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 Emi ssions Standards and Ga soline Sulfur Control Requirements (“Tier 2 sta ndards”) for passenger cars, light tr ucks, 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 appr oved or SIPs that are cu rrently under development?

In general, EPA believes that MOBILE6 should be used in SIP development as expeditiously as possible. The Clean Air Act re quires that SIP inventories and control measures be based on the most current info rmation 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 developmen t 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 w ould not require a SIP revision based on the new model. There are exceptions for certain nonattainment and mainte nance 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 mode l is now available. EPA belie ves that this is supported by existing EPA policies and case law [Delaney v. EPA , 898 F.2d 687 (9 thCir. 1990)]. Of course, States can choose to use MOBILE6 in these SIPs, for example, if it is determined that future conformity determina tions would be ensured through such a SIP revision. However, EPA does not believe that the State’s use of MOBILE 5 should be an obstacle to EPA approval for reasonable further progress, atta inment, or maintenance SIPs th at 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 w ould be unreasonable to require th e States to revise these SIPs with MOBILE6 since significant work has alr eady occurred, and EPA intends to act on these SIPs in a timely manner.

States should use MOBILE6 where SIP devel opment 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 sin ce MOBILE6's emissions es timates are based on the best information currently available. EPA also believes that the lega l 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 app licable models and data inputs, it could be difficult for EPA to3 approve a SIP developed with MOBILE5 significantly after MOBILE6 becomes available. If you have questions about which mode l 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 difficu lties 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 transportati on 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 ar e consistent with (“c onform to”) the SIP. Conformity to a SIP means that a transportati on activity will not cause or contribute to new violations; worsen existing violations; or delay timely attainment.

The transportation conformity rule (40 CFR pa rt 93) requires that conformity analyses be based on the latest motor vehicl e emissions model approved by EP A. Section 176(c)(1) of the Clean Air Act states that “....[t] he determination of conformity shall be ba sed on the most recent estimates of emissions, and such estimates shall be determined from the most recent population, employment, travel, and congesti on estimates....” When we appr ove 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 establishi ng the length of a conformity grace period in the preamble to the 1993 transporta tion 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 f actors in establishing the length of the grace period, including the degree of change in emissi ons models and the eff ects of the new model on the transportation planni ng 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 is3 Please refer to EPA’s Office of Trans portation 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 determina tions. Areas should carefully consider whether the SIP and motor vehicle emissions budget(s) should be revised with MO BILE6 before the end of the conformity grace period, since doing so may be necessary to ensu re 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 e ffective 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 withMOBILE6, and such budgets beco me applicable for conformity purposes prior to the end of the 2-year grace peri od. 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 adequatefor 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 fo r ozone and CO, the MOBILE6 grace period would end for ozone once EPA found the new MOBILE 6-based SIP budgets ad equate. 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 s upersede 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 peri od will state that the grace period will be 2 years unless new budgets becomeapplicable sooner, in which case the grace period will end once the applicability of ne w MOBILE6-based budgets becomes effective.

Implementation of grace period: During the grace period, areas can use an approved version of MOBILE5 3 for conformity determin ations 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 st arted 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 w ould 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 how4 November 3, 1999 EPA memorandum entitled, “Guidance on Motor Vehicle Emissions Budgets in One-Hour Ozone Attainment Demonstrations,” a nd November 8, 1999 EPA memorandum entitled, “1-Hour Oz one Attainment Demonstra tions 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 conf ormity determinations so, if necessary, SIPs and budgets can be revised with MOBILE6 or transportation plans and programs can bemodified 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 emi ssions 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. Regi onal conformity analyses that be gin before or during the grace period may continue to rely on an approved version of MOBILE5. Conformity determinations for transportation projects may al so be based on an approved vers ion 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 interagenc y consultation process should be used if it is unclear if a MOBILE5-based analysis was begun be fore the end of the grace period.

3. How will MOBILE6's release affect nonattain ment and maintenance areas that have included MOBILE5-based Tier 2 estimates in their SIPs?

