Particle Pollution Designations Memorandum and Data for the 2024 Revised Annual PM2.5 NAAQS
On February 7, 2024, the EPA promulgated a revised primary annual PM2.5 NAAQS, strengthening the standard from 12.0 micrograms per cubic meter (µg/m3) to 9.0 µg/m3.
Following promulgation of a new or revised NAAQS, the EPA is required under Clean Air Act Section 107(d) to designate all areas of the country with respect to the standard. This process begins with designations recommendations from states and Tribes. The information below provides states and Tribes with current data and tools that may be useful to:
- evaluate each area on a case-by-case basis, and;
- make boundary recommendations.
On this page:
- EPA's Area Designations Memorandum for the 2024 Revised Annual PM2.5 National Ambient Air Quality Standards and Tribal Guidance
- Five-Factor Analysis
- Factor 1: Air Quality Data
- Factor 2: Emissions and Emissions-Related Data
- Factor 3: Meteorology
- Factor 4: Geography/Topography
- Factor 5: Jurisdictional Boundaries
- Designations Mapping Tool
- Datasets
A. EPA's Memorandum on Initial Area Designations for the 2024 Revised Annual Fine Particle NAAQS and Tribal Guidance
States and Tribes should refer to the following Memorandum and Tribal guidance documents for area designations for the 2024 Revised Annual Fine Particle NAAQS when preparing their recommendations on area designations.
- Memorandum on the Area Designations for the 2024 Revised Annual PM2.5 NAAQS (pdf)
- Guidance to Regions for Working with Tribes during the National Ambient Air Quality Standards (NAAQS) Designations Process (pdf)
- Policy for Establishing Separate Air Quality Designations for Areas of Indian Country (pdf)
- Developing Designation Recommendations for Areas of Indian Country (April 2018) (pdf)
B. Five-Factor Analysis
The five factors identified in the 2024 Memorandum on Area Designations for the 2024 Revised Primary Annual Fine Particle NAAQS are listed below, along with data and data sources that may be useful in evaluating each area on a case-by-case basis and in making boundary recommendations. The following is not an exclusive list of factors, data, or sources of data that could be considered in assessing an area. The EPA is providing this list as a useful tool for the designations process, and it should not be construed as representing a decision by the EPA to rely solely on this list for final designation determinations.
The EPA intends, at a minimum, to evaluate these factors, data and/or data sources in making final determinations regarding area designations for the 2024 Revised Primary Annual Fine Particle NAAQS. States and Tribes should refer to the designations Memorandum and Tribal guidance documents for more complete information about these factors and the factor analysis. We recognize that consideration of the five factors for an assessment can be resource intensive. In addition to providing the relevant data to facilitate the analyses, the EPA has provided a PM2.5 Mapping Tool to assist air agencies in developing their area designations and nonattainment boundary recommendations.
Factor 1: Air Quality Data - The air quality analysis is an examination of available ambient PM2.5 air quality monitoring data, including the annual design value calculated for each area based on air quality data for a 3-year period.
For additional information about PM2.5 air quality data, visit EPA's Air Trends site.
Factor 2: Emissions and Emissions-Related Data - The emissions analysis examines emissions of identified sources of direct PM2.5, the major components of direct PM2.5 (organic mass, elemental carbon, crustal material (and/or individual trace metal compounds)), primary nitrate and primary sulfate, and precursor gaseous pollutants (e.g., SO2, NOX, total VOC, and NH3). Emissions data are derived from the 2020 National Emissions Inventory, and are given in tons per year. Emissions data indicate the potential for a source to contribute to observed violations, making it useful in assessing boundaries of nonattainment areas.
Factor 3: Meteorology - The evaluation of meteorological data helps to determine the effect on the fate and transport of emissions contributing to PM2.5 concentrations and to identify areas potentially contributing to the monitored violations. One basic meteorological analysis involves assessing potential source-receptor relationships in the area using summaries of emissions, wind speed, and wind direction data. A more sophisticated assessment involves modeling air parcel trajectories.
Factor 4: Geography/Topography - The geography/topography analysis includes an examination of physical features of the land that might define the airshed and, therefore, affect the formation and distribution of PM2.5 over an area. Mountains or other physical features may influence the fate and transport of emissions and PM2.5 concentrations. Additional analyses may consider topographical features that cause local stagnation episodes via inversions. Valley-type features can cause local cold-air drainage patterns and vertical temperature inversions that effectively “trap” air pollution. Under these conditions emissions can accumulate leading to periods of elevated PM2.5 concentrations. These air drainage patterns and inversions may be limited in extent and therefore may need to be separated from regions with more conventional air flow and PM2.5 concentration patterns.
Factor 5: Jurisdictional Boundaries - The analysis of jurisdictional boundaries identifies the planning and organizational structure of an area to provide insights into how air quality planning and enforcement in a potential nonattainment area can be carried out. Examples of jurisdictional boundaries include counties, air districts, areas of Indian country, metropolitan planning organizations (MPOs), and existing nonattainment areas.
C. PM2.5 Designations Mapping Tool
The PM2.5 Mapping Tool provides air agencies access to air quality data, emissions data, jurisdictional boundaries, and other important information to assist in designations for the 2024 Revised Primary Annual Fine Particle NAAQS.
Note: This preliminary version of the PM2.5 Designations Mapping Tool is intended for pre-planning purposes only. Additional features, data, and functionality will be added to the tool as the area designations process progresses and/or as they become available. These current datasets and mapping tool information should not be used for analyses related to designations.
D. Datasets Provided by the EPA to Support the Five-Factor Analysis
| Dataset | Availability Date |
|---|---|
|
Link to 2023 PM2.5 Design Values Note: Use Table 6a - Site Trends Ann. |
September 26, 2024 |
| November 25, 2024 | |
| 2022 County and Facility Emissions for PM2.5 Designations (xlsx) | February 6, 2025 |
|
February 6, 2025 |
|
| 2022 Vehicle Miles Traveled (xlsx) | November 25, 2024 |
|
Links to AQS wind data used for Mapping Tool wind roses: https://aqs.epa.gov/aqsweb/airdata/hourly_WIND_2021.zip |
December 2024 |
| File Name Key for Wind Rose Data (xlsx) | February 2025 |
| NWS wind data used for mapping tool wind roses (zip) | December 2024 |
| HYSPLIT output files containing hourly points used to create density maps (zip) | December 2024 |
|
HYSPLIT density maps in PNG format (suitable for inserting into documentation) |
December 2024 |
|
HYSPLIT density interactive maps in HTML format |
December 2024 |