Atmospheric gas dispersion calculator
Once a release rate is known, dispersion determines how far a flammable or toxic cloud travels before it dilutes below a concentration of concern. This module evaluates downwind concentrations for continuous releases using Gaussian plume theory with Pasquill-Gifford dispersion coefficients, and reports concentrations at user-placed receptor points.
Open the Gas Dispersion moduleWhat this calculator returns
- Downwind centreline concentration profile
- Concentration at nominated receptor coordinates
- Distance to lower flammable limit or a toxic endpoint
- Cloud footprint for plotting on a plot-plan
Required inputs
- Release rate and release height
- Wind speed at reference height and atmospheric stability class
- Surface roughness or terrain category
- Concentration of concern (LFL fraction, ERPG, or user value)
Calculation method
A continuous point-source Gaussian plume is evaluated with stability-dependent dispersion coefficients for the lateral and vertical spread.
Ground reflection is included by an image source, so near-ground receptors capture the doubled contribution.
The concentration field is scanned to find the distance at which the concentration of concern is crossed on the plume centreline.
Governing equations
C(x,y,z) = ( m / (2 pi u sy sz) ) exp( -y^2 / (2 sy^2) ) [ exp( -(z-H)^2 / (2 sz^2) ) + exp( -(z+H)^2 / (2 sz^2) ) ]Gaussian plume concentration with ground reflection.
C_centreline(x) = m / ( pi u sy sz )Ground-level centreline concentration for a ground-level release.
Nomenclature
- C
- concentration, kg/m3
- m
- continuous mass release rate, kg/s
- u
- wind speed at release height, m/s
- sy, sz
- lateral and vertical dispersion coefficients, m
- H
- effective release height, m
- x, y, z
- downwind, crosswind and vertical coordinates, m
Assumptions and limitations
- Steady wind speed and direction over the averaging period.
- Flat, unobstructed terrain; buildings and congestion are not resolved.
- Neutrally buoyant behaviour unless the dense-gas option is selected.
Reference practice
- Follows the Pasquill-Gifford framework described in the CCPS consequence analysis guidelines.
Worked example
A 0.1 kg/s ground-level methane release, wind speed 5 m/s, with dispersion coefficients sigma_y = 30 m and sigma_z = 15 m at the receptor distance.
| Step | Value | Basis |
|---|---|---|
| Plume spread | sigma_y = 30 m, sigma_z = 15 m | Set by the Pasquill stability class and downwind distance |
| Centreline concentration | 1.4e-5 kg/m3 | C = Q / (pi u sigma_y sigma_z) for a ground-level source and receptor |
| Volume fraction | about 21 ppm | Converted with the gas density of 0.68 kg/m3 |
| Comparison | Well below the 5 vol% LFL | The flammable cloud therefore ends much closer to the source than this receptor |
Concentration falls roughly with the product of the two spread parameters, so stable night-time conditions with small sigma values give the longest hazard distances for the same release rate.
Illustrative numbers only — rerun the module with the project basis of design before using any result.
Common questions
Which weather case gives the worst dispersion distance?
Stable, low-wind conditions such as Pasquill F with 2 m/s. Spread is small, so the cloud stays concentrated much further downwind.
When is a Gaussian model not appropriate?
For dense or cold vapours that slump and spread laterally near the ground; those need a dense-gas treatment rather than a passive Gaussian plume.
What release rate should be used?
The source term from the gas discharge or blowdown module, at the moment in the release history being assessed.
Related calculators
- Gas Discharge — Gas discharge rate calculator for pressurised releases
- Gas Detection Mapping — Gas detection coverage mapping
- Flare Radiation — Flare thermal radiation calculator