Skip to Content


Official Release of EMFAC2002 Motor Vehicle Emission Factor Model for Use in the State of California

Document Details

Information about this document as published in the Federal Register.

Published Document

This document has been published in the Federal Register. Use the PDF linked in the document sidebar for the official electronic format.

Start Preamble


Environmental Protection Agency (EPA).


Notice of availability.


EPA is approving and announcing the availability of the latest version of the California EMFAC model for use in state implementation plan (SIP) development in California. EMFAC2002 is the latest update to the EMFAC model for use by California state and local governments to meet Clean Air Act (CAA) requirements. EMFAC2002 calculates air pollution emission factors for passenger cars, trucks and buses. The new model is based on new and improved data and is also a more user-friendly version of the model, allowing agencies to update assumptions and run multiple scenarios for emissions analyses. Today's notice also starts a time period before EMFAC2002 is required to be used statewide in all new transportation conformity analyses in California. Since the EMFAC model is only used in California, EPA's approval of the model does not affect MOBILE model users in other states.


This determination is effective April 1, 2003. See below for further information regarding how today's approval starts a time period after which EMFAC2002 is required in new transportation conformity analyses.

Start Further Info


Karina O'Connor,, (775) 833-1276, Air Planning Office (AIR-2), Air Division, U.S. EPA, Region 9, 75 Hawthorne Street, San Francisco, California 94105-3901.

End Further Info End Preamble Start Supplemental Information


Copies of the official version of the EMFAC2002 model are available on the California Air Resources Board (CARB) Web site:​planning/​emfac2002/​emfac2002.htm (transmittal and links to support documents) and​msei/​on-road/​latest_​version.htm (model, technical support documents, etc.).

I. Background

A. What Is the EMFAC Model?

The EMFAC model (short for EMission FACtor) is a computer model that can estimate emission rates for motor vehicles for calendar years from 1970 to 2040 operating in California. Pollutant emissions for hydrocarbons, carbon monoxide, nitrogen oxides, particulate matter, lead, sulfur oxides, and carbon dioxide are output from the model. Emissions are calculated for passenger cars, eight different classes of trucks, motorcycles, urban diesel and school busses and motor homes. The EMFAC2002 model is operated with a user-friendly graphical user interface (GUI) which facilitates data input and allows the development of alternative emissions scenarios through a What If Scenarios (WIS) generator. The WIS interface can be used to incorporate updated vehicle data, adjust ambient conditions or make changes to potential emission control programs in a specific area.

EMFAC is used to calculate current and future inventories of motor vehicle emissions at the state, county, air district, air basin, or air basin within county level. EMFAC contains default vehicle activity data, and the option of modifying that data, so it can be used to estimate a motor vehicle emission inventory in tons/day for a specific day, month, or season, and as a function of ambient temperature, relative humidity, vehicle population, mileage accrual, miles of travel and speeds. Thus the model can be used to make decisions about air pollution policies and programs at the local or state level. Inventories based on EMFAC are also used to meet the federal CAA's state Start Printed Page 15721implementation plan (SIP) and transportation conformity requirements.

B. What Versions of EMFAC Are Currently in Use in California?

Most SIPs in California were developed using the vehicle emission models EMFAC7F (released by CARB September 1993) or EMFAC7G (released by CARB July 1997). EPA approved use of EMFAC7F in May 1994 and then EMFAC7G in April 1998. EMFAC7G was considered a minor update to EMFAC (similar to MOBILE5b as a minor update to MOBILE5a), thus areas with SIP motor vehicle emissions budgets developed using EMFAC7F were allowed to continue to use EMFAC7F for conformity. EMFAC7G included updated data on control measures (e.g. inspection and maintenance and truck standards), updated vehicle information (e.g. vehicle population and starts/vehicle/day) and vehicle activity data (e.g. vehicle miles per day and vehicle miles per speed distribution). Areas with SIP budgets developed using EMFAC7G were required to use EMFAC7G for regional conformity analyses.

Since the release of EMFAC7G, CARB has made several interim updates to EMFAC, EMFAC2000 and EMFAC2001. Of these models, only EMFAC2000 was submitted to EPA for approval and used in development of a SIP. Like EMFAC7G, EMFAC2000 included updated data on control measures (e.g., cleaner fuels, new vehicle standards for motorcycles, buses, and heavy-duty trucks), updated vehicle information (e.g., vehicle population, starts/vehicle/day) and updated vehicle activity data (e.g., vehicle miles per day, vehicle miles per speed distribution, and mileage accrual rates). In January 2002, EPA approved EMFAC2000 (67 FR 1464, January 11, 2002) for use only in the San Francisco Bay Area (Bay Area), for estimation of ozone precursor emission factors. EPA's approval of EMFAC2000 was limited because: (1) EMFAC2000 was an improvement on existing available models despite certain technical limitations; and (2) CARB has committed to revise the Bay Area ozone attainment SIP's motor vehicle emissions budgets with EMFAC2001 or a successor model as part of its mid-course review SIP revision in April 2004.

