1

See Clean Air Act section 172(c)(3) and 40 CFR 51.112(a)(1). 

2

The final rule on Tier 2 Motor Vehicle Emissions Standards and Gasoline Sulfur Control

Requirements (“Tier 2 standards”) for passenger cars, light trucks, and larger passenger vehicles

was published on February 10, 2000 (65 FR 6698). 

2

1. How will MOBILE6's release affect state implementation plans (SIPs) that have already

been submitted and/or approved or SIPs that are currently under development?

In general, EPA believes that MOBILE6 should be used in SIP development as

expeditiously as possible. The Clean Air Act requires that SIP inventories and control measures

be based on the most current information and applicable models that are available when a SIP is

developed.1 However, it is also important to recognize the time and level of effort that States

have already undertaken in SIP development with MOBILE5. The following paragraphs

articulate EPA’s policy for the use of MOBILE6 in the development of SIPs.

The release of MOBILE6 in most areas would not require a SIP revision based on the

new model. There are exceptions for certain nonattainment and maintenance areas that have

included interim MOBILE5-based estimates for the Tier 2 standards.2 See question 3 for more

information on the use of MOBILE6 in SIPs in these areas.

EPA believes that the Clean Air Act would not require states that have already submitted

SIPs or will submit SIPs shortly after MOBILE6's release to revise these SIPs simply because a

new motor vehicle emissions model is now available. EPA believes that this is supported by

existing EPA policies and case law [Delaney v. EPA, 898 F.2d 687 (9th Cir. 1990)]. Of course,

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 the State’s use of MOBILE5 should be an obstacle to EPA approval for

reasonable further progress, attainment, or maintenance SIPs that have been or will soon be

submitted based on MOBILE5, 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 the 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 has not

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 adequate time to incorporate the new model. MOBILE6

should be incorporated into these SIPs since MOBILE6's emissions estimates are based on the

best information currently available. EPA also believes that the legal basis for approving a

MOBILE5-based SIP is less clear the longer that MOBILE6 is in place and available for use. 

Since SIPs must be based on applicable models and data inputs, it could be difficult for EPA to
3

approve a SIP developed with MOBILE5 significantly after MOBILE6 becomes available. If you

have questions about which model should be used in your SIP, please consult with your EPA

Regional Office. 

Incorporating MOBILE6 into the SIP now could also assist areas in mitigating possible

transportation conformity difficulties in the future after the MOBILE6 conformity grace period

ends. New conformity analyses started after the grace period is over must be based on MOBILE6

(40 CFR 93.111), so having MOBILE6-based SIP motor vehicle emissions budgets in place at

that time could help ensure positive transportation conformity determinations. See question 2 for

more information on MOBILE6 and conformity. 

2. When will MOBILE6 be required for transportation conformity determinations?

Background: 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

violations; worsen existing violations; or delay timely attainment. 

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 like

MOBILE6, we establish a grace period before the model is required to be used for conformity

analyses. The conformity rule provides for a grace period for new emissions models of between

3-24 months, to be established by notification in the Federal Register. 

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 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).

Duration and starting point of conformity grace period: Upon consideration of all of

these factors, EPA and DOT have decided to establish a 2-year grace period before MOBILE6 is
3

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. 

4

required for new conformity determinations in most cases. During this grace period, areas should

use the interagency consultation process to examine how MOBILE6 will impact their future

conformity determinations. Areas should carefully consider whether the SIP and motor vehicle

emissions budget(s) should be revised with MOBILE6 before the end of the conformity grace

period, since doing so may be necessary to ensure conformity in the future. EPA intends to

publish a notice of availability in the Federal Register to announce the release of the final version

of MOBILE6 in the near future. The effective date of this Federal Register notice will constitute

the start of the conformity grace period. 

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 SIP budgets adequate. However,

MOBILE5 could continue to be used for CO conformity determinations until the end of the

MOBILE6 grace period. 

In addition, if an area revises a previously approved SIP, the revised MOBILE6 budgets

would be used for conformity purposes once EPA approves the MOBILE6 SIP revision in most

cases. In general, submitted SIPs cannot supersede approved budgets until they are approved. 

However, see question 3 for more information about when revised MOBILE6 budgets will apply

for conformity purposes if interim MOBILE5-based Tier 2 estimates were included in an

approved SIP. The Federal Register notice announcing the grace period will state that the grace

period will be 2 years unless new budgets become applicable sooner, in which case the grace

period will end once the applicability of new MOBILE6-based budgets becomes effective.

