National Environmental Law: EIA Permit Gate Simulator

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

Atmospheric Stability (Pasquill–Gifford)

D — Neutral (overcast / windy)

Permit Decision

—

Boundary conc.

0 µg/m³

Legal limit

150 µg/m³

σy at boundary

0 m

σz at boundary

0 m

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:

C(x,y,0) = Q / (2π u σy σz) · exp(-y² / 2σy²) · 2·exp(-H² / 2σz²)

where Q is the emission rate, u the wind speed, H the effective stack height, and σy, σz the horizontal and vertical plume-spread widths — both growing with downwind distance x but shrinking under calmer, more stable atmospheres (Pasquill–Gifford classes A–F). The factor of 2 models total reflection of the plume off the ground.

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, 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.

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.

Plume (concentration) Property boundary

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