All States whose attainment demonstrati ons 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 with in 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 se vere ozone nonattainment areas, EP A has implemented this policy in all other areas that have made use of Tier 2 benef its from EPA’s April 2000 MOBILE5 guidance, “MOBILE5 Information Sheet #8: Tier 2 Benefits Using MOBILE5.”5 The transportation conformity rule establishe s criteria for EPA to use in determining if submitted motor vehicle emissions budgets are ade quate for conformity pur poses prior to EPA’s approval action. For more information on ade quacy findings, see 40 CFR 93.118(e) or EPA’s May 14, 1999 guidance entitled, “Conformity Guid ance 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 ru lemaking 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 confor mity will not be determin ed during the second year unless there are adequate 5 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 be lieve that allowing areas an additiona l year to revise their budgets using MOBILE6 will not result in environmental harm as long as during that time there are no new conformity determina tions 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 th is requirement.

Any SIPs that rely on interim Tier 2 estimate s 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 progra m 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 we re 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 revi se interim MOBILE5 Tier 2 estimates with MOBILE6 are not intended to duplicate any techni cal analyses required for midcourse reviews in those areas. The MOBILE6 SIP and budget revisions ar e primarily intended to revise the motor vehicle emissi ons inventories with the new m odel. Although the overall SIP must continue to demonstrate attainment or maintenance with these revised MOBILE6 inventories as described in que stion 5 of this guidance, EPA believes that new attainmentmodeling 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 what7 November 3, 1999 EPA memorandum entitled, “Guidance on Motor Vehicle Emissions Budgets in One-Hour Ozone Attainment Demonstrations,” a nd November 8, 1999 EPA memorandum entitled, “1-Hour Oz one Attainment Demonstra tions and Tier 2/Sulfur Rulemaking.”

7 additional documentation is nece ssary to show that the MOBILE6 SIP revision demonstrates attainment.

If the State cannot demonstrate that the SIP shows attainment withthe revised MOBILE6 inventories as described in que stion 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 re view reflects that there is no emission reduction shortfall. Such a commitment, if needed, woul d 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 a nd 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, pursu ant 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 a ssumptions, for example, the revised MOBILE6 budget would supersede the original MOBILE6 budget once EPA finds it adequate for conformity purposes (assuming the original MO BILE6 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 thatSIPs and motor vehicle emissions budgets that relied on MOBILE5-based Tier 2 estimates berecalculated 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 a nd fuel standards cannot be accurately estimated until MOBILE6 is released. The MOBILE5-based Tier 2 estimates are interim approximations that were ba sed on national defau lts 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 vehicl e emissions with MOBILE6 will ensure that8 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 releas e. Affected States have included such commitments in the SIP, and will be able to star t 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. Ho wever 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 acc ount 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 emissi ons inventories and budg ets 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., atta inment, maintenance). For ex ample, if a State revises a maintenance plan to add or dele te 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 revi sions to existing SIPs and budgets continue to demonstrate the purpose of the SIP. Similarly, St ates that revise existing SIPs with MOBILE6 must show that the SIP continues to support attainment or ma intenance with the new level of motor vehicle emissions calculate d 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 furt her 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 y ear motor vehicle emission inventorie s will need to be recalculated with the latest available planning assumptions. As required by Cl ean Air Act §172(c)(3) and EPA’s regulation at 40 CFR 51.112(a), states must use the latest planning assump tions 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 dataavailable for those years. Fu ture year projection inventories must be based on the late st data available. If planning a ssumptions 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 gr owth and control strategy assumptions for non-motor vehicle sources (i.e., poi nt, area and non-road mobile s ources) are still accurate at the time that the MOBILE6 SIP revision is devel oped. Such assumptions include population and economic assumptions and any allo wable emissions relied upon for st ationary sources. If these assumptions have not changed, the State can simply re-submit the original SIP with the revised motor vehicle emission inventorie s 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 MOBILE 5-based motor vehicle e mission inventories are replaced with MOBILE6 inventories. The level of effort needed for this demonstration can vary depending upon how MOBILE6 aff ects the level of motor vehi cle emissions and whether nonmotor vehicle inventories require updating. The method used in th e original demonstration could also be a factor.

Areas can revise their motor vehicle emi ssions inventories and budgets using MOBILE6 without revising the entire SIP or completing a dditional 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 attainmen t/maintenance year inventories; and, 2) the State can document that the growth and control strategy a ssumptions for non-motor vehicle sources (i.e., poi nt, area and non-road mobile sources) continue to be valid and any minor updates do not change the overall c onclusions 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 ba se 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 lower10 than the MOBILE5 estimates for the future a ttainment year. Or, if a carbon monoxide (CO) maintenance plan relied on either a relative or absolute demonstr ation, the first criterion could be satisfied by documenting that the relative emi ssion reductions between th e 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 demonstr ation would apply to the entire inventory, rather than just the on-road mobile inventory.