Implementation of grace period: During the grace period, areas can use an approved

version of MOBILE53 for conformity determinations or choose to use MOBILE6 on a faster time

frame. When the grace period ends, MOBILE6 will become the only approved motor vehicle

emissions model for transportation conformity purposes in states outside California. In general,

this means that all new conformity analyses started after the end of the 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 and such budgets became

applicable for conformity purposes prior to the end of the generally applicable 2-year grace

period. EPA strongly encourages areas to use the consultation process to examine how
4

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.” These memoranda are available at EPA’s conformity website 

(http://www.epa.gov/otaq/, click on “conformity” button). 

5

MOBILE6 will affect future conformity determinations so, if necessary, SIPs and budgets can be

revised with MOBILE6 or transportation plans and programs can be modified prior to the end of

the grace period. 

To avoid any inconsistencies in analyses resulting from use of estimated credit, 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, so we know that motor vehicle emissions budgets based on MOBILE6 should be in place

within that time frame, at a minimum. 

Finally, the conformity rule provides some flexibility for analyses that are started before

the end of the grace period. Regional conformity analyses that begin before or during 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 analysis

was begun before or during 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. 

(40 CFR 93.111(c)). The interagency consultation process should be used if it is unclear if a

MOBILE5-based analysis was begun before the end of the grace period. 

3. How will MOBILE6's release affect nonattainment and maintenance areas that have

included MOBILE5-based Tier 2 estimates in their SIPs?

All States whose attainment demonstrations or maintenance plans include interim

MOBILE5-based estimates of the Tier 2 standards were required to commit to revise and

resubmit their motor vehicle emissions budgets within 1-2 years of the final release of MOBILE6

in order to gain SIP approval. EPA will soon publish a notice of availability in the Federal

Register to officially release the final version of MOBILE6. The effective date of this Federal

Register notice will constitute the start of the 1 or 2-year time periods for these SIP revisions. In

November of 1999, EPA issued two memoranda4

 to articulate our policy regarding States that

incorporated Tier 2 benefits into their SIPs and 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 Tier 2 benefits from EPA’s April 2000 MOBILE5

guidance, “MOBILE5 Information Sheet #8: Tier 2 Benefits Using MOBILE5.” 
5

The transportation conformity rule establishes criteria for EPA to use in determining if

submitted motor vehicle emissions budgets are adequate for conformity purposes prior to EPA’s

approval action. For more information on adequacy findings, see 40 CFR 93.118(e) or EPA’s

May 14, 1999 guidance entitled, “Conformity Guidance on Implementation of March 2, 1999

Conformity Court Decision.” 

6

This concept was initially discussed in a letter dated March 6, 2000 from John S. Seitz,

Director, Office of Air Quality Planning and Standards, to Ralph Marquez, Commissioner, Texas

Natural Resources Conservation Commission. EPA also proposed this additional option in its

July 28, 2000 supplemental notice of proposed rulemaking for certain ozone areas (65 FR

46383). 

6

EPA offered two options for revising SIPs and budgets that relied on MOBILE5 Tier 2

estimates. States could commit to revise their budgets within 1 year after MOBILE6. 

Alternatively, States could commit to revise their budgets within 2 years after MOBILE6 is

released, if the State also commits that conformity will not be determined during the second year

unless there are adequate5

 SIP budgets in place that were developed using MOBILE6.6 EPA

proposed this second option to allow States to adjust their air quality planning schedules as

appropriate. We believe that allowing areas an additional year to revise their budgets using

MOBILE6 will not result in environmental harm as long as during that time there are no new

conformity determinations that rely on the older MOBILE5 budgets. States selecting this option

also agreed to inform affected metropolitan planning organizations and their State transportation

departments of this requirement. 

Any SIPs that rely on interim Tier 2 estimates must be accompanied by one of these two

types of commitments in order for EPA to find the budgets adequate for conformity purposes,

and in order for EPA to approve the SIP. These commitments must be subject to a public

hearing and fully enforceable as part of the SIP. 

EPA has always stated that the benefits of the Tier 2 program cannot be accurately

estimated until MOBILE6 is released. The MOBILE5 Tier 2 estimates were interim

approximations based on national defaults rather than local information, and were not completely

compatible with a MOBILE5 baseline. MOBILE6 emissions estimates for an area may be

substantially different from those based on the interim MOBILE5 Tier 2 estimates. 

