Particle Pollution Designations Data for the 2024 Revised Annual PM2.5 NAAQS

On February 7, 2024, the EPA promulgated a final rule revising the primary annual fine particulate matter (PM_2.5) National Ambient Air Quality Standard (NAAQS or standard). In that action, EPA revised the standard from 12.0 micrograms per cubic meter (µg/m³) to 9.0 µg/m³. 

• Final Reconsideration of the NAAQS for Particulate Matter (pdf) (4.84 MB)

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. The 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 inform the designations process.

Data and data sources supporting a five-factor analysis may be useful in evaluating areas 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.

Factor 1: Air Quality Data - The air quality analysis is an examination of available ambient PM_2.5 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 PM_2.5 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, NO_X, total VOC, and NH₃). Emissions data are derived from the 2023 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 modeling air parcel trajectories for every violating monitoring site every six hours throughout the 2023-2025 design value period. HYSPLIT Residence Days (HRD) is a count of unique days during which back-trajectory points were located within county boundaries. HRD evaluates counties by how consistently, not just how often, they sit in the monitoring site's upwind path across the design value period. While HRD characterizes cumulative multi-day transport influence, the wind rose is a description of prevailing local flow. Another HYSPLIT back-trajectory dataset has been produced specifically for developing wind roses at violating monitoring site locations. Converting the trajectory displacement from hour 0 to 1 into a bearing and speed gives an implied near-surface wind direction and speed, which is input into the wind rose plots.       

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.

PM_2.5 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 version of the PM2.5 Designations Mapping Tool is intended for planning purposes only. 

• PM_2.5 Designations Mapping Tool

Datasets Provided by the EPA to Support a Five-Factor Analysis 

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