variability in the averaging approach, with the exception of eliminating year-specific outages or

other year-specific anomalies38 within the years used for the model-attainment test.

The emissions models also need information about the chemical species of the VOC and

PM2.5 emissions for stationary point, stationary area, and all mobile sources. These data are used

to disaggregate the total VOC and PM2.5 emissions to the chemical species expected by the air

quality model and are called speciation “factors” or “profiles”. EPA provides a starting point for

the VOC and PM2.5 speciation data, which are available at: For large or critical VOC and PM2.5 sources in

the modeling domain, States/Tribes should consider determining the individual chemical

compounds contributing to the total VOC and PM2.5. If collected, this information should then

be used to compile improved speciation profiles for the critical facilities or source categories.

These speciation profiles should be assigned to the inventory by a speciation cross-reference file,

which also needs to be created or updated from the available defaults. The cross-reference files

typically assign speciation profiles based on SCC code, though facility-specific assignments for

point source code is also possible if plant-specific data are available.

For all source sectors that are compiled at a county resolution, the emissions models also

need information about allocating the countywide emissions to individual grid cells that intersect

the county. Such sectors include stationary area, nonroad mobile, and non-link on-road mobile

sources. The spatial allocation process assigns fractions of county-total emissions to the model’s

grid cells intersecting the county based on a “surrogate” data type (e.g., population or housing

data). The appropriate types of surrogate data to use for each SCC in the inventories should be

identified for this processing step. Spatial surrogates can be created using Geographic

Information Systems (GISs) to calculate the fraction of countywide emissions to allocate to each

grid cell based on the surrogate type. These calculations can also be made using EPA’s Surrogate

Tool ( ), which is based on the

Multimedia Integrated Modeling System (MIMS) Spatial Allocator

( ). Additionally


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