States completing mid-course reviews: SIP revisions that revise interim MOBILE5 Tier 2

estimates with MOBILE6 are not intended to duplicate any technical analyses required for midcourse reviews in those areas. The MOBILE6 SIP and budget revisions are primarily intended to

revise the motor vehicle emissions inventories with the new model. Although the overall SIP

must continue to demonstrate attainment or maintenance with these revised MOBILE6

inventories as described in question 5 of this guidance, EPA believes that new attainment

modeling or additional control measures to ensure attainment may be delayed until the midcourse reviews. EPA will work with these States on a case-by-case basis to decide what
7

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.” 

7

additional documentation is necessary to show that the MOBILE6 SIP revision demonstrates

attainment.

If the State cannot demonstrate that the SIP shows attainment with the revised MOBILE6

inventories as described in question 5 of this guidance, the State can submit an enforceable

commitment to do one of the following in its mid-course review: 1) submit additional measures

needed to fill any emission reduction shortfall (if a shortfall is confirmed in the mid-course

review); or 2) document that the mid-course review reflects that there is no emission reduction

shortfall. Such a commitment, if needed, would be submitted as part of the MOBILE6 SIP

revision, and this commitment is necessary for EPA to find the revised MOBILE6-based motor

vehicle emission budgets adequate for conformity purposes.

When do budgets apply? If EPA approves SIPs and budgets that include interim

MOBILE5 Tier 2 estimates, these budgets will apply for transportation conformity purposes only

until there are revised, adequate budgets based on MOBILE6 in place. The revised MOBILE6

budgets will apply for conformity purposes as soon as they are submitted and we find them

adequate. As EPA approves these SIPs with interim MOBILE5 Tier 2 estimates, we are limiting

the duration of these approvals because we are approving the SIPs and their budgets based solely

on the fact that the States have committed to revise them. Therefore, once we have confirmed

that the revised MOBILE6 budgets are adequate, they must be used instead of the approved

MOBILE5 budgets for conformity purposes, pursuant to EPA’s conformity rules and the

limitations imposed by these SIP approval actions. In addition, if a state subsequently revises a

MOBILE6 budget to incorporate new planning assumptions, for example, the revised MOBILE6

budget would supersede the original MOBILE6 budget once EPA finds it adequate for

conformity purposes (assuming the original MOBILE6 budget has not yet been approved). 

4. Why must some areas taking credit for Tier 2 standards revise their SIPs within 1-2 years

of MOBILE6's release while other areas can take the full 2-year conformity grace period?

EPA believes it is critical that SIPs and motor vehicle emissions budgets that relied on

MOBILE5-based Tier 2 estimates be recalculated as expeditiously as possible to ensure that

public health is protected and that Clean Air Act transportation conformity goals are achieved. 

As discussed above, the benefits of the Tier 2 vehicle and fuel standards cannot be accurately

estimated until MOBILE6 is released. The MOBILE5-based Tier 2 estimates are interim

approximations that were based on national defaults rather than local information.7

 MOBILE6

emissions estimates for an area may be substantially different from those based on the MOBILE5

Tier 2 benefits. Recalculating motor vehicle emissions with MOBILE6 will ensure that
8

attainment or maintenance continues to be demonstrated by the SIP. Therefore, EPA will only

approve SIPs based on interim MOBILE5 Tier 2 estimates if States commit to revise SIPs and

budgets within 1-2 years of MOBILE6's release. Affected States have included such

commitments in the SIP, and will be able to start the SIP development process promptly once the

official release of MOBILE6 is announced in the Federal Register. 

In contrast, States that did not rely on interim Tier 2 estimates in their SIPs are not

required to revise their SIPs with MOBILE6. However these states may choose to revise their

SIPs and budgets with MOBILE6 to assist in passing conformity in the future. Such revisions are

not required because they do not need to account for the limitations of the interim MOBILE5

Tier 2 estimates. 

States that incorporated MOBILE5-based Tier 2 estimates are committing to revise their

SIPs, and consequently, will be able to start the SIP development process immediately once

MOBILE6 is released. However, States that have not made such commitments will require

additional time to decide if a MOBILE6 SIP revision is necessary to ensure future conformity

determinations. EPA considered this additional time when we decided to establish a 2-year grace

period before MOBILE6 is required in new conformity determinations for most areas that have

not committed to revise their SIPs in the short-term. 

