Glossary of Air Pollution Terms and Abbreviations

This is a glossary of words, phrases, acronyms and abbreviations used in air pollution control and air pollution monitoring. To find a particular term or abbreviation, you can navigate using the letter links below, or you can use the Find (in Page) function of your web browser (Ctrl+F in most browsers). In this glossary, abbreviations and acronyms are alphabetized as if they were words; for example, "AIRS" comes after "Air Toxic" rather than before "accuracy" in the list.

All of the terms and abbreviations are nouns, unless the definition is marked with (v) for verb or (adj) for adjective.. Terms or abbreviations that are defined only within the definition of another term look like this. Within a definition, links that look like this point to other definitions in this glossary.. Links like this are hyperlinks to other pages or other websites.. Links to other websites will open in a new window.

Alphabetical Index:

Symbols | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | XYZ

See also:

  • Regulation 1.02 for the definitions of terms used in APCD regulations.
  • Regulation 1.03 for the meanings of acronyms and abbreviations used in APCD regulations.

Symbols and Greek Letters

Note: The symbols may not appear correctly in older web browsers.

µ (µ) (Greek letter mu):

Micro-: metric prefix for one millionth (10-6).

µg (µg):

Micrograms: millionths of grams (10-6 g).

µg/m3; (µg/m³):

Micrograms per cubic meter.

µm (µm):

Micrometers: 10-6 m. Also called microns.





A quantitative measure of the magnitude of error (how close the results of a measurement are to an accepted reference value): the correctness of an analysis system. Accuracy includes both precision and (lack of) bias.

acid deposition ("acid rain"): 

A complex chemical and atmospheric phenomenon occurring when emissions of sulfur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere, often far from original sources, and then deposited on the earth in either a wet or dry form. The wet forms, popularly called "acid rain", can fall as rain, snow, or fog. The dry forms are acidic gases or particulates.


Asbestos-containing material: a substance (usually a construction material) that includes some form of asbestos.


In an air pollution control context, usually refers to the federal Clean Air Act.


Available control technology


Asbestos Contractor Tracking System, a US EPA database system for keeping track of asbestos removal contractors and sites. APCD used ACTS from about 1988 until 2009.


Asbestos Hazards Emergency Response Act, Title 2 of the Toxics Substances Control Act.


Architectural and industrial maintenance (coatings).

air quality index (AQI):

The air quality index is a number indicating the air quality at a particular time in a particular area. The US EPA developed a uniform, standard index to make it easy to compare air quality in different parts of the country, and to avoid confusion for travelers and people who have moved from one area to another. This index was formerly called the pollutant standards index (PSI).

air toxic:

Any air pollutant for which a national ambient air quality standard does not exist (i.e., excluding ozone, carbon monoxide, PM10, sulfur dioxide, and nitrogen dioxide) that may reasonably be anticipated to cause cancer, developmental effects, reproductive dysfunctions, neurological disorders, heritable gene mutations or other serious or irreversible chronic or acute health effects in humans. See also HAP.


Aerometric Information Retrieval Subsystem, the old name of the US EPA's main air pollution database system. It includes ambient monitoring and meteorological data (stored in SAROAD before 1987) in the Air Quality Subsystem (AQS) and compliance and permit data in the AIRS Facility Subsystem (AFS). The AQS of AIRS was replaced about 2001 by the new Air Quality System (also abbreviated AQS).  Until 1999, AFS also contained source emission data, but those data are now stored in the NEI. For a while, there was also an Area and Mobile Sources subsystem, but it has been shut down.


The former organization Association of Local Air Pollution Control Officials, an organization of the leadership of city, county and regional air pollution control agencies, now part of NACAA

annual arithmetic mean:

The mean (average) of a set of values of a variable over a calendar year. The arithmetic mean is equal to the sum of all the readings divided by the number of readings.

annual geometric mean:

The geometric average of a set of values of a variable over a calendar year. The geometric mean is the nth root of the product of n readings, usually calculated as the antilogarithm of the average of the logarithms of the data points. This statistic has been used to compile total suspended particulate data, for example.


The American National Standards Institute, the private non-profit organization that administrates and coordinates the U.S. private-sector voluntary standardization system. It is the US member organization of ISO. See the ANSI website for more information.


Air Pollution Control District. Here, the Louisville Metro Air Pollution Control District (also known as the Air Pollution Control District of Jefferson County).


The Air Pollution Control District of Jefferson County (Kentucky), now called the Louisville Metro Air Pollution Control District.


Air quality index


The US EPA's Air Quality System, a database of ambient air quality and meteorological data. It replaced the Air Quality Subsystem of AIRS.


A general term for a group of fibrous minerals (primarily chrysotile, amosite and crocidolite) that have long been used as fireproof insulation and as a strengthener in pipe insulation, roofing tiles, floor tiles, wall coverings and other materials. For purposes of APCD regulations, asbestos is defined as the asbestiform varieties of serpentinite (chrysotile), riebeckite (crocidolite), cummingtonite-grunerite, amosite, anthophylite, and actinolote-tremolite. More about asbestos.


