The reduction or elimination of pollution. (EPA)

Acid Deposition

A comprehensive term for the various ways acidic compounds precipitate from the atmosphere and deposit onto surfaces. It can include: 1) wet deposition by means of acid rain, fog and snow; and, 2) dry deposition of acidic particles (aerosols). (EPA)

Acid Rain

Rain that is especially acidic (pH is less than 5.2). Principal components of acid rain typically include nitric and sulfuric acid. These may be formed by the combination of nitrogen and sulfur oxides with water vapor in the atmosphere. (EPA)


An emissions control device that removes VOCs from a gas stream as a result of the gas attaching (adsorbing) onto a solid matrix such as activated carbon. (EPA)


Particles of solid or liquid matter that can remain suspended in air from a few minutes to many months depending on the particle size and weight. (EPA)


An air pollution abatement device that removes undesirable organic gases through incineration. (EPA)

Air Pollutants

Amounts of foreign and/or natural substances occurring in the atmosphere that may result in adverse effects to humans, animals, vegetation and/or materials. (See also pollution.) (EPA)


An allowance to emit one ton of carbon dioxide equivalent during a specified period, which shall be valid only for the purposes of meeting the requirements of this Directive and shall be transferable in accordance with the provisions of this Directive (First ETS Directive)

Anaerobic Digestion

A biochemical process in which bacteria break down biodegradable organic material, such as manure, in an oxygen-free environment. Temperature, moisture, nutrient content and pH, can be controlled through the use of an airtight chamber (digester). The break-down of the organic material results in biogas, a mixture of methane (CH4), carbon dioxide (CO2) and trace amounts of other gases. (EPA)


A type of hydrocarbon, such as benzene or toluene. Some aromatics are toxic. (EPA)


The gaseous mass or envelope of air surrounding the Earth. From ground-level up, the atmosphere is further subdivided into the troposphere, stratosphere, mesosphere and the thermosphere. (EPA)

Available techniques

Techniques developed on a scale which allows implementation in the relevant industrial sector, under economically and technically viable conditions, taking into consideration the costs and advantages, whether or not the techniques are used or produced inside the Member State in question, as long as they are reasonably accessible to the operator. (Industrial Emissions Directive)

Best Available Technique (BAT)

The most effective and advanced stage in the development of activities and their methods of operation which indicates the practical suitability of particular techniques for providing the basis for emission limit values and other permit conditions designed to prevent and, where that is not practicable, to reduce emissions and the impact on the environment as a whole. (Industrial Emissions Directive)

Biogenic Source

Biological sources such as plants and animals that emit air pollutants such as volatile organic compounds. Examples of biogenic sources include animal management operations and oak and pine tree forests. (See also natural sources.) (EPA)


An enforceable limit on total emissions for the facilities covered under the cap-and-trade program. The cap is set for each compliance period of the program by the state and emissions are reduced as the cap declines over time. (EPA)


Cap-and-trade is a regulatory approach used to control pollution by setting a firm cap on allowed emissions while employing market mechanisms to achieve emissions reductions while driving costs down. In a cap-and-trade program, a limit, or cap is put on the amount of greenhouse gases that can be emitted. (EPA)

Carbon Dioxide (CO2)

A colorless, odorless gas that occurs naturally in the Earth's atmosphere. Significant quantities are also emitted into the air by fossil fuel combustion. (EPA)

Carbon Dioxide Equivalent (CO2E)

The amount of carbon dioxide by weight that would produce the same global warming impact as a given weight of another greenhouse gas, based on the best available science, including from the Intergovernmental Panel on Climate Change. (EPA)

Carbon Monoxide (CO)

A colorless, odorless gas resulting from the incomplete combustion of hydrocarbon fuels. CO interferes with the blood's ability to carry oxygen to the body's tissues and results in numerous adverse health effects. (EPA)


A substance that can increase or decrease the rate of a chemical reaction between the other chemical species without being consumed in the process. (EPA)

Climate Change

See Global Warming


Any oxidation of fuels, regardless of the way in which the heat, electrical or mechanical energy produced by this process is used, and any other directly associated activities, including waste gas scrubbing. (ETS Directive)


Equipment that removes grease, dirt, or unwanted materials from any part or product. Degreasers typically use aqueous or nonaqueous solvents, as liquid baths or condensing vapors, to remove such material. (EPA)


Solid particulate matter that can become airborne. (EPA)

Emission Limit Values (ELV)

The mass, expressed in terms of certain specific parameters, concentration and/or level of an emission, which may not be exceeded during one or more periods of time. (Industrial Emissions Directive)