5. When existing attainment and maintenance SIPs and motor vehicle emissions budgets are

revised with MOBILE6, what do States need to submit to show that the SIP’s purpose

continues to be demonstrated? 

General policy: EPA will rely on its existing SIP policy and past experience in answering

this question. Whenever motor vehicle emissions inventories and budgets in attainment or

maintenance SIPs are revised, it is important to ensure that the SIP continues to demonstrate its

Clean Air Act purpose (e.g., attainment, maintenance). For example, if a State revises a

maintenance plan to add or delete control measures, the State needs to show in its revised SIP

that maintenance continues to be demonstrated with the new mix of control measures. EPA has

always required under the Clean Air Act that revisions to existing SIPs and budgets continue to

demonstrate the purpose of the SIP. Similarly, States that revise existing SIPs with MOBILE6

must show that the SIP continues to support attainment or maintenance with the new level of

motor vehicle emissions calculated by the new model. 

The transportation conformity rule (40 CFR 93.118(e)(4)(iv)) requires that “the motor

vehicle emissions budget(s), when considered together with all other emissions sources, is

consistent with applicable requirements for reasonable further progress, attainment, or

maintenance (whichever is relevant to the given implementation plan submission).” This

criterion must be satisfied before EPA can find submitted budgets adequate for use in the

conformity process. The following paragraphs articulate EPA’s policy for existing SIPs that are

revised with MOBILE6, including ideas for how to streamline these revisions whenever possible. 
9

This policy will apply to all SIP revisions completed with MOBILE6, including revisions to SIPs

and budgets that relied on interim MOBILE5-based Tier 2 estimates.

Use of latest planning assumptions: If SIPs are revised with MOBILE6, base year and

attainment/maintenance year motor vehicle emission inventories will need to be recalculated with

the latest available planning assumptions. As required by Clean Air Act §172(c)(3) and EPA’s

regulation at 40 CFR 51.112(a), states must use the latest planning assumptions available at the

time that the SIP is developed, including but not limited to the latest information for vehicle

miles traveled (VMT), speeds, fleet mix, and SIP control measures. Base year and historical year

inventories should use the best data available for those years. Future year projection inventories

must be based on the latest data available. If planning assumptions have not changed since the

original SIP was submitted, the State should document this in its new SIP submission. 

In addition, States must consider whether growth and control strategy assumptions for

non-motor vehicle sources (i.e., point, area and non-road mobile sources) are still accurate at the

time that the MOBILE6 SIP revision is developed. Such assumptions include population and

economic assumptions and any allowable emissions relied upon for stationary sources. If these

assumptions have not changed, the State can simply re-submit the original SIP with the revised

motor vehicle emission inventories and budgets. Otherwise, the emissions categories in the SIP

that have changed must be brought up to date. 

Attainment or maintenance demonstration: As discussed above, SIP revisions based on

MOBILE6 must continue to demonstrate that the SIP still demonstrates its purpose (e.g.,

attainment or maintenance) when the MOBILE5-based motor vehicle emission inventories are

replaced with MOBILE6 inventories. The level of effort needed for this demonstration can vary

depending upon how MOBILE6 affects the level of motor vehicle emissions and whether nonmotor vehicle inventories require updating. The method used in the original demonstration could

also be a factor. 

 Areas can revise their motor vehicle emissions inventories and budgets using MOBILE6

without revising the entire SIP or completing additional modeling if: 1) the SIP continues to

demonstrate attainment or maintenance when the MOBILE5-based motor vehicle emission

inventories are replaced with MOBILE6 base year and attainment/maintenance year inventories;

and, 2) the State can document that the growth and control strategy assumptions for non-motor

vehicle sources (i.e., point, area and non-road mobile sources) continue to be valid and any minor

updates do not change the overall conclusions of the SIP. For example, consistent with EPA’s

SIP modeling guidance for various pollutants, if an ozone SIP relied on changes in emissions

from the base year to an attainment or maintenance year inventory to estimate relative changes in

monitored ozone levels, the first criterion could be satisfied by demonstrating that the relative

emission reductions between the base year and the attainment or maintenance year are the same

or greater using MOBILE6 than they were using MOBILE5. Alternatively, if an ozone

attainment SIP relied on absolute model predictions for the future attainment year, then the first

criterion could be satisfied by demonstrating that the MOBILE6 estimates are equal to or lower
10

than the MOBILE5 estimates for the future attainment year. Or, if a carbon monoxide (CO)

maintenance plan relied on either a relative or absolute demonstration, the first criterion could be

satisfied by documenting that the relative emission reductions between the base year and the

maintenance year are the same or greater using MOBILE6 as compared to MOBILE5. In any

case, if using the latest planning assumptions for emissions estimates results in changes to other

emissions categories (e.g., point or area emissions), the demonstration would apply to the entire

inventory, rather than just the on-road mobile inventory.

