1.2 NON-POINT SOURCE EMISSIONS
The ISC models include algorithms to model volume, area and open-pit sources, in addition to point sources. These non-point source options of the ISC models are used to simulate the effects of emissions from a wide variety of industrial sources. In general, the ISC volume source model is used to simulate the effects of emissions from sources such as building roof monitors and line sources (for example, conveyor belts and rail lines). The ISC area source model is used to simulate the effects of fugitive emissions from sources such as storage piles and slag dumps. The ISC open pit source model is used to simulate fugitive emissions from below-grade open pits, such as surface coal mines or stone quarries.
1.2.2 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., there is no plume rise associated with volume sources. The user also assigns initial lateral (σyo) and vertical (σzo) dimensions for the volume source. Lateral (xy) and vertical (xz) virtual distances are added to the actual downwind distance x for the σy and σz calculations. The virtual distances are calculated from solutions to the sigma equations as is done for point sources with building downwash.
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