ISCST3 Tech Guide

Gaussian Plume Air Dispersion Model

6.3.3 Point and Volume Source Emissions

As stated in Equation (1-59), deposition is modeled as the product of the near-surface concentration (from Equation (1-1)) times the deposition velocity (from Equation (1-80)). Therefore, the vertical term given in Equation (1-54) that is used to obtain the concentration at height z, subject to particle settling and deposition, can be evaluated at height zd for one particle size, and multiplied by a deposition velocity for that particle size to obtain a corresponding "vertical term" for deposition. Since more than one particle size category is typically used, the deposition for the nth size category must also include the mass fraction for the category:


where K, N, Vd, and D were defined previously (Equations (1-1), (1-54), and (1-63)). The parameter QJ is the total amount of material emitted during the time period J for which the deposition calculation is made. For example, QJ is the total amount of material emitted during a 1-hour period if an hourly deposition is calculated. To simplify the user input, and to keep the maximum compatibility between input files for concentration and deposition runs, the model takes emission inputs in grams per second (g/s), and converts to grams per hour for deposition calculations. For time periods longer than an hour, the program sums the deposition calculated for each hour to obtain the total deposition flux for the period. In the case of a volume source, the user must specify the effective emission height he and the initial source dimensions Fyo and Fzo. It should be noted that for computational purposes, the model calculates the quantity, Formulaas the "vertical term."