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Federal Register / Vol. 86, No. 112 / Monday, June 14, 2021 / Rules and Regulations III. What is ozone and how is it formed?
Ground-level ozone is a gas that is formed by the reaction of volatile organic compounds VOCs and oxides of nitrogen NOX in the atmosphere in the presence of sunlight. These precursor emissions are emitted by many types of pollution sources, including power plants and industrial emissions sources, on-road and off-road motor vehicles and engines, and smaller sources, collectively referred to as area sources. Ozone is predominately a summertime air pollutant. However, high ozone concentrations have also been observed in cold months, where a few areas in the Western United States U.S. have experienced high levels of local VOC and NOX emissions that have formed ozone when snow is on the ground and temperatures are near or below freezing. Ozone and ozone precursors can be transported to an area from sources in nearby areas or from sources located hundreds of miles away.
For purposes of determining ozone nonattainment area boundaries, the CAA requires the EPA to include areas that contribute to nearby violations of the NAAQS.

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IV. What are the 2015 ozone NAAQS
and the health and welfare concerns they address?
On October 1, 2015, the EPA revised both the primary and secondary NAAQS
for ozone to a level of 0.070 ppm annual fourth-highest daily maximum 8-hour average concentration, averaged over 3 years.3 The level of the ozone NAAQS previously set in 2008 is 0.75
ppm. The 2015 ozone NAAQS retain the same general form and averaging time as the 2008 ozone NAAQS.
The primary ozone standards provide protection for children, older adults, and people with asthma or other lung diseases, and other at-risk populations against an array of adverse health effects that include reduced lung function, increased respiratory symptoms and pulmonary inflammation; effects that contribute to emergency department visits or hospital admissions; and mortality. The secondary ozone standards protect against adverse effects to the public welfare, including those related to impacts on sensitive vegetation and forested ecosystems.
V. What are the CAA requirements for air quality designations?
After the EPA promulgates a new or revised NAAQS, the EPA is required to designate all areas in the country as 3 See 80 FR 65296; October 26, 2015, for a detailed explanation of the calculation of the 3-year, 8-hour average and 40 CFR part 50, appendix U.

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nonattainment, attainment, or unclassifiable, for that NAAQS pursuant to section 107d12 of the CAA.
Section 107d1Ai of the CAA
defines a nonattainment area as an area that does not meet the NAAQS or that contributes to a nearby area that does not meet the NAAQS. An attainment area is defined by the CAA as any area that meets the NAAQS and does not contribute to any nearby areas that do not meet the NAAQS. Unclassifiable areas are defined by the CAA as those that cannot be classified on the basis of available information as meeting or not meeting the NAAQS
Historically for ozone, the EPA has designated most areas that do not meet the definition of nonattainment as unclassifiable/attainment. This category includes areas that have air quality monitoring data meeting the NAAQS
and areas that do not have monitors but for which the EPA has no evidence that the areas may be violating the NAAQS
or contributing to a nearby violation. In the designations for the 2015 ozone NAAQS, the EPA has reversed the order of the label to be attainment/
unclassifiable to better convey the definition of the designation category and so that the category is more easily distinguished from the separate unclassifiable category. In a few instances, based on circumstances where some monitoring data are available but are not sufficient for a determination that an area is or is not attaining the NAAQS, the EPA has designated an area as unclassifiable.
The EPA notes that CAA section 107d provides the Agency with discretion to determine how best to interpret the terms in the definition of a nonattainment area e.g., contributes to and nearby for a new or revised NAAQS, given considerations such as the nature of a specific pollutant, the types of sources that may contribute to violations, the form of the standards for the pollutant, and other relevant information. In particular, the EPAs position is that the statute does not require the Agency to establish bright line tests or thresholds for what constitutes contribution or nearby for purposes of designations.4
Similarly, the EPAs position is that the statute permits the EPA to evaluate the appropriate application of the term area to include geographic areas based upon full or partial county boundaries, as may be appropriate for a particular NAAQS. For example, CAA
section 107d1Bii explicitly provides that the EPA can make 4 This view was confirmed in Catawba County v.
EPA, 571 F.3d 20 D.C. Cir. 2009.

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modifications to designation recommendations for an area or portions thereof, and under CAA
section 107d1Biv a designation remains in effect for an area or portion thereof until the EPA redesignates it.
Section 107d1B of the CAA
requires the EPA to issue initial area designations within 2 years of promulgating a new or revised NAAQS.
However, if the Administrator has insufficient information to make these designations within that time frame, the EPA has the authority to extend the deadline for designation decisions by up to 1 additional year.
By no later than 1 year after the promulgation of a new or revised NAAQS, CAA section 107d1A
provides that each state governor shall recommend air quality designations, including the appropriate boundaries for areas, to the EPA. The EPA reviews those state recommendations and is authorized to make any modifications the Administrator deems necessary. The statute does not define the term necessary, but the EPA interprets this to authorize the Administrator to modify designation recommendations that are inconsistent with the statutory language, including modification of recommended boundaries for nonattainment areas that are not supported by the facts or analysis. If the EPA intends to modify a states recommendation, section 107d1B of the CAA requires the EPA to notify the state of any such intended modifications not less than 120 days prior to the EPAs promulgation of the final designation.
These notifications are commonly known as the 120-day letters. During this period, if the state does not agree with the EPAs proposed modification, it has an opportunity to respond to the EPA and to demonstrate why it believes the modification proposed by the EPA is inappropriate. If a state fails to provide any recommendation for an area, in whole or in part, the EPA must promulgate a designation that the Administrator deems appropriate, pursuant to CAA section 107d1Bii.
Section 301d of the CAA authorizes the EPA to approve eligible Indian tribes to implement provisions of the CAA on Indian reservations and other areas within the tribes jurisdiction. The Tribal Authority Rule TAR 40 CFR
part 49, which implements section 301d of the CAA, sets forth the criteria and process for tribes to apply to the EPA for eligibility to administer CAA
programs. The designations process contained in section 107d of the CAA
is included among those provisions determined to be appropriate by the
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