For point and nonpoint toxics inventories, each source can be labeled as “major” or “section-112 area” for input to SMOKE (the following paragraph explains how the term “area” can be applied to a point-source inventory). The Clean Air Act defines major sources as those stationary facilities that emit or have the potential to emit 10 tons per year or more of any one toxic air pollutant or 25 tons per year or more of any combination of toxic air pollutants. section-112 area sources include facilities that have air toxics emissions below the major source threshold as defined in section 112 of the Clean Air Act and thus emit less than 10 tons per year of a single toxics air pollutant or less than 25 tons per year combined of multiple toxics air pollutants. Another source type exists in principle for nonpoint sources: the “other” source type; an example of this source type is wildfires. However, these source types are not labeled differently from the section-112 area sources in the nonpoint toxics inventories, so the “other” source type has not been included in SMOKE to date.

A note about the confusing use of “area” terminology to describe point sources: The designation of sources in the point inventories as section-112 area sources has no relationship whatsoever to SMOKE’s area processing category. The point sources that are section-112 area sources are still processed by SMOKE as point sources using a lat-lon location and stack parameters.

In practice, all “major” sources should appear only in the point toxics inventory, but in some cases, “major” sources have shown up in the nonpoint inventory (specifically in inventory year 1996, in the July 2001 version of that inventory). Thus, the source type designation is provided in both the point and nonpoint toxics input formats.

The major and section-112 area designations are used when applying MACT-based control factors. These control factors are assigned based on a source’s MACT code and may be applied to major sources only, to section-112 area sources only, or to both types of sources regardless of designation.