2.5 How does this NEI compare to past inventories?

Many similarities exist between the 2023 NEI approaches and past NEI approaches, notably that the data are largely compiled from data submitted by S/L/T agencies for CAPs, and that the HAP emissions are augmented by the EPA to differing degrees depending on geographical jurisdiction because they are a voluntary contribution from the partner agencies. In 2023, S/L/T participation was again somewhat more comprehensive than the previous NEI. The NEI program continues with the 2023 NEI to work towards a complete compilation of the nation’s CAPs and HAPs. The EPA provided feedback to S/L/T agencies during the compilation of the data on critical issues, such as potential outliers, missing SCCs, missing mercury (Hg) data and coke oven data, as has been done in the past, collected responses from S/L/T agencies to these issues, and improved the inventory for the release based on S/L/T agency feedback. In addition to these similarities, there are some important differences in how the 2023 NEI has been created and the resulting emissions, which are described in the following two subsections.

2.5.1 Differences in approaches

With any new inventory cycle, changes to approaches are made to improve the process of creating the inventory and the methods for estimating emissions. The key changes for the 2023 cycle are highlighted here.

To improve the process, we learned from the prior triennial inventories (for 2008, 2011, 2014, 20217, and 2020) compiled with the EIS. We made changes to state-county Federal Information Processing Series (FIPS) codes, SCC codes, and refined quality assurance checks and features that were used to assist in improvements to nonpoint data category activity data input templates. We retained the Nonpoint Survey functionality used in the 2020 NEI (introduced for the 2014 NEI) to assist with S/L/T and EPA data reconciliation for the nonpoint data.

In addition to process changes, we added estimates for the first time for several source categories and improved emissions estimation methods for several source categories. We summarize the differences in approaches by data category in the following sections.

2.5.1.1 Point data category

For point sources, there were no major methodology or process changes for 2023 NEI. We did, however, incorporate additional TRI facility matches into EIS. Additionally, a point source HAP review was conducted and corrections to data were received. More information on point source improvements and approaches is available in the point section of this TSD.

2.5.1.2 Nonpoint data category

We made a significant process update and method improvements for several stationary nonpoint sectors. The EPA creates and provides emissions estimation tools for two purposes: 1) as tools for S/L/T agencies to use themselves, and 2) to backfill emissions values where not provided by S/L/T agencies.

For all nonpoint categories, we updated the activity data to use the newest data available, at the time, to represent the 2023 inventory year; in most cases, this is year-2023 activity data. Most emission changes for all nonpoint sources not otherwise discussed in this section resulted from these activity data updates -be they from EPA or provided directly from S/L/Ts.

Process Improvements

As part of the 2017 NEI development process, we introduced “Input Templates” for S/L/Ts to provide activity data for several nonpoint data category tools. By providing templates where S/L/Ts can review the previous year’s data, the data source, and easily update values at a county or state level, that then feeds into EPA’s emissions estimation tools, assures that the calculations and methods are identical. EPA provides default Input Templates to S/L/T inventory developers for them to review and submit their own estimates if they have more local or higher quality data. We encourage S/L/Ts to submit activity data inputs rather than direct emission submittals for many nonpoint categories. EPA downloads all inputs, EPA and S/L/T-submitted, periodically during the NEI development process to generate draft emission estimates prior to finalizing the NEI.

For the 2023 NEI, we integrated the templates directly into EIS, where reporting agencies can view and edit in EIS directly or bulk download, edit, and upload to EIS. Building the templates into EIS also allows for automated quality assurance of agency submitted data versus EPA default estimates, and immediate feedback on potential critical errors. In the 2020 NEI, these templates were stored on a shared website where version control and data integrity and timely feedback were significant challenges.

Other than minor SCC changes and adding new source categories that EPA estimates for the 2023 NEI, no changes were made to the Nonpoint Survey, first introduced for the 2014 NEI development cycle.

Method Improvements

Road Dust: Road dust emissions are one of the largest sources of primary particulate matter emissions in the United States. In the 2023 NEI, EPA leveraged telematics data to update and better reflect vehicular miles traveled on paved and unpaved roads at the county-level. This update led to a significant decline in estimated primary particulate matter emissions from road dust in the 2023 NEI when compared to the 2020 NEI.

Cooking: By leveraging meat consumption statistics provided by the U.S. Department of Agriculture, EPA was able to better reflect the true activity of commercial cooking and subsequent emissions. In total, estimated emissions from commercial cooking in the 2023 NEI are significantly lower when compared to the 2020 NEI. In addition, methodological updates were implemented to better estimate emissions from residential cooking.

Roofing Asphalt: Nationwide default estimates of emissions from roofing asphalts, which include asphalt cements and emulsions used in the manufacturing of asphalt shingles, asphalt sealant, and roof tar, were introduced for the first time in the 2023 NEI.

