[Federal Register Volume 86, Number 7 (Tuesday, January 12, 2021)]
[Proposed Rules]
[Pages 2318-2337]
[FR Doc No: 2020-28885]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R09-OAR-2019-0440; FRL-10018-44-Region 9]

Clean Air Plans; 2008 8-Hour Ozone Nonattainment Area
Requirements; Western Nevada County, California

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing to
approve, or conditionally approve, all or portions of a state
implementation plan (SIP) revision submitted by the State of California
to meet Clean Air Act (CAA or ``Act'') requirements for the 2008 8-hour
ozone national ambient air quality standards (NAAQS or ``standards'')
in the Nevada County (Western part), California ozone nonattainment
area (``Western Nevada County''). The SIP revision is the ``Ozone
Attainment Plan, Western Nevada County, State Implementation Plan for
the 2008 Primary Federal 8-Hour Ozone Standard of .075 ppm'' (``2018
Western Nevada County Ozone Plan'' or ``Plan''). The 2018 Western
Nevada County Ozone Plan addresses the Serious nonattainment area
requirements for the 2008 ozone NAAQS, including the requirements for
emissions inventories, attainment demonstration, reasonable further
progress, reasonably available control measures, and contingency
measures, among others; and establishes motor vehicle emissions
budgets. The EPA is proposing to approve the 2018 Western Nevada County
Ozone Plan as meeting all the applicable ozone nonattainment area
requirements except for the contingency measures requirement, for which
the EPA is proposing conditional approval. In addition, the EPA is
beginning the adequacy process for the 2020 motor vehicle emissions
budgets in the 2018 Western Nevada County Ozone Plan through this
proposed rulemaking.

DATES: Written comments must arrive on or before February 11, 2021.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-R09-
OAR-2019-0440 at https://www.regulations.gov. For comments submitted at
Regulations.gov, follow the online instructions for submitting
comments. Once submitted, comments cannot be edited or removed from
Regulations.gov. The EPA may publish any comment received to its public
docket. Do not submit electronically any information you consider to be
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Multimedia submissions (audio,
video, etc.) must be accompanied by a written comment. The written
comment is considered the official comment and should include
discussion of all points you wish to make. The EPA will generally not
consider comments or comment contents located outside of the primary
submission (i.e., on the web, cloud, or other file sharing system). For
additional submission methods, please contact the person identified in
the FOR FURTHER INFORMATION CONTACT section. For the full EPA public
comment policy, information about CBI or multimedia submissions, and
general guidance on making effective comments, please visit https://www.epa.gov/dockets/commenting-epa-dockets. If you need assistance in a
language other than English or if you are a person with disabilities
who needs a reasonable accommodation at no cost to you, please contact
the person identified in the FOR FURTHER INFORMATION CONTACT section.

FOR FURTHER INFORMATION CONTACT: T. Khoi Nguyen, Air Planning Office
(AIR-2), EPA Region IX, 75 Hawthorne Street, San Francisco, CA 94105,
(415) 947-4120, or by email at<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:
    Throughout this document, ``we,'' ``us,'' and ``our'' refer to the
EPA.

Table of Contents

I. Regulatory Context
    A. Ozone Standards, Area Designations, and SIPs
    B. The Western Nevada County Ozone Nonattainment Area

    C. CAA and Regulatory Requirements for 2008 Ozone Nonattainment
Area SIPs
II. The 2018 Western Nevada County Ozone Plan
    A. Summary of Submission
    B. Clean Air Act Procedural Requirements for Adoption and
Submission of SIP Revisions
III. Evaluation of the 2018 Western Nevada County Ozone Plan
    A. Emissions Inventories
    B. Emissions Statements
    C. Reasonably Available Control Measures Demonstration
    D. Attainment Demonstration
    E. Rate of Progress Plan and Reasonable Further Progress
Demonstration
    F. Contingency Measures
    G. Motor Vehicle Emissions Budgets for Transportation Conformity
    H. Other Clean Air Act Requirements Applicable to Serious Ozone
Nonattainment Areas
IV. Proposed Action
V. Statutory and Executive Order Reviews

I. Regulatory Context

A. Ozone Standards, Area Designations, and SIPs

    Ground-level ozone pollution is formed from the reaction of
volatile organic compounds (VOC) and oxides of nitrogen
(NOX) in the presence of sunlight.\1\ These two pollutants,
referred to as ozone precursors, are emitted by many types of sources,
including on-and off-road motor vehicles and engines, power plants and
industrial facilities, and smaller area sources such as lawn and garden
equipment and paints.
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    \1\ The State of California refers to reactive organic gases
(ROG) rather than VOC in some of its ozone-related SIP submissions.
As a practical matter, ROG and VOC refer to the same set of chemical
constituents, and for the sake of simplicity, we refer to this set
of gases as VOC in this proposed rule.
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    Scientific evidence indicates that adverse public health effects
occur following exposure to ozone, particularly in children and adults
with lung disease. Breathing air containing ozone can reduce lung
function and inflame airways, which can increase respiratory symptoms
and aggravate asthma or other lung diseases.\2\
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    \2\ ``Fact Sheet--2008 Final Revisions to the National Ambient
Air Quality Standards for Ozone'' dated March 2008.
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    Under section 109 of the CAA, the EPA promulgates NAAQS for
pervasive air pollutants, such as ozone. The NAAQS are concentration
levels that, the attainment and maintenance of which, the EPA has
determined to be requisite to protect public health and welfare.
Section 110 of the CAA requires states to develop and submit SIPs to
implement, maintain, and enforce the NAAQS.
    In 1979, the EPA established the 1-hour ozone NAAQS of 0.12 parts
per million (ppm) (referred to herein as the ``1-hour ozone
NAAQS'').\3\ All of Nevada County was designated ``Unclassifiable/
Attainment'' for the 1-hour standard on November 15, 1990.\4\
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    \3\ 44 FR 8202 (February 8, 1979).
    \4\ 56 FR 56694 (November 6, 1991).
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    In 1997, the EPA revised the NAAQS for ozone, setting it at 0.08
ppm averaged over an 8-hour timeframe (referred to herein as the ``1997
ozone NAAQS'') to replace the existing 1-hour ozone NAAQS.\5\ In 2004,
the EPA initially designated and classified Western Nevada County as a
``Subpart 1'' nonattainment area for the 1997 ozone NAAQS.\6\ In
response to a decision of the United States Court of Appeals for the
District of Columbia Circuit vacating the EPA's subpart 1 designations,
the EPA in 2012 revised the area's classification for the 1997 ozone
NAAQS to ``Moderate,'' with an outermost attainment date of June 15,
2011.\7\ In 2011, the design value for the area was 0.079 ppm, and the
EPA published a clean data determination on December 3, 2012,
suspending attainment-related planning requirements for the 1997 ozone
NAAQS.\8\
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    \5\ 62 FR 38856 (July 18, 1997). The 1-hour ozone standard was
revoked effective June 15, 2005. See 70 FR 44470 (August 3, 2005).
    \6\ 69 FR 23857 (April 30, 2004).
    \7\ 77 FR 28423 (May 14, 2012). For more details on the revised
classification, see 77 FR 56775, 56776 (September 14, 2012).
    \8\ 77 FR 71551 (December 3, 2012).
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    In 2008, the EPA lowered the 8-hour ozone NAAQS to 0.075 ppm
(referred to herein as the ``2008 ozone NAAQS'') to replace the 1997
ozone NAAQS of 0.08 ppm.\9\ In 2012, the EPA designated Western Nevada
County as nonattainment for the 2008 ozone NAAQS and classified the
area as Marginal.\10\ Areas classified as Marginal must attain the
NAAQS within 3 years of the effective date of the nonattainment
designation. For Western Nevada County, the applicable Marginal area
attainment date was as expeditiously as practicable but no later than
July 20, 2015. The area failed to attain the 2008 ozone NAAQS by this
date, and the EPA published a reclassification to Moderate on May 4,
2016.\11\ Upon reclassification, Western Nevada County was required to
attain the 2008 ozone NAAQS as expeditiously as practicable but no
later than July 20, 2018.
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    \9\ 73 FR 16436 (March 27, 2008). The EPA further tightened the
8-hour ozone NAAQS to 0.070 ppm in 2015, but today's proposed action
relates to the requirements for the 2008 ozone NAAQS only.
Information on the 2015 ozone NAAQS is available at 80 FR 65292
(October 26, 2015).
    \10\ 77 FR 30088 (May 21, 2012).
    \11\ 81 FR 26697 (May 4, 2016).
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    In November 2018, pursuant to CAA section 181(b)(2), the EPA
proposed to determine that the Western Nevada County Moderate
nonattainment area failed to attain the 2008 ozone NAAQS by the
Moderate area attainment date.\12\ Additionally, following the EPA's
November 2018 proposal, the California Air Resources Board (CARB)
submitted a request under CAA section 181(b)(3) to voluntarily
reclassify the Western Nevada County nonattainment area from Moderate
to Serious nonattainment for the 2008 ozone standards accompanied by a
SIP revision to address planning elements for a Serious area.\13\
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    \12\ 83 FR 56781 (November 14, 2018).
    \13\ See letter dated December 2, 2018, from Richard W. Corey,
Executive Officer, CARB, to Michael Stoker, Regional Administrator,
EPA Region IX, and letter dated November 14, 2018 from Gretchen
Bennitt, Executive Director, NSAQMD, to Richard W. Corey, Executive
Officer, CARB, subject ``Submittal of the Northern Sierra Air
Quality Management District Ozone Attainment Plan for the 2008
Federal 8-hour Ozone Standard.''
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    In a final rule dated August 23, 2019, the EPA found that Western
Nevada County failed to attain the 2008 ozone NAAQS by the applicable
attainment date, and reclassified the area as Serious by operation of
law, effective September 23, 2019.\14\ Once reclassified to Serious,
the area is required to attain the standard as expeditiously as
practicable, but no later than 9 years after the initial designation as
nonattainment, i.e., July 20, 2021.
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    \14\ 84 FR 44238. The notice for this action acknowledges CARB's
request for voluntary reclassification, and notes that the EPA's
determination resulted in the same outcome as would occur with an
approval of that request.
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    The SIP revision that is the subject of today's proposed action
addresses the Serious nonattainment area requirements that apply to
Western Nevada County for the 2008 ozone NAAQS.

B. The Western Nevada County Ozone Nonattainment Area

    The Western Nevada County nonattainment area for the 2008 ozone
NAAQS consists of the portion of Nevada County west of the ridge of the
Sierra Nevada mountains. Western Nevada County encompasses an area of
approximately 800 square miles. The nonattainment area is bounded on
the north by the Middle Yuba River and most of the southern border is
defined by the Bear River. The eastern boundary is a line running
north/south that generally follows the ridge of the Sierra

Nevada mountains.\15\ The population of the Western Nevada County
nonattainment area is about 83,000 people.\16\
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    \15\ For a precise definition of the boundaries of the Western
Nevada County 2008 ozone nonattainment area, see 40 CFR 81.305.
    \16\ 2018 Western Nevada County Ozone Plan, page 12.
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    Air quality in Western Nevada County is regulated jointly by the
Northern Sierra Air Quality Management District (NSAQMD or
``District'') and CARB. The Nevada County Transportation Commission
(NCTC) is the regional transportation planning agency for the County of
Nevada. For transportation planning purposes, the area is an isolated
rural area.\17\
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    \17\ Isolated rural nonattainment and maintenance areas are
defined in 40 CFR 93.101 as areas that do not contain or are not
part of any metropolitan planning area as designated under the
transportation planning regulations.
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C. CAA and Regulatory Requirements for 2008 Ozone Nonattainment Area
SIPs

