In July 1997, the EPA revised the PM NAAQS to include ambient air concentrations of PM with an aerodynamic diameter of 2.5 μm or less (PM2.5, or fine particulate matter), again without regard to chemical composition. PM2.5 was selected because particles of this size or smaller penetrate more deeply into the lung. The new PM2.5 standard establishes a 24-hour average ambient concentration limit of 65 micrograms per cubic meter (μg/m3) and an annual mean concentration limit of 15 μg/m3 to protect human health from both acute and chronic effects associated with the respiration of fine particulate matter. To meet the new ambient concentration levels, many different sources of emissions would have to reduce their current emissions that contribute to primary and secondary PM concentrations, especially in urban areas of the eastern United States where many coal-fired power plants are located. The EPA also promulgated regional haze regulations in April 1999 that focused on the impact of PM2.5 on visibility in Class I ("pristine") areas of the United States.
On May 14, 1999, the U.S. Court of Appeals for the District of Columbia remanded the new ambient PM2.5 standards on the grounds that the EPA had interpreted provisions of the Clean Air Act in a way that was unconstitutional. The court did not vacate EPA's PM2.5 NAAQS but invited EPA to provide additional justification for the level of the standard and retained jurisdiction over future hearings on the matter. EPA has indicated that it plans to appeal this ruling.
The major components of PM2.5 are nitrates, sulfates, carbonaceous materials, and varying amounts of crustal dust. PM is defined by the federal reference method (FRM) of measurement. A national PM2.5 monitoring network, consisting of 1,500 air monitors, is being deployed to collect data. The network will also include 70 National Park Service sites in remote locations to study background concentrations and to monitor for regional haze. Air quality data will be collected for three years, beginning in 1999, to determine attainment (i.e., areas that meet the NAAQS) and non-attainment areas. In non-attainment areas, states must draft plans (state implementation plans [SIPs]) to limit emissions of specific pollutants to make the necessary improvements in air quality.
The combustion of coal to generate electricity produces both primary PM2.5 (e.g., fly ash, carbon soot, acid mist) and the gaseous precursors (e.g., sulfur dioxide and NOx) that lead to the formation of secondary fine particles (principally sulfates and nitrates). The gaseous precursors contribute substantially more to the total ambient PM2.5 mass than primary PM2.5. However, because of recent modifications in emission-control technologies for coal-fired power plants, the relationship between coal-fired boiler emissions and the concentration and composition of ambient fine particulate matter is still uncertain.
In fiscal year 1998, congressional appropriations called for the U.S. Department of Energy's Office of Fossil Energy (DOE-FE) to initiate a research program to address the key technical and scientific issues on the effects of the new standard for coal-based power systems. In response to this directive, the DOE-FE staff at DOE headquarters in Washington, D.C., and at the Federal Energy Technology Center (FETC) in Pittsburgh, Pennsylvania, developed a fine particulate research plan and program (FETC, 1999a).
Atmospheric aerosols are particles ranging in size from a few nanometers to tens of micrometers. Particles less than 2.5 μm in diameter are generally referred to as "fine" and particles between 2.5 and 10 μm in diameter as the "coarse" fraction of PM10. Fine particles with diameters of less than 0.1 μm are referred to as ''ultrafine." Atmospheric PM is either directly emitted to the atmosphere (primary PM) or formed in-situ by gas-to-particle conversion processes (secondary PM). Sources of primary particles can be natural or anthropogenic.
Significant natural sources of particulates include soil and windblown dust, sea spray, and volcanic action. Natural particles are formed from mechanical processes and are mostly coarse. However, some of these particles are small enough to belong to the fine particulate category. Terrestrial dust is also emitted as the result of human activities (e.g., from construction, resuspension of dust during driving, or agricultural activities). Smoke from forest, agricultural, and other fires also contributes to ambient fine particles.
Fuel combustion, industrial processes, transportation, and fugitive dust (e.g., from paved and unpaved roads or from construction) are the most important sources of primary anthropogenic particles. Combustion of coal, oil, or biomass results in the emission of mostly fine particles (diameters around 0.1-0.5 μm) of various compositions, depending on the fuel and combustion conditions. These particles consist mainly of carbonaceous matter, sulfates, and trace metals.
Most of the ambient fine particulate mass is