The American Society for Testing and Materials, a professional organization that develops and distributes protocols for testing and provides reference standards.


Agency for Toxic Substances and Disease Registry (US Dept. of Health and Human Services).

attainment area:

A geographic area that the US EPA has designated as meeting the NAAQS for a specified pollutant. (See nonattainment area.)


Air and Waste Management Association, a professional organization of people involved in air pollution control and solid waste management, from both the public and private sectors. For more information, see AWMA's website.

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Benchmark ambient concentration: the concentration of a toxic air contaminant that is used in determining environmental acceptability pursuant to Regulation 5.21 Environmental Acceptability for Toxic Air Contaminants. The benchmark ambient concentration for a carcinogen (BACC) is the concentration,including an averaging time frame, of a toxic air contaminant that is representative of an additional lifetime cancer risk of one in one million (1 x 10-6). The benchmark ambient concentration for a carcinogen is established pursuant to Regulation 5.20 Methodology for Determining Benchmark Ambient Concentration for a Toxic Air Contaminant,Section 3. The benchmark ambient concentration for the noncarcinogenic effects of a toxic air contaminant (BACNC) is the concentration, including an averaging time frame, of a toxic air contaminant that is likely to be without an appreciable risk of deleterious effects during a lifetime. The benchmark ambient concentration for the noncarcinogenic effects of a toxic air contaminant is established pursuant to Regulation 5.20 Section 4. A substance can have both, with different values. See APCD Regulation Part 5 for more information.


Best achievable control measure.


Best available control technology for limiting pollutant emissions, required on major new or modified stationary sources that use volatile organic compounds.


A systematic or persistent distortion of a measurement process that causes error in one direction in a set of data. An unbiased measurement is close to the true value. Bias is estimated by the signed difference of an observed value from a reference value, as a percentage of the reference value. See also accuracy and precision.


Barometric pressure: ambient atmospheric pressure.


British thermal unit, a unit of energy: 1 Btu = 1060 J.

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Clean Air Act


Amendments to the Clean Air Act, especially those of 1990.


Clean Air Act Advisory Committee. See the CAAAC website.


Compliance assurance monitoring.


California Air Resources Board: California's state air pollution control agency. See CARB's Website for more information.

carbon dioxide (CO2):

A colorless, odorless gas produced in the combustion of organic matter and in the respiration of living things. It is consumed by green plants in photosynthesis. It is also the primary greenhouse gas implicated in global warming.

carbon monoxide (CO):

A colorless, odorless gas produced in incomplete combustion. It is poisonous; it replaces oxygen in the bloodstream.


Chemical Abstracts Service, a division of the American Chemical Society. CAS numbers are used to identify chemical compounds, elements and other substances.


The Clean Air Scientific Advisory Committee of the Science Advisory Board. CASAC is a Congressionally mandated group of independent scientific and technical experts drawn from academia, industry, and the states, that advises the US EPA on air pollution issues. See also CAAAC.


Community-Based Environmental Protection, a US EPA program that provides grants for local environmental projects.


The Centers for Disease Control and Prevention, the US federal agency (within the Department of Health and Human Services) for control and prevention of diseases and epidemics, headquartered in Atlanta, Georgia. See CDC's website for more information.


Continuous emissions monitoring (at stationary sources).


Continuous emissions monitoring system, a set of instruments and a data acquisition system for monitoring the emissions from a stationary source.


Comprehensive Environmental Response, Compensation and Liability Act (commonly referred to as "Superfund").




Cubic feet per minute (ft3/min).


Code of Federal Regulations


Chemical formula for formaldehyde.


Chemical formula for methane.


The US EPA's Clearinghouse for Inventories and Emissions Factors.

chlorofluorocarbon (CFC):

A chemical compound that would be a hydrocarbon, except that at least one hydrogen atom in the molecule is replaced by a fluorine atom and the others are replaced by chlorine atoms. Many CFCs are implicated in the destruction of stratospheric ozone.

Clean Air Act:

US federal legislation (42 U.S.C. 7410 et seq.) to protect the health and welfare of the public by controlling air pollution, passed by the US Congress in 1963, with major amendments in 1967, 1970, 1977 and 1990. The US EPA has more information.


Congestion Mitigation and Air Quality Improvement Program: a federal grant program, created in ISTEA and continued in TEA-21, to support state and local projects to reduce air pollution and traffic congestion.


Compressed natural gas, an alternative fuel for motor vehicles.


Chemical symbol for carbon monoxide

CO design value:

The design value for carbon monoxide. The CO design value for a particular site for a given year, based on the current 9 ppm (8-hr average) standard, is the higher of the that year’s second maximum eight-hour averages of CO concentration and the previous year’s second maximum at that site. The CO design value for an area is the highest design value among all the NAMS/SLAMS CO sites in that area.


