PSD increments: the second ceiling

In areas that meet the NAAQS, the Clean Air Act's Prevention of Significant Deterioration (PSD) program adds a second, independent limit: air quality may not deteriorate by more than a fixed increment above a baseline, even where the NAAQS itself leaves headroom. The federal PSD rule states it as a limit on increases:

(c) Ambient air increments. In areas designated as Class I, II or III, increases in pollutant concentration over the baseline concentration shall be limited to the following:

Source · 40 CFR §52.21(c) — captured verbatim in the source library.

A PSD demonstration therefore proves two things (§52.21(k)(1)): no NAAQS violation and no increment violation. Passing one does not imply the other — the increment can be the binding constraint in clean areas.

The three area classes

Increments tighten with the area's class. Class I areas — national parks and wilderness areas — get the smallest allowable increases; Class II covers most of the country; Class III (areas designated for industrial growth) the largest:

The maximum allowable increases from §52.21(c), in µg/m³:

Source · 40 CFR §52.21(c) (Class I/II/III tables) — captured verbatim in the source library.

Note what's not here: increments exist only for these pollutants and periods. There is no 1-hour NO₂ or 1-hour SO₂ increment — the 1-hour standards are NAAQS-only demonstrations.

The short-term exceedance allowance

The increment tables carry one qualifier:

For any period other than an annual period, the applicable maximum allowable increase may be exceeded during one such period per year at any one location.

Source · 40 CFR §52.21(c), closing rule.

In modeling terms, a short-term increment demonstration is judged on the highest second-highest value at each receptor — the same "not-to-be-exceeded-more-than-once" logic as the CO and SO₂ 3-hour NAAQS forms. Annual increments may not be exceeded at all, and for NO₂ and SO₂ the annual increment is judged per year — each modeled year's annual mean must stay within it, which is why the highest individual year matters here just as it does for the NO₂ annual NAAQS.

Baseline: the increment measures change, not total

The increment is consumed by concentration increases over the baseline concentration — the air quality as of the area's baseline date, a date fixed by the first complete PSD application in the area. Increment consumption is thus a running ledger across sources: emissions increases after the baseline date consume increment; reductions restore it. The modeled quantity in an increment analysis is the change attributable to increment-consuming emissions, not the total ambient concentration.

Source · 40 CFR §52.21(b)(13)–(15) — "baseline concentration," "baseline date," and "baseline area" definitions.

The demonstration runs on Appendix W models

(l) Air quality models. (1) All estimates of ambient concentrations required under this paragraph shall be based on applicable air quality models, data bases, and other requirements specified in appendix W of part 51 of this chapter (Guideline on Air Quality Models).

Source · 40 CFR §52.21(l)(1) — captured verbatim in the source library.

For near-field demonstrations that means AERMOD, with the same SIL screening ladder described on the previous page applying to increment assessments as well (App. W §9.2.3).

In PlumeSmart

The compliance tables carry every increment above with its class and citation; increment-basis annual values for NO₂ and SO₂ are reported per modeled year, and short-term increment comparisons use the highest-second-high form automatically.