AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

6.4 Estimation of Dispersion Coefficients

CThe standard deviations for both the lateral and vertical concentration distributions (y and z respectively) result from the combined effects of: ambient turbulence a ); turbulence induced by plume buoyancy (b); and, enhancements from building wake effects (d).

Dispersion (ya,za ), induced by ambient turbulence, is known to vary significantly with height, having its strongest variation near the earth’s surface. Unlike present regulatory models, AERMOD has been designed to account for this height variation.

In our earlier AERMOD formulations for ys & za we attempted to account for vertical variations in turbulence through treatment of vertical in homogeneity. However, comparisons made using the Prairie Grass data showed this approach to be inadequate. Therefore, the current expression for za is a combination of a direct treatment of surface dispersion and the more traditional approach based on Taylor (1921) for elevated dispersion. With this, we obtained good results for all SBL comparisons. However, the results in the CBL indicated that our treatment of lateral dispersion near the surface was problematic. We corrected this with an empirical relationship for ya near the surface using the full (CBL and SBL) Prairie Grass data set. In the remainder of this section we will describe those formulations for ya & za that resulted from this empirical analysis.

In the CBL, although the ambient induced dispersion for the Direct (D) and Indirect (I) sources is treated differently than for the Penetrated (P) source, the general approach of combining the effects from ambient turbulence, buoyancy and buildings, is the same. For the Direct and Indirect sources, the total dispersion coefficients (y or z ) are calculated from the following general y z expression (Pasquill and Smith, 1983):

Equation (85)

For the penetrated source, the total dispersion is calculated as follows:

Equation (86)

and building wakes are assumed to have little influence.

For the injected source, the total dispersion is calculated by eq. (87) as a source in table conditions.

In the SBL, the total dispersion is calculated as follows.

Equation (87)