Chemical symbol for carbon dioxide.

Code of Federal Regulations (CFR):

The body of regulations written by federal government agencies, such as the EPA. Regulations relating to air pollution are found in Title 40 of the CFR, sections 50-99.


(v) To site two (or more) monitors measuring the same pollutant or other parameter at the same location. The exact separation allowed varies by the parameter monitored and the sampling method, but is generally 1-5 meters. These can be either identical sampling (or sampling and analysis) systems juxtaposed for determination of the precision of a measurement system, or two different systems put side by side to measure the relative bias between the two methods.


See collocate.


Continuous opacity monitoring system: a set of instruments and a data acquisition system for continuous instrumental measurement of the opacity of the emission from a smoke stack, etc. of a stationary source

criteria pollutant:

A pollutant for which a NAAQS has been established. Criteria pollutants currently include ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, lead, PM10 and PM2.5.


Control techniques guidelines issued by the US EPA on ways to reduce pollutant emissions from stationary sources.

cubic feet per minute (ft3/min):

A unit of flow, the volume in cubic feet that passes a point each minute. For example, the air flow in a heating duct might be measured in ft3/min (also abbreviated CFM). See also cubic meters per minute.

cubic meters per minute (m3/min):

A unit of flow, the volume in cubic meters that passes a point each minute. For example, air flow through an ambient air particulate sampler or through a stack of a power plant could be measured in m3/min.


Calendar year: a period beginning January 1 and ending December 31 (as opposed to a fiscal year or car model year). For example, CY02 means calendar year 2002.

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Division for Air Quality; see Kentucky Division for Air Quality (KyDAQ or KDAQ).

data quality objective (DQO):

A goal or target for the quality of data to be obtained in a measurement process.

De minimis:

In general, a level of emissions, etc. below which a particular process or activity is exempted or not regulated. For the STAR Program, "de minimis emissions" has a specific meaning described in APCD Regulation Part 5.

design value:

Number used by EPA to determine whether a site or area attains the standard or by how much it exceeds the standard for a particular pollutant. See also ozone design value, CO design value.


When used on this website, refers to the Louisville Metro Air Pollution Control District (formerly known as the Air Pollution Control District of Jefferson County).


(1) Department of Energy (also abbreviated DoE), usually the US DOE.
(2) District-only enforceable (permit condition): refers to a permit condition that is not federally enforceable.


Department of Transportation, usually the US DOT.


Data quality objective.

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A motor vehicle fuel consisting of 90% gasoline and 10% ethanol, commonly available at gasoline stations.


A motor vehicle fuel consisting of 85% ethanol and 15% gasoline. A number of models of flexible-fuel vehicles designed to run on this fuel are available. The fuel is available in some areas, including downtown Louisville.


A motor vehicle fuel consisting of 95% ethanol and 5% gasoline.


Environmental Council of the States.


Emergency episode monitoring station, an ambient air monitor designated for use in emergency pollution episodes.  This designation is mostly of historical interest.


Electric Generating Units or Utilities (i.e., power plants)


Emissions Inventory System, a database system used by the US EPA to track emissions from stationary sources.


Environmental justice.


Empirical Kinetic Modeling Approach, a city-specific empirical method for modeling air pollution formation and dispersion, used to fill gaps between more sophisticated dispersion models and proportional modeling techniques. The EKMA is a Lagrangian photochemical air quality simulation model that calculates ozone from its precursors: nonmethane organic compounds (NMOC) and nitrogen oxides (NOx).  See the US EPA Support Center for Regulatory Air Models.

environmental justice:

Fair treatment for people of all races, cultures, and incomes, regarding the development of environmental laws, regulations, and policies.


Environmental Protection Agency, usually the US EPA.


In air monitoring applications, Environmental Systems Corporation, a supplier of data logging hardware and software.


Ethyl alcohol, chemical symbol C2H5OH.  Ethanol is the alcohol in alcoholic beverages.  It can be used as a fuel for motor vehicles.  Vehicles can be designed to run on pure ethanol; flexible fuel vehicles can run on ethanol/gasoline blends  (see E85, E95) or on gasoline; and ordinary gasoline vehicles can run on gasoline with a small portion of ethanol (see E10).  Ethanol can be produced from corn and other crops, reducing the need for fossil fuels.

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federal reference method (FRM):

In ambient monitoring, the US EPA standard method of monitoring for a particular pollutant or other parameter.  The FRM may be specified by technique or by design.


Federally enforceable District-origin operating permit, also known as a synthetic minor permit, See Types of Permits.


Flexible-fuel vehicle.


Federal Highway Administration.


Federal implementation plan. See SIP.


Federal Information Processing Standards, a set of standard codes and formats for common data fields stored in federal databases.  NIST has more about FIPS.

flexible-fuel vehicle:

A motor vehicle that can run on more than one fuel, for example E85 and gasoline, or CNG and gasoline.


