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

Plume rise: momentum, buoyancy & the stable ceiling

Before the atmosphere disperses a plume, the plume changes its own altitude. Plume rise — how far above the stack top the plume climbs — often matters as much as stack height itself, because ground-level concentration depends strongly on the plume's final height. AERMOD computes it from two properties of the release, following the classic Briggs formulation:

The plume rise for the direct source is given by the superposition of source momentum and buoyancy effects following Briggs (1984).

Source · AERMOD Model Formulation (EPA-454/B-26-003) §5.6.1 — quoted word-for-word from the source document.

  • Momentum rise comes from the exit velocity: gases leaving a stack at 20 m/s carry upward momentum regardless of temperature. In the rise equation (eq. 91) the momentum term grows with downwind distance x.
  • Buoyancy rise comes from the exit temperature: gases hotter than the surrounding air keep accelerating upward as they drift downwind — the buoyancy term grows with x². A hot plume's rise therefore keeps building with distance, which is why buoyancy dominates for combustion sources.

Both fluxes are computed from the stack parameters you enter — exit velocity, stack diameter, exit temperature versus ambient temperature — with the effective stack radius first "corrected for stack tip downwash" (§5.6.1), the small penalty applied when low exit velocity lets the plume get pulled down into the stack's own wake.

convective hours: distance-dependent rise —momentum term ∝ x, buoyancy term ∝ x², no assumed final height (§5.2, §5.6.1)stable hours: rise levels off — rising airmeets ever-warmer surroundings and loses buoyancy (§5.6.2)inputs: exit velocity & stack radius (momentum flux) · exit vs ambient temperature (buoyancy flux) · wind speed at plume heightthe stack radius is corrected for stack-tip downwash before the rise calculation (§5.6.1)
Plume rise from momentum + buoyancy (Briggs form, §5.6) — and why stable air caps it (§5.6.2)

Daytime: no final rise, just trajectories

In the convective boundary layer (CBL) AERMOD makes a deliberate break from older models:

For buoyant releases, there is no "final" plume rise assumed. Instead, the plume or particle trajectories are determined by the addition of a distance-dependent plume rise and the random vertical displacement caused by the vertical distribution of w. Ground level concentrations first appear when the negative or downdraft velocities are sufficiently large to overcome the plume rise velocity and carry plume sections to the surface.

Source · EPA-454/B-26-003 §5.2.

Rise and turbulence compete continuously: the plume keeps climbing while downdrafts keep tugging pieces of it toward the ground. The three-plume split from the previous page carries its own rise treatments — the indirect plume's lofting adjustment near the mixed-layer top (eq. 92), and the penetrated plume's equilibrium height above the inversion (eq. 94).

Stable nights: the atmosphere caps the climb

In the stable boundary layer (SBL) the environment itself shuts rise down:

When a plume rises in an atmosphere with a positive potential temperature gradient, plume buoyancy decreases because the ambient potential temperature increases as the plume rises; thus, plume buoyancy with respect to the surroundings decreases.

Source · EPA-454/B-26-003 §5.6.2.

A plume that leaves the stack far hotter than its surroundings can lose that advantage within a few hundred meters of climb, because stable nighttime air gets warmer (in potential temperature) with height. The stable-rise equation (eq. 95) captures this with an oscillatory form governed by the Brunt-Väisälä frequency — the plume climbs, overshoots its equilibrium, and levels off. This is why the same stack can produce a high-riding, low-impact plume by day and a flat, far-carrying plume at night — and why getting the stack's temperature and velocity right matters more than almost any other source parameter.

The temperature structure the stable calculation needs is built from the meteorology: AERMOD "generates a gridded vertical profile of potential temperatures for use in the plume rise calculations," anchored at the base elevation the modeler supplies (AERMOD User's Guide §3.5.3, the PROFBASE input — see Meteorological data & representativeness).

What this means for your inputs

Plume rise is where small input errors become large concentration errors:

  • Exit temperature in the wrong units (Celsius where Kelvin is expected) silently converts a buoyant plume into a dense one.
  • Exit velocity from the wrong operating load changes both momentum rise and stack-tip downwash — the load analysis on the Emission inputs page exists precisely because "changes in the stack parameters associated with the operating conditions could lead to higher ground level concentrations" (40 CFR 51 App. W §8.2.2(d)).
  • Stack inside diameter vs. outside diameter — the rise equations use the exit area the gas actually flows through.

In PlumeSmart

Stack parameters are entered with explicit units and checked for physical plausibility, and each run's input file records exactly the exit conditions the rise calculation used — so a reviewer can trace a plume height back to the stack data that produced it.