    States must implement the 2008 ozone NAAQS under title I, part D of
the CAA, including sections 171-179B of subpart 1, ``Nonattainment
Areas in General,'' and sections 181-185 of subpart 2, ``Additional
Provisions for Ozone Nonattainment Areas.'' To assist states in
developing effective plans to address ozone nonattainment problems, in
2015, the EPA issued a SIP Requirements Rule (SRR) for the 2008 ozone
NAAQS (``2008 Ozone SRR'') that addressed implementation of the 2008
standards, including attainment dates, requirements for emissions
inventories, attainment and reasonable further progress (RFP)
demonstrations, among other SIP elements, as well as the transition
from the 1997 ozone NAAQS to the 2008 ozone NAAQS and associated anti-
backsliding requirements.\18\ The 2008 Ozone SRR is codified at 40 CFR
part 51, subpart AA. We discuss the CAA and regulatory requirements for
the elements of 2008 ozone plans relevant to this proposal in more
detail in Section III of this document.
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    \18\ 80 FR 12264 (March 6, 2015).
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    The EPA's 2008 Ozone SRR was challenged, and on February 16, 2018,
the U.S. Court of Appeals for the D.C. Circuit (``D.C. Circuit'')
published its decision in South Coast Air Quality Management. District
v. EPA (``South Coast II'') \19\ vacating portions of the 2008 Ozone
SRR. The only aspect of the South Coast II decision that relates to
this proposed action is the vacatur of the alternative baseline year
for RFP plans. More specifically, the 2008 Ozone SRR required states to
develop the baseline emissions inventory for RFP plans using the
emissions inventory for the most recent calendar year for which states
submit a triennial inventory to the EPA under subpart A, ``Air
Emissions Reporting Requirements,'' of 40 CFR part 51, which was 2011.
The 2008 Ozone SRR, however, allowed states to use an alternative year,
between 2008 and 2012, for the baseline emissions inventory provided
the state demonstrated why the alternative baseline year was
appropriate. In the South Coast II decision, the D.C. Circuit vacated
the provisions of the 2008 Ozone SRR that allowed states to use an
alternative baseline year for demonstrating RFP.
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    \19\ South Coast Air Quality Management District v. EPA, 882
F.3d 1138 (D.C. Cir. 2018). The term ``South Coast II'' is used in
reference to the 2018 court decision to distinguish it from a
decision published in 2006 also referred to as ``South Coast.'' The
earlier decision involved a challenge to the EPA's Phase 1
implementation rule for the 1997 ozone NAAQS. South Coast Air
Quality Management Dist. v. EPA, 472 F.3d 882 (D.C. Cir. 2006).
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II. The 2018 Western Nevada County Ozone Plan

A. Summary of Submission

    On December 2, 2018, CARB submitted the 2018 Western Nevada County
Ozone Plan to the EPA as a revision to the California SIP to address
the nonattainment area requirements for Western Nevada County for the
2008 ozone NAAQS.\20\ The 2018 Western Nevada County Ozone Plan
includes various chapters and appendices, described further below, plus
the District's resolution of adoption for the Plan (District Resolution
2018-07) and CARB's resolution of adoption of the Plan as a revision to
the California SIP (CARB Resolution 18-36).\21\ The Plan addresses the
CAA requirements for emissions inventories, air quality modeling
demonstrating attainment, reasonably available control measures (RACM),
RFP, and motor vehicle emissions budgets, among other requirements.
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    \20\ Letter dated December 2, 2018, from Richard Corey,
Executive Officer, CARB, to Mike Stoker, Regional Administrator,
U.S. Environmental Protection Agency Region 9.
    \21\ NSAQMD Board Resolution 2018-7, October 22, 2018; CARB
Board Resolution 18-36, 2018 Ozone Attainment Plan for Western
Nevada County.
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    The 2018 Western Nevada County Ozone Plan begins with an executive
summary, an introductory section discussing ozone pollution and the
Western Nevada County nonattainment area generally, a discussion about
specific challenges in meeting air quality standards in the area, and a
formal request to reclassify the area to Serious for the 2008 ozone
NAAQS. Chapters IV through XIII address specific planning elements for
a Serious area, including emissions inventory, transportation
conformity budgets, emissions statements, new source review (NSR),
RACM, RFP, attainment demonstration, and contingency measures. The Plan
also includes eight appendices providing additional information on
emissions inventories, CARB control measures, CARB analysis of key
mobile source regulations and programs, a mobile sources and consumer
products RACM demonstration, and the modeled attainment demonstration,
a modeling emissions inventory for the nonattainment area, a
description of the conceptual model for the nonattainment area, and
CARB's modeling protocol used for the photochemical modeling.
    Additionally, to further supplement the contingency measures
element of the 2018 Western Nevada County Ozone Plan, CARB forwarded an
October 26, 2020 letter from the District \22\ committing to adopt as a
rule the most recent Architectural Coatings Suggested Control Measure
(SCM) developed and approved by CARB to serve as a contingency measure
that would be triggered if the area fails to meet an RFP milestone for
the 2008 ozone NAAQS or to reach attainment by a July 20, 2021
attainment date. In the letter forwarding this commitment, dated
November 16, 2020, CARB commits to submit the new District rule to the
EPA as a SIP revision within 12 months of the EPA's final action on the
contingency measures element of the 2018 Western Nevada County Ozone
Plan.
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    \22\ Letter dated November 16, 2020, from Richard Corey,
Executive Officer, CARB, to John Busterud, Regional Administrator,
EPA Region IX. CARB's letter also forwarded the District's
commitment letter to the EPA. The District's letter is dated October
26, 2020, from Gretchen Bennitt, NSAQMD Air Pollution Control
Officer, to Richard Corey, CARB Executive Officer.
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    In a technical memorandum submitted by email on October 27, 2020,
CARB provided additional information related to the motor vehicle
emissions budgets in the 2018 Western Nevada County Ozone Plan.\23\
Additionally, CARB has provided a copy of the 2019 emissions inventory
for the

nonattainment area,\24\ and clarifications to emissions tables in the
Plan.\25\
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    \23\ See attachment to email dated October 27, 2020 from
Nesamani Kalandiyur, CARB, to Khoi Nguyen and Karina O'Connor, EPA
Region 9.
    \24\ Email dated May 14, 2020 from Earl Withycombe, CARB, to
Khoi Nguyen, EPA Region 9, for attachment of the 2019 emission
inventory for the nonattainment area.
    \25\ Email dated August 17, 2020 from Webster Tasat, CARB, to
Khoi Nguyen, EPA Region 9, for clarifications of the emission
tables.
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B. Clean Air Act Procedural Requirements for Adoption and Submission of
SIP Revisions

    CAA sections 110(a) and 110(l) require a state to provide
reasonable public notice and opportunity for public hearing prior to
the adoption and submission of a SIP or SIP revision. To meet this
requirement, every SIP submittal should include evidence that adequate
public notice was given and an opportunity for a public hearing was
provided consistent with the EPA's implementing regulations in 40 CFR
51.102.
    Both the District and CARB have satisfied the applicable statutory
and regulatory requirements for reasonable public notice and hearing
prior to the adoption and submittal of the 2018 Western Nevada County
Ozone Plan. On September 21, 2018, the District published a notice in
the local newspaper of a public hearing to be held on October 22, 2018,
for the adoption of the 2018 Western Nevada County Ozone Plan.\26\ The
District adopted the Plan through Resolution #2018-07 at the October
22, 2018 hearing, and directed the Executive Director to forward the
Plan to CARB for inclusion in the California SIP.\27\
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    \26\ Affidavit of Publication from Nevada County Publishing
Company including a copy of the proof of publication and of the
September 21, 2018 notice for the October 22, 2018 public hearing.
    \27\ See NSAQMD Resolution #2018-07, October 22, 2018.
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    CARB also provided public notice and opportunity for public comment
on the 2018 Western Nevada County Ozone Plan. On October 12, 2018, CARB
released for public review its Staff Report for the Plan and published
a notice of public meeting to be held on November 15, 2018, to consider
adoption.\28\ At the November 15, 2018 hearing, CARB adopted the Plan
as a revision to the California SIP, excluding those portions not
required to be submitted to the EPA, and directed the Executive Officer
to submit the Plan to the EPA for approval into the California SIP. On
December 2, 2018, the Executive Officer of CARB submitted the Plan to
the EPA, including the CARB Board resolution adopting the 2018 Western
Nevada County Ozone Plan.\29\ On June 20, 2019, the EPA determined that
certain portions of this submittal applicable to the 2008 ozone NAAQS
were complete.\30\
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    \28\ Notice of Public Meeting to Consider the Ozone Attainment
Plan for Western Nevada County, signed by Richard Corey, Executive
Officer, CARB, October 12, 2018.
    \29\ CARB Resolution 18-36.
    \30\ Letter dated June 20, 2019, from Elizabeth Adams, Director,
Air and Radiation Division, EPA Region IX, to Richard Corey,
Executive Officer, CARB. The Plan was deemed complete by operation
of law on June 2, 2019, 6 months after submittal, but the EPA
completeness finding for the following SIP elements: Contingency
measures for VOC and NOX; emissions statement; ozone
attainment demonstration; and RFP demonstration for VOC and
NOX for moderate nonattainment areas was necessary to
stop clocks for mandatory sanctions in the Western Nevada
nonattainment area under section 179(a) of the CAA resulting from a
December 11, 2017 finding of failure to submit. See 82 FR 58118.
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    Based on information provided in the SIP revision summarized above,
the EPA has determined that all hearings were properly noticed.
Therefore, we find that the submittal of the 2018 Western Nevada County
Ozone Plan meets the procedural requirements for public notice and
hearing in CAA sections 110(a) and 110(l) and 40 CFR 51.102.

III. Evaluation of the 2018 Western Nevada County Ozone Plan

A. Emissions Inventories

1. Statutory and Regulatory Requirements
    CAA sections 172(c)(3) and 182(a)(1) require states to submit for
each ozone nonattainment area a ``base year inventory'' that is a
comprehensive, accurate, current inventory of actual emissions from all
sources of the relevant pollutant or pollutants in the area. In
addition, the 2008 Ozone SRR requires that the inventory year be
selected consistent with the baseline year for the RFP demonstration,
which is the most recent calendar year for which a complete triennial
inventory is required to be submitted to the EPA under the Air
Emissions Reporting Requirements.\31\
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    \31\ 2008 Ozone SRR at 40 CFR 51.1115(a) and the Air Emissions
Reporting Requirements at 40 CFR part 51, subpart A.
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    The EPA has issued guidance on the development of base year and
future year emissions inventories for ozone and other pollutants.\32\
Emissions inventories for ozone must include emissions of VOC and
NOX and represent emissions for a typical ozone season
weekday.\33\ States should include documentation explaining how the
emissions data were calculated. In estimating mobile source emissions,
states should use the latest emissions models and planning assumptions
available at the time the SIP is developed.\34\
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    \32\ ``Emissions Inventory Guidance for Implementation of Ozone
and Particulate Matter National Ambient Air Quality Standards
(NAAQS) and Regional Haze Regulations,'' EPA-454/B-17-002, May 2017.
At the time the 2018 Western Nevada County Ozone Plan was developed,
the following EPA emissions inventory guidance applied: ``Emissions
Inventory Guidance for Implementation of Ozone and Particulate
Matter National Ambient Air Quality Standards (NAAQS) and Regional
Haze Regulations,'' EPA-454-R-05-001, August 2005.
    \33\ 40 CFR 51.1115(a) and (c), and 40 CFR 51.1100(bb) and (cc).
    \34\ 80 FR 12264, at 12290 (March 6, 2015).
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    Future baseline emissions inventories must reflect the most recent
population, employment, travel and congestion projections for the area.
In this context, future ``baseline'' emissions inventories refer to
emissions estimates for a given year and area that reflect rules and
regulations and other measures that are already adopted and that
consider expected growth. Future baseline emissions inventories are
necessary to show the projected effectiveness of SIP control measures.
Both the base year and future year inventories are necessary for
photochemical modeling to demonstrate attainment.
2. Summary of State's Submission
    The 2018 Western Nevada County Ozone Plan includes base year (2011)
and future year (2012, 2014, 2017, 2020, and 2021) baseline inventories
for NOX and VOC for the Western Nevada County ozone
nonattainment area. Documentation for the inventories are found in
Chapter IV, ``Emissions Inventory Background,'' Chapter V, ``Summary of
Emissions Inventory Methodologies,'' Appendix A, ``Emissions
Inventories for 2011, 2012, 2014, 2017, 2020 and 2021,'' and Appendix
F, ``Modeling Emission Inventory for the 8-Hour Ozone State
Implementation Plan in Western Nevada County Non-attainment Area
(WNNA).''
    The emissions inventories represent average summer day emissions,
consistent with the observation that higher ozone levels in Western
Nevada County typically occur from May through October. The 2011 base
year and future year inventories in the 2018 Western Nevada County
Ozone Plan reflect District rules and CARB regulations submitted
through November 2016.\35\ The mobile source portions of both base year
and projected future year inventories were developed using California's
EPA-approved mobile source emissions model, EMFAC2014,

