Glossary of Terms

Acid rain: This is the wet form of a complex chemical and atmospheric phenomenon called acid deposition, which occurs when emissions of sulfur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere. The substances can travel far from the original sources and be deposited on earth in either wet or dry form. As the wet form, acid rain can fall to earth as rain, snow or fog. The dry forms are acidic gases or particulates. Concerns about acid rain deposition were high in the 1980s which led to the development of new technologies (like scrubbers) that were installed to meet more stringent emissions standards under the 1990 Clean Air Act amendments. Since that time, sulfur dioxide emissions (SO2) emissions in the coal-based electricity sector have been significantly reduced.

Air pollutant: Any substance in air, either natural or artificial, that in high concentrations could harm humans, plants or animals. Pollutants may be in the form of solid particles, liquid droplets, gases or a combination thereof. Generally, they fall into two main groups: (1) those emitted directly from identifiable sources and (2) those produced in the air by interaction between two or more primary pollutants, or by reaction with normal atmospheric constituents, with or without photoactivation. Exclusive of pollen, fog and dust, which are of natural origin, about 100 contaminants have been identified. Air pollutants are often grouped in categories for ease in classification, such as solids, sulfur compounds, volatile organic chemicals, particulate matter, nitrogen compounds, oxygen compounds, halogen compounds, radioactive compound and odors.

Air pollution control device: Mechanism or equipment that cleans emissions generated by a source (e.g. an incinerator, industrial smokestack, or an automobile exhaust system) by removing pollutants that would otherwise be released to the atmosphere.

British thermal unit (Btu): Unit of heat energy equal to the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at sea level. As a matter of comparison, one watt is approximately 3.41 Btu per hour. According to the Energy Information Administration, high-grade coal contains about 10,000 Btu per pound and about 20 million Btu per short ton. A ton of crude oil (6.65 barrels) contains about 38.6 million Btu.

Carbon monoxide (CO): A colorless, odorless, poisonous gas produced by incomplete fossil fuel combustion.

Climate change: The term can refer to all forms of climatic inconsistency, but because the Earth's climate is never static, the term is more properly used to imply a significant change from one climatic condition to another. “Global warming” which is often used interchangeably with this term refers to the question as to whether a build-up of greenhouse gases in the atmosphere will result in catastrophic changes to the Earth’s climate patterns.

Coal: Coal is classified into four main types: lignite, subbituminous, bituminous and anthracite. Lignite, which is mostly found in Texas and North Dakota, has the lowest energy content with 25-35% carbon. Subbituminous coal, found in Montana and Wyoming, contains 35-45% carbon. Bituminous coal, the most abundant type contains 45-86% carbon and is found in Illinois, Kentucky, Ohio, Pennsylvania and West Virginia. Anthracite, which contains 86-97% carbon, is very rare and found only in northeastern Pennsylvania.

Clean coal technology: Any technology to reduce pollutants associated with the burning of coal that was not in widespread use prior to the Clean Air Act Amendments of 1990.

Coal gasification: A method for converting coal into a gas that can be burned to generate power or processed into chemicals and fuels.

Criteria pollutants: The 1970 amendments to the Clean Air Act required the U.S. Environmental Protection Agency to set National Ambient Air Quality Standards for certain pollutants known that at higher concentrations to be hazardous to human health. EPA has identified and set standards to protect human health and welfare for six pollutants: ozone, carbon monoxide, total suspended particulates, sulfur dioxide, lead and nitrogen oxide. The term "criteria pollutants" derives from the requirement that EPA must describe the characteristics and potential health and welfare effects of these pollutants. It is on the basis of these criteria that standards are set or revised. EPA sets emissions limits for criteria air pollutants to levels necessary to protect human health and welfare with an added margin of safety.

Emission cap-and-trade: An emissions cap is designed to limit projected growth in emissions from existing and future stationary sources. Generally, such provisions require that any emission growth from facilities under the restrictions be offset by equivalent reductions at other facilities under the same cap. A cap-and-trade system allows a facility to bank surplus emission reductions and sell them to other facilities that are above the cap, maintaining an overall emission level consistent with levels established by the National Ambient Air Quality Standards (NAAQS) and other regulations.

Fuel: A material (such as coal, coke, gas, oil, peat, wood) used to produce heat or power by burning.

Global warming: The concern that a build-up of greenhouse gases in the atmosphere (beyond those levels required to sustain temperatures to support life on the planet) will result in catastrophic changes to the earth’s climate patterns.

Greenhouse gas: A gas that acts as an insulator helps trap the sun’s rays within the earth’s atmosphere. Without the heat-trapping properties of greenhouse gases, life on earth would not be possible. Greenhouse gases are produced from both natural and human-made sources. Water vapor is the most common greenhouse gas.

Mercury (Hg): A heavy metal that can accumulate in the environment and is highly toxic when consumed in large quantities from affected fish. The burning of coal to produce electricity results in emissions of mercury, but not at levels to be considered harmful. Health concerns related to mercury and power plants stem from when airborne mercury is deposited into waterbodies and (in some cases) that mercury changes form into another substance – methylmercury. Methylmercury can build up in fish tissues and therefore become ingested by humans. To reduce this concern, the U.S. Environmental Protection Agency adopted new regulations to require coal-based power plants and other mercury emitting sources to reduce emissions by 70% over the next several years.

Ozone (O3): In the stratosphere, ozone is a natural form of oxygen that provides a protective layer shielding the earth from ultraviolet radiation. In the troposphere, ozone is a chemical oxidant and major component of photochemical smog. It can seriously impair the respiratory system and is one of the most wide-spread of all the criteria pollutants under the Clean Air Act. Ozone is produced through complex chemical reactions of nitrogen oxides, which are among the primary pollutants emitted by combustion sources; hydrocarbons, released into the atmosphere through the combustion, handling and processing of petroleum products; and sunlight. Primary emissions sources are cars (transportation), industrial boilers, and power plants.

PM-10 and PM-2.5: Particle pollutants include dust, ash, soot, lint, smoke, pollen, spores, algal cells and other suspended materials. Particulates are often the most visible form of air pollution since they reduce visibility and leave dirty deposits on windows, painted surfaces and textiles. PM-10 is a measure of particles in the atmosphere with a diameter of less than 10 or equal to a nominal 10 micrometers. PM-2.5 is a measure of smaller particles in the air. PM-10 is the pollutant particulate level standard against which EPA has measured Clean Air Act compliance. On the basis of newer scientific findings, the agency is considering regulations that will make PM-2.5 the new standard.

Smog: Air pollution resulting when ozone combines with sunlight. It is most significantly a problem in urban areas in the summer months.

Sulfur dioxide (SO2): A pungent, colorless, gas formed primarily by the combustion of fossil fuels. It becomes a pollutant when present in large amounts.

Volatile organic compound (VOC): An organic compound that participates in atmospheric photochemical reactions, except those designated by EPA as having negligible photochemical reactivity.