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Official Release of the MOBILE6 Motor Vehicle Emissions Factor Model

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Environmental Protection Agency (EPA).


Notice of availability.


EPA is approving and announcing the availability of the MOBILE6 motor vehicle emissions factor model for official use outside of California. MOBILE6 is the latest update to the MOBILE model for use by state and local governments to meet Clean Air Act requirements. Today's notice also starts time periods before MOBILE6 is required to be used in certain state implementation plan (SIP) revisions and all new transportation conformity analyses.

MOBILE6 is a major revision of the MOBILE model which calculates air pollution emission factors from passenger cars and trucks. The new model is based on new and improved data and a new understanding of vehicle emission processes. MOBILE6 is also a more user-friendly version of the model which allows users to better tailor their motor vehicle emissions estimates to local conditions.

EPA strongly encourages areas to use the interagency consultation process to examine how MOBILE6 will affect future transportation conformity determinations, so, if necessary, SIPs and motor vehicle emissions budgets can be revised with MOBILE6 or transportation plans and programs can be revised as appropriate prior to the end of the MOBILE6 conformity grace period.


EPA's approval of the MOBILE6 emissions factor model is effective January 29, 2002. See below for further information regarding how today's approval starts time periods after which MOBILE6 is required in new transportation conformity analyses and certain SIP and motor vehicle emissions budget revisions.

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If you have questions on this notice, please send an e-mail to EPA at or contact EPA at (734) 214-4636 for technical model questions. Start Printed Page 4255

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Availability of MOBILE6 and Support Materials

Copies of the official version of the MOBILE6 model are available on EPA's MOBILE Web site,​otaq/​m6.htm. The MOBILE Web site also contains the following support materials for implementing the new model: a detailed MOBILE6 User's Guide; MOBILE6 training materials; EPA's “Policy Guidance on the Use of MOBILE6 for SIP Development and Transportation Conformity'; EPA's “Technical Guidance on the Use of MOBILE6 for Emission Inventory Preparation'; and a list of Frequently Asked Questions about MOBILE6. EPA will continue to update this website in the future as other MOBILE6 support materials are developed.

Individuals who wish to receive EPA announcements related to the MOBILE model should subscribe to the EPA-MOBILENEWS e-mail listserver. To subscribe to the EPA-MOBILENEWS listserver, write the following in the body of the e-mail message: subscribe EPA-MOBILENEWS FIRSTNAME LASTNAME where FIRSTNAME and LASTNAME is your name (for example: John Smith) and send the e-mail to the EPA Listserver at

Your e-mail address will then be added to the list of subscribers and a confirmation message will be sent to your e-mail address. Whenever a message is posted to the EPA-MOBILENEWS listserver by the listserver owner (the Assessment and Standards Division of the EPA Office of Transportation and Air Quality), a copy of that message will be sent to every person who has subscribed.

You can remove yourself from the list by sending another message to the listserver address. This message must be sent from the same e-mail address that you used to subscribe, and should contain the message: unsubscribe EPA-MOBILENEWS

Availability of Related SIP Policies

In November 1999, EPA issued two memoranda articulating the policy for use of interim MOBILE5-based Tier 2 estimates and subsequent MOBILE6 SIP revisions. These memoranda are discussed in question 3 of EPA's “Policy Guidance on the Use of MOBILE6 for SIP Development and Transportation Conformity.” Copies of the memoranda are available at EPA's transportation conformity Web site,​otaq/​transp/​traqconf.htm.

I. What Is MOBILE6?

MOBILE is an EPA emissions factor model for estimating pollution from on-road motor vehicles in states outside of California. MOBILE calculates emissions of volatile organic compounds (VOCs), nitrogen oxides (NOX) and carbon monoxide (CO) from passenger cars, motorcycles, buses, and light-duty and heavy-duty trucks. The model accounts for the emission impacts of factors such as changes in vehicle emission standards, changes in vehicle populations and activity, and variation in local conditions such as temperature, humidity, fuel quality, and air quality programs.

MOBILE is used to calculate current and future inventories of motor vehicle emissions at the national and local level. These inventories are used to make decisions about air pollution policies and programs at the local, state and national level. Inventories based on MOBILE are also used to meet the federal Clean Air Act's state implementation plan (SIP) and transportation conformity requirements.

MOBILE6 is the first major update of the MOBILE model since 1993. The MOBILE model was first developed in 1978. It has been updated many times to reflect changes in the vehicle fleet and fuels, to incorporate EPA's growing understanding of vehicle emissions, and to cover new emissions regulations and modeling needs. Although some minor updates were made in 1996 with the release of MOBILE5b, MOBILE6 is the first major revision to MOBILE since MOBILE5a was released in 1993.

