ISCST3 Tech Guide

Gaussian Plume Air Dispersion Model

6.2.2 The Short-Term Volume Source Model

The ISC models use a virtual point source algorithm to model the effects of volume sources, which means that an imaginary or virtual point source is located at a certain distance upwind of the volume source (called the virtual distance) to account for the initial size of the volume source plume. Therefore, Equation (1-1) is also used to calculate concentrations produced by volume source emissions.

There are two types of volume sources: surface-based sources, which may also be modeled as area sources, and elevated sources. An example of a surface-based source is a surface rail line. The effective emission height he for a surface-based source is usually set equal to zero. An example of an elevated source is an elevated rail line with an effective emission height he set equal to the height of the rail line. If the volume source is elevated, the user assigns the effective emission height he, i.e., t here is no plume rise associated with volume sources. The user also assigns initial lateral (Fyo) and vertical (Fzo) dimensions for the volume source. Lateral (xy) and vertical (xz) virtual distances are added to the actual downwind distance x for the Fy and Fz calculations. The virtual distances are calculated from solutions to the sigma equations as is done for point sources with building downwash.

The volume source model is used to simulate the effects of emissions from sources such as building roof monitors and for line sources (for example, conveyor belts and rail lines). The north-south and east-west dimensions of each volume source used in the model must be the same. Table 1-6 summarizes the general procedures suggested for estimating initial lateral (Fyo) and vertical (Fzo) dimensions for single volume sources and for multiple volume sources used to represent a line source. In the case of a long and narrow line source such as a rail line, it may not be practical to divide the source into N volume sources, where N is given by the length of the line source divided by its width. The user can obtain an approximate representation of the line source by placing a smaller number of volume sources at equal intervals along the line source, as shown in Figure 1-8. In general, the spacing between individual volume sources should not be greater than twice the width of the line source. However, a larger spacing can be used if the ratio of the minimum source-receptor separation and the spacing between individual volume sources is greater than about 3. In these cases, concentrations calculated using fewer than N volume sources to represent the line source converge to the concentrations calculated using N volume sources to represent the line source as long as sufficient volume sources are used to preserve the horizontal geometry of the line source.

Figure 1-8 illustrates representations of a curved line source by multiple volume sources. Emissions from a line source or narrow volume source represented by multiple volume sources are divided equally among the individual sources unless there is a known spatial variation in emissions. Setting the initial lateral dimension Fyo equal to W/2.15 in Figure 1-8(a) or 2W/2.15 in Figure 1-8(b) results in overlapping Gaussian distributions for the individual sources. If the wind direction is normal to a straight line source that is represented by multiple volume sources, the initial crosswind concentration distribution is uniform except at the edges of the line source. The doubling of Fyo by the user in the approximate line-source representation in Figure 1-8(b) is offset by the fact that the emission rates for the individual volume sources are also doubled by the user.

TABLE 6

Type of Source

Procedure for Obtaining
Initial Dimension

(a) Initial Lateral Dimensions (Fyo)

Single Volume Source

Fyo = length of side divided by 4.3

Line Source Represented by Adjacent
Volume Sources (see Figure 1-8(a))

Fyo = length of side divided by 2.15

Line Source Represented by Separated
Volume Sources (see Figure 1-8(b))

Fyo = center to center distance
divided by 2.15

(b) Initial Vertical Dimensions (Fzo)

Surface-Based Source (he - 0)

Fzo = vertical dimension of source
divided by 2.15

Elevated Source (he > 0) on or
Adjacent to a Building

Fzo = building height divided by 2.15

Elevated Source (he > 0) not on or
Adjacent to a Building

Fzo = vertical dimension of source
divided by 4.3