PlumeSmartEPA Regulatory Air Dispersion Modeling
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Air Modeling GuideCovers AERMOD 24142 & 26135 — every excerpt version-stamped · current NAAQS

Emission inputs: what rate goes into the model

The single most consequential number in a demonstration is the emission rate — and in regulatory modeling it is not simply "what the source emits." Appendix W prescribes different emission inputs for different sources in the same analysis, and reviewers check this table early. The governing rule for a Prevention of Significant Deterioration (PSD) demonstration against the National Ambient Air Quality Standards (NAAQS):

For the purposes of demonstrating NAAQS compliance in a PSD assessment, the regulatory modeling of inert pollutants shall use the emissions input data shown in Table 8-2 for short and long-term NAAQS. The new or modifying stationary point source shall be modeled with "allowable" emissions in the regulatory dispersion modeling. As part of a cumulative impact analysis, Table 8-2 allows for the model user to account for actual operations in developing the emissions inputs for dispersion modeling of nearby sources, while other sources are best represented by air quality monitoring data.

Source · 40 CFR 51 App. W §8.2.2(c) — quoted word-for-word from the current regulation text (eCFR, 2026-08-01 edition).

Table 8-2's three-factor structure

Every emission input in the table is a product of three factors — an emissions limit (lb/MMBtu), an operating level (MMBtu/hr), and an operating factor (hours) — with "analogous terminology (e.g., lb/throughput)" for non-fuel-burning sources (Table 8-2, fn 1):

emissions limit(lb/MMBtu)×operating level(MMBtu/hr)×operating factor(hr/yr · hr/day)project sourcemaximum allowable or federallyenforceable permit limitdesign capacity (also model50% / 75% loads — fn 2)continuous — 8760 hr/yr;all hours of the met recordnearby sourcesmaximum allowable or federallyenforceable permit limitACTUAL level when operating,most recent 2 yearsannual: actual 2-yr factor ·short-term: all hoursother sourcesnot modeled from emissions at all — "represented by air quality monitoring dataunless adequate data do not exist" (the background monitor)project source = worst legal case (allowable · design · continuous) — nearby = allowable limit at real operationsconstrained schedules: model only permitted operating hours; never average emissions across non-operating hours (fn 3)
Table 8-2's three-factor structure — every modeled emission input is limit × operating level × operating factor
  • The project source is modeled at its worst legal case: maximum allowable (or federally enforceable permit) limit, at design capacity, operating continuously — 8760 hr/yr for annual standards, every hour of the met record for short-term ones. The demonstration protects the standard against what the permit allows, not what the plant happens to do today.
  • Nearby sources get realism on one axis only: still the maximum allowable emission limit, but at the actual operating level averaged over the most recent 2 years (annual), or a "temporarily representative level when actually operating" for short-term standards — the axis where padding every neighbor's emissions to a theoretical maximum would stack conservatism into an unbuildable analysis.
  • Other sources aren't modeled from emissions at all — their ambient impact "can be represented by air quality monitoring data unless adequate data do not exist" (Table 8-2). That is the background monitor's job (see Background concentrations & representative monitors).

Source · 40 CFR 51 App. W, Table 8-2 — the full table, including its footnotes, is reproduced from the regulation.

One footnote deserves bold type, because getting it wrong understates every short-term impact: for a source with a federally enforceable limited schedule, "only these hours will be modeled with emissions from the source. Modeled emissions should not be averaged across non-operating time periods" (Table 8-2, fn 3). A source permitted for 8 hours a day is modeled at full rate for those 8 hours and zero for the rest — never at one-third rate around the clock.

The load analysis: capacity is not always the worst case

Appendix W requires checking that the modeled operating condition is the one that actually maximizes impact:

As a minimum, the source should be modeled using the design capacity (100 percent load). … Where the source operates at substantially less than design capacity, and the changes in the stack parameters associated with the operating conditions could lead to higher ground level concentrations, loads such as 50 percent and 75 percent of capacity should also be modeled.

Source · 40 CFR 51 App. W §8.2.2(d).

The physics behind the rule: lower load usually means lower exit velocity and temperature — weaker plume rise — so a smaller emission rate can produce a higher ground-level concentration. The same paragraph settles malfunctions: they "are not considered to be a normal operating condition" and generally stay out of allowable emissions — unless the excess results from "poor maintenance, careless operation, or other preventable conditions."

Where the numbers come from

The rate itself needs provenance a reviewer can check. Appendix W's list (§8.2.1(d)): source-specific stack tests, vendor or trade-association data, and the U.S. Environmental Protection Agency's (EPA) compiled emission-factor library — AP-42, which "also provides an indication of the quality and amount of data on which many of the factors are based." And beyond the rate, §8.2.1(b) lists the physical inputs that must come with it: plant layout, stack height and diameter, operating conditions, controls — "required inputs to air quality models … needed to determine maximum potential impacts."

In PlumeSmart

Each source carries its emission rate with units and basis, stack parameters, and an optional variable-emissions schedule, so a limited-schedule source can be modeled at its permitted hours rather than smeared across the day; project-source versus inventory-source roles are kept distinct in cumulative analyses.