Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration

Office of Planning, Environment, & Realty (HEP)
PlanningEnvironmentReal Estate

HEP Events Guidance Publications Awards Contacts

Final PM2.5 Project-Level Conformity Analysis for the Ohio River Bridges Project in Louisville, Kentucky

IV. PM2.5 Requirements Background

What is Fine Particulate Matter (PM2.5)?

Particulate matter (PM) is the term for particles and liquid droplets suspended in the air. Motor vehicles (i.e., cars, trucks, and buses) emit direct PM from their tailpipes, as well as from normal brake and tire wear. Vehicles cause dust from paved and unpaved roads to be re-entrained, or re-suspended, in the atmosphere. Construction of highway and transit projects may cause dust. Finally, gases in vehicle exhaust may react in the atmosphere to form particulate matter.

Particles come in a wide variety of sizes and have been historically assessed based on size, typically measured by the diameter of the particle in micrometers. PM2.5 or fine particulate matter refers to particles that are 2.5 micrometers or less in diameter. In comparison, a human hair is about 70 micrometers in diameter and a grain of sand is about 90 micrometers in diameter. The National Ambient Air Quality Standards for fine particulate matter include an annual standard of 15.0 micrograms per cubic meter (ug/m3) and a 24-hour standard of 65 ug/m3. The annual standard is based on a 3-year average of annual mean PM2.5 concentrations; the 24-hour standard is based on a 3-year average of the 98th percentile of 24-hour concentrations.

Statutory and Regulatory Requirements for PM Hot-Spot Analyses

On March 10, 2006, EPA issued amendments to the Transportation Conformity Rule to address localized impacts of particulate matter: PM2.5 and PM10 Hot-Spot Analyses in Project-level Transportation Conformity Determinations for the New PM2.5 and Existing PM10 National Ambient Air Quality Standards (71 FR 12468). These rule amendments require the assessment of localized air quality impacts of federally-funded or approved transportation projects in PM10 and PM2.5 nonattainment and maintenance areas deemed to be projects of air quality concern5. This assessment of localized impacts or "hot-spot analysis" examines potential air quality impacts on a scale smaller than an entire nonattainment or maintenance area. Such an analysis is a means of demonstrating that a transportation project meets Clean Air Act Amendment conformity requirements to support state and local air quality goals.

The LMA area is currently designated nonattainment for PM2.5 because of violations of the annual PM2.5 standard. However, project level conformity analyses are required to address both the annual and hourly forms of the standard. As noted below, regional emissions are projected to decline; this project will reduce emissions at the worst-case location. Further, since none of the area monitors are showing a violation of the 24-hour standard, it is reasonable to assume that if this project can be shown to not cause a new violation or worsen an existing violation for the annual standard, it will not cause a new violation for the 24-hour standard.

Qualitative hot-spot analysis is required for these projects until EPA releases its future quantitative modeling guidance and announces that quantitative PM2.5 hot-spot analyses are required under 40 CFR §93.123(b)(4). EPA requires hot-spot findings to be based on directly emitted PM2.5, since secondary particles take several hours to form in the atmosphere giving emissions time to disperse beyond the immediate area of concern. The Conformity Rule requires PM2.5 hot-spot analyses to include road dust emissions only if such emissions have been found significant by EPA or the state air agency prior to the PM2.5 SIP or as part of an adequate PM2.5 SIP motor vehicle emissions budget (40 CFR §93.102(b)(3)). Emissions resulting from construction of the project are not required to be considered in the hot-spot analysis if such emissions are considered temporary according to 40 CFR §93.123(c)(5).

V. PM2.5 Regional Conformity Determination

Section 176(c) of the Clean Air Act Amendment and the federal conformity rule require that transportation plans and programs conform to the intent of the state air quality implementation plan (SIP) through a regional emissions analysis in PM2.5 nonattainment areas. The Louisville area 2030 Long Range Transportation Plan (LRTP) and the 2006-2008 Transportation Improvement Program (TIP) have been determined to conform to the intent of the SIP. The US Department of Transportation made a PM2.5 conformity determination on the LRTP and the TIP on March 3, 2006, and thus both the LRTP and TIP are currently conforming in accordance with 40 CFR 93.114. The current regional conformity determination is consistent with the final conformity rule found in 40 CFR Parts 51 and 93. The ORB project was included in the regional mobile source emissions analysis and there have been no significant changes in the project's design concept, scope, or implementation schedule, as used in the conformity analysis. Therefore, the project comes from a conforming plan and program in accordance with 40 CFR 93.115.



5 Criteria for identifying projects of air quality concern is described in 40 CFR 93.123(b)(1).
Updated: 07/06/2011
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000