Federal Motor Vehicle Control Program: All federal actions to control pollution from motor vehicles, for example establishing and enforcing tailpipe and evaporative emission standards for new vehicles, development of testing methods, and guidance to states that operate inspection and maintenance programs.


A chemical compound, the simplest aldehyde, chemical symbol CH2O.  Formaldehyde is a common pollutant, a VOC.


The Federal Register, a US Government periodical listing new and proposed regulations, hearings, comment periods and other executive-branch actions.


Federal reference method.


Federal Transit Authority.


Fourier-transform infrared: an analysis technique for deriving a spectrum from an infrared signal. This technique can be used in open-path analyzers to measure concentrations of pollutants in the ambient air.

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geographic information system (GIS):

A system comprised of computer software and geographic data that enables analysis of those data and generation of maps.


Greenhouse gas.


Geographic information system.

global warming:

See greenhouse gas.

global warming potential (GWP):

A measure of how much a particular chemical compound contributes to global warming relative to other compounds.

gram (g):

A metric unit of mass: one thousandth of a kilogram. For comparison, 28.3 grams weigh one ounce.

greenhouse gas:

A gas that, in the Earth's atmosphere, allows visible light to pass through and warm the Earth, but reflects infrared light emitted by the Earth's surface back to to the surface, thus maintaining the temperature of the atmosphere.  This is called the greenhouse effect, because the gases act like the glass in a greenhouse.  An excess of greenhouse gases could cause the planet to warm above the current temperature.  This is called global warming.


Global warming potential.



Hazardous air pollutant: One of 188 substances (originally 189) identified as air toxics by section 112(b) of the federal Clean Air Act.  APCD regulation 5.14 also lists these substances.










Hazardous organic NESHAPs.

hydrocarbon (HC):

A hydrocarbon is a chemical compound in which each molecule consists of a backbone of carbon atoms bound to one another, with hydrogen atoms bound to each carbon atom. Methane, hexane, octane, and benzene are examples of hydrocarbons.

hydrochlorocarbon (HCC):

A chemical compound that would be a hydrocarbon except that one or more hydrogen atoms in each molecule is replaced by a chlorine atom.

hydrochlorofluorocarbon (HCFC):

A chemical compound that would be a hydrocarbon except that one or more hydrogen atoms in each molecule is replaced by a chlorine atom and one or more hydrogen atom is replaced by a fluorine atom. Some HCFCs are implicated in the destruction of stratospheric ozone.

hydrofluorocarbon (HFC):

A chemical compounds that would be a hydrocarbon except that one or more hydrogen atoms in each molecule is replaced by a fluorine atom.

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Inspection and maintenance programs for motor vehicles. Our 1984-2003 VET program was an example of an I/M program. The US EPA has more information.


Incorporation by reference (of a regulation, etc.)


Indiana Department of Environmental Management.


Intermediate implementation policy.


Interagency monitoring of protected visual environments, a multi-federal agency (EPA, National Park Service and others) monitoring program for visibility and particulate matter (PM10 and PM2.5). See the IMPROVE website (hosted by Colorado State University).


Intermodal Surface Transportation Efficiency Act, a federal law enabling various transportation enhancements, superseded in 1998 by TEA-21.

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The Air Pollution Control District of Jefferson County (Kentucky). More commonly abbreviated APCDJC or APCD. Now known as the Louisville Metro Air Pollution Control District.



Kentuckiana Air Education, APCD's community outreach and education program for the Louisville metro area. See the KAIRE website.



The Division for Air Quality of the Energy and Environment Cabinet of the State of Kentucky.


The Kentuckiana Regional Planning and Development Agency, an association of local governments in a region consisting of seven counties in Kentucky and two in Indiana. KIPDA is the metropolitan planning organization for the five-county Louisville metropolitan area and the Area Development District for seven counties in Kentucky. More information can be found on KIPDA's website.


Kentuckiana Ozone Prevention Coalition, now called the KAIRE Network.


Kentucky Revised Statutes: Kentucky State law.


Kentucky Transportation Cabinet.


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Lake (Michigan) Air Directors Consortium, an organization of air pollution control agencies around the lower half of Lake Michigan.


Lowest Achievable Emission Rate, required on major new or modified stationary sources in nonattainment areas.

lead (Pb):

A heavy metal, element 82. Lead is toxic if ingested or inhaled. In high concentrations, it can cause permanent brain damage. Since the 1970s, its use in paint and as an additive to gasoline have been greatly reduced or eliminated by federal laws and regulations, resulting in a dramatic drop in ambient air concentrations.


Louisville and Jefferson County Information Consortium

Louisville and Jefferson County Information Consortium (LOJIC):

An organization of public agencies and governments in Jefferson County, Kentucky to collect, organize and provide geographic data via a geographic information system. See LOJIC's website.


Liquefied propane gas, a fuel.


