Air Dispersion Primer

Deposition begins when material reaches the ground

Material from the lower stack reaches the ground before that of the taller stack. Consequently the material from the taller stack is more dilute when it first reaches the ground since it is mixed with a larger volume of air

Limited Mixing

Temperature inversion above the ground acts like a lid so that materials are trapped between the lid and the ground and both small and taller stacks tend to have similar concentrations once both plumes have expanded enough to reach the lid. Initially the small stack will have a more concentrated plume because it is mixed over a smaller depth

Stability & Plume Cross-sections

stableneutral

unstable

In a stable atmosphere the plume spreads very little in the vertical direction; concentrations aloft at the same distance downwind are higher, but they may not occur at ground level. These conditions usually occur at night with clear skies and low winds.

With unstable conditions the plume spreads rapidly in the vertical direction and concentrations aloft are lower for the same distance downwind, but the highest ground-level occur closer to the stack because the plumes reaches the ground more quickly.

Terrain EffectsContaminants moving with cold air flow along the slope until the cold air reaches a point where colder air has pooled in the valley bottom. Once there is colder air below the contaminants they tend to be isolated from the ground

Buildings Change Dispersion

∙Eddies form behind buildings and mix contaminants in the wake of the buildings

∙Material emitted from stacks can be brought to the ground if the stacks are not significantly higher than the buildings

∙Material can be dispersed in much different directions than expected (see following examples)

Sometimes quick turn around, not accuracy, is of utmost importance.

For many cases, more time is available and accurate answers are required.

Diagnostic-Empirical Computational Fluid Dynamics

Fast: less than a secondAccuracy: poorUniversality: poorMeteorology: crude

Medium Speed: 1 - 10 minutesAccuracy: fairUniversality: okMeteorology: very simplified

Slow: hours to daysAccuracy: goodUniversality: very goodMeteorology: closer to real

LANL-modified EPA INPUFF QUIC LANL HIGRAD

Gaussian Puff

Model Fidelity vs. Application

Buildings produce different dispersion than predicted with simpler models.

The High-Rise Experiment

U

S

Ohba, M., W. H. Snyder and R. E. Lawson, 1993: “Study in Prediction of Gas Concentrations Around Twin High-Rise Buildings using Wind-Tunnel Techniques,” USEPA

● Compare measured concentrations with those estimated by QUIC-Plume

● Compare local mixing estimations with Non-local estimations and measurements

Non-Local Mixing Dispersion

Non-Local Mixing Dispersion

Effect of bldgs

Eddies mix the pollutants behind the buildings in very complex and fluctuating flows

Effects of multiple stacks

Plumes from multiple sources overlap making the concentration less sensitive to the wind direction