What is the Gaussian plume model for stack dispersion?

The Gaussian plume model is the standard EPA methodology for estimating ground-level pollutant concentrations from elevated point sources (stacks). It assumes the plume spreads in a Gaussian (bell-curve) distribution both laterally and vertically as it travels downwind, with dispersion rates determined by atmospheric stability class (A through F) and distance.

How is effective stack height calculated?

Effective stack height equals the physical stack height plus plume rise. Plume rise is calculated using Briggs equations accounting for buoyancy (hot exhaust gas rising due to temperature difference) and momentum (exit velocity). For most industrial sources, buoyancy dominates. Effective heights are typically 1.5 to 3 times the physical stack height.

What is GEP stack height and building downwash?

Good Engineering Practice (GEP) stack height per 40 CFR 51.100 is H + 1.5L, where H is building height and L is the lesser of building height or projected width. If a stack is shorter than GEP height, building wake effects can bring the plume down to ground level (downwash), significantly increasing ground-level concentrations near the source.

What atmospheric stability class should I use?

For worst-case screening analysis, use Stability Class F (very stable) with low wind speed (1-2 m/s) for rural sites, or Class D (neutral) for urban sites. Class A (very unstable) gives the highest short-distance concentrations. EPA SCREEN3 methodology tests all stability classes and reports the maximum concentration.

Stack Height / Atmospheric Dispersion Calculator

Emission Source

Emission Rate

Pollutant

Stack Parameters

Stack Height

Stack Inside Diameter

Exit Gas Velocity

ft/s

Exit Gas Temperature

°F

Meteorological Conditions

Ambient Temperature

°F

Wind Speed at Stack Height

mph

Atmospheric Stability Class

Terrain Type

Receptor & Downwash

Receptor Distance (0 = find max GLC)

Enter 0 to calculate maximum ground-level concentration and its distance

Building Downwash

Stack Tip Downwash Correction

Understanding Atmospheric Dispersion

Gaussian Plume Model
The standard EPA method for estimating pollutant concentrations downwind of a stack. Assumes steady-state emission, uniform wind, and Gaussian concentration profiles in both crosswind and vertical directions.

Stability Classes
A (very unstable) through F (very stable). Unstable conditions disperse pollutants rapidly but create high near-field concentrations. Stable conditions produce narrow plumes that travel farther before dispersing.

Key Applications:
Permit applications (PSD/NSR), stack height determination, NAAQS compliance demonstrations, emergency planning for H2S releases, flare stack siting, compressor station air quality assessments.

📚 Learn the Theory

Understand Gaussian dispersion, stability classes, Briggs plume rise, and EPA modeling guidance

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Results

Max Ground-Level Concentration

μg/m³

Effective Stack Height -

Plume Rise -

Distance of Max GLC -

Concentration at Receptor -

Buoyancy Flux (Fb) -

Momentum Flux (Fm) -

σy at Max GLC -

σz at Max GLC -

GEP Stack Height -

Stability Class Used -

Formula

C = Q / (2πσyσz u) × exp(−H² / 2σz²)

C = Ground-level concentration (μg/m³)

Q = Emission rate (g/s)

σy, σz = Pasquill-Gifford dispersion coefficients (m)

u = Wind speed at effective stack height (m/s)

He = Effective stack height = Hs + Δh (m)

Δh = Briggs plume rise (m)

Standards & References

EPA AP-42
Compilation of Air Pollutant Emission Factors
40 CFR 51.100
GEP Stack Height, Good Engineering Practice
EPA SCREEN3
Screening-level air dispersion model
Pasquill (1961) & Gifford (1961)
Atmospheric dispersion coefficients
Briggs (1969, 1971, 1975)
Plume rise equations for buoyant sources
40 CFR 50
National Ambient Air Quality Standards (NAAQS)

Engineering Notes

Screening model: This is a Tier-1 screening tool equivalent to SCREEN3. For permit applications, use refined models (AERMOD, CALPUFF).
Plume rise: Briggs equations assume final plume rise. Actual rise develops over distance; near-field concentrations may be higher.
Building downwash: Stacks shorter than GEP height should use downwash corrections. This calculator applies the Huber-Snyder adjustment.
Stack tip downwash: When exit velocity is less than 1.5 × wind speed, the plume bends down behind the stack tip.
Stability Class F: Use for worst-case analysis at rural sites. Urban sites rarely experience F stability due to heat island effect.
Terrain: Complex terrain with nearby hills or valleys requires specialized modeling (CTSCREEN, AERMOD with terrain).

Quick Reference — NAAQS Standards

SO2: 75 ppb (196 μg/m³) 1-hr
NO2: 100 ppb (188 μg/m³) 1-hr
CO: 35 ppm (40,000 μg/m³) 1-hr
PM10: 150 μg/m³ 24-hr
PM2.5: 35 μg/m³ 24-hr
H2S: 30 μg/m³ (state typical, no federal)