C. Why Have Transportation Agencies Stopped Using EMFAC7F and EMFAC7G for Regional Conformity Emissions Analyses?

On May 2, 2002, the Federal Highway Administration (FHWA) and the Federal Transit Administration (FTA) sent a letter to CARB indicating that, after December 31, 2002, they would not continue to make conformity determinations that require a new regional emissions analysis, unless the analysis incorporates more recent vehicle data. The letter indicated that vehicle data embedded in the EMFAC7F and EMFAC7G versions of the EMFAC emission factor model were about ten years old. Consistent with the U.S. DOT/EPA January 18, 2001, guidance on latest planning assumptions, new vehicle registration data must be used when it is available for conformity purposes. Since newer vehicle data was available but was not included in the older versions of EMFAC, EMFAC7F and EMFAC7G have not been used in any new regional emissions analyses since December 31, 2002.

D. Why Is EPA Announcing Its Approval of the EMFAC Model?

CAA section 172(c)(3) and 40 CFR 51.112(a)(1) require that SIP inventories be based on the most current and applicable models that are available at the time the SIP is developed. CAA section 176(c)(1) requires that the latest emissions estimates be used in conformity analyses. EPA approves models that fulfill these requirements.

Under 40 CFR 93.111(a), EPA must approve new versions of EMFAC for SIP purposes before they can be used in conformity analyses. In a December 20, 2002 letter, CARB requested that EPA approve EMFAC2002 for use in developing SIPs and in determining conformity of transportation plans in California. EMFAC2002 is a significant change from previous EMFAC models and is capable of calculating motor vehicle emissions for all California areas. EMFAC2002 is being approved as the latest emissions model for statewide use in SIP development (rather than as an interim update to the EMFAC model as EMFAC7G was an interim update to EMFAC7F). Since the EMFAC model is only used in California, EPA's statewide approval of the model does not affect MOBILE emissions factor model users in other states.

II. EPA Action

A. What Version of EMFAC Is EPA Approving?

In this notice, EPA is approving and announcing that EMFAC2002 is available to use in statewide California SIP development. EMFAC2002 was developed by CARB and transmitted for approval to EPA on December 20, 2002.

On January 28, 2003, CARB also transmitted a methodology to adjust vehicle activity data used by EMFAC2002 when updated data is available. Since the transportation conformity rule (40 CFR 93.110) requires areas to use the latest information for estimating vehicle activity, we are also approving the CARB methodology for updating vehicle activity data in EMFAC2002.

CARB's methodology, “Recommended Methods for Use of EMFAC2002 to Develop Motor Vehicle Emission Budgets and Assess Conformity,” explains how vehicle activity data should be updated. The methodology explains how each parameter associated with vehicle activity was originally developed in EMFAC, how each parameter is related, and how each can be updated when new data becomes available. These relationships are important when adjusting vehicle trips or VMT. For example, VMT in EMFAC2002 is directly related to vehicle population [1] and mileage accrual rate. Similarly, start and evaporative vehicle emissions are also related to vehicle population levels. If new VMT data is available, CARB suggests modifying the input vehicle population levels, instead of directly inputting new VMT data so that start and evaporative emissions are revised appropriately. Updated vehicle activity data (vehicle miles traveled) can also be input to EMFAC using the WIS interface. As CARB states, local circumstances may alternatively support adjustment of mileage accrual rates, subject to the interagency consultation process. In addition, CARB intends to periodically update vehicle population information, including vehicle classes by model year, in EMFAC2002.

