EMFAC2011

Overview

September 19, 2011
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Summary

EMFAC2011 represents the next step in the ongoing improvement of the EMFAC series of 

emissions estimation models. The EMFAC2011 release is needed to support the Air Resources 

Board’s (ARB) regulatory and air quality planning efforts and to meet the Federal Highway 

Administration’s transportation planning requirements. EMFAC2011 includes the latest data on 

California’s car and truck fleets and travel activity. The model also reflects the emissions 

benefits of ARB’s recent rulemakings including on-road diesel fleet rules, Pavley Clean Car 

Standards, and the Low Carbon Fuel standard.

In order to incorporate the new detailed data and methods to estimate emissions from diesel 

trucks and buses and future improvements, staff used a modular emissions modeling approach 

for EMFAC2011 that departs from past EMFAC versions. The first module, named EMFAC-LDV, 

estimates passenger vehicles emissions. A second module, called EMFAC-HD, estimates 

emissions from diesel trucks and buses. A third module integrates the output of EMFAC-LDV 

and EMFAC-HD and provides users with the ability to conduct scenario assessments for air 

quality and transportation planning. The third module is called EMFAC-SG. Together the three 

modules comprise EMFAC2011.

The sections below describe and summarize the major updates to this version of the model. 

The reader is directed to the Technical Support Document for a complete discussion of all the 

updates, revisions, and additions to EMFAC2011, and to the new ARB on-line data tool. Most 

importantly, the on-line tool is designed to cover most data needs for both summarized and 

detailed emissions and emission rates. The vast majority of EMFAC users will no longer have to 

download, learn, and run the model to get the data they need. Much simple data queries 

across the internet will suffice.

New Data

The most important improvement in EMFAC2011 is the integration of the new data and 

methods to estimate emissions from diesel trucks and buses. EMFAC2011 uses the same diesel 

truck and bus vehicle populations, miles traveled and other emissions-related factors 

developed for the Truck and Bus Rule approved by the Air Resources Board in 2010. 

EMFAC2011 still provides the same traditional emissions rate detail for these vehicles that user 

are familiar with in previous EMFAC versions. The model includes the emissions benefits of the 

truck and bus rule and the previously adopted rules for other on-road diesel equipment. 

Finally, the impacts of the recession on emissions that were quantified as part of the truck and 

bus rulemaking are included.

Light-duty motor vehicle fleet age, vehicle type, and vehicle population in EMFAC2011 is based 

on 2009 California Department of Motor Vehicles data. These data along with the new diesel 
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truck and bus data, satisfies guidance issued by the U.S. Department of Transportation, Federal 

Highway Administration that requires that vehicle fleet data used in transportation conformity 

analyses be no older than 5 years.

As in previous releases of EMFAC, travel activity data (which includes vehicle miles traveled, 

trips, and distributions of vehicle miles traveled by speed and time period) are provided by 

metropolitan planning organizations (MPOs) that conduct travel demand modeling for the 

transportation planning process. For EMFAC2011, updated travel activity data was provided by 

the Southern California Association of Governments (SCAG) for the greater Los Angeles region, 

by the San Diego Association of Governments (SANDAG) for San Diego County, by the eight 

San Joaquin Valley MPOs, by the Metropolitan Transportation Commission (MTC) for the 

Bay Area, and by the Sacramento Area Council of Governments (SACOG) for the greater 

Sacramento region.

New Modular Model Structure

The new EMFAC2011 comprises a suite of three modules that estimates emissions from diesel 

trucks, buses, and gasoline powered vehicles. Staff has moved to a modular model structure to 

accommodate more detailed information about the truck and bus fleet than has been in prior 

EMFAC versions. The modular structure will also more easily accommodate future model 

enhancements that are necessary to support on-going program development associated with 

criteria and greenhouse gas emissions.

The first module, named EMFAC-LDV, is used as the basis for estimating emissions from 

gasoline powered on-road vehicles, diesel vehicles below 14,000 pounds gross vehicle weight 

rating, and urban transit buses. The algorithms used in EMFAC-LDV for passenger cars are the 

same as used in EMFAC2007

The second module, called EMFAC-HD, is the basis for emissions estimates for diesel trucks and 

buses with a gross vehicle weight rating greater than 14,000 pounds operating in California. 

This model is based upon the Statewide Truck and Bus Rule emissions inventory that was 

developed between 2007 and 2010 and approved by the Air Resources Board in December 

2010. The truck and bus population and vehicle miles traveled estimates in EMFAC2011-HD are 

identical to those presented to the Board. Emission factors in the Statewide Truck and Bus Rule 

inventory were based on statewide, annual average emission factors and adjusted for 

improvements to fuel economy, medium-heavy duty truck emission factors, and other new 

information. EMFAC-HD emissions estimates include all of the improvements made as part of 

the Statewide Truck and Bus Rule inventory and then add the traditional detailed EMFAC 

methods that provide emission rates for annual, summer, and winter seasons on a regional 

basis. 
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Finally, EMFAC2011 contains a new simplified tool, called EMFAC-SG, that provides air quality 

planners, transportation planners, and other EMFAC users a tool for assessing emissions under 

different future growth scenarios. This includes conformity analyses of transportation plans 

and programs with the State Implementation Plans required by federal law, State 

Implementation Plan inventories, alternative growth scenarios associated with regional 

transportation planning for greenhouse gas reductions (SB375), and the like.

EMFAC-SG takes the output from EMFAC-LDV and EMFAC-HD and applies scaling factors to 

estimate emissions consistent with user-defined vehicle miles of travel (VMT) and speeds. The 

EMFAC-SG module also estimates the benefits of Pavley and Low Carbon Fuel Standard 

regulations.

EMFAC2011’s enhanced modular structure and computation requirements can make running 

the model more resource-intensive than the previous EMFAC release. Therefore, ARB is 

enhancing data availability by providing a new database through the ARB mobile source 

emissions inventory web site that provides regional population, activity, emissions, and 

emission rates at varying levels of detail. Most current EMFAC users will find the new database 

sufficient for obtaining emissions data to support California Environmental Quality Act (CEQA) 

assessments and other types of analyses for which the EMFAC model is currently used and will 

not need to run the model. The model executable files will also remain publicly available on 

our website. Figure 1 provides a schematic that describes how the modules fit together. 

Figure 1: EMFAC2011 Schematic

EMFAC-LDV Truck Model Activity

EMFAC-HD

EMFAC-SG

Internet Data Access Downloadable Models
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Beyond EMFAC2011

EMFAC2011 is a transition step to a future EMFAC emissions modeling system under 

development to meet the increasingly demanding needs for data in both the regulatory and 

planning arenas. Some of the areas of focus for the future EMFAC system that staff is 

developing over the next two to three years include:

Population, activity, and forecasting:

 Improved capture of recession and fuel price spikes.

 Reflect planned GHG emissions standards and their impact on future year fleet mix.

New Smog Check Algorithms

 Reflect the new Smog Check mandated by State law starting in 2013.

Flexibility for air quality planning better support for long-term scenario assessment

 Scenario development support for tighter National Ambient Air Quality Standards.

 SB375 –Model structure to better support for smart growth planning.

 Better conformity methods to support MPO conformity determinations.

Improved documentation and public availability of model output:

Future model output will be available in raw and processed form over the internet in ways that 

require minimal processing to understand.