A link is defined as a straight segment of roadway having a constant width, height, traffic volume, and vehicle emission factor. The location of the link is specified by the endpoint coordinates of its centerline. The location of a receptor is specified in terms of X, Y, Z coordinates. The program automatically sums the contributions from each link to each receptor. After this is completed for all receptors, a background value may be added.
Surface roughness is assumed to be uniform throughout the study area.
The meteorological variables of atmospheric stability, wind speed, and wind direction are also taken as constant over the study area. Deposition velocity is a measure of the rate at which a pollutant can be adsorbed or assimilated by a surface. It involves a molecular diffusive process through the laminar sublayer covering the surface. Settling velocity is the rate at which a particle falls with respect to its immediate surroundings. A composite vehicle emission factor in grams per vehicle-mile must be provided for each link.