfur diesel. Last, the electricity mix for 2005 and 2030 were ad-



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