Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration
Office of Planning, Environment, & Realty (HEP)

PM2.5 CONFORMITY DETERMINATION for Baltimore Beltway I-695: MD 140 to MD 702

C. 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. In addition, vehicles cause dust from paved and unpaved roads to be re-entrained, or re-suspended, in the atmosphere. Also, highway and transit project construction may cause dust. Finally, gases in vehicle exhaust may react in the atmosphere to form PM.

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 in diameter or less. (Note: 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 (15.0 micrograms per cubic meter (ug/m3)) and a 24-hour standard (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 Hotspot 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 concern2. This assessment of localized impacts (i.e., "hotspot 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 conformity requirements to support State and local air quality goals.

Qualitative hotspot analysis is required for these projects before EPA releases its future quantitative modeling guidance and announces that quantitative PM2.5 hotspot analyses are required under 40 CFR §93.123(b)(4). EPA requires hotspot 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 hotspot analysis if such emissions are considered temporary according to 40 CFR §93.123(c)(5).

Section 176(c) of the Clean Air Act and the federal conformity rule requires 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 I-695: MD140 to MD702 project is located in the Baltimore, MD PM2.5 nonattainment area and is under the jurisdiction of the Baltimore Regional Transportation Board (BRTB). The Baltimore Regional Transportation Board is the federally recognized Metropolitan Planning Organization (MPO) for transportation planning in the Baltimore region. Members of the Baltimore Metropolitan Council (BMC) Board serve on the BRTB, and the BMC provides technical and staff support to the BRTB. The 2004 Baltimore Regional Transportation Plan and the FY 2007-2011 Transportation Improvement Program have been determined to conform to the intent of the SIP3. The US Department of Transportation made a Conformity Determination on the CLRP and the TIP on November 8, 2006, and thus there is a currently conforming transportation plan and TIP in accordance with 40 CFR 93.114. The current conformity determination is consistent with the final conformity rule found in 40 CFR Parts 51 and 93. The I-695: MD 140 to MD 702 project was included in the regional emissions analysis and there have been no significant changes in the project's design concept or scope, as used in the conformity analyses. Therefore, this project comes from a conforming plan and program in accordance with 40 CFR 93.115. Conformity to the purpose of the SIP means that the transportation activity will not cause new air quality violations, worsen existing violations, or delay timely attainment of the relevant national ambient air quality standards (NAAQS or "standards").

2 Criteria for identifying projects of air quality concern is described in 40 CFR 93.123(b)(1), as amended.
3 Conformity Determination of the 2004 Baltimore Regional Transportation Plan and the FY 2007-2011 Transportation Improvement Program, Baltimore Regional Transportation Board (BRTB), August 2006.
Updated: 7/6/2011
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000