EPA produced 48 technical reports explaining the data and analysis behind the MOBILE6 estimates and the methods in the model. State and local governments, industry, academia, and the general public were previously offered an opportunity to comment on MOBILE6 technical reports, which are currently posted on EPA's MOBILE6 Web site​otaq/​m6.htm.

MOBILE6 provides many more options for users to incorporate local inputs than were possible in MOBILE5a or MOBILE5b. These new options are provided for implementers to use if desired, and MOBILE6 defaults are appropriate when local information is not available for MOBILE6 purposes. Users now have the option to adapt MOBILE to local conditions and model special situations that are not reflected in the model's defaults. MOBILE6 also has an updated structure that allows users to create result files with emissions by hour of the day, and to segregate start and running emissions. The new output uses standard database formats to allow users to easily post-process their results. These features will be useful for entering the emissions data into air quality models and other tools that make use of motor vehicle emission inventories. For further information regarding operating MOBILE6, please refer to the MOBILE6 User's Guide and EPA's “Technical Guidance on the Use of MOBILE6 for Emission Inventory Preparation.” Please see Availability of MOBILE6 and Support Materials for how to obtain these documents.

II. SIP Policy for MOBILE6

EPA has articulated its policy regarding the use of MOBILE6 in SIP development in its “Policy Guidance on the Use of MOBILE6 for SIP Development and Transportation Conformity.” Today's notice highlights certain aspects of the guidance, but state and local governments should refer to the guidance for more detailed information on how and when to use MOBILE6 in attainment and maintenance SIPs, inventory updates, and other SIP submission requirements. See Availability of Related SIP Policies to obtain the MOBILE6 policy guidance.

Although MOBILE6 should be used in SIP development as expeditiously as possible, EPA also recognizes the time and level of effort that States have already undertaken in SIP development with MOBILE5. States that have already submitted SIPs or will submit SIPs shortly after EPA's approval of MOBILE6 are not required to revise these SIPs simply because a new motor vehicle emissions model is now available. States can choose to use MOBILE6 in these SIPs, for example, if it is determined that future conformity determinations would be ensured through such a SIP revision. However, EPA does not believe that a State's use of MOBILE5 should be an obstacle to EPA approval for SIPs that have been or will soon be submitted, assuming that such SIPs are otherwise approvable and significant SIP work has already occurred (e.g., attainment modeling for an attainment SIP has already been completed with MOBILE5). It would be unreasonable to require States to revise these SIPs with MOBILE6 since significant work has already occurred, and EPA intends to act on these SIPs in a timely manner.

States should use MOBILE6 where SIP development is in its initial stages or hasn't progressed far enough along that switching to MOBILE6 would create a significantly adverse impact on State resources. For example, SIPs that will be submitted later in 2002 should be based on MOBILE6 since there is Start Printed Page 4256adequate time to incorporate the new model's results. MOBILE6 should be incorporated into these SIPs since MOBILE6's emissions estimates are based on the best information currently available, as required by Clean Air Act section 172(c)(3) and 40 CFR 51.112(a)(1).

In addition, SIPs that EPA has already approved are not required to be revised in most areas now that EPA has approved MOBILE6. As discussed below, there are exceptions for certain nonattainment and maintenance areas that have included interim MOBILE5-based estimates of the federal Tier 2 vehicle and fuel standards (65 FR 6698).

In November of 1999, EPA issued two memoranda [1] to articulate its policy regarding States that incorporated MOBILE5-based interim Tier 2 standard benefits into their SIPs and motor vehicle emissions budgets (“budgets”). Although these memoranda primarily targeted certain serious and severe ozone nonattainment areas, EPA has implemented this policy in all other areas that have made use of federal Tier 2 benefits in air quality plans from EPA's April 2000 MOBILE5 guidance, “MOBILE5 Information Sheet #8: Tier 2 Benefits Using MOBILE5.”

All States whose attainment demonstrations or maintenance plans include interim estimates of the Tier 2 standards have committed to revise and resubmit their budgets within either 1 or 2 years of the final release of MOBILE6 in order to gain SIP approval. States that committed to revise their budgets within 2 years after MOBILE6 is released also committed that conformity will not be determined during the second year unless there are adequate SIP budgets in place that were developed using MOBILE6. The effective date of today's Federal Register notice will constitute the start of the 1 or 2-year time periods for these SIP revisions. SIP revisions are due by January 29, 2003, for States that committed to revise budgets within one year of MOBILE6's release. SIP revisions are due by January 29, 2004, for States that committed to revise budgets within two years of MOBILE6's release.