If both of the above criteria are met, the St ate can simply re-submit the original SIP with the revised MOBILE6 motor vehicle emissions invent ories. If either criteria are not met, the emissions categories in the SIP that have change d must be brought up to date. Any changes in mobile or non-mobile control strategies, including stationary sour ce inventories, must be factored in to both base and future year inventories to dete rmine if they would indicate a nonattainment problem. However, a State would not necessarily have to revisea 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 t echnique used for attain ment or maintenance demonstrations, a more rigorous reassessment of the SIP’s demons tration 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 comple te a mid-course review cannot de monstrate 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 re view reflects that there is no emission reduction shortfall. Such a commitment, if needed, woul d 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 re view areas, other co mmitments (as described above) that are necessary to show that the SI P revision demonstrates its intended purpose (e.g., attainment or maintenance). Fo r example, EPA is available to discuss whether additional SIP documentation for validating or updating non-motor vehicle emissi ons 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 ar ea. 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 so me 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 needfor additional contro ls, 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 mainte nance SIPs. Likewise, MOBILE6 users should not immediately assume that increases or decreases in emissions in any si ngle 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 tr ansportation improvement program (TIP). Areas are encouraged, through the inte ragency consultation process, to consider whether MOBILE6 will have any potential impact on their future conformity determinations. Areas should determine whether the SIP and motor vehicl e emissions budgets should be updated using MOBILE6 or whether the transportation plan/T IP 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-o f-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 cons ideration of the benefits of the NOx SIP call reductions. EPA received comments on our December 16, 1999 proposedapprovals 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 source12 compliance date as late as May 31, 2004. Becausethe NOx reductions are critical to attainment for these areas, EPA is contemplating an extens ion of the mid-course re view submission date to December 2004 for States affected by the NOx SIP ca ll. It is expected that MOBILE6 will be in use in 2003 in SIP development a nd conformity, and a number of Stat es 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 us e of MOBILE5 in any emissions-based analyses in the 2003-2004 time frame is appropriate.

The December 1999 notices of proposed rule making (NPRMs) on th e 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 a ssessment should follow EPA’s t echnical 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 mode ls. However, EPA stated that these changes, in and of themselves, would not require r ecalculation 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 bene fit 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 addre ss interstate transport. When EPA evaluates each State’s NOx emissions reports, EPA will fo cus on whether the State has implemented the measures to the level that its approved NOx budget de monstration had shown would, based on the applicable base case inventories, achieve the Statewide budget levels.

9. What role will MOBILE6 play in EP A's 1999 National Emissions Inventory?13 The National Emission Inventory (NEI) is a data base of criteri a and hazardous air pollutant (HAP) emission estimates for all types of sources, coveri ng every county in the United States. It is used for a vari ety of EPA purposes related to pub lic 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 i nventories for purposes of regional haze assessment and planning.

EPA is planning to prepare th ree versions of the 1999 Nationa l 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 rel eased 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 growthand control factors to 1996 estimates many of which came from state agen cies. Version 1.5, a minor revision of this inventory to reflect the final 1999 vehicl e miles traveled (VMT) reports, with an extension to cover the year 2000, ha s been posted on EPA’s FTP server ( ftp://ftp.epa.gov/Emis Inventory/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 sub mitted 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 ve hicles. A draft edition, not intended for actual use, is now in a public review pe riod 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 th is 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 estima tes will be short-lived. We therefore have advised the public thatcomments 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 f iles; instead, they were available as part of Version 1.5 of the 1999 NEI.

We expect our first MOBILE6-based estimate s to be released publicly in June 2002. June 2002 is the planned release date for th e 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 s econd version. If our confidence in how we14 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 estimate s have not yet underg one 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 theMOBILE5 estimates in the inventory, and present the MOBILE6-based estimates as stilldraft alternative estimates for those interested. In the la tter 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 th at wish to have their MOBILE6 highway vehicle emissions inventory incorporated into the 1999 Na tional Emission Inventory will have adequate opportunity to s ubmit their inventory in time for it to be incorporated into the June 2003 final edition of the third versi on 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 haza rdous 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. Th e HAP estimates for 1996 that are now available were based on an appr oximate 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 in cludes HAP emissions, we are not planning on generating 1999 HAP emission estimates with MOBTOX. The Oct ober 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 wh ich it was originally de signed. For some of these input options, it may take some time fo r 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.