Attainment, nonattainment & unclassifiable areas

The National Ambient Air Quality Standards (NAAQS) are enforced geographically: every part of the country carries a designation for each pollutant, and that designation decides which permitting program — and therefore which modeling demonstration — applies to a new or modified source there.

The three designations

The Clean Air Act (CAA) defines them directly. After each new or revised NAAQS, each state must submit a list of its areas, designating as:

(i) nonattainment, any area that does not meet (or that contributes to ambient air quality in a nearby area that does not meet) the national primary or secondary ambient air quality standard for the pollutant,

(ii) attainment, any area (other than an area identified in clause (i)) that meets the national primary or secondary ambient air quality standard for the pollutant, or

(iii) unclassifiable, any area that cannot be classified on the basis of available information as meeting or not meeting the national primary or secondary ambient air quality standard for the pollutant.

Source · Clean Air Act §107(d)(1)(A), 42 U.S.C. §7407(d)(1)(A).

Two details in the statute's own words are worth noticing: an area can be nonattainment for contributing to a violation in a nearby area, not only for violating locally; and "unclassifiable" is a real designation — areas without enough data default there, and are treated like attainment for permitting purposes (see below).

Where designations live

The designations themselves are codified — county by county, area by area — in 40 CFR Part 81, Subpart C:

(a) Attainment status designations as approved or designated by the Environmental Protection Agency (EPA) pursuant to section 107 of the CAA are listed in this subpart. Area designations are subject to revision whenever sufficient data become available to warrant a redesignation.

Source · 40 CFR §81.300(a). EPA publishes the working compilation as the Green Book.

Designations are pollutant-by-pollutant: the same county can be attainment for SO₂ and nonattainment for ozone at the same time, and each pollutant's designation controls that pollutant's permitting path.

Maintenance areas — attainment, with a plan attached

A fourth category shows up constantly in the Green Book and is easy to misread: a maintenance area. It is a former nonattainment area that EPA has since redesignated to attainment. The statute only allows that redesignation once the state has an approved plan for staying there:

Each State which submits a request under section 7407(d) of this title for redesignation of a nonattainment area … to an area which has attained the standard shall also submit a revision of the applicable State implementation plan to provide for the maintenance of the national primary ambient air quality standard for such air pollutant in the area concerned for at least 10 years after the redesignation.

Source · Clean Air Act §175A(a), 42 U.S.C. §7505a(a).

For permitting purposes a maintenance area is an attainment area — PSD applies, not nonattainment NSR. What differs is that an approved maintenance plan governs the area, and the reviewing agency may attach requirements under it. So "maintenance" is not a softer flavour of nonattainment; it is attainment, plus an obligation. Carbon monoxide is the clearest illustration: every CO area in the country that was ever designated nonattainment has since been redesignated, so CO appears in the Green Book only as maintenance.

The Green Book records this with a redesignation date — the date the redesignation took effect. An area with no redesignation date has never been redesignated and is still nonattainment. That single field is the whole distinction between the two permitting paths.

Which program — and which modeling demonstration — follows

The Prevention of Significant Deterioration (PSD) rule states its own scope:

(i) The requirements of this section apply to the construction of any new major stationary source … or any project at an existing major stationary source in an area designated as attainment or unclassifiable under sections 107(d)(1)(A)(ii) or (iii) of the Act.

Source · 40 CFR §52.21(a)(2)(i).

So the modeling framework this Part of the guide describes — the Significant Impact Level (SIL) screen, the cumulative NAAQS demonstration, the PSD increments — is the attainment/unclassifiable-area framework. Major sources in nonattainment areas go through the separate nonattainment New Source Review program (Clean Air Act title I, part D), whose currency is emission offsets and lowest-achievable-emission-rate technology rather than an ambient modeling demonstration against the NAAQS.

Why the designation matters inside the model inputs, too

The designation is not just a program switch. It reaches the analysis itself: background monitor selection happens against the area's actual air quality (App. W §8.3), and increment baselines are tracked per baseline area (§52.21(b)(13)–(15)) — both defined over the same geography the designations carve up.

In PlumeSmart

The platform carries EPA's Green Book designations (county designations and nonattainment-area boundaries, refreshed per EPA release) and shows the regulatory setting for a project location — which pollutants are in attainment where you are modeling, with the vintage of the designation data stated.