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Federal Register / Vol. 86, No. 150 / Monday, August 9, 2021 / Proposed Rules
Nevada October 2018 herein, referred to as the 2018 Ozone Maintenance Plan Revision.2 The 2020 Ozone Maintenance Plan Revision revises certain budgets from the 2018
Ozone Maintenance Plan Revision to prevent interference with reasonable further progress or attainment of the 2008 and 2015 ozone NAAQS. If the EPA takes final action to approve the 2020 Ozone Maintenance Plan Revision, the revised budgets will replace Clark Countys existing budgets for the plan horizon year 2022 for the 1997 ozone NAAQS. At that time, the previouslyapproved budgets would no longer be applicable for transportation conformity purposes, and the revised budgets would need to be used beginning on the publication date of the EPAs final approval in the Federal Register.3
II. Background A. NAAQS, SIPs, Designations, and Clark County 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. Under CAA section 107d, the EPA must designate all areas of the country as attainment, nonattainment or unclassifiable for new or revised NAAQS. Section 110 of the CAA
requires states to develop and submit SIPs to implement, maintain, and enforce the NAAQS. Once a nonattainment area has attained the NAAQS, the state may request redesignation of the area from nonattainment to attainment, and the EPA grants such requests if the criteria in CAA section 107d3E are met, including the approval of a maintenance plan under CAA section 175A that demonstrates how the area will maintain the NAAQS for at least 10
years after the redesignation. Such former nonattainment areas that have been redesignated to attainment are referred to as maintenance areas.
In 1997, the EPA replaced the 1-hour ozone 4 NAAQS at a level of 0.12 parts per million ppm with an 8-hour ozone NAAQS at a level of 0.08 ppm herein,
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FR 44699 August 27, 2019.
CFR 93.118f2v.
4 Ground-level ozone pollution is formed from the reaction of volatile organic compounds VOC and oxides of nitrogen NOX in the presence of sunlight. 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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the 1997 ozone NAAQS.5 In 2004, the EPA designated a portion of Clark County as a Subpart 1 nonattainment area for the 1997 ozone NAAQS.6 In 2011, the EPA determined that the Clark County 8-hour ozone nonattainment area had attained the 1997 8-hour ozone NAAQS, based on complete, qualityassured, and certified ambient air monitoring data that showed the area monitored attainment of the 1997 ozone NAAQS for the 20072009 monitoring period.7
In light of ambient monitoring data showing that the Clark County ozone nonattainment area had attained the 1997 ozone NAAQS, NDEP submitted a request to redesignate the Clark County ozone area from nonattainment to attainment and submitted the Ozone Redesignation Request and Maintenance Plan, Clark County, Nevada March 2011 herein, the 2011 Ozone Maintenance Plan to the EPA for approval as a revision to the Clark County portion of the Nevada SIP.
Prepared by the Clark County Department of Air Quality and Environmental Management currently named Department of Environment and Sustainability DES,8 the 2011
Ozone Maintenance Plan includes the required elements for maintenance plans, including an attainment inventory, maintenance demonstration, monitoring network, verification of continued attainment, contingency plan, and budgets.9 The 2011 Ozone 5 62

FR 38856 July 18, 1997 and 40 CFR 50.10.
FR 23858 April 30, 2004 and 69 FR 55956
September 17, 2004. The Clark County ozone nonattainment area for the 1997 ozone NAAQS
includes a significant portion of the unincorporated portions of central and southern Clark County, as well as the cities of Las Vegas, Henderson, North Las Vegas, and Boulder City. The Subpart 1
classification meant that the area was subject solely to the general nonattainment area requirements under subpart 1 of part D of title I of the CAA
rather than to the requirements under both subparts 1 and the ozone-specific requirements under subpart 2. Several years later, in response to litigation over the designations for the 1997 ozone NAAQS, the EPA revised the classification of the Clark County ozone nonattainment area from Subpart 1 to Subpart 2/Marginal. 77 FR 28424
May 14, 2012.
7 76 FR 17343 March 29, 2011.
8 In the State of Nevada, NDEP is the Governors designee for adoption and submittal of SIPs and SIP
revisions to the EPA. In Clark County, the Clark County DES is responsible under state law for regulation of most types of stationary sources within the county and for development of local air quality plans. Once adopted by the Clark County Board of County Commissioners, such county plans are forwarded to NDEP for adoption and submittal to the EPA as revisions to the Nevada SIP.
9 Under the EPAs transportation conformity rule, at 40 CFR 93.101, budgets are defined as the portions of the total allowable emissions that are allocated to on-road vehicle use that, together with emissions from other sources in the area, will provide for RFP, attainment or maintenance. The 6 69

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Maintenance Plan demonstrates maintenance of the 1997 ozone NAAQS
through year 2022 by reference to emissions inventories developed for years 2015 and 2022 that show emissions of volatile organic compounds VOC and oxides of nitrogen NOX in those years would not exceed the level of the corresponding emissions of the 2008 attainment inventory. The 2011 Ozone Maintenance Plan established budgets for NOX and VOC for years 2008, 2015, and 2022. The budgets were derived from the on-road motor vehicle emissions estimates prepared using the EPAs then-current on-road vehicle emissions model, MOBILE6.2, and the most recent vehicle mix and activity data available from the Regional Transportation Commission of Southern Nevada. In 2013, the EPA approved the 2011 Ozone Maintenance Plan and redesignated the Clark County ozone nonattainment area to attainment for the 1997 ozone NAAQS.10
Through adoption of the 2011 Ozone Maintenance Plan, Clark County DES
committed to maintaining an ambient air quality monitoring network to verify the continued attainment of the 1997
ozone NAAQS in the Clark County ozone maintenance area.11 At the present time, 10 monitoring sites continuously monitor ambient concentrations of ozone within the maintenance area. Since 2008, i.e., the year used for the attainment inventory in the 2011 Ozone Maintenance Plan, ambient ozone concentrations in Clark County have decreased. As shown in Table 1, 8-hour ozone design values have decreased from 0.082 ppm in 2008
to 0.073 ppm in 2019.12 In more recent years, the design value has remained relatively steady, varying little from year to year. Table 1 shows that Clark County has maintained the 1997 ozone NAAQS
through the first seven years 2013
through 2019 of the first maintenance period.
budgets serve as a ceiling on emissions from an areas planned transportation system.
10 78 FR 1149 January 8, 2013.
11 2011 Ozone Maintenance Plan, 611.
12 Under EPA regulations at 40 CFR 50.10 and appendix I, the 1997 ozone NAAQS is attained at a site when the 3-year average of the annual fourthhighest daily maximum 8-hour average ozone concentration is less than or equal to 0.08 ppm.
This 3-year average is referred to as the design value. When the design value is less than or equal to 0.084 ppm based on the rounding convention in 40 CFR part 50, appendix I at each monitoring site within the area, then the area is meeting the 1997
ozone NAAQS. The highest design value among the various ozone monitoring sites represents the design value for the area.

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