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Federal Register / Vol. 86, No. 7 / Tuesday, January 12, 2021 / Proposed Rules
County Ozone Plan provides for the implementation of all RACM as required by CAA section 172c1 and 40 CFR
51.1112c.
D. Attainment Demonstration 1. Statutory and Regulatory Requirements Section 182c2A of the CAA
requires that a plan for an ozone nonattainment area classified Serious or above include a demonstration that the plan . . . will provide for attainment of the ozone NAAQS by the applicable attainment date. This attainment demonstration must be based on photochemical grid modeling or any other analytical method determined . . . to be at least as effective. The attainment demonstration predicts future ambient concentrations for comparison to the NAAQS, making use of available information on measured concentrations, meteorology, and current and projected emissions inventories of ozone precursors, including the effect of control measures in the Plan.
Areas classified Serious for the 2008
ozone NAAQS must demonstrate attainment as expeditiously as practicable, but no later than 9 years after the effective date of designation as nonattainment. Western Nevada County was designated as a Marginal nonattainment area for the 2008 ozone NAAQS effective July 20, 2012.61 It was subsequently reclassified to Moderate,62
and then to Serious,63 and accordingly must demonstrate attainment of the standards by no later than July 20, 2021.64 An attainment demonstration must show attainment of the standards for a full calendar year before the attainment date, so in practice, Serious nonattainment areas must demonstrate attainment for the attainment year 2020.
The EPAs recommended procedures for modeling ozone as part of an attainment demonstration are contained in Modeling Guidance for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional Haze Modeling Guidance.65 The Modeling Guidance 61 77

FR 30088 May 21, 2012.
FR 26697 April 4, 2015.
63 84 FR 44238 August 23, 2019.
64 Nine years after the initial designation, 84 FR
44244.
65 Modeling Guidance for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional Haze, EPA 454/R18009, EPA
OAQPS, November 2018; available at https
www.epa.gov/scram/state-implementation-plan-sipattainment-demonstration-guidance. See also December 2014 draft of this guidance, available at https www3.epa.gov/ttn/scram/guidance/guide/
Draft-O3-PM-RH-Modeling_Guidance-2014.pdf. The December 2014 draft guidance was available during
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includes recommendations for a modeling protocol, model input preparation, model performance evaluation, use of model output for the numerical NAAQS attainment test, and modeling documentation. Air quality modeling is performed using meteorology and emissions from a base year, and the predicted concentrations from this base case modeling are compared to air quality monitoring data from that year to evaluate model performance. Once the model performance is determined to be acceptable, future year emissions are simulated with the model. The relative or percent change in modeled concentration due to future emissions reductions provides a relative response factor RRF. Each monitoring sites RRF
is applied to its monitored base year design value to give the future design value for comparison to the NAAQS.
The Modeling Guidance also recommends supplemental air quality analyses, which may be used as part of a weight of evidence WOE analysis. A
WOE analysis corroborates the attainment demonstration by considering evidence other than the main air quality modeling attainment test, such as trends and additional monitoring and modeling analyses.
Unlike the RFP demonstration and the emissions inventory requirements, the 2008 SRR does not specify that a specific year must be used for the modeled base year for the attainment demonstration. The Modeling Guidance also does not require a particular year to be used as the base year for 8-hour ozone plans.66 The Modeling Guidance states that the most recent year of the National Emissions Inventory may be appropriate for use as the base year for modeling, but that other years may be more appropriate when considering meteorology, transport patterns, exceptional events, or other factors that may vary from year to year.67 Therefore, the base year used for the attainment demonstration need not be the same year used to meet the requirements for emissions inventories and RFP.
With respect to the list of adopted measures, CAA section 172c6
requires that nonattainment area plans include enforceable emissions limitations, and such other control development of the Plan; the final version differs mainly in organization, and in updates to the regional haze portion and to other document references. Additional EPA modeling guidance can be found in 40 CFR 51 Appendix W, Guideline on Air Quality Models, 82 FR 5182 January 17, 2017;
available at https www.epa.gov/scram/clean-airact-permit-modeling-guidance.
66 Modeling Guidance section 2.7.1, 35.
67 Id.

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measures, means or techniques including economic incentives such as fees, marketable permits, and auctions of emission rights, as well as schedules and timetables for compliance, as may be necessary or appropriate to provide for timely attainment of the NAAQS.68
Under the 2008 Ozone SRR, all control measures needed for attainment must be implemented no later than the beginning of the attainment year ozone season.69 The attainment year ozone season is defined as the ozone season immediately preceding a nonattainment areas maximum attainment date; 70 in the case of the Western Nevada County area, the attainment year is 2020.
2. Summary of the States Submission a. Photochemical Modeling CARB performed the air quality modeling for the Western Nevada Ozone Plan, and has included documentation of this modeling within the Plan and the Staff Report that accompanied CARBs submittal of the 2018 Ozone Plan CARB Staff Report.71 The modeling relies on a 2012 base year and projects design values for 2020. The Plans modeling protocol is in Appendix H of the 2018 Western Nevada County Ozone Plan and contains all the elements recommended in the Modeling Guidance, with the exception of a conceptual description and a WOE
analysis, which appear in the CARB
Staff Report.72 The area is dominated by transport of ozone and precursors from the Sacramento Metro nonattainment area, which has a much higher population and emissions about twenty times larger.73 Concentrations at Western Nevada Countys single monitor, Grass Valley, have paralleled those in the eastern portions of the Sacramento area for the past two decades. The Western Nevada County area has multiple valleys extending from southwest to northeast into the higher elevations of the Sierra Nevada mountain range. Upslope-downslope 68 See
also CAA section 110a2A.
CFR 51.1108d.
70 40 CFR 51.1100h.
71 Staff Report: CARB Review of the Ozone Attainment Plan for Western Nevada County, CARB, October 12, 2018.
72 CARB Staff Report, 2 and 20.
73 The summer 2020 emissions inventories for the Sacramento nonattainment area and Western Nevada Nonattainment NOX are 63.2 and 3.1 tpd, respectively; VOC emissions are 86.8 and 4.3 tpd, respectively, 2018 Western Nevada County Ozone Plan, E27. The 2020 Sacramento County population is 1,543,522, about 14 times the size of the Nevada County population of 104,343, Almanac of Emissions & Air Quality, California Air Resources Board, 2013, Appendix C, available at https ww2.arb.ca.gov/our-work/programs/
resource-center/technical-assistance/air-qualityand-emissions-data/almanac.
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