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4. PM2.5 Requirements Background

A. 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. The Birmingham area was designated non-attainment of the annual standard (effective date April 5, 2005).

B. Statutory and Regulatory Requirements for PM Hot Spot Analysis

The Clean Air Act Section 176(c) of the CAA requires that federally supported highway and transit project activities are consistent with state air quality goals, found in the state implementation plan (SIP). This process is called Transportation Conformity. Conformity to the SIP means that transportation activities will not cause new violations of the national ambient air quality standards (NAAQS or "standards"), worsen existing violations of the standard, or delay timely attainment of the relevant standard. Transportation conformity is determined by meeting the requirements of the Transportation Conformity Rule (40 CFR Part 93).

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 concern1. 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.

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).

1 Criteria for identifying projects of air quality concern is described in 40 CFR 93.123(b) (1).
Updated: 7/6/2011
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