Local temperature and pressure (actual conditions), as opposed to standard temperature and pressure (STP).

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Cubic meters.


Maximum achievable control technologies: ways to reduce emissions of toxic pollutants as much as technologically possible.

mercury (Hg):

A metal, element 80 (CAS number 7439-97-6), mercury is the only metal that is liquid at room temperature and pressure. Mercury has long been used in thermometers, barometers and other instruments. However, mercury is poisonous and is somewhat volatile (it has a measurable vapor pressure at room temperature). The Clean Air Act lists mercury and compounds containing mercury as hazardous air pollutants.


The simplest hydrocarbon, chemical symbol CH4; a molecule of methane consists of one carbon atom with four hydrogen atoms attached. Since the US EPA has determined that methane has negligible photochemical activity (it participates very little in the production of ozone in the ambient air), it is excluded from the US EPA definition of volatile organic compounds.

metropolitan area (MA):

A geographic area, consisting of one or more counties (or equivalent units or county divisions in New England) including a core city and developed areas closely integrated with that city, as determined by the US Census Bureau. A MA is classified as either an metropolitan statistical area (MSA) or a consolidated metropolitan statistical area (CMSA) composed of two or more primary metropolitan statistical areas (PMSAs). For example, the Louisville metropolitan area is an MSA consisting of Jefferson, Bullitt, and Oldham counties of Kentucky and Clark, Floyd, Harrison and Scott counties of Indiana. See map of MSAs in Kentucky and southern Indiana at the Kentucky State Data Center.

metropolitan planning organization.

An organization that coordinates planning and development, especially of transportation, within a metropolitan area. The US DOT has more information. KIPDA is the MPO for the Louisville metro area.

microgram (µg):

One millionth of a gram: 1 µg = 10-6 g = 0.001 mg.

micrograms per cubic meter (µg/m3):

The mass in micrograms of a substance contained within a cubic meter of another substance or vacuum. This is the standard unit of measure for the mass density (concentration) of particles suspended in air; also sometimes used for the concentration of gases in air.

mm Hg:

Millimeters of mercury, a unit of pressure also called a torr, the amount of (atmospheric or gas) pressure that can support 1 mm of mercury; 1 mm Hg = 133 Pa = 0.0394 in. Hg = 0.0193 lb/in2 (psi).

mobile source:

A motor vehicle, viewed as a mobile source of air pollution.

monitoring planning area:

A geographic area, usually represented by a single air monitoring reporting organization, within which ambient air quality monitors are to be sited.

monitoring scale:

The spatial monitoring scale is size of the area represented by an ambient air monitor:

  1. Collocated-scale (1 to 10 m): Collocated monitors are intended to measure the same air.
  2. Micro-scale (10 to 100 m): Micro-scale monitors show significant difference between monitors separated by 10 to 50 m.
  3. Middle-scale (100 to 1000 m): Middle-scale monitors show significant differences between locations that are 1 km apart.
  4. Neighborhood-scale (1 to 10 km): Neighborhood-scale monitors do not show significant differences in pollutant concentrations with spacing of a few kilometers.
  5. Urban-scale (10 to 100 km): Urban-scale monitors show consistency among measurements with monitor separations of at least 10 km.
  6. Regional-scale (100 to 1000 km): Regional-scale background monitors show consistency among measurements for monitor separations of a few hundred kilometers.
  7. Continental-scale (1000 to 10,000 km): Continental-scale background monitors show little variation even when they are separated by more that 1000 km.
  8. Global-scale (> 10,000 km): Monitors are intended to quantify concentrations transported between different continents as well as naturally-emitted particles and precursors from sea spray, volcanoes, and windblown dust.


Monitoring planning area.


Metropolitan planning organization.


Metropolitan statistical area


Methyl tertiary-butyl ether, an oxygenate added to gasoline to reduce emissions of pollutants when the fuel is burned.


Model year for production automobiles.

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National Association of Clean Air Agencies, formerly STAPPA and ALAPCO.


National ambient air quality standards.


North American Industry Classification System, a system developed jointly by the US, Canada and Mexico, for classifying industries and other businesses. NAICS replaced SIC. The system assigns two-digit numbers to broad categories and adds more digits for increasingly finer classes within those categories. The US Census Bureau has more information.


National air monitoring station.

National air monitoring station (NAMS):

A SLAMS that has been selected by the US EPA as part of a national network. A NAMS must meet more stringent criteria for monitor siting, equipment type, and quality assurance. NAMS monitors also must submit detailed quarterly and annual monitoring results to the EPA.

National ambient air quality standard (NAAQS):

An air quality standard set by the US EPA as required by the Clean Air Act. The primary standard for a pollutant is the maximum concentration of that pollutant (over a specified averaging period) that should be observed within a community for the protection of human health. The secondary standard is designed to protect the community welfare. A table of NAAQS for the criteria pollutants is available.

