Background concentrations & representative monitors

A modeled impact alone never answers a compliance question. The Guideline on Air Quality Models (40 CFR Part 51, Appendix W) requires the design concentration — the number compared against a NAAQS or PSD increment — to be built as a cumulative total: the modeled contribution plus a background term representing everything the model didn't include.

What background must — and must not — contain

Appendix W defines the split directly:

Background concentrations are essential in constructing the design concentration, or total air quality concentration, as part of a cumulative impact analysis for NAAQS and PSD increments (section 9.2.3). Background air quality should not include the ambient impacts of the project source under consideration.

The remainder divides into two components:

i. Nearby sources: These are individual sources located in the vicinity of the source(s) under consideration for emissions limits that are not adequately represented by ambient monitoring data. Typically, sources that cause a significant concentration gradient in the vicinity of the source(s) under consideration for emissions limits are not adequately represented by background ambient monitoring. The ambient contributions from these nearby sources are thereby accounted for by explicitly modeling their emissions (section 8.2).

ii. Other sources: That portion of the background attributable to natural sources, other unidentified sources in the vicinity of the project, and regional transport contributions from more distant sources (domestic and international). The ambient contributions from these sources are typically accounted for through use of ambient monitoring data or, in some cases, regional-scale photochemical grid modeling results.

Source · 40 CFR 51 App. W §8.3.1(a) — captured verbatim in the source library.

Three consequences worth reading twice:

• The project source is never in the background. Its impact enters only through the model.
• A nearby source with a significant concentration gradient is modeled, not monitored — a monitor a few kilometers away cannot represent the steep spatial variation close to such a source (§8.3.1(a)(i), pointing to the §8.2 emission-input rules).
• The monitor's job is everything else: natural, unidentified, and regional/distant contributions (§8.3.1(a)(ii)).

Choosing the monitor

For areas with an isolated source, Appendix W's recommendations (§8.3.2) are specific:

The EPA recommends use of the most recent quality assured air quality monitoring data collected in the vicinity of the source to determine the background concentration for the averaging times of concern. In most cases, the EPA recommends using data from the monitor closest to and upwind of the project area. If several monitors are available, preference should be given to the monitor with characteristics that are most similar to the project area.

Source · 40 CFR 51 App. W §8.3.2(b).

The data itself must hold up to the same scrutiny as compliance monitoring:

The monitoring network used for developing background concentrations is expected to conform to the same quality assurance and other requirements as those networks established for PSD purposes. … These data should be adequately representative of the area to inform calculation of the design concentration for comparison to the applicable NAAQS (section 9.2.2).

Source · 40 CFR 51 App. W §8.3.1(b).

Monitoring agencies certify their data annually — the previous calendar year's ambient and quality-assurance data must be completely submitted to EPA's Air Quality System (AQS) and certified accurate by May 1 of each year (40 CFR §58.15). AQS is where background candidates come from.

Source · 40 CFR §58.15(a) — captured verbatim in the source library.

Refinements: temporal pairing and multiple monitors

A single annual background value added everywhere is the simplest treatment — and often conservatively high for short-term standards. Appendix W allows two refinements when the situation warrants them:

iii. For short-term standards, the diurnal or seasonal patterns of the air quality monitoring data may differ significantly from the patterns associated with the modeled concentrations. When this occurs, it may be appropriate to pair the air quality monitoring data in a temporal manner that reflects these patterns (e.g., pairing by season and/or hour of day).

iv. For situations where monitored air quality concentrations vary across the modeling domain, it may be appropriate to consider air quality monitoring data from multiple monitors within the project area.

Source · 40 CFR 51 App. W §8.3.2(c)(iii)–(iv).

In AERMOD terms, a temporally varying background enters through the BACKGRND keyword on the SO pathway, with temporal profiles (season, month, hour-of-day, and combinations) and optional wind-sector variation; a uniform background can equivalently be added to the modeled design value after the run. The AERMOD User's Guide documents the keyword, its temporal options, and background sectors in §3.3.8.

Source · AERMOD User's Guide (EPA-454/B-26-001) §3.3.8 (BACKGRND).

In PlumeSmart

The platform carries both treatments: a uniform background is combined with the modeled design value in the standard's own form, and temporally varying backgrounds (season, month, hour-of-day, sector) generate the corresponding BACKGRND records. Monitor candidates are pulled with their AQS identities so the choice is documented for review.