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Particulate Matter

Particulate matter is composed of solid particles or liquid droplets that are small enough to remain suspended in the air.

PM2.5 refers to particles that are 2.5 microns or less in diameter, roughly 1/28th the diameter of a human hair. PM2.5 results from fuel combustion (e.g., from motor vehicles, power generation, and industrial facilities), residential fireplaces and wood stoves. In addition, PM2.5 can be formed in the atmosphere from gases such as sulfur dioxide, nitrogen oxides, and volatile organic compounds. PM2.5 can penetrate the human respiratory system's natural defenses and damage the respiratory tract when inhaled. Numerous scientific studies have linked particle pollution exposure to a variety of problems, including:


National Ambient Air Quality Standards

As required by the Clean Air Act, National Ambient Air Quality Standards (NAAQS) have been established for the following major air pollutants. These pollutants, known as criteria pollutants, are: carbon monoxide, nitrogen dioxide, ozone, particulate matter (PM10 and PM2.5), sulfur dioxide and lead.

The Federal standards for PM2.5 are summarized in Table 1. The primary standards have been established to protect the public health. The secondary standards are intended to protect the nation's welfare and account for air pollutant effects on soil, water, visibility, materials, vegetation and other aspects of the general welfare.

Table 1
National Ambient Air Quality Standards
Pollutant Averaging Time Federal Standards
Primary Secondary
Particulate Matter as PM2.5 Annual arithmetic mean1 15.0 μg/m3 Same as primary standard
24 hour2 65 μg/m3
1 To attain this standard, the 3-year average of the weighted annual mean PM2.5 concentrations from single or multiple community-oriented monitors must not exceed 15.0 μg/m3.
2 To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor with an area must not exceed 65 μg/m3.
Source: EPA (May 24, 2006)

Statutory and Regulatory Requirements for PM Nonattainment Areas

On March 10, 2006, the EPA issued amendments to the Transportation Conformity Rule to address localized impacts of particulate matter, entitled "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 amendments require the assessment of localized air quality impacts of federally-funded or approved transportation projects that are deemed to be projects of air quality concern that are located in PM2.5 nonattainment and maintenance areas. This assessment of localized impacts (i.e., "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 conformity requirements to support state and local air quality goals.

EPA requires hot-spot findings to be based on directly emitted PM2.5. This is because secondary particles take several hours to form in the atmosphere, giving emissions time to disperse beyond the immediate area of concern. A 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 CFR93.123(b)(4). The Conformity Rule requires PM2.5 hot-spot analyses to include road dust emissions only if such emissions have been found significant by the 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).

PM2.5 Regional Conformity Determination

Section 176(c) of the Clean Air Act and the federal conformity rule require that transportation plans and programs conform to the intent of the state implementation plan (SIP) through a regional emissions analysis in PM2.5 nonattainment areas. The National Capital Region 2005 Constrained Long Range Transportation Plan (CLRP) and the 2006-2011 Metropolitan Transportation Improvement Program (TIP) have been determined to conform to the intent of the SIP. The CLRP is a comprehensive plan of transportation projects and strategies that the Transportation Planning Board realistically anticipates can be implemented over the next 30 years. The TIP is a 6-year program that describes the time-frame for federal funds to be obligated to state and local projects. The U.S. Department of Transportation made a PM2.5 conformity determination on the CLRP and the TIP on February 21, 2006; thus, there are 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.

All project phases remaining for the entire Woodrow Wilson Bridge Project were included in the regional emissions analysis and there have been no significant changes in the project's design concept or scope from that used in the conformity analyses. The project, therefore, comes from a conforming plan and program in accordance with 40 CFR 93.115.

Updated: 7/6/2011
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