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Federal Register / Vol. 86, No. 30 / Wednesday, February 17, 2021 / Proposed Rules 2. Maintenance Demonstration CAA section 175A requires a state seeking redesignation to attainment to submit a SIP revision to provide for the maintenance of the NAAQS in the area for at least 10 years after the redesignation. A state can make this demonstration by either showing that future emissions of a pollutant or its precursors will not exceed the level of the attainment inventory, or by modeling to show that the future mix of sources and emissions rates will not cause a violation of the NAAQS. See Calcagni Memo, pages 910. Idaho elected to demonstrate maintenance of the 2006 PM2.5 NAAQS for at least ten years following redesignation using the attainment inventory method.
IDEQ developed projected inventories, provided in Tables 3
through 5 of this document, to show that the Logan UT-ID area will remain in attainment through the year 2031.
These projected inventories, covering an interim year of 2026 and a horizon year of 2031, show that future emissions of NOX, SO2, VOCs, ammonia, and direct PM2.5 throughout the NAA will remain at or below the 2017 attainment-level emissions for the 2006 24-hour PM2.5
NAAQS. As these inventories show, emissions from NOX, SO2, VOCs and NH3 are projected to decrease between 2017 and 2031 Table 5 of this document. The emissions of direct filterable PM2.5 are projected to decline by 5.4% by 2031 Table 5 of this document.
Although emissions from condensable PM2.5 increase by 6.1% by 2031, Idaho adequately demonstrated that this increase will not prevent maintenance of the NAAQS through 2031. The condensable fraction of PM2.5 is 0.6% of the total PM2.5-Primary levels projected for 2031. As depicted in Table 5 of this document, the total condensable PM2.5
emissions are projected to increase by 0.71 pounds per winter day between 2017 and 2031, while total filterable PM2.5 emissions are projected to decrease by 119.72 pounds per winter day over the same time period. Overall, total PM2.5 sum of filterable and condensable PM2.5 is projected to decrease by 5.3% from 2017 to 2031.
EPA has reviewed the documentation provided by Idaho for developing the 2026 and 2031 emissions inventories for the Logan UT-ID PM2.5 NAA. Based on our review, EPA finds that the emissions inventories were prepared in accordance with EPA requirements.
These inventories indicate a decrease in PM2.5 and precursor emissions National Ambient Air Quality Standards NAAQS
and Regional Haze Regulations, May 2017.

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throughout the maintenance period, therefore EPA is proposing to determine that the projected emissions inventories in the Idaho maintenance plan sufficiently demonstrate that the Logan UT-ID PM2.5 NAA will continue to maintain the 2006 24-hour PM2.5
standard throughout the maintenance period.
3. Monitoring Network Once a NAA has been redesignated to attainment, the state must continue to operate an appropriate air quality monitoring network, in accordance with 40 CFR part 58, to verify the attainment status of the area.18 The maintenance plan should contain provisions for continued operation of air quality monitors that will provide such verification. In the maintenance plan, IDEQ noted that it currently operates a regulatory monitor the Preston monitor in the Idaho portion of the Logan, UTID PM2.5 NAA, and committed to continue operating a regulatory monitoring network in Franklin County in order to verify continued attainment of the PM2.5 NAAQS and track the progress of the maintenance plan. IDEQ
also stated that it will work with EPA
each year through the air monitoring network review process per 40 CFR
part 58 to determine the adequacy of the monitoring network.19 EPA
proposes to determine that the maintenance plan contains adequate provisions for continued operation of an air quality monitoring network to verify maintenance of the 2006 PM2.5 NAAQS.
4. Verification of Continued Attainment As stated in Section III.D.3 of this document, in its maintenance plan, Idaho commits to continue to operate a regulatory monitoring network in order to verify continued attainment of the PM2.5 NAAQS in the Idaho portion of the Logan UT-ID area. Idaho is also required to periodically update the emissions inventory for Franklin County in accordance with the Annual Air Emissions Reporting Requirements Rule AERR during the maintenance plan period. This includes developing annual inventories for major point sources and a comprehensive periodic inventory covering all source categories every three years.
18 As stated, Utah and Idaho signed an MOU to collectively meet the monitoring requirements of 40
CFR part 58, Appendix D in the Logan UT-ID MSA
on September 1, 2020.
19 See EPAs November 9, 2020 approval of Idahos 2020 Annual Monitoring Network Plan, in the docket for this action.

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5. Contingency Plan CAA section 175Ad requires that a maintenance plan also include contingency provisions, as necessary, to promptly correct any violation of the NAAQS that occurs after redesignation of the area to attainment. For the purposes of CAA section 175A, a state is not required to have fully adopted contingency measures that will take effect without further action by the state in order for the maintenance plan to be approved. However, the contingency plan is an enforceable part of the SIP
and should ensure that contingency measures are adopted expeditiously once they are triggered. The plan should discuss the measures to be adopted and a schedule and procedure for adoption and implementation. The contingency plan must require that the state will implement all measures contained in the Part D nonattainment plan for the area prior to redesignation. The state should also identify the specific indicators, or triggers, which will be used to determine when the contingency plan will be implemented.
Idahos maintenance plan outlines the procedures for the adoption and implementation of contingency measures to further reduce emissions should a violation occur. Idahos contingency measures include a warning level response and an action level response. An initial warning level response is triggered for the 2006 24hour PM2.5 NAAQS when the 98th percentile 24-hour PM2.5 concentration for a single calendar year reaches 35.5
mg/m3 or greater within the area. An action level response will be prompted by any one of the following: 1 A two year average of the 98th percentile reaches 35.5 mg/m3 or greater within the area; or 2 a violation of the standard occurs in the area i.e. a three-year average of the 98th percentile reaches 35.5 mg/m3 or greater.
Regardless of which level of response is triggered, the State will evaluate all appropriate data including air quality data, evaluation of wood smoke programs and information on wildfires or winter power outages to determine the cause of the exceedance. IDEQ will perform this evaluation within six months of the end of the year in which the NAAQS is exceeded or violated.
Should a warning level response be triggered, and IDEQ determines that additional emissions reductions are necessary, the State will adopt and implement contingency measures as expeditiously as possible and no later than 18 months from the determination of a single year exceedance based on quality assured data. Should an action
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