National Emissions Inventory (NEI):

The US EPA's inventories of emissions from point sources in the US. The data are stored in the Emissions Inventory System (EIS).




National Emissions Inventory.


National emission standards for hazardous air pollutants: federal limits on the amounts of toxic air pollutants it is acceptable for a source to emit.


National Emissions Trends Inventory: The name for an old US EPA's emissions inventory database, which contains emissions inventories for 1985 through 1998. NET was incorporated into the NEI.


The National Institute of Standards and Technology, formerly called the National Bureau of Standards (NBS).

nitric oxide (NO):

The simplest oxide of nitrogen (see NOx). It is usually measured in the process of measuring NO2.

nitrogen dioxide (NO2):

A criteria pollutant, nitrogen dioxide can irritate the lungs and lower resistance to respiratory infections such as influenza. EPA’s health-based national ambient air quality standard for NO2 is 0.053 ppm, measured as an annual arithmetic mean concentration.

nitrogen oxides, oxides of nitrogen (NOx):

Chemical compounds composed of nitrogen and oxygen. NOx can react with volatile organic compounds in the presence of heat and sunlight to form ozone. NOx is also a major precursor to acid rain. NOx is emitted from motor vehicles, chemical plants, refineries, factories, consumer and commercial products, and other industrial sources. Nationwide, approximately 45% of NOx emissions come from mobile sources, 35% from electric utilities, and 15% from industrial fuel combustion. For ambient monitoring purposes, the concentration of NOx is the total concentration of NO2 and NO. NOy is NOx plus N2O and similar species.


Nitric oxide.


Nitrogen dioxide.

nonattainment area, non-attainment area:

A geographic area that the US EPA has designated as not meeting the NAAQS for a specified pollutant. (See attainment area.) 


Oxides of nitrogen.


See oxides of nitrogen.

Nonpoint Sources:

Also referred to as "Area Sources” this category represents numerous facilities or activities that individually release small amounts of a given pollutant, but collectively can result in significant amounts of emissions. For example, dry cleaners, vehicle refinishing, gasoline dispensing facilities, and residential heating will not typically qualify as point sources, but collectively the various emissions from these sources are classified as area sources.

Nonroad Sources:

Nonroad mobile source include vehicles that do not typically operate on roads or highways and are often referred to as off-road or off-highway vehicles, these include construction and lawn equipment, aircraft, locomotives, marine, airport ground support equipment and many other engine types.


Notice of Violation


New source performance standards: EPA criteria for emission limits on new stationary sources.


New source review: The process of reviewing proposed new stationary sources to determine whether they can be built without exceeding allowed levels of pollutant emissions.


National Toxics Inventory: An old US EPA database for tracking emissions of toxic air pollutants.  NTI was incorporated into the NEI.

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On-board diagnostics.

on-board diagnostics (OBD):

A self-troubleshooting system for motor vehicles, consisting of a microprocessor and various sensors to monitor parameters such as oxygen level.

Onroad Sources:

Mobile emission sources consisting of automobiles, trucks, motorcycles, and other motor vehicles traveling on public roadways.


The opacity of a material (solid, liquid or gas) is the fraction of transmitted light obscured by the substance. For air pollution purposes, as in testing the smoke from a stack or the exhaust of a vehicle, opacity is usually measured as a percent, where 0% opacity means completely transparent and 100% opacity means completely opaque.

open path:

(adj) Refers to ambient air analyzers that measure pollutant concentrations along the path of a beam of infrared, visible or ultraviolet light, as opposed to sampling analyzers, which pull air through a probe line or inlet into an analysis system.


Ozone Transport Assessment Group, a national partnership including the US EPA, the Environmental Council of the States (ECOS) and industry, that examined regional aspects of ozone pollution and regional transport of ozone, from 1995-1997.


US EPA Office of Transportation and Air Quality (formerly the Office of Mobile sources)



A chemical compound that can be added to gasoline (or other motor fuel) to increase the oxygen content of the fuel, reducing "knocking" and reducing pollutants emitted from the exhaust.  Ethanol and MTBE are examples of commonly-used oxygenates.

ozone (O3):

A colorless gas, a highly reactive compound in which each molecule consists of three oxygen atoms. Ozone is "good up high, bad nearby": In the stratosphere, ozone forms a protective layer high above the earth against ultraviolet light. But at ground level it is increasingly harmful to breathe as concentrations rise and contributes to or aggravates various heart and lung conditions, such as asthma. Ground-level ozone also interferes with the ability of plants (including crops) to produce and store food. Ozone is a criteria pollutant and is the prime ingredient of summertime smog in most cities. Ozone is not emitted directly into the air, but is formed in reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of heat and sunlight.

ozone design value:

The design value for ozone. The ozone design value for a particular site for a given year, based on the 0.075 ppm 8-hour standard, the design value is the 3-year average of the annual fourth highest daily maximum eight-hour average ozone concentration. The ozone design value for an area is the highest design value among all the NAMS/SLAMS ozone monitors in that area.

