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Federal Register / Vol. 86, No. 33 / Monday, February 22, 2021 / Proposed Rules contribute significantly to PM2.5 levels that exceed the NAAQS in the nonattainment area.18 The provisions of subpart 4 do not define the term precursor for purposes of PM2.5, nor do they explicitly require the control of any specifically identified PM2.5
precursor. The statutory definition of air pollutant, however, provides that the term includes any precursors to the formation of any air pollutant, to the extent the Administrator has identified such precursor or precursors for the particular purpose for which the term air pollutant is used. 19 EPA has identified SO2, NOX, VOCs, and ammonia as precursors to the formation of PM2.5.20 Accordingly, the attainment plan requirements of subpart 4 apply to emissions of all four precursors and direct PM2.5 from all types of stationary, area, and mobile sources, except as otherwise provided in the Act e.g., CAA section 189e.
A large number of chemical reactions, often non-linear in nature, can convert gaseous SO2, NOX, VOCs, and ammonia to PM2.5, making them precursors to PM2.5.21 Formation of secondary PM2.5
also depends on atmospheric conditions, including solar radiation, temperature, and relative humidity, and the interactions of precursors with particles and with cloud or fog droplets.22 According to the Fairbanks Serious Plan, total wintertime PM2.5
concentrations in the Fairbanks PM2.5
Nonattainment Area are a function of both primary PM2.5 emissions and secondary PM2.5 formed from precursors see Chapter III.D.7.8.17.
Section 189e of the Act requires that the control requirements for major stationary sources of direct PM10 23 also apply to major stationary sources of PM10 precursors, except where the Administrator determines that such sources do not contribute significantly to PM10 levels that exceed the standard in the area. Section 189e contains the only express exception to the control requirements under subpart 4 e.g., requirements for reasonably available
control measures RACM and reasonably available control technology RACT, BACM and BACT, Most Stringent Measures MSM, and New Source Review NSR for sources of direct PM2.5 and PM2.5 precursor emissions. Although section 189e explicitly addresses only major stationary sources, EPA interprets the Act as authorizing it also to determine, under appropriate circumstances, that regulation of specific PM2.5 precursors from other source categories in a given nonattainment area is not necessary.24
For example, under EPAs longstanding interpretation of the control requirements that apply to stationary, area, and mobile sources of PM10
precursors in the nonattainment area under CAA section 172c1 and subpart 4,25 a state may demonstrate in a SIP submission that control of a certain precursor pollutant is not necessary in light of its insignificant contribution to ambient PM10 levels in the nonattainment area.26
Under the PM2.5 SIP Requirements Rule, a state may elect to submit to EPA
a comprehensive precursor demonstration for a specific nonattainment area to show that emissions of a particular precursor from all existing sources located in the nonattainment area do not contribute significantly to PM2.5 levels that exceed the NAAQS at issue in the nonattainment in the area.27 If EPA
determines that the contribution of the precursor to PM2.5 levels in the area is not significant and approves the demonstration, the state is not required to control emissions of the relevant precursor from existing sources in the attainment plan.28
In addition, in May 2019, EPA issued the PM2.5 Precursor Demonstration Guidance PM2.5 Precursor Guidance, which provides recommendations to states for analyzing nonattainment area PM2.5 emissions and developing such optional precursor demonstrations, consistent with the PM2.5 SIP Requirements Rule.29

18 40 CFR 51.1006; See 81 FR 58010, August 24, 2016, at pp. 5801758020.
19 CAA section 302g.
20 81 FR 58010, August 24, 2016, at p. 58015.
21 Air Quality Criteria for Particulate Matter EPA/600/P99/002aF, EPA, October 2004, Ch. 3.
22 Regulatory Impact Analysis for the Final Revisions to the National Ambient Air Quality Standards for Particulate Matter EPA/452/R12
005, EPA, December 2012, 21.
23 The requirements for attainment plans for the 2006 24-hour PM2.5 NAAQS include the general nonattainment area planning requirements in CAA
section 172 of title I, part D, subpart 1 and the additional planning requirements specific to particulate matter in CAA sections 188 and 189 of title I, part D, subpart 4. 81 FR 58010, August 24, 2016, at pp. 5801258014.

24 81 FR 58010, August 24, 2016, at pp. 58018
58019.
25 General Preamble, 57 FR 13498, April 16, 1992, at pp. 1353942.
26 Courts have upheld this approach to the requirements of subpart 4 for PM10. See, e.g., Assoc.
of Irritated Residents v. EPA, et al., 423 F.3d 989
9th Cir. 2005.
27 40 CFR 51.1006a1.
28 40 CFR 51.1006a1.
29 PM
2.5 Precursor Demonstration Guidance, EPA454/R19004, May 2019, including Memo dated May 30, 2019 from Scott Mathias, Acting Director, Air Quality Policy Division and Richard Wayland, Director, Air Quality Assessment Division, Office of Air Quality Planning and Standards OAQPS, EPA to Regional Air Division Directors, Regions 110, EPA.

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EPA evaluated the Fairbanks Serious Plan in accordance with the presumption embodied within subpart 4
that the State must address all PM2.5
precursors in the evaluation and implementation of potential control measures, unless the State adequately demonstrates that emissions of a particular precursor or precursors do not contribute significantly to ambient PM2.5 levels that exceed the PM2.5
NAAQS in the nonattainment area. In reviewing any determination by the state to exclude a PM2.5 precursor from the required evaluation of potential control measures, we considered both the magnitude of the precursors contribution to ambient PM2.5
concentrations in the nonattainment area and the sensitivity of ambient PM2.5
concentrations in the area to reductions in emissions of that precursor.30
2. Summary of States Submission Alaska includes its PM2.5 precursor analysis in Chapter III.D.7.8, section 7.8.12, of the Fairbanks Serious Plan.
The State provides both concentrationbased and sensitivity-based analyses of precursor contributions to ambient PM2.5 concentrations in the Fairbanks PM2.5 Nonattainment Area. These analyses led the State to conclude that SO2 and ammonia emissions contribute significantly to ambient PM2.5 levels that exceed the PM2.5 NAAQS in the Fairbanks PM2.5 Nonattainment Area, while NOX and VOCs do not contribute significantly to such exceedances, as discussed below. Consistent with this conclusion, the State focused the control strategy and attainment demonstration on sources of PM2.5, SO2, and ammonia emissions. Importantly, Alaskas precursor analysis did not address nonattainment NSR
requirements. The State made the prior determination to regulate all four EPAidentified legal precursors to PM2.5 in the nonattainment NSR regulations applicable to the Fairbanks PM2.5
Nonattainment Area. The EPA approved Alaskas October 25, 2018, SIP revision as meeting the nonattainment NSR
requirements triggered upon reclassification of the area to Serious August 29, 2019, 84 FR 45419.
Alaska applied a tiered approach to the precursor demonstrations in the Fairbanks PM2.5 Nonattainment Area.
The tiered analysis included: 1 A
concentration-based analysis of ambient data; 2 a concentration-based analysis using air quality modeling zero-out;
and 3 sensitivity-based analysis using air quality modeling. For the concentration-based analysis using 30 40

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CFR 51.1006a1i and ii.

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