If both of the above criteria are met, the State can simply re-submit the original SIP with

the revised MOBILE6 motor vehicle emissions inventories. If either criteria are not met, the

emissions categories in the SIP that have changed must be brought up to date. Any changes in

mobile or non-mobile control strategies, including stationary source inventories, must be factored

in to both base and future year inventories to determine if they would indicate a nonattainment

problem. However, a State would not necessarily have to revise a non-mobile emissions

inventory category just to account for a regulatory or permit change that reduces these emissions

in an attainment or maintenance year relative to the existing SIP.

 

It should be noted that regardless of the technique used for attainment or maintenance

demonstrations, a more rigorous reassessment of the SIP’s demonstration may be necessary if a

State decides to reallocate possible excess emission reductions to the motor vehicle emissions

budget as a safety margin. In other words, the State will need to assess how its original

attainment demonstration is impacted by using MOBILE6 vs. MOBILE5 before it reallocates any

apparent motor vehicle emission reductions resulting from the use of MOBILE6.

States completing mid-course reviews: As described in question 3 of this guidance, if a

State that has committed to complete a mid-course review cannot demonstrate that the SIP shows

attainment with the revised MOBILE6 inventories, the State can submit an enforceable

commitment to do one of the following in its mid-course review: 1) submit additional measures

needed to fill any emission reduction shortfall (if a shortfall is confirmed in the mid-course

review); or 2) document that the mid-course review reflects that there is no emission reduction

shortfall. Such a commitment, if needed, would be submitted as part of the MOBILE6 SIP

revision, and this commitment is necessary for EPA to find the revised MOBILE6-based motor

vehicle emission budgets adequate for conformity purposes.

EPA assistance: States are expected to consult with their EPA Regional Office prior to

submitting MOBILE6 SIP revisions. Early consultation can limit delays in EPA’s adequacy or

approval processes. EPA will work with States on a case-by-case basis to decide what additional

documentation, analyses, and for mid-course review areas, other commitments (as described

above) that are necessary to show that the SIP revision demonstrates its intended purpose (e.g.,

attainment or maintenance). For example, EPA is available to discuss whether additional SIP

documentation for validating or updating non-motor vehicle emissions inventories or air quality

modeling is needed. EPA will consider issuing additional SIP guidance in the future if additional

issues and questions arise. 
11

6. How will MOBILE6 affect the development of future attainment or maintenance SIPs? 

The answer to this question depends upon the unique circumstances of each

nonattainment or maintenance area. The emissions comparisons depend very heavily on the

pollutants of concern, the dates of concern, and on existing local regulations, traffic patterns,

fleet age, and mix of cars and trucks. In some cases, a change from MOBILE5 to MOBILE6 may

result in increased emissions estimates, while in other cases it may result in decreased emissions

estimates for various time periods.

Moreover, because of the complex chemistry and meteorology involved in air pollution,

the policy consequences of changes in highway vehicle emissions may not be clear until multiple

years are examined and the new emissions levels are applied to an air quality model. Relative

differences in emissions over time from MOBILE5 to MOBILE6 may be as important, or more

important than differences in any one year. As a result, an estimate of higher emissions under

MOBILE6 may not necessarily result in a need for additional controls, if the reduction in

emissions over time in MOBILE6 is greater than the reduction in MOBILE5. Therefore, it is

impossible to make general predictions about the implications of using MOBILE6 in

nonattainment or maintenance SIPs. Likewise, MOBILE6 users should not immediately assume

that increases or decreases in emissions in any single year imply the need for more or fewer SIP

control measures until those changes in emissions have been put in the complete SIP context.

