AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

6.2.1 Direct Source Contribution to Concentration Calculations in the CBL

Following Weil et al. (1997), the concentration distribution for the horizontal state contribution to the direct plume is given by:

Equation (69)

Here, dj and zj are the effective source height and vertical dispersion parameter corresponding dj zj to each of the two distributions in eq. (65). The dispersion parameters (y , zl & z2 ) resulting from the total turbulence are calculated using eqs. (85), and (98) thru (103). The subscripts 1 & 2 refer to the updraft & downdraft plumes respectively, and

Equation (70)

with a1 and a2 given by eq. (72). 1 2

In obtaining Eq. (69), we use an "image" plume to satisfy the no-flux condition at the ground, i.e., an image plume from a source at zr = -hs, which results in the exponential terms containing zr + dj on the right-hand side of Eq.(69). The image source at zr = -hs results in a positive flux of material at zr = zi. To satisfy the no-flux condition there, an image source is introduced at zr = 2 zi + hs, which then leads to a series of image sources at zr = 2 zi - hs, 4zi + hs , -4zi - hs , etc.

The height of the direct plume is given by the following expression

Equation (71)

The second term in eq. (71) is the plume rise due to convection. h is calculated using eq. (116), and

Equation (72)

Recall that wT is an effective vertical turbulence component and is calculated from eq. (35). The parameters appearing in eq. (72) are given by.

Equation (73)

and R is assumed to be 2.0 (Weil et al., 1997).