for estimating on-road motor vehicle emissions.\36\
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    \35\ ``Staff Report: CARB Review of the Ozone Attainment Plan
for Western Nevada County,'' CARB, October 12, 2018, page 6 (``CARB
Staff Report'').
    \36\ EMFAC is short for EMission FACtor. In December 2015, the
EPA approved EMFAC2014 for SIP development and transportation
conformity purposes in California. 80 FR 77337 (December 14, 2015).
EMFAC2014 was the most recently approved version of the EMFAC model
that was available at the time of preparation of the Western Nevada
County Ozone Attainment Plan. The EPA recently approved an updated
version of the EMFAC model, EMFAC2017, for future SIP development
and transportation purposes in California. 84 FR 41717 (August 15,
2019).
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    Emissions estimates of VOC and NOX in the 2018 Western
Nevada County Ozone Plan are grouped into three categories: (1)
Stationary point sources, (2) areawide sources, (3) on-road and other
mobile sources. Stationary point sources refer to larger sources that
have a fixed geographic location, such as power plants, industrial
engines, and oil storage tanks. This inventory includes emissions from
stationary internal combustion engines and gasoline dispensing
facilities; these are not inventoried individually but estimated as a
group and reported as an aggregated total. Areawide sources are
emissions sources occurring over a wide geographic area, such as
consumer products and architectural coatings. The on-road sources
include light-duty automobiles, light-, medium-, and heavy-duty trucks,
and motorcycles. Other mobile (off-road) sources include aircraft,
recreational boats, and off-road equipment.
    For the 2018 Western Nevada County Ozone Plan, stationary point
source emissions for the 2011 base year emissions inventory are based
on reported data from all stationary point sources in Western Nevada
County using the District's annual emissions reporting program, which
applies under District Rule 513, ``Emissions Statements and
Recordkeeping,'' to stationary sources that emit VOC or
NOX.\37\ Areawide sources include smaller emissions sources
distributed across the nonattainment area. CARB and the District
estimate emissions for areawide sources using the most recent models
and methodologies, including publicly available emission factors and
activity information. CARB also reviewed the growth profiles for point
and areawide source categories and updated them as necessary to ensure
that the emission projections are based on data that reflect historical
trends, current conditions, and recent economic and demographic
forecasts. Growth forecasts for most point and areawide sources were
developed by CARB.
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    \37\ The Air Pollution Control Officer of the NSAQMD may waive
the applicability of the reporting required by District Rule 513 for
certain classes or categories of sources with actual emissions or
potential to emit less than 10 tons per year of actual facility-wide
VOC or NOX emissions if the emissions for the class or
category of source are included in the base year and periodic
emission inventories and the emissions are calculated using emission
factors established by the EPA or other methods acceptable to the
EPA. As described in Section B of this document, this approach is
consistent with CAA section 182(a)(3)(B)(ii).
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    On-road emissions inventories in the 2018 Western Nevada County
Ozone Plan are based on 2012 travel activity data provided by the
California Department of Motor Vehicles. CARB provided emissions
inventories for off-road equipment, including locomotives, pleasure
craft and recreational vehicles, in-use off-road equipment, transport
refrigeration units, cargo handling equipment, diesel agricultural
equipment, and fuel storage and handling. Emissions from off-road
sources were estimated using a suite of category-specific models or,
where a new model was not available, the OFFROAD2007 model. A detailed
list of the updates made to specific emissions inventory categories can
be found in Chapter V.
    CARB developed the emission forecasts in the 2018 Western Nevada
County Ozone Plan by applying growth and control profiles to the base
year inventory. Growth profiles for stationary point and areawide
sources are derived from surrogates such as economic activity, fuel
usage, population, housing units, etc. Growth projections were obtained
from government entities with expertise in developing forecasts for
specific sectors, and from econometric models. Control profiles, which
account for emissions reductions resulting from adopted rules and
regulations, are derived from data provided by the regulatory agencies
responsible for the affected emission categories.\38\
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    \38\ 2018 Western Nevada County Ozone Plan, page 23.
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    Table 1 provides a summary of the District's 2011 base year, 2012
baseline year for modeling, and 2020 attainment year baseline VOC and
NOX emissions estimates in tons per day (tpd) for an average
summer day. All inventory years in the 2018 Western Nevada County Ozone
Plan are derived from the 2011 base year inventory, except that 2012 is
used as the baseline year for attainment modeling. These inventories
provide the basis for the control measure analysis and the attainment
demonstration in the Plan. Based on the inventory for 2011, mobile
sources are the predominant sources for both VOC and NOX
emissions. For a more detailed discussion of the inventories, see
Appendix A of the Plan.

                 Table 1--Western Nevada 2011 Base Year, 2012 Baseline Year for Modeling, and 2020 Attainment Year Emissions Inventories
                                                            [Summer planning inventory, tpd]
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                       2011                            2012                            2020
                        Category                         -----------------------------------------------------------------------------------------------
                                                                VOC             NOX             VOC             NOX             VOC             NOX
--------------------------------------------------------------------------------------------------------------------------------------------------------
Stationary..............................................          0.7620          0.0999          0.7006          0.0997          0.7843          0.0918
Area Sources............................................          1.4109          0.1452          1.3946          0.1349          1.5150          0.1377
On-Road and Other Mobile Sources........................          3.3227          5.4415          3.1131          4.9124          1.9559          2.8886
                                                         -----------------------------------------------------------------------------------------------
    Total for Western Nevada County Nonattainment Area..          5.4956          5.6866          5.2083          5.1470          4.2552          3.1181
--------------------------------------------------------------------------------------------------------------------------------------------------------
Source: 2018 Western Nevada County Ozone Plan, Appendix A. The sum of the emissions values may not equal the total shown due to rounding of the numbers.

3. The EPA's Review of the State's Submission
    We have reviewed the 2011 base year emissions inventory in the 2018
Western Nevada County Ozone Plan and the inventory methodologies used
by the District and CARB for consistency with CAA requirements and EPA
guidance. First, as required by EPA regulation, we find that the 2011
inventory includes estimates for VOC and NOX for a typical
ozone season

weekday, and that the Plan provides adequate documentation explaining
how the emissions are calculated.
    Second, we find that the 2011 base year emissions inventory in the
Plan reflects appropriate emissions models and methodologies, and,
therefore, represents a comprehensive, accurate, and current inventory
of actual emissions during that year in the Western Nevada County
nonattainment area. Therefore, the EPA is proposing to approve the 2011
emissions inventory in the 2018 Western Nevada County Ozone Plan as
meeting the requirements for a base year inventory set forth in CAA
section 182(a)(1) and 40 CFR 51.1115.
    With respect to future year baseline projections, we have reviewed
the growth and control factors and find them acceptable and conclude
that the future baseline emissions projections in the 2018 Western
Nevada County Ozone Plan reflect appropriate calculation methods and
the latest planning assumptions.
    Furthermore, we note that the future year baseline projections take
into account emissions reductions from control measures in adopted
state and local rules and regulations. As a general matter, the EPA
will approve a SIP revision that takes emissions reduction credit for
such control measures only where the EPA has approved the control
measures as part of the SIP. See Appendix B of the 2018 Western Nevada
County Ozone Plan, ``CARB Control Measures, 1985 to 2016,'' 2018
Western Nevada County Ozone Plan for the list of control measures.
    With respect to mobile sources, the EPA has taken action in recent
years to approve CARB mobile source regulations into the California
SIP.\39\ We therefore find that the future year baseline projections in
the 2018 Western Nevada County Ozone Plan are properly supported by
SIP-approved stationary and mobile source control measures.
---------------------------------------------------------------------------

    \39\ See 81 FR 39424 (June 16, 2016), 82 FR 14446 (March 21,
2017), and 83 FR 23232 (May 18, 2018).
---------------------------------------------------------------------------

B. Emissions Statements

1. Statutory and Regulatory Requirements
    Section 182(a)(3)(B)(i) of the Act requires each state to submit a
SIP revision requiring owners or operators of stationary sources of VOC
or NOX to provide the state with statements of actual
emissions from such sources. Statements must be submitted at least
every year and must contain a certification that the information
contained in the statement is accurate to the best knowledge of the
individual certifying the statement. Section 182(a)(3)(B)(ii) of the
Act allows states to waive the emissions statement requirement for any
class or category of stationary sources that emit less than 25 tpy of
VOC or NOX, if the state provides an inventory of emissions
from such class or category of sources as part of the base year or
periodic inventories required under CAA sections 182(a)(1) and
182(a)(3)(A), based on the use of emission factors established by the
EPA or other methods acceptable to the EPA.
    The preamble of the 2008 Ozone SRR states that if an area has a
previously approved emissions statement rule for the 1997 ozone NAAQS
or the 1-hour ozone NAAQS that covers all portions of the nonattainment
area for the 2008 ozone NAAQS, such rule should be sufficient for
purposes of the emissions statement requirement for the 2008 ozone
NAAQS.\40\ The state should review the existing rule to ensure it is
adequate and, if so, may rely on it to meet the emission statement
requirement for the 2008 ozone NAAQS. Where an existing emissions
statement program is still adequate to meet the requirements of this
rule, states can provide the rationale for that determination to the
EPA in a written statement in the SIP to meet this requirement. States
should identify the various requirements and how each is met by the
existing emissions statement program. Where an emissions statement
requirement is modified for any reason, states must provide the
revision to the emissions statement as part of its SIP.
---------------------------------------------------------------------------

    \40\ 80 FR 12264, at 12291 (March 6, 2015).
---------------------------------------------------------------------------

2. Summary of the State's Submission
    The 2018 Western Nevada County Ozone Plan addresses compliance with
the emissions statement requirement in CAA section 182(a)(3)(B) for the
2008 ozone NAAQS by reference to District Rule 513, ``Emission
Statements and Recordkeeping,'' which, among other things, requires
emissions reporting from all stationary sources of NOX and
VOC greater than or equal to 10 tpy. The EPA approved District Rule 513
as a revision to the California SIP on June 21, 2017, finding that Rule
513 fulfills the relevant emissions statement requirements of CAA
section 182(a)(3)(B)(i).\41\
---------------------------------------------------------------------------

    \41\ 82 FR 28240.
---------------------------------------------------------------------------

3. The EPA's Review of the State's Submission
    We find that District Rule 513 applies within the entire ozone
nonattainment area; applies to all stationary sources emitting
NOX and VOC, except those emitting less than 10 tpy for
which the District has waived the requirement (consistent with CAA
section 182(a)(3)(B)(ii)); and requires reporting, on an annual basis,
of total emissions of VOC and NOX. Also, as required under
CAA section 182(a)(3)(B), District Rule 513 requires certification that
the information provided to the District is accurate to the best
knowledge of the individual certifying the emissions data.
    Therefore, for the reasons described in the preceding paragraph, we
propose to find that District Rule meets the emissions statement
requirements for the 2008 ozone NAAQS under CAA section 182(a)(3)(B).

