Atmospheric fine particles


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.


Substances that may form these particles come from power plants, industrial facilities, agricultural practices, 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. These compounds are commonly found in fine particles. However, particle composition varies in response to the types of human activities and natural sources in areas as small as cities. Therefore, it is not surprising that fine particle composition varies greatly 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 in that area.

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


According to recent analyses by EPA, U.S. fine particle concentrations have been declining over the past three decades 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 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, comprehensive studies conducted in Atlanta funded by the Electric Power Research Institute (EPRI) and others have 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) in 2005, requiring 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.

On July 6, 2010, EPA proposed the Transport Rule that would require 31 states and the District of Columbia to reduce power plant emissions that contribute to ozone and fine particle pollution in other states. This proposed rule would replace EPA's CAIR. A December 2008 court decision kept the requirements of CAIR in place temporarily but directed EPA to issue a new rule to implement the Clean Air Act requirements concerning the transport of air pollution across state boundaries. This action responds to the court's concerns.

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.

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 thought to be very small. EPA standards for fine particles are mass-based; the annual average standard is 15 ug/m3 of air, based on measurement, while the maximum allowable daily standard is 35 ug/m3. EPA and the states are using 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 65 percent the NOx and SO2 emissions from our existing coal-fueled power plants by 2013 on a system-wide basis. At our Pleasant Prairie Power Plant, we installed wet scrubbers designed to reduce SO2 emissions and selective catalytic reduction units designed to reduce NOx emissions. This was 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, based on annual average measurements. 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 resources

NETL: PM Emissions Control