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Multi-Pollutant Emissions Benefits of Transportation Strategies-FHWA

Appendix C: Overview of Emissions Factors and Assumptions Used in Sample Calculations

Methodological Procedures

Most strategies can be analyzed in multiple ways, and variations of these approaches are available. The methods described in this report are generally simple sketch planning approaches that involve two main processes: 1) estimating the travel, speed, or vehicle changes associated with the strategy; and 2) estimating emissions impacts. For those strategies where sketch planning was not appropriate, we used the COMMUTER Model and the National Inventory Model (NMIM) Model.

The most challenging part of each methodology is developing the values for changes in travel activity, speeds, or vehicle stock. Various approaches can be used for these inputs, such as using models (e.g., travel demand forecasting models, COMMUTER Model), original data collection (e.g., surveys), past experience/studies of similar projects, standardized factors drawn from the literature, or professional judgment.

Application of Emissions Factors

Unless otherwise noted in each of the strategy review sections, all of the on-road strategies analyzed and presented in this report use emissions factors generated from MOBILE6.2 to determine emissions impacts. Factors generated for PM-2.5, PM-10, CO, NOx, VOCs, SOx, and NH3 were used to estimate the emissions impacts of documented strategies. Standard defaults have been applied, covering years 2006, 2010, and 2020. Emissions were generated for start (trip-based factors assuming 100 percent cold start) and running emissions (per mile factors). The recognition of a difference between trip starts emissions and running emissions is significant, since emissions control equipment does not function as effectively from a "cold start" causing the release of more pollutants during the first few miles of a trip.

MOBILE6.2 allows the calculation of several different types of vehicle emissions. This is important for some strategies because they affect running and trip emissions differently, as noted in this report. In areas that are concerned with VOC emissions, MOBILE6.2 modeling can be tailored to account for only the emissions that are affected by a particular strategy. For example, a trip reduction strategy not only reduces running emissions, but also start and "hot soak" emissions. Strategies that shorten trip length but do not eliminate trips affect running exhaust and running loss emissions, but not start, hot soak or diurnal soak emissions. Finally, most modeling should employ the "NO REFUELING" command in MOBILE6.2, since refueling emissions are associated with gas stations and are not normally affected by the types of stategies outlined in this document. Guidance is available from EPA and FHWA for those wishing to refine their analyses.

This section discusses the emissions factors used to perform the sample calculations.

MOBILE6.2 Emissions Factors

The major input parameters used to generate running mobile emissions factors for CO, PM-10, PM-2.5, NOx, VOCs, SOx, and NH3 are shown in the table below.

Table C-1. Major Input Parameters for MOBILE6.2 Emissions Factor Modeling

Parameter or Variable Values or Sources
Vehicle Fleet and Activity Inputs
VMT mix EPA national average (default)
Mileage accrual rates EPA national average (default)
Vehicle model year (registration) distribution EPA national average (default)
Diesel sales fractions EPA national average (default)
Soak time distribution EPA national average (default), or
All soak times >720 minutes (corresponds to 100percent cold starts).
Starts per day distribution EPA national average (default), or
Zero starts per day (for running emissions only)
Region Low altitude
Vehicle speeds Varied 2.5 mph and 3-65 mph by integers, with single average speed per scenario.
Roadway facility (functional classes) Arterial (allows use of specific average speeds)
Seasonal/Meteorological Inputs
Month of evaluation July
Temperatures for all pollutants Minimum 68.0° F
Maximum94.0° F
(Representative summer temperatures only. Actual source for these values is high-ozone-day data from Boston, MA nonattainment area SIP.)
Absolute humidity MOBILE6.2 default
Fuel Inputs
ASTM Class MOBILE6.2 default
Oxygenated fuels No (MOBILE6.2 default)
Reformulated gasoline No (MOBILE6.2 default)
Gasoline RVP 8.7 psi
(Representative summer RVP only. Actual source for this value is Philadelphia, PA nonattainment area SIP.)
Diesel fuel sulfur content 15 ppm
State Program Inputs
Inspection/Maintenance (I/M) Program No program (MOBILE6.2 default)
Low Emitting Vehicle (LEV) Program No program (MOBILE6.2 default)
Anti-tampering program (ATP) No program (MOBILE6.2 default)
Stage II refueling controls Not modeled (NO REFUELING command used).
Other Inputs
Particulate matter emissions parameters EPA national average (default)
All other inputs EPA national average (default)
Updated: 6/28/2017
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