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

Ambient air & receptor placement

A dispersion model computes concentrations wherever you put receptors — so where you are required to put them is itself a regulatory question, with two anchors: the definition of ambient air (where the standards apply) and Appendix W's rules for the modeling domain and receptor network.

Ambient air: where the standards apply

The National Ambient Air Quality Standards (NAAQS) protect, literally, the ambient air — and the U.S. Environmental Protection Agency (EPA) defines that term in one sentence:

Ambient air means that portion of the atmosphere, external to buildings, to which the general public has access.

Source · 40 CFR §50.1(e) — quoted word-for-word from the current regulation text (eCFR, 2026-08-01 edition).

The practical consequence: locations where the general public cannot go — typically the facility's own property inside a secured boundary — are not ambient air, and compliance receptors are not placed there. Everywhere else, including the fence line itself, a receptor may be the controlling one. Whether a particular boundary actually excludes the public is a site-specific determination made with the reviewing agency, so document the basis (fencing, physical barriers, access control) rather than assuming it.

The modeling domain: how far out to model

For a NAAQS or PSD increments assessment, the modeling domain or project's impact area shall include all locations where the emissions of a pollutant from the new or modifying source(s) may cause a significant ambient impact. This impact area is defined as an area with a radius extending from the new or modifying source to: (1) the most distant location where air quality modeling predicts a significant ambient impact will occur, or (2) the nominal 50 km distance considered applicable for Gaussian dispersion models, whichever is less.

Source · 40 CFR 51 App. W §8.1.2(a). The 50 km ceiling is also the transport-distance limit of AERMOD's steady-state formulation (Addendum A, A.1).

So the domain is impact-defined, not arbitrary: it reaches as far as the project's impacts stay significant (the Significant Impact Level (SIL) concept from earlier in this Part), and never beyond the 50-kilometer range where a steady-state Gaussian plume model stops being the right tool.

The receptor network: density where it matters

facility(public excluded)modeling domain — to the farthest significant impact, or 50 km, whichever is lesswhere receptors goeverywhere the general publichas access — "ambient air,"§50.1(e) — starting at thefence linedensity beats count: resolvethe maximum-impact areas ofthe project, of nearby sources,and of all sources combined(§9.2.2(d))two-pass practice: moderate gridfirst, then densify where pass 1shows the peaks
Receptors cover ambient air, densest where the peaks live (App W §9.2.2(d)) — the domain reaches to the significant-impact distance, at most 50 km (§8.1.2)

Appendix W's receptor guidance is refreshingly direct about what counts:

In designing a receptor network, the emphasis should be placed on receptor density and location, not total number of receptors. Typically, the density of receptor sites should be progressively more resolved near the new or modifying source, areas of interest, and areas with the highest concentrations with sufficient detail to determine where possible violations of a NAAQS or PSD increments are most likely to occur. … Locations of particular importance include: (1) the area of maximum impact of the point source; (2) the area of maximum impact of nearby sources; and (3) the area where all sources combine to cause maximum impact.

Source · 40 CFR 51 App. W §9.2.2(d).

The same paragraph endorses the standard two-pass workflow: model once with a moderate network resolved near the source, then "modify the receptor network from the first model run with a denser array of receptors in areas showing potential for high concentrations" — while noting the EPA "neither anticipates nor encourages" endless refinement iterations.

Two details reviewers check at the receptor stage

  • No rounding before the comparison. "Modeled concentrations should not be rounded before comparing the resulting design concentration to the NAAQS or PSD increments" (§9.2.2(b)) — the design value is compared at full precision, at every receptor.
  • The increment forms ride along. At each receptor, short-term Prevention of Significant Deterioration (PSD) increments are judged on the highest-second-highest modeled increase, and annual increments may not be exceeded at all (§9.2.2(c)) — the forms covered in PSD increments: the second ceiling.

In PlumeSmart

Receptor tools support the workflow the Guideline describes: fence-line and multi-tier grids with denser spacing near the facility, discrete receptors at places of interest, and elevations assigned through the terrain processor — with design values reported unrounded at every receptor so the controlling location is explicit.