B. What Pollutants Can EMFAC2002 Estimate?

EPA is approving the model to estimate regional emissions of hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOX), particulate matter (PM), lead, sulfur oxides and carbon dioxide. However, EMFAC2002 will only be used in transportation conformity for pollutants and precursors that affect transportation emissions and are identified in air quality plans as significant. Transportation-related pollutants and precursors that are currently covered by the conformity rule are HC, NOX, CO, and PM-10. EPA is also approving EMFAC2002 to estimate hotspot emissions for carbon monoxide. Start Printed Page 15722

C. Why Does EPA Consider EMFAC2002 as a Major Update to EMFAC?

EMFAC2002 includes significant changes to its model interface, new data and methodologies regarding calculation of vehicle emissions, revisions to implementation data for control measures, and corrections to technical errors mentioned in our prior approval of EMFAC2000. The new WIS interface makes EMFAC2002 both more user friendly and allows users to change input data updated for factors that had previously been embedded within the model (e.g. fleet vehicle data). EMFAC2002 includes updated data estimating idle emissions from heavy duty trucks and school buses. Base emission rates for light-duty vehicles are now drawn from the real-world unified drive cycle. EMFAC2002 accounts for gross evaporative emissions (i.e., “liquid leakers”) and updates particulate emissions to account for smoking vehicles. The model also accounts for recent amendments to California's low emissions vehicle and fuels regulations, as well as updated heavy-duty truck standards. Full chassis dynamometer tests, rather than engine dynamometer tests, are now the source of heavy-duty truck emissions factors. CARB's web site describes these and other model changes at​planning/​emfac2002/​encl_​a.pdf.

D. How Were Stakeholders and the Public Involved in the EMFAC Development Process?

Since 1999, CARB has held a series of public workshops to discuss proposed model updates and receive comments on interim versions of the new model. In the most recent workshops, held June 11 and 13, 2002, CARB described the latest EMFAC changes under consideration and sought public input. Those changes are reflected in the final EMFAC2002 model released in October 2002. Two additional public information briefings were held on November 6 and 7, 2002, to share the emissions estimates resulting from use of the final model with updated travel activity, as well as plans for transmittal of EMFAC2002 to U.S. EPA. CARB has also discussed both the model and the activity update methodology with affected transportation and air agencies as part of the interagency consultation process.

CARB also released a series of technical memos, that describe each update to the model, and a public information document that summarizes the changes from earlier versions of the model. The technical memos are available on CARB's Web site at:​msei/​on-road/​latest_​revisions.htm. Each memo describes the model update, the reason for the change, how the change was incorporated into the EMFAC model, and the resulting emissions impact. The public information document is also available on the CARB Web site at:​msei/​on-road/​briefs/​2002.pdf. This document summarizes the major changes to the EMFAC model and contains tables showing the impacts of the changes both statewide and by county for VOC, CO, NOX and PM. The detailed description of all major model updates since EMFAC7F is an enclosure to the transmittal package and available on the CARB Web site at:​planning/​emfac2002/​encl_​a.pdf.

E. Will a Transportation Conformity Grace Period Be Set by This Approval?

Yes. The transportation conformity rule (40 CFR 93.111) requires that conformity analyses be based on the latest motor vehicle emissions model approved by EPA for SIP purposes for a state or area. Section 176(c)(1) of the CAA states that “* * * [t]he determination of conformity shall be based on the most recent estimates of emissions, and such estimates shall be determined from the most recent population, employment, travel, and congestion estimates* * *.” When we approve a new emissions model such as EMFAC2002, a grace period is established before the model is required for conformity analyses. However, areas have the option of using the new model prior to the end of the grace period. The conformity rule provides for a grace period for new emissions models of between 3 to 24 months.

In consultation with the DOT, EPA considers many factors in establishing the length of the grace period, including the degree of change in emissions models and the effects of the new model on the transportation planning process (40 CFR 93.111).

Upon consideration of all of these factors, EPA is establishing a 3-month grace period before EMFAC2002 is required for new conformity analyses. The grace period begins today and ends on June 30, 2003. As discussed earlier in the notice, several prior versions of EMFAC (EMFAC7F, EMFAC7G) are no longer used in California for new regional emissions analyses for transportation plan and transportation improvement program (TIP) conformity determinations. Therefore it is appropriate to set a short grace period since all areas in California will need to use EMFAC2002 to begin any new regional conformity analyses. A longer grace period would provide no practical benefit for transportation plan and TIP conformity determinations, since older EMFAC models cannot be used in new regional analyses.

When the grace period ends on June 30, 2003, EMFAC2002 will become the only approved motor vehicle emissions model for new regional and hot-spot transportation conformity analyses across California. In general, this means that all new VOC, NOX, PM-10, and CO regional conformity analyses and CO hot-spot analyses started after the end of the 3-month grace period must be based on EMFAC2002, even if the SIP is based on an earlier version of the EMFAC model.