C. Reasonably Available Control Measures Demonstration

1. Statutory and Regulatory Requirements
    CAA section 172(c)(1) requires that each attainment plan provide
for the implementation of all RACM as expeditiously as practicable
(including such reductions in emissions from existing sources in the
area as may be obtained through implementation of reasonably available
control technology (RACT)), and also provide for attainment of the
NAAQS. The 2008 Ozone SRR requires that, for each nonattainment area
required to submit an attainment demonstration, the state concurrently
submit a SIP revision demonstrating that it has adopted all RACM
necessary to demonstrate attainment as expeditiously as practicable and
to meet any RFP requirements.\42\
---------------------------------------------------------------------------

    \42\ 40 CFR 51.1112(c).
---------------------------------------------------------------------------

    The EPA has previously provided guidance interpreting the RACM
requirement in the General Preamble for the Implementation of the CAA
Amendments of 1990 (``General Preamble'') and in a memorandum entitled
``Guidance on the Reasonably Available Control Measure Requirement and
Attainment Demonstration Submissions for Ozone Nonattainment Areas.''
\43\ In short, to address the requirement to adopt all RACM, states
should consider all potentially reasonable control measures for source
categories in the nonattainment area to determine whether they are
reasonably available for implementation in that area and whether they
would, if implemented individually or collectively, advance the area's

attainment date by one year or more.\44\ Any measures that are
necessary to meet these requirements that are not already either
federally promulgated, or part of the state's SIP, must be submitted in
enforceable form as part of the state's attainment plan for the
area.\45\
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    \43\ See General Preamble, 57 FR 13498 at 13560 (April 16, 1992)
and memorandum dated November 30, 1999, from John Seitz, Director,
OAQPS, to Regional Air Directors, titled ``Guidance on the
Reasonably Available Control Measure Requirement and Attainment
Demonstration Submissions for Ozone Nonattainment Areas.''
    \44\ Id. See also 44 FR 20372 (April 4, 1979), and memorandum
dated December 14, 2000, from John S. Seitz, Director, OAQPS, to
Regional Air Directors, titled ``Additional Submission on RACM From
States with Severe One-Hour Ozone Nonattainment Area SIPs.''
    \45\ For ozone nonattainment areas classified as Moderate or
above, CAA section 182(b)(2) also requires implementation of RACT
for all major sources of VOC and for each VOC source category for
which the EPA has issued a control techniques guideline. CAA section
182(f) requires that RACT under section 182(b)(2) also apply to
major stationary sources of NOX. In Serious areas, a
major source is a stationary source that emits or has the potential
to emit at least 50 tpy of VOC or NOX (see CAA section
182(c) and (f)). Under the 2008 Ozone SRR, states were required to
submit SIP revisions meeting the RACT requirements of CAA sections
182(b)(2) and 182(f) no later than 24 months after the effective
date of designation for the 2008 Ozone NAAQS and to implement the
required RACT measures as expeditiously as practicable but no later
than January 1 of the 5th year after the effective date of
designation (see 40 CFR 51.1112(a)). California submitted the CAA
section 182 RACT SIP for Western Nevada County for the 2008 ozone
NAAQS on June 7, 2018. Although Western Nevada County was classified
as Moderate nonattainment for the 2008 ozone NAAQS at the time of
submittal, the RACT SIP evaluated the area for compliance with
applicable RACT requirements based on the 50 tpy Serious major
source thresholds, in anticipation of the area's reclassification to
the higher classification. The EPA found this submission complete on
November 29, 2018 (see letter dated November 29, 2018 from Elizabeth
Adams, Acting Director, Air Divison, EPA Region IX, to Richard
Corey, Executive Officer, California Air Resources Board, and
finalized the RACT SIP submission on January 15, 2020 (85 FR 2313).
---------------------------------------------------------------------------

2. Summary of the State's Submission
    For the 2018 Western Nevada County Ozone Plan, the District and
CARB each undertook a process to identify and evaluate potential RACM
that could contribute to expeditious attainment of the 2008 ozone NAAQS
in Western Nevada County. We describe each agency's efforts below.
a. District's RACM Analysis
    The District's RACM demonstration for the 2008 ozone NAAQS is
described in Chapter X, ``Reasonably Available Control Measures
Demonstration,'' of the 2018 Western Nevada County Ozone Plan. This
discussion summarizes the District's analysis of potential additional
control measures for stationary sources conducted in the District's
RACT SIP,\46\ and describes additional controls in place for
``areawide'' source categories, such as architectural and automotive
coatings. Chapter X and Appendices B-D discuss CARB's mobile source and
consumer products RACM assessment. The District concludes that there
are no additional control measures reasonably available in the area
that can advance attainment by a year or more.
---------------------------------------------------------------------------

    \46\ The EPA approved the District's RACT SIP on January 15,
2020. 85 FR 2313.
---------------------------------------------------------------------------

    The District's RACM analysis builds upon a foundation of District
rules developed for earlier ozone plans and approved as part of the
SIP.\47\ The District has adopted rules to address various source
categories of NOX and VOC. We provide a list of the
District's NOX and VOC rules approved into the California
SIP in Table 1 of our December 3, 2020 memorandum to file in the docket
for this proposed action. The SIP-approved District VOC or
NOX rules listed in Table 1 of our memorandum establish
emission limits or other types of emissions controls for a wide range
of sources, including incinerator burning, orchard or citrus heaters,
fossil fuel steam generator facilities, gas stations, and more. These
rules have already provided significant and ongoing reductions toward
attainment of the 2008 ozone NAAQS by 2021.
---------------------------------------------------------------------------

    \47\ 2018 Western Nevada County Ozone Plan, page 42.
---------------------------------------------------------------------------

    Tables 2 and 3 of the December 3, 2020 memorandum provide a
crosswalk of the area's top-emitting stationary and area source
categories of NOX and VOC with related District control
rules. As shown in these tables, the area's 2020 stationary and area
source emissions inventory includes about 0.23 tpd of NOX
and 2.20 tpd of VOC. The top NOX source categories for this
year are residential fuel combustion (0.13 tpd; 4.26 percent of 2020
inventory) and service/commercial fuel combustion (0.04 tpd; 1.25
percent of 2020 inventory); all other categories each represent less
than 1 percent of the 2020 inventory.\48\ The top VOC source categories
for this year are consumer products (0.44 tpd; 10.28 percent of 2020
inventory), asphalt paving/roofing (0.38 tpd; 8.98 percent of 2020
inventory), and architectural coatings (0.32 tpd; 7.55 percent of 2020
inventory).
---------------------------------------------------------------------------

    \48\ For a further breakdown of the area's NOX and
VOC sources, see Table 3 of the EPA's December 3, 2020 memorandum to
file.
---------------------------------------------------------------------------

    The District's October 26, 2020 commitment letter for contingency
measures includes further analysis of potential additional controls for
regulated high-emission source categories. As mentioned above, the two
largest NOX source categories are residential fuel
combustion and service/commercial fuel combustion. For residential fuel
combustion, the District evaluated Sacramento Metropolitan Air Quality
Management District (SMAQMD) Rule 414 for water heaters, boilers, and
process heaters rated less than a million BTU per hour. Based on its
analysis, and considering especially the low population in the
nonattainment area, the District concluded that potential cumulative
reductions in NOX from a similar rule in the District would
produce only about 0.0005 tpd each year, and that these reductions
would occur too slowly to make any meaningful difference in attainment.
For service/commercial fuel combustion, the District evaluated SMAQMD
Rule 419 for miscellaneous combustion units. The District concluded
that emission reductions from applying Rule 419 controls in the area
would be approximately zero, because applying the rule would not be
feasible for two of the three sources in the nonattainment area that
would be subject to the rule and would not result in a more stringent
emissions limit for the last applicable source in the nonattainment
area. For VOC reductions, the District evaluated state measures for
architectural coatings and automotive coatings,\49\ and found that
reductions would be equivalent to 0.010 tpd and 0.003 tpd,
respectively. The District found that the estimated reductions for
automotive coatings was negligible and not cost effective but committed
to adopting a rule for architectural coatings as a contingency
measure.\50\
---------------------------------------------------------------------------

    \49\ Architectural coatings is Western Nevada County's third
largest VOC source category. The largest VOC source categories in
the area are consumer products and asphalt paving/roofing, and they
are already regulated, respectively, by multiple CARB regulations
and District Rule 227. See Table 3 of our December 3, 2020
memorandum to file.
    \50\ The emission reductions from the adopting an architectural
coatings rule for VOC (0.010 tpd) is less than the value needed to
advance attainment by a year for VOC (0.075 tpd), as calculated
below in Section III.C.3.
---------------------------------------------------------------------------

    Transportation Control Measures (TCMs) are projects that reduce air
pollutants from transportation sources by reducing vehicle use, traffic
congestion, or vehicle miles traveled. The Nevada County Regional
Transportation Plan 2015-2035 (``Transportation Plan''), prepared by
NCTC in January 2018, summarizes and highlights TCMs in Nevada County,
including the Western portion of Nevada County, and is included in the
docket for this action. Sample measures in Western Nevada County are
included within the TCM categories of CAA section 108(f)(1)(A). They
include proposed bikeways, for example, in Grass Valley,\51\ a 511
traveler

information system that provides information on ridesharing and directs
drivers to other regional resources for carpools and vanpools,\52\ and
programs for improved public transit,\53\ including improvements and
maintenance for bus stops and shelters.
---------------------------------------------------------------------------

    \51\ Transportation Plan, Appendix D, page D-1.
    \52\ Transportation Plan, page 126.
    \53\ Transportation Plan, Tables 42-46.
---------------------------------------------------------------------------

    As explained above, the District identified potential candidate
measures for RACM based upon categories with high NOX and
VOC emissions and relevant local or state measures. This analysis was
included in the District's commitment letter for contingency measures
and is further described in Section III.F.2. Based on its evaluation of
all available measures and the NOX-limited nature of the
nonattainment area, the District concludes that the District's existing
rules for stationary and area sources are generally as stringent as, or
more stringent than the analogous rules in other districts. Further,
the District concludes that, based on its comprehensive review and
evaluation of potential candidate measures, the District meets the RACM
requirement for the 2008 ozone NAAQS for all sources under the
District's jurisdiction.
b. CARB's RACM Analysis
    CARB's RACM analysis is contained in Chapter X as well as
Appendices B-D of the 2018 Western Nevada County Ozone Plan. CARB's
RACM analysis provides a general description of CARB's existing mobile
source programs. A more detailed description of CARB's mobile source
control program, including a comprehensive table listing on- and off-
road mobile source regulatory actions taken by CARB since 1985, is
contained in Appendix A. The RACM assessment contains CARB's evaluation
of mobile source and other statewide control measures that reduce
emissions of NOX and VOC in Western Nevada County.
    Source categories for which CARB has primary responsibility for
reducing emissions in California include most new and existing on- and
off-road engines and vehicles, motor vehicle fuels, and consumer
products.
    Given the need for substantial emissions reductions from mobile and
area sources to meet the NAAQS in California nonattainment areas, CARB
has established stringent control measures for on-road and off-road
mobile sources and the fuels that power them. California has authority
under CAA section 209 (subject to a waiver by the EPA) to adopt and
implement new emission standards for many categories of on-road
vehicles and engines, and new and in-use off-road vehicles and engines.
    CARB's mobile source program extends beyond regulations that are
subject to the waiver or authorization process set forth in CAA section
209 to include standards and other requirements to control emissions
from in-use heavy-duty trucks and buses, gasoline and diesel fuel
specifications, and many other types of mobile sources. Generally,
these regulations have been submitted and approved as revisions to the
California SIP.\54\
---------------------------------------------------------------------------

    \54\ See, e.g., the EPA's approval of standards and other
requirements to control emissions from in-use heavy-duty diesel-
powered trucks, at 77 FR 20308 (April 4, 2012), revisions to the
California on-road reformulated gasoline and diesel fuel regulations
at 75 FR 26653 (May 12, 2010), and revisions to the California motor
vehicle inspection and maintenance program at 75 FR 38023 (July 1,
2010).
---------------------------------------------------------------------------

    CARB's Consumer Products Program has established regulations that
limit VOC emissions from 129 consumer product categories, which apply
in Western Nevada County.\55\ The EPA has approved many CARB measures
into the California SIP that limit VOC emissions from a wide array of
products, including antiperspirants and deodorants, aerosol coating
products, and other consumer products.\56\
---------------------------------------------------------------------------

    \55\ 2018 Western Nevada County Ozone Plan, page 51 and Appendix
D, ``Reasonably Available Control Measures Assessment for Mobile
Sources and Consumer Products.''
    \56\ CARB's consumer product measures are found in Title 17
California Code of Regulations section 94500 et seq. The compilation
of such measures that have been approved into the California SIP,
including Federal Register citations, is available at: https://www.epa.gov/sips-ca/epa-approved-regulations-california-sip. EPA's
most recent approval of amendments to California's consumer products
regulations was in 2014. 79 FR 62346 (October 17, 2014).
---------------------------------------------------------------------------

    CARB's RACM analysis determines that, with the current mobile
source program and proposed measures, there are no additional RACM that
would advance attainment of the 2008 ozone NAAQS in Western Nevada
County. As a result, CARB concludes that California's mobile source
programs fully meet the RACM requirement.\57\
---------------------------------------------------------------------------

    \57\ Appendix VI-A, Attachment VI-A-3, page VI-A-106.
---------------------------------------------------------------------------

