In reality, however, the spatial allocation of emissions in a modeling inventory only approximates the actual location of emissions. Before any spatial allocation can be performed, several characteristics of the inventory must be identified. First, the modeling grid domain must be established. A modeling grid domain is a rectangular area that encompasses all the desired emission sources to be modeled. After defining the necessary grid characteristics, the spatial allocation of emissions can be performed.
Spatial allocation can refer to both the horizontal and vertical allocation.
Horizontal spatial allocation refers to assigning emissions to their proper grid cell prior to modeling activities. The specific method of allocation will vary depending upon the source type emissions being allocated. Point sources are allocated to grid cells using the latitude and longitude reported for each source. When location data are not reported, default locations using ZIP code or county centroids are used. Vertical spatial allocation refers to assigning emissions to their proper layer in the atmosphere prior to modeling activities.