Residential Wood Combustion: We used Residential Energy Consumption Survey (RECS) microdata analysis to better allocate state level wood consumption from urban to rural counties. Emission factors are distribution profiles for woodstoves were also updated. Nationally, PM2.5 emissions are similar to the 2020 NEI; however, emissions are distributed more realistically to rural counties than urban counties compared to the 2020 NEI.

Structure Fires and Motor Vehicle Fires: A new source category that EPA estimates for the 2023 NEI, we leveraged U.S. Fire Administration (USFA) National Fire Incident Reporting System (NFIRS) data with fuel loading and emission factor assumptions from a 2022 study [ref 1]. We also accounted for the August 2023 wildfire urban interface Lahaina (Maui County Hawaii) fires [ref 2]. While CAP emissions are very small nationally, due to the type of materials combusted for these sources, air toxics such as polycylic aromatic hydrocarbons (PAHs) are more significant and impact national trends for the “Miscellaneous” tier.

Campfires: A new source category that EPA estimates for the 2023 NEI, campfires data from U.S. Campgrounds provide most public campground and campsite data and combined with assumptions on burn rates and existing emissions factors, yield a modest approximately 14,000-ton contribution to national PM2.5 estimates.

Other Ammonia: These are a new set of sources that EPA estimates for the 2023 NEI that includes direct ammonia (NH3) from dog, cat, and deer waste, and human processes such as respiration, perspiration, cigarette smoke, and infant diaper waste. In previous NEIs, these sources were estimated by only a couple states. Nationally, these sources contribute approximately 128,000 tons of NH3, a small portion of the more than 4 million tons of overall NH3 (dominated by livestock waste and fertilizer application); however, these sources are concentrated more in urban areas.

Oil and Gas Exploration and Production: County emissions estimates of abandoned oil and gas wells were added as a new source in this NEI. It is estimated that there were about 4 million abandoned wells in the US in 2023. These county estimates included plugged and unplugged wells which added about 175,000 tons of VOC nationally.

NH3 from soils and vegetation: Prior to the 2023 NEI, these estimates were classified entirely as anthropogenic “Agricultural fertilizer application” NH3 emissions. While the methodology for computing these estimates is unchanged for the 2023 NEI, the air quality model (CMAQ v5.4) used to estimate the NH3 estimates was modified to output non-agricultural land biogenic NH3 emissions separately from agricultural land emissions. Therefore, while the national total NH3 from this model is approximately 1.8 million tons of NH3 in both 2020 and 2023, for 2023, approximately 0.7 million tons are now classified as biogenic, with the remaining 1.1 million tons being agricultural land. This impacts national NH3 trends for both agriculture fertilizer (Miscellaneous Tier) and biogenics, despite national total NH3 for these sum processes being similar in 2020 and 2023.

Soil NOX: Prior to the 2023 NEI, the nitric oxide (NO) emissions output from the Biogenic Emission Inventory System (BEIS) model as strictly biogenic. Starting with the 2023 NEI, BEIS output for NO is separated between vegetation on non-agricultural lands (biogenic) versus that related to fertilizer application on agricultural lands. Therefore, while the underlying model methodology is unchanged and overall NOX (NO) estimates from BEIS are very similar, 1.03 million tons in both the 2020 and 2023 NEI, where these emissions were entirely “biogenic” in the 2020 NEI, the 2023 now apportions approximately 589,000 tons as anthropogenic agricultural fertilizer application sector (Miscellaneous Tier), and only 442,000 tons as biogenic.

Fires: Emissions from wildland fires are variable from year to year but are typically a significant source of primary particulate matter (PM) in the United States. In the 2023 NEI emission factors for PM increased nationwide resulting in comparable total PM emissions to the 2020 NEI despite 2023 having substantially lower US fire activity. The 2023 NEI also introduced region and landcover specific SCCs rather than aggregate SCCs. Additionally, two new fire sources were added for the 2023 NEI: pile burns and agricultural ditch burns.

Similar to the 2020 NEI, all fires data are included in the nonpoint data category for the 2023 NEI. This is simply a format issue as the underlying methodology for computing wildland fires (wildfires and prescribed burning) are still developed using satellite data for location and day-specific fires, but for the NEI, are subsequently aggregated to the county-level.

2.5.1.3 Onroad and nonroad data categories

For mobile sources, onroad methodology used an updated version of the MOVES model (MOVES5) with updated mobile source activity data such as vehicle miles travelled (VMT), age distributions, and fuel type mix, and improved idling computations; we also received new telematics data from StreetLight Data, Inc. For both onroad and nonroad, we relied on model inputs provided by S/L/T agencies and other sources, except for California and Tribes, who submitted emissions estimates. A key improvement for onroad mobile sources is NH3 emission factor updates and new vehicle registration data. Nonroad mobile methods are essentially unchanged from those in the 2020 NEI. The nonroad mobile onroad mobile sections of this TSD provide more detail on these improvements.