3. The EPA's Review of the State's Submission
    As described above and in our December 3, 2020 memorandum to file
in the docket for this proposed action, the District has implemented
rules to reduce VOC and NOX emissions from stationary
sources in the Western Nevada nonattainment area. For the 2018 Western
Nevada County Ozone Plan, the District indicates that its ozone
precursor control strategy focuses on NOX emission
reductions due to the NOX-limited nature of the
nonattainment area.\58\
---------------------------------------------------------------------------

    \58\ 2018 Western Nevada County Ozone Plan, page 41. As
explained in Section III.D.2.a of this document, Western Nevada
County is ``NOX limited'' because ozone formation in the
area is driven primarily by NOX emissions. As a result,
reducing NOX emissions is more effective for reducing
ozone than reducing VOC emissions.
---------------------------------------------------------------------------

    The District evaluated a range of potentially available measures
and was unable to find a combination of potential additional control
measures for RACM. The EPA further calculated the additional reductions
that would be necessary to advance attainment by a year. Subtracting
the District's 2020 attainment year emissions inventory from the 2019
emissions inventory yields a difference of 0.21 tpd NOX and
0.075 tpd VOC, equivalent to the reductions needed to advance
attainment by a year.\59\ Based on our review of the District's
analysis, we agree that no additional control measures are available
for stationary and area source categories in the nonattainment area
that would provide the emissions reductions needed to advance
attainment by a year.
---------------------------------------------------------------------------

    \59\ The Plan's RACM analysis incorrectly identifies the
necessary year-to-year reductions as 0.06 tpd of VOC and 0.23 tpd of
NOX, based on a comparison of 2020 and 2021 inventories.
Given the small discrepancy in these numbers, relative to the
emission reductions available in the area, we find that the
District's RACM analysis is adequately supported.
---------------------------------------------------------------------------

    With respect to mobile sources, CARB's current program addresses
the full range of mobile sources in the Western Nevada County
nonattainment area through regulatory programs for both new and in-use
vehicles. With respect to TCMs, we find that the TCMs being implemented
in Western Nevada County (i.e., the TCMs described in the
Transportation Plan) are inclusive of all TCM RACM to be reasonably
justified and supported.
    We also find that CARB's consumer products program comprehensively
addresses emissions from consumer products in the Western Nevada County
nonattainment area. CARB measures are more stringent than the EPA's
consumer products regulation promulgated in 1998,\60\ and generally
exceed the controls in place throughout other areas of the country.
---------------------------------------------------------------------------

    \60\ 63 FR 48819 (September 11, 1998).
---------------------------------------------------------------------------

    Based on our review of these RACM analyses and the District's and
CARB's adopted rules, we propose to find that there are, at this time,
no additional RACM (including RACT) that would advance attainment of
the 2008 ozone NAAQS in Western Nevada County. For the foregoing
reasons, we propose to find that the 2018 Western Nevada

County Ozone Plan provides for the implementation of all RACM as
required by CAA section 172(c)(1) and 40 CFR 51.1112(c).

D. Attainment Demonstration

1. Statutory and Regulatory Requirements
    Section 182(c)(2)(A) 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.
---------------------------------------------------------------------------

    \61\ 77 FR 30088 (May 21, 2012).
    \62\ 81 FR 26697 (April 4, 2015).
    \63\ 84 FR 44238 (August 23, 2019).
    \64\ Nine years after the initial designation, 84 FR 44244.
---------------------------------------------------------------------------

    The EPA's 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 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 site's 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.
---------------------------------------------------------------------------

    \65\ ``Modeling Guidance for Demonstrating Attainment of Air
Quality Goals for Ozone, PM2.5, and Regional Haze,'' EPA
454/R-18-009, EPA OAQPS, November 2018; available at https://www.epa.gov/scram/state-implementation-plan-sip-attainment-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 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-air-act-permit-modeling-guidance.
---------------------------------------------------------------------------

    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.
---------------------------------------------------------------------------

    \66\ Modeling Guidance section 2.7.1, 35.
    \67\ Id.
---------------------------------------------------------------------------

    With respect to the list of adopted measures, CAA section 172(c)(6)
requires that nonattainment area plans include enforceable emissions
limitations, and such other control 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 area's maximum attainment date; \70\ in the case of the
Western Nevada County area, the attainment year is 2020.
---------------------------------------------------------------------------

    \68\ See also CAA section 110(a)(2)(A).
    \69\ 40 CFR 51.1108(d).
    \70\ 40 CFR 51.1100(h).
---------------------------------------------------------------------------

2. Summary of the State's 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 CARB's 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 Plan's 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
County's 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

flows in those valleys lead to recirculation of pollutants, and the
Sierra crest tends to block flow further east; both of these enhance
ozone concentrations. The area is mainly rural, with generally low
NOX emissions and relatively high VOC emissions, so that
ozone formation there is expected to be NOX-limited.\74\ The
recirculation and the lack of NOX emissions prevents the
removal of ozone through the NOX titration process. This
allows carryover of pollution from the previous day, leading to high
ozone values that persist through the night at the start of the
following morning, unlike the typical pattern for areas with ozone
caused by locally generated emissions.\75\
---------------------------------------------------------------------------

    \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, E-27. 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-quality-and-emissions-data/almanac.
    \74\ Ozone is generally NOX-limited in rural areas
and downwind suburban areas. See pages 24 and 38 of CARB Staff
Report and also Chapter 2.1 Ozone Chemistry, ``Final Ozone NAAQS
Regulatory Impact Analysis,'' March 2008, EPA Office of Air Quality
Planning and Standards, available at https://www3.epa.gov/ttnecas1/regdata/RIAs/452_R_08_003.pdf. The term ``NOX-limited''
can mean either that reducing NOX emissions decrease
ozone (as opposed to increasing it); or that reducing NOX
is much more effective at decreasing ozone than is reducing VOC. As
discussed below and on page 42 of CARB Staff Report, ozone in
Western Nevada County are decreased by reducing NOX
emissions.
    \75\ 2018 Western Nevada Ozone Plan, page H-16.
---------------------------------------------------------------------------

    The modeling and the modeled attainment demonstration are described
in Chapter XII of the 2018 Western Nevada County Ozone Plan and in more
detail in Appendix E, which provides a description of model input
preparation procedures and various model configuration options.
Appendix F of the 2018 Western Nevada County Ozone Plan provides the
coordinates of the modeling domain and thoroughly describes the
development of the modeling emissions inventory, including its chemical
speciation, its spatial and temporal allocation, its temperature
dependence, and quality assurance procedures. The modeling analysis
uses version 5 of the Community Multiscale Air Quality (CMAQ)
photochemical model developed by the EPA, using the 2007 version of the
Statewide Air Pollution Research Center (SAPRC07) chemical mechanism.
The CMAQ modeling domain covers most of California, nested within a
domain covering the entire state. To prepare meteorological inputs for
CMAQ, CARB used the Weather and Research Forecasting model version 3.6
(WRF) from the National Center for Atmospheric Research. The WRF domain
covers the entire state of California, nested within a domain covering
most of the western United States. The modeling used inputs prepared
from routinely available meteorological and air quality data collected
during 2012. Those data cover May through September, a period that
spans the period of highest ozone concentrations in Western Nevada
County. The Modeling Guidance recognizes both CMAQ and WRF as
technically sound, state-of-the-art models. The areal extent and the
horizontal and vertical resolution used in these models is adequate for
modeling Western Nevada County ozone.
    The WRF meteorological model results and performance statistics are
described in Appendix E.\76\ The performance evaluation focuses on a
smaller area than the full domain but encompassing the Western Nevada
County nonattainment area and the greater Sacramento area, with special
attention on the winds for high ozone days. There is a slight
overprediction of wind speeds and underprediction of temperatures in
the eastern portion of the nonattainment area, but overall, modeled
wind speed, wind direction, and temperature all track observations very
well, as shown in scatter and time series plots. The modeling
replicates some important meteorological features such as the upslope-
downslope flows in the Sierra Nevada foothills, and the ``Schulz eddy''
known to occur in the greater Sacramento area. The 2018 Western Nevada
County Ozone Plan states that the bias and error are relatively small
and are comparable to those seen in previous meteorological modeling of
central California and cited in the Plan. In summary, the 2018 Western
Nevada County Ozone Plan's meteorological modeling performance
statistics appear satisfactory.
---------------------------------------------------------------------------

    \76\ Appendix E, section 3.2, E-17; also, refer to supplemental
figures S.1-S.11, E-48.
---------------------------------------------------------------------------

    Ozone model performance statistics are described in the 2018
Western Nevada County Ozone Plan at Appendix E.\77\ Appendix E includes
tables of statistics recommended in the Modeling Guidance for 8-hour
and 1-hour daily maximum ozone concentrations. Predicted concentrations
have a small negative bias (underprediction) of 4.1 ppb.\78\ This
compares well to the range of 2.7 to 10.8 ppb seen in a previous
modeling exercise for central California that is cited in the Plan;
bias and error are both at the low end of those seen in a comparative
study of 69 modeling exercises.\79\ The Plan's supplemental figures
with hourly time series show good performance; although some individual
daily ozone peaks are missed in May and September, there are days for
which the modeled highest concentration is close to the value of the
highest observed concentration. This supports the adequacy of the model
for use in the attainment demonstration.
---------------------------------------------------------------------------

    \77\ Appendix E, section 5.2, E-32; also, refer to supplemental
figures S.12-S.16, E-55.
    \78\ Because only the relative response to emissions changes
(RRF) from the modeling is used, the underprediction of absolute
ozone concentrations does not mean that future concentrations will
be underestimated.
    \79\ Simon, H., Baker, K.R., Phillips, S, 2012, Compilation and
Interpretation of Photochemical Model Performance Statistics
Published Between 2006 and 2012, Atmos. Environ., 61, 124-139.
doi:10.1016/j.atmosenv.2012.07.012.
---------------------------------------------------------------------------

    As noted in the 2018 Western Nevada County Ozone Plan's modeling
protocol, the Modeling Guidance recognizes that limited time and
resources can constrain the extent of the diagnostic and dynamic
evaluation of model performance undertaken.\80\ The Plan describes a
dynamic evaluation \81\ in which model predictions of ozone
concentrations for weekdays and weekends were compared to each other
and to observed concentrations. This evaluation provides useful
information on how well the model simulates the effect of emissions
changes, since NOX emissions are lower on weekends than on
weekdays, but the days are otherwise similar. The modeled ozone
decreased in response to the weekend NOX reductions, which
matches the observed decrease, and indicates that the model is
simulating the chemistry correctly. The Plan also contains results of
an analysis of weekday and weekend ozone concentrations during the
2000-2015 period. It notes a shift over the years toward lower ozone on
weekends, especially after 2010, showing that lower NOX
emissions lead to lower ozone concentrations.\82\ Both the modeling and
the observed weekday-weekend trends show that ozone responds to
NOX emissions reductions, i.e., that ozone formation is
NOX-limited. The modeled 2012 base year is also
NOX-limited, with the weekday-weekend difference comparable
to those seen historically. This match lends confidence to the
modeling.
---------------------------------------------------------------------------

    \80\ 2018 Western Nevada County Ozone Plan, Appendix H,
``Modeling Protocol,'' H-31; Modeling Guidance, 63.
    \81\ See ``Diagnostic Evaluation'' in Appendix E section 5.2.1,
E-36.
    \82\ 2018 Western Nevada County Ozone Plan, Appendix E, E-40.
---------------------------------------------------------------------------

    After accepting the model performance for the 2012 base case, CARB
used the model to develop RRFs for the attainment demonstration.\83\
This entailed running the model with the same meteorological inputs as
before, but with emissions inventories to reflect

the expected changes between the 2012 base year and the 2020 future
year. These modeling inventories exclude ``emissions events which are
either random and/or cannot be projected to the future . . . wildfires,
and events such as the [San Francisco Bay Area] Chevron refinery
fire.'' \84\ The future inventories project the base year with these
exclusions into the future by including the effect of economic growth
and emissions control measures.
---------------------------------------------------------------------------

    \83\ Id. at 57, and Appendix H, ``Modeling Protocol,'' section
10.3, H-34.
    \84\ Id. at Appendix H, H-33; and, Appendix F, ``Modeling
Emissions Inventory,'' F-35. To include the fires in the base year
but not the future year would effectively credit the Plan's control
measures with eliminating emissions from the fire.
---------------------------------------------------------------------------

