ISCST3 Tech Guide

Gaussian Plume Air Dispersion Model

6.1.2 Downwind and Crosswind Distances

The ISC model uses either a polar or a Cartesian receptor network as specified by the user. The model allows for the use of both types of receptors and for multiple networks in a single run. All receptor points are converted to Cartesian (X,Y) coordinates prior to performing the dispersion calculations. In the polar coordinate system, the radial coordinate of the point (r, 2) is measured from the user-specified origin and the angular coordinate 2 is measured clockwise from the north. In the Cartesian coordinate system, the X axis is positive to the east of the user-specified origin and the Y axis is positive to the north. For either type of receptor network, the user must define the location of each source with respect to the origin of the grid using Cartesian coordinates. In the polar coordinate system, assuming the origin is at X = Xo, Y = Yo, the X and Y coordinates of a receptor at the point (r, 2) are given by:

Equation (1)

If the X and Y coordinates of the source are X(S) and Y(S), the downwind distance x to the receptor, along the direction of plume travel, is given by:

Equation (2)

where WD is the direction from which the wind is blowing. The downwind distance is used in calculating the distance-dependent plume rise (see Section 1.1.4) and the dispersion parameters (see Section 1.1.5). If any receptor is located within 1 meter of a point source or within 1 meter of the effective radius of a volume source, a warning message is printed and no concentrations are calculated for the source-receptor combination. The crosswind distance y to the receptor from the plume centerline is given by:

Equation (3)

The crosswind distance is used in Equation (1-1).