2.5.2 Differences in emissions between 2023 and 2020 NEI

This section presents a comparison from the 2020 NEI to the 2023 NEI. Table 2.4 compares CAP emissions for the 2023 minus 2020 NEI for seven highly aggregated emission sectors. Table 2.5 compares emissions for select HAPs for the 2023 minus 2020 NEI for the same seven highly aggregated emission sectors. Emissions from the biogenic (natural) sources are excluded, and the wildfire sector is shown separately for CAPs and HAPs. While Pb is a CAP for the purposes of the NAAQS, due to toxic attributes and inclusion in previous national air toxics assessments, it is reviewed here with the HAPs. The HAPs selected for comparison are based on their national scope of interest as defined by air toxics screening assessments. With a couple of notable exceptions, CAP emissions are lower overall in 2023 than in 2020. Some specific sector/pollutants increased in 2023 from 2020.

The increase in fuel combustion for VOC is driven by new emission factors for residential wood combustion. Conversely, the decrease in coal-fired electric generating unit (EGU) emissions account for the decrease in overall NOX and SO2 fuel combustion.

Increases in oil and gas exploration and production, particularly those in the Permian basin, account for the increase in NOX and VOC industrial processes.

Miscellaneous NOX increase is entirely driven by the aforementioned change in accounting of the biogenic model soil nitric oxide (NO) emissions output from the Biogenic Emission Inventory System (BEIS) from strictly biogenic in the 2020 NEI to those related to fertilizer application on agricultural lands. The underlying model methodology is unchanged and overall NOX (NO) estimates from BEIS are very similar, 1.03 million tons in both the 2020 and 2023 NEI; however, these emissions were entirely “biogenic” in the 2020 NEI and the 2023 now apportions approximately 589,000 tons as anthropogenic agricultural fertilizer application sector (broad sector “Miscellaneous”), and only 442,000 tons as biogenic. In contrast, the large decrease in Miscellaneous NH3 are primarily the result of agricultural fertilizer application estimates now separating out NH3 emissions to reflect the component from non-agricultural land biogenic NH3 emissions in the 2023 NEI.

Increases in nonroad mobile CO and SO2 are driven primarily by post-pandemic increased aircraft and commercial marine vessel activity.

There were comparatively less wildfire activity in 2023 than 2020, more than offsetting the increase in PM emission factors in the 2023 methodology, explaining the significant decreases in wildfire emissions for 2023.

Table 2.4: 2023 and 2020 NEI CAP emissions and broad sector changes (2023 minus 2020) in tons.

Broad Sector	CO	NH3	NOX	PM10	PM2.5	SO2	VOC
Fuel Combustion	-536682	-10258	-132407	-114617	-100261	-196264	122203
Highway Vehicles	-131058	96476	-388172	-34871	-7436	1950	-98018
Industrial Processes	186239	2538	155112	24920	-2885	4643	361828
Miscellaneous	-511971	-860758	553979	-3132360	-83640	-477	-18314
Nonroad Mobile	225890	47	3174	-10021	-9792	6202	-73099
Total 2023 NEI, excluding wildfires	42049923	4391849	7761013	11535014	3941875	1519748	12299534
Total 2020 NEI, excluding wildfires	42817505	5163803	7569326	14801964	4145890	1703694	12004934
Total Difference, excluding wildfires	-767582	-771955	191687	-3266950	-204015	-183946	294600
Wildfires	-12830823	-256302	-178225	-586231	-791000	-90901	-2251568

Table 2.5: 2023 and 2030 NEI select HAP emissions and broad sector changes (2023 minus 2020) in tons.

Broad Sector	Acrolein	Benzene	Ethylene Oxide	Formaldehyde	Hexavalent Chromium	Lead
Fuel Combustion	892	2881	-0.11	12367	-2.04	10
Highway Vehicles	-204	-3958	NA	-2554	-0.01	–
Industrial Processes	1687	4675	-27.00	15761	3.83	-12
Miscellaneous	-234	1149	10.00	1158	-0.59	7.37
Nonroad Mobile	-195	-809	NA	-3270	-0.05	72
Total 2023 NEI, excluding wildfires	38267	130723	75.00	298152	26.00	689
Total 2020 NEI, excluding wildfires	36322	126785	92.00	274688	25.00	613
Total Difference, excluding wildfires	1945	3938	-17.00	23463	1.14	77
Wildfires	-27803	-18523	NA	-160217	NA	-4.26