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Emissions Modeling Clearinghouse
Spatial Allocation
Another step of modeling inventory development is the spatial allocation of emissions. Because air quality modeling strives to replicate the actual physical and chemical processes that occur in an inventory domain, it is important that the physical location of emissions be determined as accurately as possible. In an ideal situation, the physical location of all emissions would be known exactly. 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. For horizontal allocation, emission models generally use gridded data on spatial surrogates such as population, land use and other data with known geographic distributions for allocating emissions to grid cells.
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. This data is available in the state and county Federal Information Processing Standards (FIPS) codes file.
Vertical spatial allocation refers to assigning emissions to their proper layer in the atmosphere prior to modeling activities. Vertical allocation is limited to those emissions that are released from an elevated height with a significant upward velocity and/or buoyancy. For all practical purposes, vertical allocation is limited to significant point sources with stacks.
Related Spatial Allocation Files
- "Old" surrogates
These files were developed prior to 2002 for use with 1996 base year inventories and inventories derived (e.g. future year projections) from the 1996 base year. They include data from the 1990 census, the North America Land Cover Characteristics, and other spatial sources that produce the 15 spatial surrogates EPA has been using for modeling with the 1996 and derived inventories. They are based on Albers projection. - "New" surrogates
These files were initially completed in the summer of 2003 for use with 1999 base year inventories and inventories derived from the 1999 base year. They include data from the 2000 census, National Land Cover Characteristics Data, and other spatial sources that produce the approximately 65 spatial surrogates that EPA plans to use for modeling the 1999 and derived inventories. They are based on a Lambert Conformal projection. - Spatial Surrogate Tool
We are providing shapefiles for input into the Spatial Surrogate Tool, which is stand-alone tool for generating spatial surrogates that are inputs to emission models such as SMOKE. This tool allows users to generate, merge or gapfill surrogate ratios for a user-specified grid, regardless of the type of operating system used and without needing third-party software.