The direct or indirect release of substances, vibrations, heat or noise from individual or diffuse sources in the installation into air, water or land. (Industrial Emissions Directive)

Environmental quality standard

The set of requirements which must be fulfilled at a given time by a given environment or particular part thereof, as set out in Union law. (Industrial Emissions Directive)

Fly Ash

Air borne solid particles that result from the burning of coal and other solid fuel. (EPA)

Fugitive Emissions

Emissions not caught by a capture system; which are often due to equipment leaks, evaporative processes and windblown disturbances. (EPA)

General binding rules

Emission limit values or other conditions, at least at sector level, that are adopted with the intention of being used directly to set permit conditions (Industrial Emissions Directive)

Greenhouse Gases (GHG)

Atmospheric gases such as carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, ozone and water vapor that slow the passage of re-radiated heat through the Earth's atmosphere.


Compounds containing various combinations of hydrogen and carbon atoms. They may be emitted into the air by natural sources (e.g., trees) and as a result of fossil and vegetative fuel combustion, fuel volatilization and solvent use. Hydrocarbons are a major contributor to smog. (EPA)

Inert Gas

A gas that does not react with the substances coming in contact with it. (EPA)

Intergovernmental Panel on Climate Change (IPCC)

A scientific intergovernmental body set up by the World Meteorological Organization (WMO) and by the United Nations Environment Programme (UNEP) to provide the decision-makers and others interested in climate change with an objective source of information about climate change. (EPA)

Internal Combustion Engine

An engine in which both the heat energy and the ensuing mechanical energy are produced inside the engine. Includes gas turbines, spark ignition gas and compression ignition diesel engines. (EPA)


A layer of warm air in the atmosphere that prevents the rise of cooling air and traps pollutants beneath it. (EPA)

Low NOx Burners

One of several combustion technologies used to reduce emissions of nitrogen oxides. (EPA)


The layer of the Earth's atmosphere above the stratosphere and below the thermosphere. (EPA)


Million tons of carbon dioxide equivalents. (EPA)

Natural Sources

Non-manmade emission sources, including biological and geological sources, wildfires and windblown dust. (EPA)

Nitric Oxide (NO)

A Precursor of ozone, NO2 and nitrate; nitric oxide is usually emitted from combustion processes. Nitric oxide is converted to nitrogen dioxide (NO2) in the atmosphere and then becomes involved in the photochemical processes and/or particulate formation. (EPA)

Nitrogen Oxides (Oxides of Nitrogen, NOx)

A general term pertaining to compounds of nitric oxide (NO), nitrogen dioxide (NO2) and other oxides of nitrogen. Nitrogen oxides are typically created during combustion processes and are major contributors to smog formation and acid deposition. NO2 is a criteria air pollutant and may result in numerous adverse health effects. (EPA)

Non-Industrial Source

Any of a large number of sources -- such as mobile, area-wide, indirect and natural sources -- which emit substances into the atmosphere. (EPA)

Non-Methane Hydrocarbon (NMHC)

The sum of all hydrocarbon air pollutants except methane. NMHCs are significant precursors to ozone formation. (EPA)


Offsets are tradable credits that represent greenhouse gas emissions reductions that are made in areas or sectors not covered by a cap-and-trade program. Under a greenhouse gas cap-and-trade program, covered entities could buy offset credits in lieu of buying allowances or reducing their greenhouse gas emissions on-site. One offset credit would be equal to one metric ton of greenhouse gas emissions. Offsets must meet rigorous criteria that demonstrate that the emissions reductions are real, permanent, verifiable, enforceable and quantifiable. (EPA)

Organic Compounds

A large group of chemical compounds containing mainly carbon, hydrogen, nitrogen and oxygen. All living organisms are made up of organic compounds. (EPA)


A strong smelling, pale blue, reactive toxic chemical gas consisting of three oxygen atoms. It is a product of the photochemical process involving the sun's energy and ozone precursors, such as hydrocarbons and oxides of nitrogen. Ozone exists in the upper atmosphere ozone layer (stratospheric ozone) as well as at the Earth's surface in the troposphere (ozone). Ozone in the troposphere causes numerous adverse health effects and is a criteria air pollutant. It is a major component of smog. (EPA)

Ozone Depletion

The reduction in the stratospheric ozone layer. Stratospheric ozone shields the Earth from ultraviolet radiation. The breakdown of certain chlorine and/or bromine-containing compounds that catalytically destroy ozone molecules in the stratosphere can cause a reduction in the ozone layer. (EPA)