F. Can Areas Use Any Other Models During the Grace Period?

Yes, in limited cases. The only area in California with motor vehicle emission budgets developed with EMFAC2000 and approved to use EMFAC2000 is the Bay Area. During the grace period, the Bay Area can continue to use EMFAC2000 for regional analyses for ozone precursors or choose to use EMFAC2002 on a shorter time frame. DOT's May 2, 2002 memo did not preclude the continuing use of EMFAC2000 for use in new regional conformity determinations. In addition, the Bay Area could proceed with transportation plan and TIP conformity analyses for ozone precursors based on EMFAC2000 if the analysis was begun before or during the grace period. (40 CFR 93.111(c))

Conformity determinations for transportation projects can also be based on EMFAC7F if the analysis was begun before the end of the grace period, and if the final environmental document for the project is issued no more than three years after the issuance of the draft environmental document (see 40 CFR 93.111(c)). The interagency consultation process should be used if it is unclear whether an EMFAC7F or EMFAC7G based analysis was begun before the end of the grace period.

G. Related SIP Development Actions

The Bay Area 2001 ozone SIP included a commitment from CARB to revise the SIP with the latest technical information as part of its mid-course review in April 2004. This commitment was referenced in EPA's approval of EMFAC2000 (67 FR 1464, January 11, 2002), with the understanding that CARB would submit revisions to the EMFAC model (e.g., EMFAC2002) in early 2003, so that the Bay Area SIP revision would occur within one year of EPA's approval of EMFAC2002. This is Start Printed Page 15723consistent with EPA's past practice where an older version of the MOBILE model has been used prior to the release of a newer version that includes technical corrections in emission estimates. We understand that the Bay Area has begun work on the plan revision, that the EMFAC2002 model will be used in development of the motor vehicle emission inventories in the plan and that the SIP should be submitted to EPA by the April 2004 deadline.

In addition, on June 14, 2002, CARB submitted a letter indicating the State's intention to submit comprehensive revisions to the progress, attainment, and maintenance SIPs and the budgets for most areas in California. These SIP revisions, would reflect, among other new information, the State's revised motor vehicle emissions factors using EMFAC2002 and updated information on vehicle fleet, age distribution, and activity levels (letter from Michael P. Kenny, CARB, to Wayne Nastri, EPA). In this letter, CARB acknowledged that the previously approved budgets had become outdated and made clear its intention to update these budgets as part of a comprehensive update to the plans. CARB also requested that EPA limit the duration of its prior approvals of the emission budgets.

In response, on July 16, 2002 (67 FR 46618), EPA proposed to limit our approvals of the existing SIP budgets to last only until the effective date of our adequacy finding for new budgets that replace the existing approved budgets for the same pollutant, CAA requirement, and year. That proposal was finalized on November 15, 2002 (67 FR 69139), limiting our prior approvals of the emission budgets. Normally, new budgets that replace existing budgets in approved plans cannot be used until the corresponding plans have been fully approved as part of the SIP (see 40 CFR 93.118(e)). However, since approval of the existing budgets now expires when we determine that the new budgets are adequate, the updated budgets can be employed in transportation conformity determinations within a few months of their submission, rather than only when the SIP is finally approved, which could take as long as 18 months.

Consistent with the June 2002 letter, CARB has already begun submitting revised SIPs. Ozone maintenance plans, new and revised, have already been submitted for San Diego and Santa Barbara. A revised statewide CO maintenance plan and revised regional air quality plans for San Joaquin Valley, South Coast, Coachella Valley, and Ventura County are also expected within the next year. Motor vehicle emission budgets from all of these plans would go into effect upon determination of a positive adequacy finding, rather than after a full plan approval and be available for conformity analyses using EMFAC2002.

H. Summary of EPA Actions

EPA is approving EMFAC2002 as submitted by CARB on December 20, 2002 with the following limitations and conditions.

(1) The approval is limited to California.

(2) The approval is Statewide and applies to estimation of hydrocarbon, carbon monoxide, nitrogen oxides, and particulate matter emissions, lead, sulfur oxides and carbon dioxide. However, EMFAC2002 will only be used in transportation conformity for pollutants and precursors that affect transportation emissions and are identified in air quality plans as significant. EPA is also approving EMFAC2002 to estimate hotspot emissions for carbon monoxide.

(3) A 3-month statewide conformity grace period will be established beginning April 1, 2003 and ends June 30, 2003.

Start Signature

Dated: March 20, 2003.

Wayne Nastri,

Regional Administrator, EPA Region IX.

End Signature End Supplemental Information


1.  Vehicle population includes vehicle type (class) and age distribution.

Back to Citation

[FR Doc. 03-7815 Filed 3-31-03; 8:45 am]