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Official Release of EMFAC2007 Motor Vehicle Emission Factor Model for Use in the State of California

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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. EMFAC2007 is the latest update to the EMFAC model for use by California state and local governments to meet Clean Air Act (CAA) requirements. The new model, which is based on new and improved data, calculates air pollution emission factors for passenger cars, trucks, motorcycles, motor homes and buses. Today's notice also sets the date after which EMFAC2007 is required to be used statewide in all new regional emissions analyses and carbon monoxide (CO) hot-spot analyses for transportation conformity determinations in California. EPA also intends to approve EMFAC2007 in the future for particulate matter (PM10 and PM2.5) hot-spot analyses in California, once EPA issues quantitative hot-spot modeling guidance for such analyses. Since the EMFAC model is used only in California, EPA's approval of the model does not affect MOBILE model users in other states.


This determination is effective January 18, 2008. EMFAC2007 must be used for all new regional emissions Start Printed Page 3465analyses and CO hot-spot analyses that are started on or after April 18, 2008.

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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. For questions regarding the future development of quantitative PM10 and PM2.5 hot-spot modeling guidance, contact Meg Patulski,, (734) 214-4842, Transportation and Regional Programs Division, U.S. EPA's Office of Transportation and Air Quality, 2000 Traverwood Road, Ann Arbor, MI 48105.

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Copies of the official version of the EMFAC2007 model are available on the California Air Resources Board (CARB) Web site:​msei/​onroad/​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 on-road mobile sources (“motor vehicles”) for calendar years from 1970 to 2040 operating in California. Pollutant emissions for hydrocarbons (HC), CO, nitrogen oxides (NOX), PM10, PM2.5, lead, and sulfur oxides are output from the model. Emissions are calculated for thirteen different vehicles classes comprised of passenger cars, various types of trucks and buses, motorcycles, and motor homes. The EMFAC2007 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, air district, air basin, or 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 year, 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 SIP and transportation conformity requirements. Transportation conformity is required under CAA section 176(c) to ensure that federally supported transportation plans, transportation improvement programs (TIPs), and highway and transit projects are consistent with (“conform to”) the purpose of the SIP.

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

Most SIPs in California were developed using EMFAC2002 (released by CARB in October 2002) or EMFAC2000 (released by CARB in May 2000). EPA approved use of EMFAC2000 in January 2002 only for the San Francisco Bay Area (67 FR 1464). EPA approved EMFAC2002 in April 2003 (68 FR 15720) for all areas in California.

EMFAC2002 was considered a major update to previous versions of EMFAC and most SIPs in California were updated with EMFAC2002 in the 2003-2006 timeframe. EMFAC2002 included significant changes to its model interface, new data and methodologies regarding calculation of motor vehicle emissions, revision to implementation data for control measures, and corrections to technical errors mentioned in our prior approval of EMFAC2000. Areas with SIP motor vehicle emissions budgets developed using earlier versions of EMFAC were required to use EMFAC2002 for regional conformity analyses at the end of the grace period provided by 40 CFR 93.111(b).

C. 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, accurate, 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 transportation conformity analyses. In an April 18, 2007 letter, CARB requested that EPA approve EMFAC2007 for use in developing SIPs and in determining conformity in California. EMFAC2007 is a significant change from previous EMFAC models and is capable of calculating motor vehicle emissions for all California areas. EMFAC2007 is being approved as the latest emissions model for statewide use in SIP development. 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.

D. Why Have Transportation Agencies Stopped Using EMFAC2002 for Regional Conformity Emissions Analyses?

On February 1, 2007, the Federal Highway Administration (FHWA) sent a letter to all of the Metropolitan Planning Organizations in California and Caltrans Districts indicating that, after August 1, 2007, they would not be able to start any new transportation conformity determinations that require a new regional emissions analysis, unless the analysis incorporates more recent vehicle data consistent with the latest planning assumptions requirements in the conformity rule (40 CFR 93.110). The letter indicated that updated vehicle data was available in EMFAC2007, which was released to the public in November 2006. The letter indicated that FHWA, the Federal Transit Administration (FTA), and EPA were providing agencies with a 6-month transition period, beginning on February 1, 2007 before the new vehicle fleet data was required in conformity determinations. Consistent with the EPA/USDOT January 18, 2001, guidance on latest planning assumptions and EPA's July 1, 2004 final rule (69 FR 40052-40056), new vehicle registration data must be used when it is available at the start of new conformity analyses. Since newer vehicle data was available but was not included in the older version of EMFAC, EMFAC2002 has not been used in any new regional emissions analyses that were started since August 1, 2007.

II. EPA Action

A. What Version of EMFAC Is EPA Approving?

In this notice, EPA is approving and announcing that EMFAC2007 is available to use in statewide California SIP development and for regional emissions analyses and carbon monoxide (CO) hot-spot analyses for transportation conformity analyses. EMFAC2007 was developed by CARB and transmitted for approval to EPA on April 18, 2007.

