ISCST3 Tech Guide

Gaussian Plume Air Dispersion Model

7. The ICS Long-term Dispersion Model Equations

This section describes the ISC Long-Term model equations. Where the technical information is the same, this section refers to the ISC Short-Term model description in Section 1 for details. The long-term model provides options for modeling the same types of sources as provided by the short-term model. The information provided below follows the same order as used for the short-term model equations.

The ISC long-term model uses input meteorological data that have been summarized into joint frequencies of occurrence for particular wind speed classes, wind direction sectors, and stability categories. These summaries, called STAR summaries for STability ARray, may include frequency distributions over a monthly, seasonal or annual basis. The long term model has the option of calculating concentration or dry deposition values for each separate STAR summary input and/or for the combined period covered by all available STAR summaries. Since the wind direction input is the frequency of occurrence over a sector, with no information on the distribution of winds within the sector, the ISC long-term model uses a Gaussian sector-average plume equation as the basis for modeling pollutant emissions on a long-term basis.