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Air Modeling GuideCovers AERMOD 24142 & 26135 — every excerpt version-stamped · current NAAQS

The criteria pollutants & the NAAQS

The National Ambient Air Quality Standards (NAAQS) are set forth in 40 CFR Part 50 — one section per pollutant and standard. Six pollutants carry these standards: nitrogen dioxide, sulfur dioxide, particulate matter (as PM2.5 and PM10), carbon monoxide, ozone, and lead. Every regulatory dispersion-modeling demonstration compares its result against one or more of them.

Primary vs. secondary standards

The regulation defines the two kinds of standard directly:

National primary ambient air quality standards define levels of air quality which the Administrator judges are necessary, with an adequate margin of safety, to protect the public health. National secondary ambient air quality standards define levels of air quality which the Administrator judges necessary to protect the public welfare from any known or anticipated adverse effects of a pollutant.

Source · 40 CFR §50.2(b) — captured from eCFR 2026-08-17.

A pollutant can carry both kinds at the same or different levels — or only one: SO₂'s 1-hour standard is primary-only (§50.17), its 3-hour standard secondary-only (§50.5).

The six pollutants and their current standards

Nitrogen dioxide (NO₂) — 1-hour primary: 100 ppb, as the 3-year average of the annual 98th percentile of the daily maximum 1-hour concentration. Annual primary: 53 ppb, met when the annual average "in a calendar year" is at or below the level — the highest individual year governs. In modeling, NO₂ has dedicated chemistry options for the conversion of NOₓ to NO₂ — ARM2, OLM, PVMRM, GRSM, and TTRM/TTRM2 — described in the AERMOD User's Guide (§3.2.5.1 for the ozone inputs these options take; §3.3.6.3 for ARM2's ambient ratios), with the regulatory framework in Appendix W §4.2.3.4, Models for Nitrogen Dioxide.

Source · 40 CFR §50.11(a), (b), (e), (f) · AERMOD User's Guide (EPA-454/B-26-001, 26135) §3.2.5.1, §3.3.6.3 · 40 CFR 51 App. W §4.2.3.4.

Sulfur dioxide (SO₂) — 1-hour primary: 75 ppb, as the 3-year average of the annual 99th percentile of the daily maximum 1-hour concentration. 3-hour secondary: 0.5 ppm, not to be exceeded more than once per calendar year. The pre-2010 annual (0.030 ppm) and 24-hour (0.14 ppm) primary standards at §50.4 remain codified and continue to apply in certain areas until the 1-hour standard is fully implemented there (§50.4(e)).

Source · 40 CFR §50.17(a), (b) · §50.5(a) · §50.4.

Fine particulate matter (PM2.5) — measured "as PM2.5 (particles with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers)." Annual primary: 9.0 µg/m³ annual arithmetic mean (the 2024 revision), determined per Part 50 Appendix N. 24-hour primary: 35 µg/m³ as the 98th percentile of 24-hour concentrations, also per Appendix N. The prior 2012 standards at §50.18 (annual 12.0 µg/m³) remain codified for reference but are not the current primary annual standard.

Source · 40 CFR §50.20(a), (b), (c) and Part 50 Appendix N.

Coarse particulate matter (PM10) — 24-hour, primary and secondary: 150 µg/m³, attained "when the expected number of days per calendar year with a 24-hour average concentration above 150 µg/m³ … is equal to or less than one," determined per Part 50 Appendix K.

Source · 40 CFR §50.6(a).

Carbon monoxide (CO) — primary standards: 9 ppm (10 mg/m³) as an 8-hour average and 35 ppm (40 mg/m³) as a 1-hour average, each not to be exceeded more than once per year.

Source · 40 CFR §50.8(a).

Ozone (O₃) — 8-hour primary and secondary: 0.070 ppm, as the 3-year average of the annual fourth-highest daily maximum 8-hour concentration. Ozone is not modeled with AERMOD: Appendix W addresses it separately under §5.0, Models for Ozone and Secondarily Formed Particulate Matter (recommended approaches in §5.3). In an AERMOD analysis, ambient ozone appears only as an input — the oxidant concentration the NO₂ chemistry options consume (AERMOD UG §3.2.5.1).

Source · 40 CFR §50.19(a), (b) · 40 CFR 51 App. W §5.0, §5.3.

Lead (Pb) — primary and secondary: 0.15 µg/m³, met "when the maximum arithmetic 3-month mean concentration for a 3-year period" is at or below the level. The 1978 standard (1.5 µg/m³, calendar-quarter mean) at §50.12 remains applicable to an area until one year after that area's designation under the current standard (§50.12(b)). Appendix W carries a dedicated section on lead modeling (§4.2.3.2, Models for Lead).

Source · 40 CFR §50.16(a), (b) · §50.12 · App. W §4.2.3.2.

What "meeting the NAAQS" means in a modeling demonstration

The comparison is never a raw model output against these numbers: each standard's form (the statistics quoted above — percentiles of daily maxima, calendar-year means, expected exceedances) defines the design value that gets compared, and Appendix W directs that background concentrations be combined with the modeled result for the comparison (App. W §8.3). The next pages walk both halves:

  • Averaging periods & design-value forms — how each statistic is built.
  • Background concentrations & representative monitors — App. W §8.3's rules for the other half of the sum.

In PlumeSmart

Every standard above — level, form, and citation — is built into the platform's compliance tables; each reported design value is computed in its standard's own form and labeled with its regulatory basis.