    The 2018 Western Nevada County Ozone Plan carries out the
attainment test procedure consistent with the Modeling Guidance. The
RRF is calculated as the ratio of future to base year concentrations;
these are then applied to the 2013 weighted design values for the Grass
Valley monitor to arrive at a future year design value.\85\ Typically
the RRFs would be applied to a weighted design value for 2012, the
model base year,\86\ but in this case CARB used the somewhat higher
value for 2013, considering the upward trend design values starting in
2013.\87\ The predicted 2020 ozone design value is 67 ppb or 0.067 ppm,
well below the level of the 2008 8-hour ozone NAAQS of 0.075 ppm.
---------------------------------------------------------------------------

    \85\ Id. at 57, and Appendix H, ``Modeling Protocol,'' section
10.3, H-34. The combination of years used is illustrated in Appendix
E, Table 1, E-11.
    \86\ The Modeling Guidance recommends that RRFs be applied to
the average of three three-year design values, for the base year and
the two subsequent years. This amounts to a 5-year weighted average
of individual year 4th high concentrations, centered on the base
year, and so is referred to as a weighted design value.
    \87\ 2018 Western Nevada County Ozone Plan, Appendix E, E-10;
also Plan, 58.
---------------------------------------------------------------------------

    Finally, the 2018 Western Nevada County Ozone Plan modeling
includes an ``Unmonitored Area Analysis'' (UAA) to assess whether
locations without a monitor are able to reach attainment; the standard
attainment test procedure covers only locations with a monitor.\88\ The
Modeling Guidance describes a procedure utilizing ``gradient adjusted
spatial fields,'' as well as the EPA software used to carry it out.\89\
This procedure uses a form of interpolation, combining monitored
concentrations and modeled gradients (modeled changes in concentration
with distance from a monitor) to estimate future concentrations at
locations without a monitor. The 2018 Western Nevada County Ozone Plan
describes a UAA carried out using software developed by CARB and
implemented in ``R,'' \90\ using a procedure virtually the same as that
outlined in the Modeling Guidance. The Plan states that the 2020
results show concentrations below 75 ppb at all locations in the
nonattainment area; it did not examine the surrounding area. Because
the results are well below the 2008 ozone NAAQS level of 75 ppb, the
UAA supports the demonstration that all locations in Western Nevada
County will attain the NAAQS in 2020.
---------------------------------------------------------------------------

    \88\ 2018 Western Nevada County Ozone Plan, Appendix E, section
5.4, E-41.
    \89\ Modeling Guidance section 4.7, 138.
    \90\ The R Project for Statistical Computing, https://www.r-project.org.
---------------------------------------------------------------------------

    In addition to the formal attainment demonstration, the Plan also
contains a WOE analysis within Appendix A to the CARB Staff Report. It
mainly shows the long-term downward trends that continue through 2017,
the latest year available prior to development of the 2018 Western
Nevada County Ozone Plan. As described in the WOE, Western Nevada
County has shown a general downward trend in measured ozone
concentrations and number of days above the ozone NAAQS but has
recently seen increases in 2017 and 2018. Atypical high ozone
concentrations were observed in 2017, though CARB's staff analysis does
not point to specific anthropogenic or biogenic emission increases or
meteorology as likely causes for the unusual number of exceedances.
Additionally, the area may have experienced higher than normal ozone
concentrations in 2018 due to wildfire impacts in the surrounding areas
during the summer and fall months. Despite the recent exceptions, there
are strong downward trends in emissions of ozone and of the ozone
precursors NOX and VOC, both within the Western Nevada
County area and in the upwind Sacramento and San Francisco Bay
areas.\91\ These all show the substantial air quality progress made in
the Western Nevada County Area and add support to the attainment
demonstration for 2020.
---------------------------------------------------------------------------

    \91\ 2018 Western Nevada County Ozone Plan, page 41.
---------------------------------------------------------------------------

b. Control Strategy
    The control strategy for attainment of the 2008 ozone NAAQS is
detailed in Chapter IV of the 2018 Western Nevada County Ozone Plan.
The Plan's strategy relies primarily on emissions reductions from
control measures that have been adopted by the Districts and CARB prior
to the submittal of the Plan. The District has adopted rules for
reducing emissions from a broad scope of stationary and area sources
into its RACT SIP. Additionally, a detailed description of the mobile
source control programs and a comprehensive list of CARB regulations
are included in Appendices B and C of the Plan. CARB's comprehensive
strategy to reduce emissions from mobile sources consists of emissions
standards for new vehicles, in-use programs to reduce emissions from
existing vehicle and equipment fleets, cleaner fuels, and incentive
programs to accelerate the penetration of the cleanest vehicles beyond
that achieved by regulations alone.
    As Table 2 and Table 3 show, the vast majority of emissions
reductions relied upon by the Plan's control strategy are from the on-
and off-road mobile source inventory and can be largely attributed to
control measures adopted by CARB, subsequently approved by the EPA, and
cited in detail in Section III.C. Generally, the bulk of the emissions
reductions on which the control strategies rely is expected to come
from already-adopted measures, which are discussed in Section III.C of
this document. For the 2008 ozone NAAQS, already-adopted measures are
expected to achieve all of the reductions needed from the 2012 base
year to attain the NAAQS in 2020.

                         Table 2--2012 and 2020 NOX Emissions for Western Nevada County
                                        [Summer planning inventory, tpd]
----------------------------------------------------------------------------------------------------------------
                                                                             Emissions
            Source category                   2012            2020        difference from    Percentage of total
                                                                            2012 to 2020    emissions change (%)
----------------------------------------------------------------------------------------------------------------
Stationary Sources.....................           0.106           0.096             -0.010                  -9.4
Area Sources...........................           0.135           0.138             +0.003                   2.2
On-Road Mobile Sources.................           3.976           2.160             -1.816                 -45.7
Other Mobile Sources...................           0.944           0.738             -0.206                 -21.8
                                        ------------------------------------------------------------------------

    Total..............................           5.160           3.131             -2.029                 -39.3
----------------------------------------------------------------------------------------------------------------
Source: 2018 Western Nevada County Ozone Plan, Chapter XII, Table 11, 57. The sum of the emissions values may
  not equal the total shown due to rounding.

                  Table 3--2012 and 2020 Anthropogenic VOC Emissions for Western Nevada County
                                        [Summer planning inventory, tpd]
----------------------------------------------------------------------------------------------------------------
                                                                             Emissions
            Source category                   2012            2020        difference from    Percentage of total
                                                                            2012 to 2020    emissions change (%)
----------------------------------------------------------------------------------------------------------------
Stationary Sources.....................           0.702           0.785             +0.083                  11.8
Area Sources...........................           1.394           1.515             +0.121                   8.7
On-Road Mobile Sources.................           1.793           1.007             -0.786                 -43.8
Other Mobile Sources...................           1.327           0.958             -0.369                 -27.8
                                        ------------------------------------------------------------------------
    Total..............................           5.215           4.265             -0.950                 -18.2
----------------------------------------------------------------------------------------------------------------
Source: 2018 Western Nevada County Ozone Plan, Chapter XII, Table 11, 57. The sum of the emissions values may
  not equal the total shown due to rounding.

c. Attainment Demonstration
    Chapter XII of the Plan describes the attainment demonstration in
general terms, including photochemical modeling results, while Appendix
E to the Plan provides more detail concerning photochemical modeling.
Other aspects of this demonstration are included throughout the Plan,
including emissions inventory forecasts included in Appendix A and the
control strategy described in Chapter IV. The WOE analysis in Appendix
A to the CARB Staff Report includes additional supporting information
to complement the photochemical modeling and to provide context for
this attainment demonstration, such as analyses of anthropogenic
emissions, ambient ozone data, and meteorological analyses.
3. The EPA's Review of the State's Submission
a. Photochemical Modeling
    To approve a SIP's attainment demonstration, the EPA must make
several findings. First, we must find that the demonstration's
technical bases, including the emissions inventories and air quality
modeling, are adequate. As discussed above in Section III.A of this
document, we are proposing to approve the base year emissions inventory
and to find that the future year emissions projections in the 2018
Western Nevada County Ozone Plan reflect appropriate calculation
methods and that the latest planning assumptions are properly supported
by SIP-approved stationary and mobile source measures.
    The modeling followed the Modeling Guidance in essentially all
respects, and both the meteorological and the photochemical models
showed good performance. One difference between CARB's modeling and the
Modeling Guidance was that the state applied RRFs to a weighted design
value based on the year 2013, instead of 2012, as would be typical for
modeling of a 2012 base year. The Modeling Guidance recognizes that
there is no one correct method for choosing base design values,\92\ and
provides for other calculations with appropriate justification, such as
consideration of unusual meteorological conditions. As noted above, the
state's choice of 2013 was based on design values increasing relative
to 2012. Since a higher starting point base design value will yield a
higher 2020 attainment year design value, the state's use of 2013 adds
conservatism to the attainment demonstration.
---------------------------------------------------------------------------

    \92\ Modeling Guidance, section 4.1.1, ``Establishing the Base
Design Value,'' 103.
---------------------------------------------------------------------------

    An important difference from the Modeling Guidance is that the
state presented a model performance evaluation only for the single
monitoring site in the nonattainment area, in Grass Valley. The
Modeling Guidance recommends a performance evaluation using all
available ambient monitoring data.\93\ This is of particular importance
for the Western Nevada County area. As described in the conceptual
description in the Plan discussed above, ozone in the area is largely
due to emissions in and transport from the upwind Sacramento area. The
chemical evolution of the pollutant plume as it travels from Sacramento
to Nevada County necessitates evaluation at more than a single downwind
location. This means that the submitted modeling performance evaluation
alone may not be adequate for assessing the performance model, which is
influenced by emissions from a much larger area, with various
meteorological and terrain impacts. However, because the 2012 modeling
exercise in the Plan was essentially the same as that undertaken for
the 2017 Sacramento Regional Ozone Plan, the EPA is relying on the
latter plan's more complete model performance evaluation. As discussed
in the technical support document \94\ accompanying the EPA's proposed
action on the Sacramento plan, the state followed EPA recommended
modeling procedures and the modeling had good performance. That was
shown in statistical and dynamic performance analyses that covered a
larger portion of the modeling domain than the analyses in the
submittal for the 2018 Western Nevada County Ozone Plan, encompassing
the Western Nevada County as well as the Sacramento area. Overall, the
EPA therefore considers the modeling in the 2018 Western Nevada

County Ozone Plan to be adequate for establishing modeling performance.
---------------------------------------------------------------------------

    \93\ Modeling Guidance, section 3.1, ``Overview of Model
Performance Evaluation,'' 68.
    \94\ Docket EPA-R09-OAR-2020-0425, item A-85, ``Modeling
Technical Support Document (TSD) for the `2017 Sacramento Regional
Ozone Plan', 2008 8-hour ozone National Ambient Air Quality
Standard,'' September 14, 2020, Air and Radiation Division, EPA
Region IX.
---------------------------------------------------------------------------

    The modeling shows that existing control measures from CARB and the
Districts are sufficient to attain the 2008 8-hour ozone NAAQS by 2020
at all monitoring sites in the Western Nevada County area. The Plan
follows the procedures recommended in the EPA Modeling Guidance,
properly incorporates all modeling and input preparation procedures,
tests, and performance analyses called for in the modeling protocol,
demonstrates good model performance, and responds to emission changes
consistent with observations. Therefore, based on the documentation
included in the modeling performance analysis, UAA, and WOE analysis,
the EPA finds that the photochemical modeling is adequate for purposes
of supporting the attainment demonstration.
b. Control Strategy
    As discussed above, the 2018 Western Nevada County Ozone Plan
relies on previously adopted measures to achieve all of the emissions
reductions needed to attain the 2008 ozone NAAQS in 2020. For the
reasons described above, we find that the emissions reductions that are
relied on for attainment are creditable and are sufficient to provide
for attainment.
c. Attainment Demonstration
    The 2018 Western Nevada County Ozone Plan follows the modeling
procedures recommended in the EPA's Modeling Guidance and shows
excellent performance in simulating observed ozone concentrations in
the 2012 base year. Given the extensive discussion of modeling
procedures, tests, and performance analyses called for in the modeling
protocol, the good model performance, and the model response to
emissions changes consistent with observations, the EPA finds that the
modeling is adequate for purposes of supporting the attainment
demonstration. Based on our review of the 2018 Western Nevada County
Ozone Plan and our proposed findings that the photochemical modeling
and control strategy are acceptable and demonstrate attainment by the
applicable attainment date, we propose to approve the attainment
demonstration for the 2008 ozone NAAQS in the Western Nevada County
Ozone Plan as meeting the requirements of CAA section 182(c)(2)(A) and
40 CFR 51.1108.

