ISCST3 Tech Guide

Gaussian Plume Air Dispersion Model

6. The ISC Short-term Dispersion Model Equations

The Industrial Source Complex (ISC) Short Term model provides options to model emissions from a wide range of sources that might be present at a typical industrial source complex. The basis of the model is the straight-line, steady-state Gaussian plume equation, which is used with some modifications to model simple point source emissions from stacks, emissions from stacks that experience the effects of aerodynamic downwash due to nearby buildings, isolated vents, multiple vents, storage piles, conveyor belts, and the like. Emission sources are categorized into four basic types of sources, i.e., point sources, volume sources, area sources, and open pit sources. The volume source option and the area source option may also be used to simulate line sources. The algorithms used to model each of these source types are described in detail in the following sections. The point source algorithms are described in Section 1.1. The volume, area and open pit source model algorithms are described in Section 1.2. Section 1.3 gives the optional algorithms for calculating dry deposition for point, volume, area and open pit sources, and Section 1.4 describes the optional algorithms for calculating wet deposition. Sections 1.1 through 1.4 describe calculations for simple terrain (defined as terrain elevations below the release height). The modifications to these calculations to account for complex terrain are described in Section 1.5, and the treatment of intermediate terrain is discussed in Section 1.6.

The ISC Short Term model accepts hourly meteorological data records to define the conditions for plume rise, transport, diffusion, and deposition. The model estimates the concentration or deposition value for each source and receptor combination for each hour of input meteorology, and calculates user-selected short-term averages. For deposition values, either the dry deposition flux, the wet deposition flux, or the total deposition flux may be estimated. The total deposition flux is simply the sum of the dry and wet deposition fluxes at a particular receptor location. The user also has the option of selecting averages for the entire period of input meteorology.