AirWare:    urban air quality
assessment and management

Gaussian plume models

The model which is most frequently used as the basis for more complex air pollution calculations is the Gaussian plume model. This model assumes that over short periods of time, say one or a few hours, steady state conditions exists with regard to air pollutant emissions and meteorological driving forces, i.e., stable wind speed and direction, stability class, mixing height, and temperature.

This limits the applicability of the Gaussian approximation to short episode (in the order of hours), local scales (up to 30 km), and situations that are not goverened by complex topography or market surface temperature distributiuons, e.g., in coastal areas causing sea-breeze phenomena.

Conceptually, the model is symbolized by this figure. Air pollution is represented by an idealized plume coming from the top of a stack. Because the plume is being produced by a fuel burning process, the hot plume will be thrust upward some distance above the top of the stack, to the effective stack height. The actual value of this vertical displacement will primarily depend on the stack gas exit velocity and temperature, and the temperature of the surrounding air.

Once the plume has reached effective stack height, dispersion will begin in three dimensions. Dispersion in the downwind direction will be proportional to the mean wind speed and in that direction. Dispersion in the cross-wind direction and in the vertical direction will be governed by the Gaussian plume equations. Model coefficients are related to atmospheric stability, and distance from the source. The model assumes that dispersion in these two dimensions will take the form of a normal Gaussian curve, with the maximum concentration in the center of the plume.

The solution of the model involves a semi-empirical solution of a set of partial differential equations.