Description
Fine particles are defined by the U.S. Environmental Protection Agency (EPA) as compounds that have a diameter of less than or equal to 2.5 micrometers. Thirty of these particles laid end to end would equal the width of a human hair. Fine particles are composed of many organic and inorganic compounds, including sulfate, nitrate, organic carbon and elemental carbon, earthen dust and biological materials (pollen pieces), among others.

Sources
Substances that may form these particles come from power plants, industrial facilities, motor vehicles, jet aircraft, paint spray operations and gasoline stations, as well as natural sources such as volcanoes, plowed fields, trees, sea spray and vegetation matter.

Power Plants
Coal-fueled power plants emit sulfur dioxide (SO2) and nitrogen oxide (NOx) gases that are converted downwind to sulfate and nitrate compounds by chemical reactions in the atmosphere. Because particle sources vary by the types of human activities and natural sources in an area, fine particle composition varies across the U.S.

For example, coal-fueled power plants are more numerous in the eastern half of the U.S. Because these plants emit SO2, sulfate comprises a larger proportion of the particle mass in that area. In western states, the more numerous sources of nitrogen-containing gases, including NOx and ammonia, result in nitrates, organic compounds and earthen dust comprising the largest portion of fine particles.

Researchers are trying to determine how particle composition varies among regions within the U.S. and how emission controls of certain sources may change the regional composition of fine particles in the air. This is a difficult task because thousands of sources may exist in any one area, and atmospheric variability, such as clouds, humidity, temperature and wind speed and direction, can impact fine particle formation.

Trends
According to recent analyses by EPA, U.S. fine particle concentrations have been declining over the past decade due to control of mobile, industrial, and utility emissions sources. However, numerous counties in the eastern and western regions, but especially California, exceed the federal standard for fine particles in the air.

Health Effects of Fine Particles

• Current Studies
  Fine particles have been linked to incremental increases in mortality in urban areas; increases in respiratory problems; and contribution to regional haze that reduces visibility in national parks. Epidemiologic studies suggesting a link between fine particles and higher mortality rates in some U.S. cities led EPA to establish new Air Quality Standards for fine particles in July 1997.
  
  However, a comprehensive study conducted in Atlanta funded by the Electric Power Research Institute (EPRI) and others has detected a link between the elemental and organic carbon components of these particles and human health effects. EPA also has released a study of diesel soot (black carbon or elemental carbon) in which diesel soot has been linked to asthma, increased risks to other breathing impairments and certain cancers. By contrast, the EPRI research and some European research found no linkages between the sulfate and nitrate components of these particles and human health effects.
  
  Health experts are conducting detailed toxicological studies of fine particles produced by a variety of combustion sources to determine which sources may be contributing to health problems that may be associated with fine particles.

• Limiting Fine Particles
  Since sulfate and nitrate compounds comprise a large fraction of individual fine particles, EPA has developed regulations for reducing SO2 and NOx emissions from power plants and mobile sources, the two most prominent emissions sources that give rise to sulfate and nitrate components of fine particles. Specifically, EPA published the Clean Air Interstate Rule (CAIR) on May 12, 2005. CAIR requires NOx and SO2 emission reductions from utility boilers located in a 28-state region within the eastern U.S., including Wisconsin and Michigan. The regulations also require western regions to prepare detailed plans to reduce levels of fine particles.
  
  The Clean Air Act also requires EPA to revisit its pollutant standards, including those for fine particles, every five years. During this process, EPA must review published research concerning the health effects attributable to these pollutants and decide whether its standards adequately protect human health. EPA is currently reviewing its standards for atmospheric particles.
  
  
• Useful Resources Concerning Health Effects of Fine Particles
  EPA: Particulate Matter – National Ambient Air Quality Standards
  The Annapolis Center: Publications
  EPRI: Power Plants and Particulate Matter: Formation and Direct Emissions

Fine Particle Emission Controls for Power Plants 

• Power Plant Contributions to Fine Particles in the Air
  While power plants emit sulfur and nitrogen oxides that are converted into components of fine particles, actual power plant contributions to fine particle mass in the atmosphere are not well understood. EPA standards for fine particles are mass-based; the annual average standard is 15 ug/m³ of air, based on measurement. EPA and the states plan to use state-of-the-science models to establish reasonable relationships between the many sources and atmospheric particles actually measured in urban/regional locations. These models will help assure that EPA’s standards are achieved and maintained in the most cost-effective manner.
  
  
• We Energies Control Measures
  We entered into a consent decree with EPA to reduce by more than 6 percent the NOx and SO2 emissions from our existing coal-fueled power plants over the next 10 years on a system-wide basis. At our Pleasant Prairie Power Plant, we are installing a wet scrubber designed to reduce SO2 emissions, and a second Selective Catalytic Reduction (SCR) unit designed to reduce NOx emissions. This is the first power plant in Wisconsin to employ either of these technologies. Installation of this equipment, and the resulting reduction in plant emissions, was a key factor in EPA designating Southeast Wisconsin as an attainment area for fine particulates. Our commitment to system-wide SO2 and NOx emission reductions is expected to reduce our impact on the fine particle formation, as well as ground-level ozone, in the region.
  
  
• Other Useful Resources
  NETL: PM Emissions Control
  EPA: Particulate Matter Research Program