E. Rate of Progress Plan and Reasonable Further Progress Demonstration

1. Statutory and Regulatory Requirements
    Requirements for RFP for ozone nonattainment areas are specified in
CAA sections 172(c)(2), 182(b)(1), and 182(c)(2)(B). CAA section
172(c)(2) requires that plans for nonattainment areas provide for RFP,
which is defined at CAA section 171(1) as such annual incremental
reductions in emissions of the relevant air pollutant as are required
under part D, ``Plan Requirements for Nonattainment Areas,'' or may
reasonably be required by the EPA for the purpose of ensuring
attainment of the applicable NAAQS by the applicable date. CAA section
182(b)(1) specifically requires that ozone nonattainment areas that are
classified as Moderate or above demonstrate a 15 percent reduction in
VOC within the first six years of the planning period. The EPA has
typically referred to section 182(b)(1) as the Rate of Progress (ROP)
requirement. For ozone nonattainment areas classified as Serious or
higher, section 182(c)(2)(B) requires reductions averaged over each
consecutive 3-year period, beginning 6 years after the baseline year
until the attainment date, of at least 3 percent of baseline emissions
per year. CAA section 182(c)(2)(B)(ii) allows an amount less than 3
percent of such baseline emissions each year if the state demonstrates
to the EPA that a plan includes all measures that can feasibly be
implemented in the area in light of technological achievability. To
meet CAA sections 172(c)(2) and 182(c)(2)(B) RFP requirements, the
state may substitute NOX emissions reductions for VOC
reductions.\95\
---------------------------------------------------------------------------

    \95\ 40 CFR 51.1110(a)(2)(i)(C) and 40 CFR 51.1110(a)(2)(ii)(B);
and 70 FR 12264, at 12271 (March 6, 2015).
---------------------------------------------------------------------------

    The 2008 Ozone SRR provides that areas classified Moderate or
higher for the 2008 8-hour ozone standard will have met the ROP
requirements of CAA section 182(b)(1) if the area has a fully approved
15 percent ROP plan for the 1979 1-hour or 1997 8-hour ozone standards,
provided the boundaries of the ozone nonattainment areas are the
same.\96\ Western Nevada County does not have a fully approved 15
percent ROP plan for either the 1979 1-hour or the 1997 8-hour ozone
standards.\97\ Therefore, the 15 percent ROP requirement of section
182(b)(1) remains applicable to Western Nevada County, and the area
must show a 15 percent reduction in VOC within the first six years of
the planning period.
---------------------------------------------------------------------------

    \96\ 70 FR 12264, 12271 (March 6, 2015). For more information
about how the RFP requirement of section 172(c)(2) applies in such
areas, see 84 FR 28132, 28157 (June 17, 2019).
    \97\ As explained above in Section I, for the 1979 1-hour ozone
NAAQS, the EPA classified Western Nevada County as Unclassifiable/
Attainment and, thus, it was not subject to the ROP requirement. 62
FR 38856 (July 18, 1997). For the 1997 8-hour ozone NAAQS, the EPA
initially designated Western Nevada County as a ``Subpart 1''
nonattainment area and later reclassified the area to Moderate,
triggering the ROP requirement, but subsequently issued a clean data
determination, which suspended attainment-related planning
requirements, including the ROP requirement. 69 FR 23857 (April 30,
2004); 77 FR 28423 (May 14, 2012); 77 FR 71551 (December 3, 2012).
---------------------------------------------------------------------------

    Except as specifically provided in CAA section 182(b)(1)(C),
emissions reductions from all SIP-approved, federally promulgated, or
otherwise SIP-creditable measures that occur after the baseline year
are creditable for purposes of demonstrating that the RFP targets are
met. Because the EPA has determined that the passage of time has caused
the effect of certain exclusions to be de minimis, the RFP
demonstration is no longer required to calculate and specifically
exclude reductions from measures related to motor vehicle exhaust or
evaporative emissions promulgated by January 1, 1990; regulations
concerning Reid vapor pressure promulgated by November 15, 1990;
measures to correct previous RACT requirements; and, measures required
to correct previous inspection and maintenance (I/M) programs.\98\
---------------------------------------------------------------------------

    \98\ 40 CFR 51.1110(a)(7).
---------------------------------------------------------------------------

    The 2008 Ozone SRR requires the RFP baseline year to be the most
recent calendar year for which a complete triennial inventory was
required to be submitted to the EPA. For the purposes of developing RFP
demonstrations for the 2008 ozone NAAQS, the applicable triennial
inventory year is 2011. As discussed previously, the 2008 Ozone SRR
provided states with the opportunity to use an alternative baseline
year for RFP,\99\ but this provision was vacated by the D.C. Circuit in
the South Coast II decision.
---------------------------------------------------------------------------

    \99\ See 40 CFR 51.1110(b).
---------------------------------------------------------------------------

2. Summary of the State's Submission
    Documentation for the Western Nevada County RFP baseline and
milestone emissions inventories is found in the 2018 Western Nevada
County Ozone Plan on pages 21-34, 54-56, and in Appendix A. Consistent
with the South Coast II decision, CARB's RFP demonstration for Western
Nevada County uses a 2011 RFP baseline emissions inventory.\100\ To
develop the 2011 RFP baseline inventory, CARB relied on actual
emissions reported from industrial point sources for year 2011 and
backcasted emissions from smaller stationary sources and area sources
from

2012 to 2011 using the same growth and control factors used for future
years.\101\ The Plan indicates that the 2012 inventory base year for
modeling and the 2011 baseline year inventory for RFP are consistent
with each other since they both use actual emissions for stationary
sources and the same growth profiles. Emissions estimates in the
baseline emissions inventory reflect District and CARB rules submitted
to the EPA through November 2016.
---------------------------------------------------------------------------

    \101\ 2018 Western Nevada County Ozone Plan, page 23.
---------------------------------------------------------------------------

    The RFP demonstration for Western Nevada County for the 2008 ozone
NAAQS is shown in Table 10 of the 2018 Western Nevada County Ozone
Plan, which is reproduced as Table 4 below. As Western Nevada County is
a Serious nonattainment area without a previously approved ROP plan,
the Plan demonstrates a reduction in VOC of 15 percent from baseline
emissions within six years of the RFP baseline year period, consistent
with CAA 182(b)(1). The Plan shows an additional 3 percent reduction of
VOC or NOX emissions, averaged over each consecutive 3-year
period until the attainment year. The RFP demonstration calculates
future year VOC targets from the 2011 baseline, consistent with CAA
182(c)(2)(B)(i), and it substitutes NOX reductions for VOC
reductions beginning in milestone year 2020 to meet VOC emission
targets as allowed under CAA section 182(c)(2)(C).\102\ CARB concludes
that the RFP demonstration meets the applicable requirements for each
milestone year as well as the attainment year.
---------------------------------------------------------------------------

    \102\ See also 40 CFR 51.1110(a)(2)(i)(C) and 40 CFR
51.1110(a)(2)(ii)(B); and 70 FR 12264, at 12271 (March 6, 2015). The
District's RFP demonstration substitutes NOX reductions
for VOC reductions on a percentage basis. See EPA, NOX
Substitution Guidance (December 1993).

       Table 4--2008 Ozone RFP Demonstration Western Nevada County
               [Summer planning inventory, tpd or percent]
------------------------------------------------------------------------
                                                    VOC
                                  --------------------------------------
                                       2011         2017         2020
------------------------------------------------------------------------
Baseline VOC.....................         5.50         4.50         4.20
Required change since 2011 (VOC    ...........           15           24
 or NOX), %......................
Target VOC level.................  ...........          4.7          4.2
Apparent shortfall (-)/surplus     ...........         +0.2         -0.1
 (+) in VOC......................
Apparent shortfall (-)/surplus     ...........         +3.2         -1.4
 (+) in VOC, %...................
Actual VOC shortfall (-)/surplus   ...........         +3.2         -1.4
 (+), %..........................
------------------------------------------------------------------------

                                                    NOX
                                  --------------------------------------
                                       2011         2017         2020
------------------------------------------------------------------------
Baseline NOX.....................         5.69         3.74         2.89
Change in NOX since 2011.........  ...........         1.95          2.8
Change in NOX since 2011, %......  ...........           34           49
NOX reductions used for VOC        ...........  ...........          3.1
 substitution through last
 milestone year, %...............
NOX reductions since 2011          ...........           34           49
 available for VOC substitution
 in this milestone year, %.......
NOX reductions since 2011 used     ...........            0          3.1
 for VOC substitution in this
 milestone year, %...............
NOX reductions since 2011 surplus  ...........           34         45.9
 after meeting VOC substitution
 needs in this milestone year, %.
Total shortfall for RFP..........  ...........            0            0
RFP met?.........................  ...........          Yes          Yes
------------------------------------------------------------------------
Source: 2018 Western Nevada County Ozone Plan, Table 10, p. 55.

3. The EPA's Review of the State's Submission
    Based on our review of the emissions inventory documentation in the
2018 Western Nevada County Ozone Plan, we find that CARB and the
District have used the most recent planning and activity assumptions,
emissions models, and methodologies in developing the RFP baseline and
milestone year emissions inventories. We have also reviewed the
calculations in Table 10 of the Plan and presented in Table 4 above and
find that the District and CARB have used an appropriate calculation
method to demonstrate RFP.
    We have also reviewed the comparison of the VOC emission reductions
against the 15 percent ROP requirement. As shown in Table 4, the RFP
demonstration shows that Western Nevada County meets the 15 percent
reduction in VOC emissions with an additional 3.2 percent surplus in
VOC emissions reductions from 2011 to 2017. Such reductions satisfy the
ROP requirement for Western Nevada County for the 2008 ozone NAAQS. As
a result, we find that the District and CARB have met the ROP
requirements of CAA section 182(b)(1) for Western Nevada County with
respect to the 2008 ozone NAAQS.
    We find that the District's use of substitution of NOX
reductions for VOC reductions in this demonstration is appropriate
under CAA section 182(c)(2)(C). As described in Section III.D.2.a of
this document, ozone formation in Western Nevada County is
NOX-limited, and the substituted NOX reductions
are expected to achieve an equal or greater reduction in ozone
concentrations as would result from the VOC emissions reductions
described in CAA section 182(c)(2)(B).\103\
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    \103\ As discussed above, modeling for the Sacramento
nonattainment area used a modeling domain that encompassed the
Western Nevada nonattainment area, and was used to create an
isopleth diagram showing ozone for various levels of NOX
and VOC emissions. Sacramento Regional 2008 NAAQS 8-hour Attainment
and Reasonable Further Progress Plan (``2017 Sacramento Regional
Ozone Plan''), July 24, 2017, Appendix B-4, p.B-158, Figure 16,
available at https://ww2.arb.ca.gov/resources/documents/2017-sacramento-regional-2008-8-hour-ozone-attainment-and-further-reasonable. The EPA used this information to estimate the
sensitivity of ozone to NOX reductions and to VOC
reductions, and found NOX reductions to be 23 times as
effective at reducing ozone as VOC reductions, on a tonnage basis,
and 13 times as effective on a percentage basis. Docket EPA-R09-OAR-
2020-0425, item A-86, ``Assessment of Sacramento Metro NAA
Conformity Motor Vehicle Emissions Budget Consistency with O3 NAAQS
Attainment,'' September 14, 2020, Air and Radiation Division, EPA
Region IX.

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    For these reasons, we have determined that the 2018 Western Nevada
County Ozone Plan demonstrates RFP in each milestone year and the
attainment year, consistent with applicable CAA requirements and EPA
guidance. We therefore propose to approve the RFP demonstrations for
the Western Nevada County nonattainment area for the 2008 ozone NAAQS
under sections 172(c)(2), 182(b)(1) and 182(c)(2)(B) of the CAA and 40
CFR 51.1110(a)(2)(ii).

