1.5 How is the NEI created?

The “Air Emissions Reporting Rule” (AERR) is the regulation that requires state and local agencies to submit CAP emissions, and the Emissions Inventory System is the data system used to collect, QA, and compile those submittals as well as EPA augmentation data. Most S/L/T air agencies also provide voluntary submissions of HAP emissions. The 2008 NEI was the first inventory compiled using the AERR, rather than its predecessor, the Consolidated Emissions Reporting Rule (CERR). The 2023 NEI is the sixth AERR-based inventory, and improvements in the 2023 NEI process reflect lessons learned by the S/L/T air agencies and EPA from prior NEI efforts. The AERR requires agencies to report all sources of emissions, except fires and biogenic sources. Reporting of open fire sources, such as wildfires, is encouraged, but not required. Sources are divided into large groups called “data categories”: stationary sources are “point” or “nonpoint” (county totals) and mobile sources are either onroad (cars and trucks driven on roads) or nonroad (off-road vehicles, recreational marine vessels, and nonroad equipment such as lawn and garden equipment). One nuance about the NEI data categories: while locomotives and commercial marine vessels are nonroad mobile sources, but are developed, in format, as part of the nonpoint data category; near and ground-level aircraft estimates are also nonroad sources but are compiled at discrete airport locations and therefore reside in the point data category.

The AERR has emissions thresholds above which States must report stationary emissions as “point” sources, with the remainder of the stationary emissions reported as “nonpoint” sources.

The AERR changed the way these reporting thresholds work, as compared to the CERR, by changing these thresholds to “potential to emit” thresholds rather than actual emissions thresholds. While the criteria for which sources to report is now based on potential emissions, in both the CERR and the AERR, the reported emissions are actual emissions. The AERR requires emissions reporting for point sources every year, with additional requirements every third year in the form of lower point source emissions thresholds, and 2023 is one of these third-year inventories.

Table 1.1 provides the potential-to-emit reporting thresholds that applied for the 2023 NEI cycle. “Type B” is the terminology in the rule that represents the lower emissions thresholds required for point sources in the triennial years. The reporting thresholds are sources with the potential to emit 100 tons/year or more for most criteria pollutants, except for CO (1000 tons/year), and, updated starting with the 2014 inventory, Pb (0.5 tons/year, actual emissions). As shown in the table, special requirements apply to nonattainment area (NAA) sources, where even lower thresholds apply. The relevant ozone (O3), CO, and PM10 nonattainment areas that applied during the year that the S/L/T agencies submitted their data for the 2023 NEI are available on the “Nonattainment Areas for Criteria Pollutants (Green Book) web site”. Note that while the AERR establishes the minimum requirements for State and local air agencies to report their stationary sources as discrete point sources, many agencies have gone beyond those minimums for many years past, i.e., they report many smaller-emitting sources as discrete point sources.

Table 1.1: Point source reporting thresholds, as potential to emit, for CAPs in the AERR. Note that the potential to emit is shown in tons per year, as defined in 40 CFR part 70, with the exception of lead.

Pollutant	Type B Source Threshold	Thresholds within NAAs
SO2	≥100	≥100
		PM2.5 (serious) ≥70
VOC	≥100	O3 (moderate) ≥ 100
		O3 (serious) ≥ 50
		O3 (severe) ≥ 25
		O3 (extreme) ≥ 10
		PM2.5 (serious) ≥70
NOx	≥100	≥100
		O3 (serious) ≥ 50
		O3 (severe) ≥ 25
		O3 (extreme) ≥ 10
		PM2.5 (serious) ≥70
CO	≥1000	O3 (all areas) ≥ 100
		CO (all areas) ≥ 100
Lead	≥0.5 (actual)	≥0.5 (actual)
Primary PM10	≥100	≥100
		PM10 (serious) ≥70
Primary PM2.5	≥100	≥100
		PM2.5 (serious) ≥70
NH3	≥100	≥100
		PM2.5 (serious) ≥70

Based on the AERR requirements, S/L/T air agencies submit emissions or model inputs of point, nonpoint, onroad mobile, nonroad mobile, and fires emissions sources. Except for California, reporting agencies were required to submit model inputs for onroad and nonroad mobile sources instead of emissions. For the 2023 NEI, all these emissions and inputs were required to be submitted to the EPA per the AERR by December 31, 2024 (with an extension given through January 15, 2025). Once the initial NEI reporting period closed, the EPA provided feedback on data quality such as suspected outliers and missing data by comparing to previously established emissions ranges and past inventories. In addition, the EPA augmented the S/L/T data using various sources of data and augmentation procedures. This documentation provides a detailed account of EPA’s quality assurance and augmentation methods.