On January 28, 2003, CARB also transmitted a methodology (“Recommended Methods for Use of EMFAC2002 to Develop Motor Vehicle Emission Budgets and Assess Conformity”) to adjust vehicle activity data used by EMFAC2007 when updated data is available. EPA previously approved this CARB Start Printed Page 3466methodology on April 1, 2003 (68 FR 15720, at 15721) for use with EMFAC2002, since the transportation conformity rule (40 CFR 93.110) requires areas to use the latest information for estimating vehicle activity. With EMFAC2007, CARB has not made any changes to the methodology to adjust vehicle activity data, and therefore the existing approved methodology will continue to be approved with no change. However, the methodology has been included in the updates to Chapter 8 of the EMFAC user's guide. Therefore, CARB's methodology should continue to be used to update vehicle activity data in EMFAC2007, as described in the EMFAC user's guide.

B. What Analyses Can EMFAC2007 Be Used For?

EPA is approving the model to estimate regional emissions of HC, CO, NOX, PM10, PM2.5, lead, and sulfur oxides. However, EMFAC2007 will only be used in transportation conformity for pollutants and precursors that affect transportation emissions, e.g., HC, NOX, CO, PM10 and PM2.5.

EPA is also approving EMFAC2007 to estimate CO emissions for hot-spot analyses involving individual transportation projects. A hot-spot analysis is defined in 40 CFR 93.101 as an estimation of likely future localized pollutant concentrations resulting from a new transportation project and a comparison of those concentrations to the relevant air quality standard. This analysis is conducted on a smaller scale, e.g., for a congested roadway intersection.

EPA also notes that today's approval action does not impact what methodology is required for calculating re-entrained road dust or ammonia emissions for regional PM10 and PM2.5 SIP and transportation conformity analyses. EMFAC2007's PM10 and PM2.5 estimates do not include such emissions. When applicable, PM10 and PM2.5 nonattainment and maintenance areas are required to use EPA's AP-42 road dust method for calculating road dust emissions, unless a local method is approved by EPA.[1] EMFAC2007 does not estimate ammonia emissions; air quality and transportation agencies should contact the regional office if ammonia emission estimates are needed.

C. Is EMFAC2007 Being Approved for PM 10 and PM 2.5 Hot-spot Analyses At This Time?

On March 10, 2006, EPA published a final rule that established the transportation conformity procedures for analyzing PM10 and PM2.5 hot-spot impacts of new projects involving significant levels of on-road diesel traffic (71 FR 12468). Quantitative PM10 and PM2.5 hot-spot analyses have not yet been required for the projects of local air quality concern described in 40 CFR 93.123(b)(1) due to the need to develop EPA modeling guidance and appropriate methods (71 FR at 12498-12502). Section 93.123(b)(4) of the conformity rule states that the requirements for quantitative PM10 and PM2.5 hot-spot analyses will not take effect until EPA releases modeling guidance and announces in the Federal Register that these requirements are in effect. At present, project sponsors are currently following EPA and FHWA's qualitative hot-spot modeling guidance for conformity determinations involving projects of local air quality concern.[2] EMFAC2007 is capable of assessing project-level emissions for PM10 and PM2.5 hot-spot analyses, but as described in the March 2006 final rule and 40 CFR 93.123(b)(4), EPA believes that quantitative PM hot-spot modeling guidance is also necessary before quantitative PM hot-spot analyses can be required. Therefore, since such guidance is currently not available, EPA is not approving EMFAC2007 for PM10 and PM2.5 hot-spot conformity analyses at this time.

Over the next year, EPA intends to develop quantitative PM10 and PM2.5 hot-spot modeling guidance, which will contain information on how to use EMFAC2007 and how to apply the appropriate dispersion models for such analyses in California. EPA will announce the availability of the guidance and approval of EMFAC2007 for quantitative PM hot-spot analyses when the guidance is completed for California. EPA will consult with FHWA, FTA, CARB, the California state department of transportation (Caltrans), other stakeholders, and the public in the development of this guidance, as described in the March 2006 final rule. Please see the FOR FURTHER INFORMATION CONTACT section if you have questions regarding the development of conformity guidance for quantitative PM10 and PM2.5 hot-spot analyses in California.

D. Why Does EPA Consider EMFAC2007 as a Major Update to EMFAC?

EMFAC2007 includes new data and methodologies regarding calculation of motor vehicle emissions and revisions to implementation data for control measures. EMFAC2007 includes updated data supporting new emission factors and speed correction factors for estimating emissions from heavy-heavy duty diesel trucks. The model includes modifications to the algorithms for inspection and maintenance as well as corrections for heavy-duty truck gas cap benefits from the inspection and maintenance program. Impacts of ethanol permeation and updates to fuel correction factors are included as well as revisions to particulate brake wear emissions. EMFAC2007 incorporates new temperature and humidity profiles. In addition to these changes, which impact emission factors for each area in California, EMFAC incorporates new mileage accrual rates and speed distributions, a redistribution of heavy-duty diesel truck vehicle miles traveled (VMT) and updated VMT for all vehicle classes. CARB's web site describes these and other model changes at:​msei/​workshops.htm#work20061115.

