National Environmental Law: EIA Permit Gate Simulator (2D)

Facility & Site

Emission rate Q 10 g/s

Stack height H 25 m

Wind speed u 4.0 m/s

Property boundary D 500 m

Profile probe distance 500 m

Atmospheric Stability (Pasquill–Gifford)

D — Neutral (overcast / windy)

Permit Decision

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Boundary conc.

0 µg/m³

Legal limit

150 µg/m³

σy at boundary

0 m

σz at boundary

0 m

Probe point

click plan view

Probe conc.

—

How it works

National environmental-permitting law rarely bans emissions outright — it gates them behind a modeled compliance test. A regulator (or the applicant's own Environmental Impact Assessment) runs an atmospheric dispersion model and compares the predicted ground-level concentration at the property boundary against a statutory ambient limit. This simulator uses the standard Gaussian plume equation, in its general form (valid at any height z, not just the ground):

C(x,y,z) = Q / (2π u σy σz) · exp(-y² / 2σy²) · [exp(-(z-H)² / 2σz²) + exp(-(z+H)² / 2σz²)]

The second bracket term is the mirror-image source used to model total reflection of the plume off the ground (no deposition/absorption). Setting z = 0 reduces the bracket to 2·exp(-H² / 2σz²) — the ground-level special case regulators actually check, and what the plan-view and decay-chart panels use. The profile panel below sweeps z directly so you can see the full vertical Gaussian shape the ground-level number is sampled from.

Emission rate / stack height / wind speed — the physical inputs a facility's permit application must disclose.
Property boundary D — the nearest point the public can legally be exposed, drawn as a circle around the facility and evaluated directly downwind (y = 0) where concentration peaks.
Stability class — how turbulent the atmosphere is; unstable air (A) disperses the plume fast, stable air (F) lets it stay narrow and travel further before spreading.
Permit verdict — Approved when boundary concentration is below the legal limit, Conditional with extra monitoring when it is within 50% over, and Denied — full EIS required beyond that, mirroring how real permitting authorities use a modeled-compliance test as the gate.
Plan view — drag to pan, scroll/pinch to zoom, click to drop a probe and read the concentration at any point on the map, not just the boundary.

The σy/σz growth-rate coefficients here are a simplified, pedagogical Pasquill–Gifford-style parameterization — real regulatory submissions use validated tools such as AERMOD, but the underlying physics and the legal decision logic are the same.

Plan view (top-down)

Low concentration Approaching limit Over legal limit Property boundary

drag: pan · wheel/pinch: zoom · click: probe