An increase in emissions due to MOBILE6 may affect an area’s ability to demonstrate

conformity for their transportation plan and transportation improvement program (TIP). Areas

are encouraged, through the interagency consultation process, to consider whether MOBILE6

will have any potential impact on their future conformity determinations. Areas should

determine whether the SIP and motor vehicle emissions budgets should be updated using

MOBILE6 or whether the transportation plan/TIP should be modified during the MOBILE6

conformity grace period to facilitate future conformity determinations. After the grace period

ends, all new conformity analyses must be based on MOBILE6 even if the SIP was based on an

earlier MOBILE model. Please see question 2 for further information on MOBILE6 and

conformity. 

7. How will MOBILE6 play a role in mid-course reviews required in certain serious and

severe ozone nonattainment areas?

States that relied on a weight-of-evidence test in their attainment demonstrations for the

1-hour ozone standard had originally committed to submit a mid-course review by the end of

2003, a date that would allow consideration of the benefits of the NOx SIP call reductions. EPA

received comments on our December 16, 1999 proposed approvals of these SIPs (64 FR 70318)

recommending that the submission deadline be extended. Moreover, in the NOx SIP call

litigation, the court issued an order requiring EPA to allow states to establish a source
12

compliance date as late as May 31, 2004. Because the NOx reductions are critical to attainment

for these areas, EPA is contemplating an extension of the mid-course review submission date to

December 2004 for States affected by the NOx SIP call. It is expected that MOBILE6 will be in

use in 2003 in SIP development and conformity, and a number of States will have already revised

their SIPs and motor vehicle emissions budgets by that time. Therefore, EPA expects States to

use MOBILE6 in any emission-related analyses for their mid-course review. EPA does not

believe that the continued use of MOBILE5 in any emissions-based analyses in the 2003-2004

time frame is appropriate. 

The December 1999 notices of proposed rulemaking (NPRMs) on the 1-hour attainment

SIPs for serious ozone areas with proposed attainment dates of 2004 or earlier (Atlanta and

Western Massachusetts) acknowledged that to approve attainment SIPs for the serious areas

requesting an attainment date extension to a year prior to 2005, a review that occurs at a midpoint

prior to the attainment date would be impractical in terms of timing. Therefore, for these areas,

EPA requested the State’s commitment to a mid-course review be a commitment to perform an

early attainment assessment to be submitted by the end of the attainment year. EPA expects that

such an early attainment assessment should follow EPA’s technical guidance for mid-course

reviews. This early attainment assessment will help guide the State and EPA in determining

what further action might be required if the area does not attain by its attainment date.

8. How will MOBILE6's release affect the NOx SIP call submissions or how States show

compliance with their emission reduction rules that are designed to meet their NOx

budgets? 

In the NOx SIP call, EPA recognized that projections of emissions from mobile sources

would change as EPA improved its emissions models. However, EPA stated that these changes,

in and of themselves, would not require recalculation of the NOx budgets (63 FR at 57419-20;

October 27, 1998). The EPA does not intend to recalculate and repromulgate a State’s NOx

budget due to the availability of MOBILE6. Regarding determining compliance, the NOx SIP

call stated that States will neither be penalized by any new emission calculation technique nor

will they be allowed to benefit from such a new technique (63 FR at 57427; October 27, 1998

and 62 FR at 60365-66; November 7, 1997). The 2007 Statewide NOx budgets are an

accounting mechanism to ensure that States have adopted and are implementing controls

designed to achieve the necessary emissions reductions to address interstate transport. When

EPA evaluates each State’s NOx emissions reports, EPA will focus on whether the State has

implemented the measures to the level that its approved NOx budget demonstration had shown

would, based on the applicable base case inventories, achieve the Statewide budget levels. 

9. What role will MOBILE6 play in EPA's 1999 National Emissions Inventory?
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The National Emission Inventory (NEI) is a data base of criteria and hazardous air

pollutant (HAP) emission estimates for all types of sources, covering every county in the United

States. It is used for a variety of EPA purposes related to public information, air quality trends

analysis, and regulatory impact analysis. EPA also welcomes others to use the NEI. For

example, most Regional Planning Organizations will use it as a starting point for the

development of more refined regional emissions inventories for purposes of regional haze

assessment and planning. 

EPA is planning to prepare three versions of the 1999 National Emissions Inventory. The

following paragraphs address how criteria pollutants from highway vehicles will be estimated for

each version.