Ozone Season: 

March - October

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Photochemical assessment monitoring station

parts per billion (ppb):

A unit of measurement of concentration: a concentration of 1 ppb by volume (also symbolized ppbv) means one unit of volume of one substance (such as a pollutant) in every billion (109) of the same unit of volume of another substance (such as air). This unit (ppb) is often used for the concentrations of ozone, sulfur dioxide, and nitrogen dioxide in ambient air.

parts per million (ppm):

A unit of measurement of concentration: a concentration of 1 ppm by volume (also symbolized ppmv) means one unit of volume of one substance (such as a pollutant) in every million (106) of the same unit of volume of another substance (such as air); 1 ppm = 1000 ppb. This unit (ppm) is often used for the concentrations of ozone, sulfur dioxide, carbon monoxide, and nitrogen dioxide in ambient air.


Lead (the metal).


Poly-chlorinated biphenyl, a class of organic chemicals, primarily used as solvents in manufacturing processes. PCBs have been shown to induce cancer.


(1) Portable document format: the Adobe Acrobat file format, used to provide documents to users who may not have the same word processing or spreadsheet software as the document's author(s). 

(2) Probability density function


Periodic emission inventory

photochemical assessment monitoring station (PAMS):

A monitoring site that continuously monitors for various VOCs, as well as ozone and NOx. "Serious" ozone non-attainment areas are required to have PAMS.

PM10, PM10:

The amount of solid or liquid particulate matter 10 microns and less in aerodynamic diameter suspended in the atmosphere or in a sample of ambient air. PM10 is a component of TSP. The small PM10 particles penetrate to deep portions of the lung, affecting sensitive population groups such as children and those with respiratory diseases.

PM2.5, PM2.5:

Particulate matter with an aerodynamic diameter of 2.5 microns and less. PM2.5 is a component of PM10. The smaller PM2.5 particles penetrate to even deeper portions of the lung, reaching individual alveoli. High concentrations of PM2.5 can aggravate lung conditions and result in premature mortality for sufferers of heart and lung conditions.


Primary Metropolitan Statistical Area


Parameter occurrence code: Used in the US EPA's air quality databases to identify multiple monitors of the same pollutant (or other parameter) at the same site.

Point Sources:

Large stationary sources of emissions that have specific locations and release pollutants in quantities above a certain emission threshold

pollutant standards index (PSI):

Old term for a number to indicate the relative air quality in reports issued to the community. This term has been replaced by air quality index (AQI).

pollutant standards index station (PSIS):

An ambient air pollution monitor used in determining the daily Air Quality Index for an area.


(1) (lb) A unit of weight (see mass) used in the US.  1 lb  = 4.45 N = the weight of 0.454 kg.
(2) (£) The primary unit of currency in the United Kingdom.


parts per billion

ppd, PPD:

Pounds per day, a measure of pollutant emissions.


parts per million

ppsd, PPSD:

Pounds per summer day, a measure of pollutant emissions.


The reproducibility of an analysis system (quality assurance term). See also accuracy and bias.

Primary Metropolitan Statistical Area (PMSA):

PMSAs are single counties or groups of counties, the component metropolitan portions of a mega-metropolitan area. They are similar to MSAs with the additional characteristic of having a degree of integration with surrounding metropolitan areas.

Primary Standard:


Probability Density Function:

For air pollution purposes, a mathematical function that describes the number of suspended particles at given times and points in space with specified chemical composition and particle size.


Prevention of significant deterioration: An EPA program to prevent "backsliding" in the control of emissions from a stationary source in an area that is in attainment for the pollutants in question.


Pollutant Standards Index. See Air Quality Index.


Pounds per square inch (lb/in2), a unit of pressure; 1 psi = 51.7 mm Hg (torr) = 6 890 Pa.


Pounds per square inch absolute: unit of pressure in psi measured relative to vacuum, as opposed to a relative pressure difference.


Pollutant Standards Index Station.


Potential to emit: capacity (usually measured in tons per year) of a pollution source to release a particular pollutant or class of pollutants.

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Quality assurance


QA. (Since QC is part of QA, this is a redundant abbreviation.)


Quality control

quality assurance (QA):

This term has been used in more than one way, but the generally accepted definition now is: a management system to assure quality in an organization's products or measurement processes. QA incorporates quality control.

quality control (QC):

Technical procedures to ensure that a measurement process or product meets predetermined data quality objectives. QC is part of quality assurance.

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Readily achievable control technologies: ways to reduce pollutant emissions, required on existing stationary sources in nonattainment areas.

receptor site zone of representation:

Pollutant concentrations measured at any receptor (a point where pollution concentration is to be measured or calculated) result from contributions of emissions from nearby and distant sources. The zone of representation of a monitoring site depends on the relative amounts contributed by sources on different spatial monitoring scales.