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

Since 2005, CARB has held a series of public workshops to discuss proposed model updates and receive comments on interim versions of the new model. Three workshops were held to solicit input from stakeholders and the public in March of 2005. Then, in workshops held on September 11 and November 15 and 16, 2006, CARB described the latest EMFAC changes under consideration and sought public input. Those changes are reflected in the final EMFAC2007 model submitted to EPA in April 2007. One additional public information briefing was held on December 15, 2006, to share the emissions estimates resulting from use of the final model with updated travel activity, as well as plans for transmittal of EMFAC2007 to U.S. EPA. CARB has also discussed both the model and the impact of the changes to emission totals for the state at the workshops.

CARB also released a series of technical memos that describe each update to the model, and public presentations that summarize the changes from earlier versions of the model. The technical memos are available on CARB's Web site at:​msei/​supportdocs.htm. Each memo describes the model update, Start Printed Page 3467the reason for the change, how the change was incorporated into the EMFAC model, and the resulting emissions impact. All presentations from the public workshops are available on the CARB Web site at:​msei/​workshops.htm#work200612. These presentations summarize the major changes to the EMFAC model and contain tables showing the impacts of the changes both statewide and by county for HC, CO, NOX, PM10, and PM2.5.

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

Yes. The transportation conformity rule (40 CFR part 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 EPA approves a new emissions model such as EMFAC2007, 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 FHWA and FTA, 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 EMFAC2007 is required for the following conformity analyses:

  • All new HC, NOX, PM10, PM2.5 and CO regional emissions analyses (e.g., supporting transportation plan and TIP conformity determinations); and
  • All new CO hot-spot analyses supporting project-level conformity determinations.

The grace period begins today and ends on April 18, 2008. As discussed earlier in the notice, the last version of EMFAC (EMFAC2002) is no longer used in California for new regional emissions analyses for transportation plan and TIP conformity determinations. Therefore it is appropriate to set a short grace period since all areas in California will need to use EMFAC2007 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 due to the latest planning assumptions requirements in the conformity rule (40 CFR 93.110).

When the grace period ends on April 18, 2008, EMFAC2007 will become the only approved motor vehicle emissions model for all new regional and CO hot-spot transportation conformity analyses across California. In general, this means that all new HC, NOX, PM10, PM2.5, and CO regional conformity analyses and CO hot-spot analyses started after the end of the 3-month grace period must be based on EMFAC2007, even if the SIP is based on an earlier version of the EMFAC model.

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

Yes, in limited cases. CO hot-spot analyses for project-level conformity determinations can be based on EMFAC2002 if the analysis was begun before the end of the grace period, started before August 1, 2007 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 EMFAC2002 based analysis is covered by the circumstances described above.

H. Future Updates to EMFAC

On January 31, 2006, CARB submitted a letter to EPA and to the California Division of the FHWA indicating the State's intention to update future revisions to EMFAC. These EMFAC updates would reflect, among other new information, updated vehicle fleet data every three years. In California, Metropolitan Planning Organizations and Air Districts have not been able to update vehicle fleet data embedded into EMFAC. The EPA/USDOT January 18, 2001, guidance on latest planning assumptions and EPA's July 2004 final rule, indicate that new vehicle registration data must be used when it is available prior to the start of new conformity analyses and that states should update the data at least every five years. The State reaffirmed their commitment to keeping the latest planning assumptions included in EMFAC updated on a three year cycle in the April 18, 2007 EMFAC submittal letter. The next update to the planning assumptions in EMFAC is expected in 2010, which would most likely also include updates to the emissions factors of the model as well.

III. Summary of EPA Actions

EPA is approving EMFAC2007 as submitted by CARB on April 18, 2007 with the following limitations and conditions:

(1) The approval is limited to California.

(2) The approval is Statewide and applies to estimation of emissions of hydrocarbon, carbon monoxide, nitrogen oxides, particulate matter (PM10 and PM2.5), lead, and sulfur oxides. However, EMFAC2007 will only be used in transportation conformity analyses for pollutants and precursors that affect regional on-road mobile emissions and are applicable in a given nonattainment or maintenance area. EPA is also approving EMFAC2007 to estimate hot-spot emissions for carbon monoxide conformity analyses.

(3) A 3-month statewide transportation conformity grace period will be established beginning January 18, 2008 and ending April 18, 2008 for the transportation conformity uses described in (2) above.

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Dated: January 10, 2008.

Jane Diamond,

Acting Regional Administrator, Region IX.

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1.  For further information, see EPA's August 2, 2007 memorandum entitled, “Policy Guidance on the Use of the November 1, 2006, Update to AP-42 for Re-entrained Road Dust for SIP Development and Transportation Conformity.”

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2.  “Transportation Conformity Guidance for Qualitative Hot-Spot Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas” jointly published by EPA and FHWA in March 2006.

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[FR Doc. E8-876 Filed 1-17-08; 8:45 am]