Consequence Analysis 2.2

Dispersion Model Consider the instantaneous release of a fixed mass of material into an infinite expanse of air. Assuming no reaction or molecular diffusion,

Time-Averaged Dispersion Model Define eddy diffusivity

Time-Averaged Dispersion Model Assume the air is incompressible

Time-Averaged Dispersion Model

Case 1.1: Steady-State Continuous Point Release with No Wind

Case 1.1: Steady-State Continuous Point Release with No Wind Assume constant release rate Boundary condition

Case 1.2: Unsteady-State Continuous Point Release with No Wind

Case 1.3: Steady-State Continuous Point Release with Wind

Case 1.3: Steady-State Continuous Point Release with Wind

Case 1.3: Steady-State Plume with Source on Ground Case 1.3: Steady-State Plume with Source at Height H above Ground

Case 2.1: Puff with No Wind

Case 2.2: Puff with Wind Case 2.3: Puff with No Wind with Source on Ground

Neutral and Positively Buoyant Plume and Puff Models Neutral and positively buoyant plume and puff models are used to predict concentration and time profiles of flammable or toxic materials downwind of a source based on the concept of Gaussian distribution. Atmospheric diffusion is a random mixing process driven by turbulence in the atmosphere. The concentration of any point down wind of a source is well approximated by a Gaussian concentration profile in both the horizontal and vertical dimensions.

Dispersion Coefficients The dispersion coefficients represent the standard deviations of concentrations in the downwind, crosswind and vertical directions respectively. They are much easier to obtain experimentally than eddy diffusivities.

Dispersion Coefficients Example formulas as a function of x (meters) in urban D stability conditions are

Pasquill-Gifford Model Case 2.2: Puff. Instantaneous point source at ground level, coordinate fixed at release point. Constant wind in x direction with velocity u. The total integrated dose

Pasquill-Gifford Model The standard formula for dispersion from an elevated point source assuming no ground absorption or reaction is

Gaussian Model for Puff

Simplified Approach