III. Transportation Conformity Policy for MOBILE6

Transportation conformity is a Clean Air Act requirement to ensure that federally supported highway and transit activities are consistent with (“conform to”) the SIP. Conformity to a SIP means that a transportation activity will not cause or contribute to new air pollution violations; worsen existing violations; or delay timely attainment of federal air quality standards.

The transportation conformity rule (40 CFR part 93) requires that conformity analyses be based on the latest motor vehicle emissions model approved by EPA. Section 176(c)(1) of the Clean Air Act 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 MOBILE6, a grace period is established before the model is required for conformity analyses. The conformity rule provides for a grace period for new emissions models of between 3-24 months.

EPA articulated its intentions for establishing the length of a conformity grace period in the preamble to the 1993 transportation conformity rule (58 FR 62211):

EPA and [the Department of Transportation (DOT)] will consider extending the grace period if the effects of the new emissions model are so significant that previous SIP demonstrations of what emission levels are consistent with attainment would be substantially affected. In such cases, States should have an opportunity to revise their SIPs before MPOs must use the model's new emissions factors.

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 2-year grace period, which begins today and ends on January 29, 2004, before MOBILE6 is required for new conformity analyses in most cases. During this grace period, areas should use the interagency consultation process to examine how MOBILE6 will impact their future conformity determinations.

However, the grace period will be shorter than 2 years for a given pollutant if an area revises its SIP and budgets with MOBILE6 and such budgets become applicable for conformity purposes prior to the end of the 2-year grace period. For example, if an area revises a previously submitted (but not approved) MOBILE5-based ozone SIP with MOBILE6 and EPA finds the revised MOBILE6 budgets adequate for conformity, such budgets would apply for conformity on the effective date of the Federal Register notice announcing EPA's adequacy finding. In this example, if an area was in nonattainment for ozone and CO, the MOBILE6 grace period would end for ozone once EPA found the new MOBILE6-based ozone SIP budgets adequate, but MOBILE5 could continue to be used for CO conformity determinations until the end of the general MOBILE6 grace period.

During the grace period, areas can use an approved version of MOBILE5 [2] for conformity determinations or choose to use MOBILE6 on a faster time frame. When the grace period ends on January 29, 2004, MOBILE6 will become the only approved motor vehicle emissions model for new transportation conformity analyses outside of California. In general, this means that all new VOC, NOX, and CO conformity analyses started after the end of the 2-year grace period must be based on MOBILE6, even if the SIP is based on an earlier version of the MOBILE model. As discussed above, the grace period for new conformity analyses would be shorter for a given pollutant if an area revised its SIP and budgets with MOBILE6 for such pollutant and such budgets became applicable for conformity purposes prior to the end of the 2-year grace period. EPA strongly encourages areas to use the consultation process to examine how MOBILE6 will affect future conformity determinations, so, if necessary, SIPs and budgets can be revised with MOBILE6 or transportation plans and programs can be revised as appropriate prior to the end of the grace period.

For consistency purposes, EPA encourages areas that have incorporated interim MOBILE5-based Tier 2 estimates into their SIPs to continue to use MOBILE5 (instead of MOBILE6) for conformity analyses until new MOBILE6 budgets are submitted and found adequate (unless the grace period ends before this occurs). These areas have committed to submit SIP revisions within 1-2 years of MOBILE6's release, therefore conformity budgets based on MOBILE6 should be in place by the end of the grace period.

Finally, the conformity rule provides some flexibility for analyses that are started before or during the grace period. Regional conformity analyses Start Printed Page 4257that began before the end of the grace period may continue to rely on an approved version of MOBILE5. Conformity determinations for transportation projects may also be based on an approved version of MOBILE5 if the regional 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 a MOBILE5-based analysis was begun before the end of the grace period.

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Dated: January 23, 2002.

Margo Tsirigotis Oge,

Director, Office of Transportation and Air Quality, United States Environmental Protection Agency.

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1.  November 3, 1999 EPA memorandum entitled, “Guidance on Motor Vehicle Emissions Budgets in One-Hour Ozone Attainment Demonstrations,” and November 8, 1999 EPA memorandum entitled, “1-Hour Ozone Attainment Demonstrations and Tier 2/Sulfur Rulemaking.” Please see Availability of Related SIP Policies for how to obtain these memoranda.

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2.  Please refer to EPA's Office of Transportation and Air Quality's August 11, 1997 memorandum entitled, “Summary of Comments on and Guidance for Use of MOBILE5b,” which describes our policy on when MOBILE5a or 5b can be used in conformity determinations.

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[FR Doc. 02-2125 Filed 1-28-02; 8:45 am]