 Version 1 and 1.5 - EPA has already released a first version of the 1999 NEI, containing

highway vehicle emission estimates that were developed by EPA with MOBILE5. Other

emission source categories were estimated by applying growth and control factors to 1996

estimates many of which came from state agencies. Version 1.5, a minor revision of this

inventory to reflect the final 1999 vehicle miles traveled (VMT) reports, with an

extension to cover the year 2000, has been posted on EPA’s FTP server

(ftp://ftp.epa.gov/EmisInventory/net_99v15/) and will be released in one or more

summary formats approximately December 31, 2001.

 Version 2 - EPA has begun work on Version 2 of the 1999 NEI. Version 2 will be the

first that incorporates data submitted by state and local agencies on actual emissions in

1999. For the most part, these agencies submitted data on point sources, with some states

also supplying estimates for some area and non-road mobile source categories. Some

states supplied VMT estimates for highway vehicles. A draft edition, not intended for

actual use, is now in a public review period of October 1, 2001 through January 31, 2002. 

(See ftp://ftp.epa.gov/EmisInventory/draftnei99ver2/ and

http://www.epa.gov/ttn/chief/net/index.html#draft.) State estimates of highway vehicle

emissions were not incorporated into this draft edition. Instead, EPA has used 

MOBILE5 to estimate highway vehicle emissions for all states (except California, which

has submitted estimates using its own emissions model). In light of the eventual

transition to MOBILE6, these MOBILE5 estimates will be short-lived. We therefore

have advised the public that comments on these MOBILE5 estimates should be focused

on issues that would also be relevant to the eventual development of MOBILE6

estimates. To emphasize the short-lived nature of these highway estimates, they were not

incorporated into the draft Version 2 data files; instead, they were available as part of

Version 1.5 of the 1999 NEI. 

 We expect our first MOBILE6-based estimates to be released publicly in June 2002. 

June 2002 is the planned release date for the final edition of the second version of the

1999 NEI. We will need to decide whether the MOBILE5 or the MOBILE6 estimates

will appear in this final edition of the second version. If our confidence in how we
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prepared the MOBILE6 estimates is high we likely will formally incorporate the

MOBILE6-based estimates into the final edition of the second version. If we do this, we

will warn users that the MOBILE6 estimates have not yet undergone public review as

other parts of the NEI will have by then. If our confidence is lower and we believe more

public review is appropriate before giving them status in the NEI, we may keep the

MOBILE5 estimates in the inventory, and present the MOBILE6-based estimates as stilldraft alternative estimates for those interested. In the latter situation, we would also

regard the MOBILE6-based estimates as the first step in the development of the highway

vehicle portion of the third version of the NEI.

 Version 3 - We fully expect that all states that wish to have their MOBILE6 highway

vehicle emissions inventory incorporated into the 1999 National Emission Inventory will

have adequate opportunity to submit their inventory in time for it to be incorporated into

the June 2003 final edition of the third version of the 1999 NEI. For states that do not

submit their own estimates, EPA will use MOBILE6 to make estimates. These may have

improvements over the estimates available in June 2002. The draft edition of the third

version will be released in October 2002.

Version 1 of the 1999 NEI did not include any estimates of hazardous air pollutants

(HAP). Our approach to HAP emissions for highway vehicles for the second and third versions

of the 1999 NEI is not fully settled and may be affected by the timing of a new version of

MOBILE6 that addresses these pollutants. The HAP estimates for 1996 that are now available

were based on an approximate modeling approach (MOBTOX) that was intermediate between

MOBILE5 and MOBILE6. This approach is laborious to apply. In light of the expected release

in 2002 of a final version of MOBILE6 that includes HAP emissions, we are not planning on

generating 1999 HAP emission estimates with MOBTOX. The October 2001 draft edition of the

second version of the 1999 NEI does not contain any estimates of HAP emissions from highway

vehicles. 

10. Will EPA provide new guidance on how specific features of MOBILE6 should be used in

the creation of emission inventories for SIP and conformity submissions?

MOBILE6 incorporates significant changes in internal structure, underlying assumptions,

and input and output options compared to MOBILE5. In particular, MOBILE6 has much greater

input flexibility than MOBILE5, including many more input options. These new options are

designed to allow users to more accurately model local conditions or to expand the use of

MOBILE6 beyond the regional-scale modeling for which it was originally designed. For some of

these input options, it may take some time for users to develop reliable sources of local

information. EPA is releasing a separate document (“Technical Guidance on the Use of

MOBILE6 for Emission Inventory Preparation”) that provides detailed guidance on the use of

MOBILE6 in creating motor vehicle emissions estimates for SIPs and transportation conformity

determinations.