Ozone Layer

A layer of ozone in the lower portion of the stratosphere, which helps to filter out harmful ultraviolet rays from the sun. (EPA)

Ozone Precursors

Chemicals such as non-methane hydrocarbons and oxides of nitrogen, occurring either naturally or as a result of human activities, which contribute to the formation of ozone, a major component of smog. (EPA)

Particulate Matter (PM)

Any material, except pure water, that exists in the solid or liquid state in the atmosphere. The size of particulate matter can vary from coarse, wind-blown dust particles to fine particle combustion products. For more information, see ARB's PM brochure. (EPA)


A written authorisation to operate all or part of an installation or combustion plant, waste incineration plant or waste co-incineration plant. (Industrial Emissions Directive)


Refers to the length of time a compound stays in the atmosphere, once introduced. A compound may persist for less than a second or indefinitely. (EPA)


See Particulate Matter.


The direct or indirect introduction, as a result of human activity, of substances, vibrations, heat or noise into the air, water or land which may be harmful to human health or the quality of the environment, result in damage to material property, or impair or interfere with amenities and other legitimate uses of the environment. (Industrial Emissions Directive)

Primary Particles

Particles that are directly emitted from combustion and fugitive dust sources. (Compare with Secondary Particle.) (EPA)

Ringelmann Chart

A series of charts, numbered 0 to 5, that simulate various smoke densities by presenting different percentages of black. A Ringelmann No. 1 is equivalent to 20 percent black; a Ringelmann No. 5 is 100 percent black. They are used for measuring the opacity or equivalent obscuration of smoke arising from stacks and other sources by matching the actual effluent with the various numbers, or densities, indicated by the charts. (EPA)


An air pollution control device that uses a high energy liquid spray to remove aerosol and gaseous pollutants from an air stream. The gases are removed either by absorption or chemical reaction. (EPA)

Secondary Particle

Particles that are formed in the atmosphere. Secondary particles are products of the chemical reactions between gases, such as nitrates, sulfur oxides, ammonia and organic products. (EPA)

Selective Catalytic Reduction (SCR) System

An emission control system that reduces NOx emissions through the catalytic reduction of NOx in diesel exhaust to N2 and H2O by injecting nitrogen-containing compounds into the exhaust stream, such as ammonia or urea. (EPA)


A combination of smoke and other particulates, ozone, hydrocarbons, nitrogen oxides and other chemically reactive compounds which, under certain conditions of weather and sunlight, may result in a murky brown haze that causes adverse health effects. (EPA)


Any place or object from which air pollutants are released. Sources that are fixed in space are stationary sources and sources that move are mobile sources. (EPA)


The layer of the Earth's atmosphere above the troposphere and below the mesosphere. It contains the ozone layer in its lower portion. The stratospheric layer mixes relatively slowly; pollutants that enter it may remain for long periods of time. (EPA)

Sulfur Dioxide (SO2)

A strong smelling, colorless gas that is formed by the combustion of fossil fuels. Power plants, which may use coal or oil high in sulfur content, can be major sources of SO2 and other sulfur oxides contribute to the problem of acid deposition. SO2 is a criteria air pollutant. (EPA)

Sulfur Oxides

Pungent, colorless gases (sulfates are solids) formed primarily by the combustion of sulfur-containing fossil fuels, especially coal and oil. Considered major air pollutants, sulfur oxides may impact human health and damage vegetation. (EPA)


The outermost layer of the Earth's atmosphere. The temperature of this layer varies from many hundreds to thousands of degrees Celsius. (EPA)

Tonne of carbon dioxide equivalent

One metric ton of carbon dioxide (CO2) or an amount of any other greenhouse gas with an equivalent global-warming potential. (First ETS Directive)


The layer of the Earth's atmosphere nearest to the surface of the Earth. (EPA)

Upstream Emissions

Emissions from processes that take place up to when the fuel enters a vehicle---typically during extraction, production, distribution and dispensing of the fuel. (EPA)


The gaseous phase of liquids or solids at atmospheric temperature and pressure. (EPA)

Vapor Density

The vapor density is expressed in grams per liter (g/L) and is compared to the density of air (air=1). (EPA)

Vapor Pressure

The pressure, often expressed in millimeters of mercury (mm Hg) or pounds per square inch (PSI), that is characteristic at any given temperature of a vapor in equilibrium with its liquid or solid form. (EPA)

Vapor Recovery Systems

Mechanical systems that collect and recover chemical vapors resulting from transfer of gasoline from operations such as tank-to-truck systems at refineries, tanker-to-pipeline systems at offshore oil operations and pump-to-vehicle systems at gasoline stations. (EPA)