F. Contingency Measures

1. Statutory and Regulatory Requirements
    Under the CAA, 8-hour ozone nonattainment areas classified under
subpart 2 as Moderate or above must include in their SIPs contingency
measures consistent with sections 172(c)(9) and 182(c)(9). Contingency
measures are additional controls or measures to be implemented in the
event the area fails to make reasonable further progress or to attain
the NAAQS by the attainment date. The SIP should contain trigger
mechanisms for the contingency measures, specify a schedule for
implementation, and indicate that the measure will be implemented
without significant further action by the state or the EPA.\104\
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    \104\ 70 FR 71612 (November 29, 2005). See also 2008 Ozone SRR,
80 FR 12264, at 12285 (March 6, 2015).
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    Neither the CAA nor the EPA's implementing regulations establish a
specific level of emissions reductions that implementation of
contingency measures must achieve, but the EPA's 2008 Ozone SRR
reiterates the EPA's policy that contingency measures should generally
provide for emissions reductions approximately equivalent to one year's
worth progress, amounting to reductions of 3 percent of the baseline
emissions inventory for the nonattainment area.\105\
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    \105\ 80 FR 12264 at 12285 (March 6, 2015).
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    It has been the EPA's longstanding interpretation of CAA section
172(c)(9) that states may rely on federal measures (e.g., federal
mobile source measures based on the incremental turnover of the motor
vehicle fleet each year) and local measures already scheduled for
implementation that provide emissions reductions in excess of those
needed to provide for RFP or expeditious attainment. The key is that
the Act requires that contingency measures provide for additional
emissions reductions that are not relied on for RFP or attainment and
that are not included in the RFP or attainment demonstrations as
meeting part or all of the contingency measures requirements. The
purpose of contingency measures is to provide continued emissions
reductions while the plan is being revised to meet the missed milestone
or attainment date.
    The EPA has approved numerous SIPs under this interpretation, i.e.,
SIPs that use as contingency measures one or more federal or local
measures that are in place and provide reductions that are in excess of
the reductions required by the attainment demonstration or RFP
plan,\106\ and there is case law supporting the EPA's interpretation in
this regard.\107\ However, in Bahr v. EPA, the United States Court of
Appeals for the Ninth Circuit (``Ninth Circuit'') rejected the EPA's
interpretation of CAA section 172(c)(9) as allowing for early
implementation of contingency measures.\108\ The Ninth Circuit
concluded that contingency measures must take effect at the time the
area fails to make RFP or attain by the applicable attainment date, not
before.\109\ Thus, within the geographic jurisdiction of the Ninth
Circuit, states cannot rely on early-implemented measures to comply
with the contingency measures requirements under CAA section 172(c)(9)
and 182(c)(9).\110\
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    \106\ See, e.g., 62 FR 15844 (April 3, 1997) (direct final rule
approving an Indiana ozone SIP revision); 62 FR 66279 (December 18,
1997) (final rule approving an Illinois ozone SIP revision); 66 FR
30811 (June 8, 2001) (direct final rule approving a Rhode Island
ozone SIP revision); 66 FR 586 (January 3, 2001) (final rule
approving District of Columbia, Maryland, and Virginia ozone SIP
revisions); and 66 FR 634 (January 3, 2001) (final rule approving a
Connecticut ozone SIP revision).
    \107\ See, e.g., LEAN v. EPA, 382 F.3d 575 (5th Cir. 2004)
(upholding contingency measures that were previously required and
implemented where they were in excess of the attainment
demonstration and RFP SIP).
    \108\ Bahr v. EPA, 836 F.3d 1218, at 1235-1237 (9th Cir. 2016).
    \109\ Id. at 1235-1237.
    \110\ The Bahr v. EPA decision involved a challenge to an EPA
approval of contingency measures under the general nonattainment
area plan provisions for contingency measures in CAA section
172(c)(9), but, given the similarity between the statutory language
in section 172(c)(9) and the ozone-specific contingency measures
provision in section 182(c)(9), we find that the decision affects
how both sections of the Act must be interpreted.
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2. Summary of the State's Submission
    In the 2018 Western Nevada County Ozone Plan, CARB calculates the
extent of surplus emission reductions (i.e., surplus to meeting the RFP
milestone requirement for a given milestone year) in the milestone
years and estimates the incremental emissions reductions in the year
following the attainment year.\111\ In light of the Bahr v. EPA
decision, however, the 2018 Western Nevada County Ozone Plan does not
rely on the surplus or incremental emissions reductions to comply with
the contingency measures requirements of sections 172(c)(9) and
182(c)(9) but, rather, to provide context in which to evaluate the
adequacy of Bahr-compliant (i.e., to take effect if triggered)
contingency measures for the 2008 ozone NAAQS.
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    \111\ CARB Staff Report, Section D, 10-11.
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    To comply with sections 172(c)(9) and 182(c)(9), as interpreted in
the Bahr v. EPA decision, the state must develop, adopt and submit a
contingency measure to be triggered upon a failure to meet RFP
milestones or failure to attain the NAAQS by the applicable attainment
date regardless of the extent to which already-implemented measures
would achieve surplus emissions reductions beyond those necessary to
meet RFP milestones and beyond those predicted to achieve attainment of
the NAAQS. Therefore, to fully address the contingency measures
requirement for the 2008 ozone NAAQS in the Western Nevada
nonattainment area, the District has committed to develop, adopt and
submit a contingency measure to CARB in sufficient time to allow CARB
to submit the contingency measure as a SIP revision to the EPA within
12 months of the EPA's final conditional approval of the contingency
measures element of the 2018 Western Nevada County Ozone Plan.\112\
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    \112\ Letter dated October 26, 2020, from Gretchen Bennitt,
Executive Director, NSAQMD, to Richard Corey, Executive Officer,
CARB.
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    The District's commitment is to adopt the 2019 (or most recent)
Architectural Coatings Suggested Control Measure (SCM), developed and
approved by CARB, as a rule to take effect upon adoption throughout the
nonattainment area upon a determination that the Western Nevada County
nonattainment area failed to meet an RFP milestone or failed to attain
the 2008 ozone NAAQS by the applicable attainment date. The District
estimates that adoption of this new measure will yield an estimated
0.010 tpd in VOC emissions reductions in the nonattainment area. The
District also evaluated three other additional categories for potential
contingency measures. The categories are automotive coatings, water
heaters, boilers, and process heaters rated less than a million BTU per
hour, and miscellaneous combustion units. However, the District
determined that these categories did not yield meaningful
reductions.\113\
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    \113\ Appendix to Letter dated October 26, 2020, from Gretchen
Bennitt, Executive Director, NSAQMD, to Richard Corey, Executive
Officer, CARB.
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    CARB attached the District's commitment to a letter committing CARB
to adopt and submit the revised

NSAQMD rule to the EPA within one year of the EPA's final conditional
approval of the contingency measures element of the 2018 Western Nevada
County Ozone Plan.\114\
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    \114\ Letter dated November 16, 2020, from Richard W. Corey,
Executive Officer, CARB, to John Busterud, Regional Administrator,
EPA Region IX.
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3. The EPA's Review of the State's Submission
    CAA sections 172(c)(9) and 182(c)(9) require contingency measures
to address potential failures to achieve RFP milestones or to attain
the NAAQS by the applicable attainment date through implementation of
additional emissions controls in the event the area fails to make RFP
or to attain the NAAQS by the applicable attainment date. Contingency
measures must provide for the implementation of additional emissions
controls, if triggered, without significant further action by the state
or the EPA. For the purposes of evaluating the adequacy of the
emissions reductions from the contingency measures (once adopted and
submitted), we find it useful to distinguish between contingency
measures to address potential failure to achieve RFP milestones (``RFP
contingency measures'') and contingency measures to address potential
failure to attain the NAAQS (``attainment contingency measures'').
    With respect to the RFP contingency measures requirement, we have
reviewed the surplus emissions estimates in each of the RFP milestone
years, as shown in CARB's Staff Report, and find that the calculations
are correct. We therefore agree that the 2018 Western Nevada County
Ozone Plan provides surplus emissions reductions well beyond those
necessary to demonstrate RFP in all of the RFP milestone years. While
such surplus emissions reductions in the RFP milestone years do not
represent contingency measures themselves, we believe they are relevant
in evaluating the adequacy of RFP contingency measures that are
submitted (or will be submitted) to meet the requirements of sections
172(c)(9) and 182(c)(9).
    The attainment year for the 2008 ozone NAAQS in Western Nevada
County coincides with the 2020 RFP milestone, and thus, we have
reviewed the emissions reductions estimated by the District for the
committed contingency measures in light of the facts and circumstances
in Western Nevada County in the year following the attainment year, to
determine whether there will be sufficient continued progress in that
area in the event the area fails to achieve the 2020 RFP milestone or
fails to attain the 2008 ozone NAAQS by the 2020 attainment year.\115\
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    \115\ CAA section 182(g)(2) provides that states must submit RFP
milestone compliance demonstrations within 90 days after the date on
which an applicable milestone occurs, except where the milestone and
attainment date are the same and the standard has been attained.
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    As discussed above, 2018 Western Nevada County Ozone Plan provides
estimates of emissions reductions that are surplus of the reductions
necessary for RFP or attainment, but does not include measures that
would implement additional emissions controls, if triggered, without
significant further action by the state or the EPA. However, CARB and
the District have submitted commitments to adopt and submit a revised
District rule with the necessary provisions as a SIP revision within
one year of the EPA's final action on the contingency measures element
of the Plan. The specific revisions the District has committed to make,
such as tightening control efficiencies or establishing content limits,
upon a failure to achieve a milestone or a failure to attain, would
comply with the requirements in CAA sections 172(c)(9) and 182(c)(9)
because the additional controls would be undertaken if the area fails
to achieve a milestone or fails to attain, and would take effect
without significant further action by the State or the EPA.
    We find that the contingency measures described in the District and
CARB's commitment letters would provide adequate emissions reductions
when triggered. Neither the CAA nor the EPA's implementing regulations
for the ozone NAAQS establish a specific amount of emissions reductions
that implementation of contingency measures must achieve, but we
generally expect that contingency measures should provide for emissions
reductions approximately equivalent to one year's worth of RFP, which,
for ozone, amounts to reductions of 3 percent of the RFP baseline year
emissions inventory for the nonattainment area. For the 2008 ozone
NAAQS in Western Nevada County, one year's worth of RFP is
approximately 0.16 tpd of VOC or 0.17 tpd of NOX
reductions.\116\ The District's commitment letter estimates the
potential additional emission reductions from its contingency measure
commitment at 0.010 tpd VOC. However, emissions in the year following
the attainment year (2021) in Western Nevada County are expected to be
approximately 0.048 tpd lower for VOC and 0.23 tpd lower for
NOX than in the attainment year (2020).\117\ The downward
trend in emissions reflects the continuing benefits of already-
implemented measures and is primarily the result of vehicle turnover,
which refers to the ongoing replacement by individuals, companies, and
government agencies of older, more polluting vehicles and engines with
newer vehicles and engines. While the continuing reductions from such
already-implemented measures do not constitute contingency measures
themselves, they provide context in which we evaluate the adequacy of
the contingency measures submitted (or, in this case, to be submitted)
to fulfill the requirements of CAA sections 172(c)(9) and 182(c)(9).
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    \116\ One year's worth of RFP for Western Nevada County
corresponds to 3 percent of the 2011 RFP baseline year inventories
for VOC (5.496 tpd) and NOX (5.687 tpd).
    \117\ Estimates for the emissions reductions in the year
following the attainment year are based on the emissions inventories
for Western Nevada County in Appendix A of the Plan. The estimate of
the reductions in emissions of 0.048 tpd of VOC and 0.23 tpd of
NOX in 2021 (relative to 2020) amounts to approximately
29 percent and 132 percent of one year's worth of progress,
respectively in this area based on the 2011 RFP baseline inventory.
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    In this instance, we find that the emissions reductions from the
to-be-adopted contingency measures together with the reductions
expected to occur due to already-implemented measures are consistent
with our guidance recommending that contingency measures provide for
one year's worth of progress in the event of a failure to meet an RFP
milestone or a failure to attain the NAAQS by the applicable attainment
date. Therefore, in light of the year-to-year reductions in the VOC and
NOX inventories, we find that the contingency measures