Plume Models

One of the simplest ways of estimating smoke concentrations is to assume that plumes diffuse in a Gaussian pattern along the centerline of a steady wind trajectory. Plume models usually assume steady-state conditions during the life of the plume, which means relatively constant emission rates, wind speed, and wind direction. For this reason, they can be used only to estimate concentrations relatively near the source or for a short duration. Their steady-state approximation also restricts plume models to conditions that do not include the influence of topography or significant changes in land use, such as flow from a forest to grassland or across a land-water boundary.

Gaussian plume models have a great benefit in places and circumstances that restrict the amount of available input data. They can be run fast and have simple but realistic output that can be easily interpreted. Many regulatory guidelines from the EPA are based on Gaussian plume models.

Plume models typically are in Lagrangian coordinates that follow particles or parcels as they move, assigning the positions in space of a particle or parcel at some arbitrarily selected moment. (Lagrangian coordinates are used by plume, puff, and particle models.) Examples adapted for wildland biomass smoke include VSMOKE (Harms and others 1997; Lavdas 1996) and SASEM (Riebau and others 1988; Sestak and Riebau 1988). Both models follow regulatory guidelines in their development and offer a simple screening tool for examining potential concentrations at receptor locations from straight-line trajectories relatively near the source. However, SASEM directly compares downwind concentrations with ambient standards and calculates visibility impairment in a simple manner. It is also used as a State regulatory model in Wyoming, Colorado, New Mexico, and Arizona, and has been recommended for use by the EPA.

Plume rise models developed for other applications might be useful if adapted to fire environments. For example, ALOFT-FT (A Large Outdoor Fire Plume Trajectory Model - Flat Terrain), developed for oil-spill fires (Walton and others 1996), is a computer-based model to predict the downwind distribution of smoke particulate and combustion products from large outdoor fires. It solves the fundamental fluid dynamic equations for the smoke plume and its surroundings with flat terrain. The program contains a graphical user interface for input and output, and a database of fuel and smoke emission parameters that can be modified by the user. The output can be displayed as downwind, crosswind, and vertical smoke concentration contours.