Reformulated gasoline, a gasoline formulation designed to reduce pollutant emissions by incorporating an oxygenate, usually MTBE, into the fuel.


Regional transport of ozone: the movement of ozone from area to area.


Actual time as a percentage of possible time for a process, such as ambient air monitoring. For ambient air monitoring and CEMS or COMS, calibrations, calibration checks, audits, and malfunctions are not considered runtime; therefore, 100% runtime is usually not possible for continuous air monitoring systems.


Reid vapor pressure, a measure of the volatility of gasoline or a similar fuel.

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s, sec:

Seconds (unit of time).


Superfund Amendments and Reauthorization Act.


Storage and retrieval of aerometric data. This acronym was the name of the US EPA’s main air pollution database system from about 1977 to 1987. It was replaced by AIRS (now AQS).


Spatial Averaging Zone

secondary particulate:

Particles that usually form over several hours or days and attain aerodynamic diameters between 0.1 and 1 µm. Several of these particles, notably those containing ammonium nitrate, are volatile and transfer mass between the gas and particle phase to maintain a chemical equilibrium.

secondary standard:



(1) Supplemental environmental project, a project undertaken by a stationary source as part of an agreement with a regulatory authority such as the APCD.
(2) Special emphasis program.


Standard industrial classification: an old federal system of classifying industries and other businesses. The system assigned two-digit numbers to broad categories and 4-digit numbers beginning with those 2-digit numbers to finer classes within those categories. SIC has been replaced by NAICS, but SIC numbers are still in common use.


State implementation plan

siting criteria:

US EPA regulations and guidance for locating an air monitoring system within the community to serve a particular purpose.


State and Local Air Monitoring Station


A combination of the words "smoke" and "fog", the term originally referred to suspended particulate air pollution (TSP/PM10/PM2.5). These days, it is used more often to refer to the pollution that forms over urbanized areas on sunny summer days, consisting primarily of invisible ozone and haze-forming PM2.5.

spatial uniformity:

The extent to which concentrations (do not) vary over a specified area.

spatial averaging zone (SAZ):

Spatial averaging zones are defined in the state implementation plan. SAZs have dimensions of 10 to 100 km with boundaries defined by existing political demarcations (e.g. aggregates of zip codes, census tracts) with population attributes.

special purpose monitor (SPM):

SPMs, or "other sites", are ambient air monitoring sites that are not SLAMS. An SPM may or may not be used to determine compliance. Their purpose is to understand the nature and causes of excessive concentrations measured at population-oriented compliance sites. They are not subject to the same siting criteria as SLAMS.


special purpose monitor


State and Territorial Air Pollution Program Administrators, the former organization of the leadership of state, territorial and tribal air pollution control agencies, now part of NACAA.

STAR Program

Strategic Toxic Air Reduction Program, an initiative of APCD adopted in 2005 to reduce levels of a number of toxic air pollutants in Louisville's air. For more information, see STAR Program.

state and local air monitoring station (SLAMS):

An ambient monitoring station (monitor) designed and operated by a state or local air pollution control agency to meet its SIP requirements. The monitoring objective of a SLAMS could include (1) maximum concentrations; (2) representative concentrations in areas of high population density; (3) the impact on ambient pollution levels of significant sources or source categories; (4) general background concentration levels; (5) the extent of regional pollutant transport among populated areas, and (6) welfare-related impacts in rural and remote areas (such as visibility impairment and effects on vegetation). Only population-oriented SLAMS acquire data for determining compliance with standards. All SLAMSs must meet monitor siting criteria and be operated under quality-assurance requirements specified by the US EPA. See also NAMS.

state implementation plan (SIP):

A plan describing how a state will attain and maintain national ambient air quality standards (NAAQS): how it will clean up polluted areas and keep them clean. The Clean Air Act requires each state to develop a SIP. The state must involve the public in approving the plan before it is submitted to the US EPA. If the EPA finds a plan unacceptable, it can promulgate and enforce a Federal Implementation Plan (FIP) in that state. Information about Louisville's part of the Kentucky SIP is available on this website. 

stationary source:

An actual or potential fixed source of air pollution, such as a power plant, chemical factory or landfill.


The layer of the Earth's atmosphere above the troposphere (the surface layer) and below the mesosphere, ranging from about 10 to 50 km above the ground. See ozone.


(adj) Of or relating to the stratosphere. See ozone.

sulfur dioxide (SO2):

A heavy, pungent, colorless air pollutant formed primarily by the combustion of fossil fuels. It is a respiratory irritant, especially for asthmatics.

sulfur oxides, oxides of sulfur (SOx):

Covalent compounds of sulfur and oxygen, including SO2 and SO3 (sulfur trioxide). Formed primarily by the combustion of fossil fuels. SOx and NOx are the major precursors to the formation of acid rain and are significant components of PM2.5.

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Toxic air contaminant; an air toxic regulated under the STAR Program.


Toxic air pollutant. See air toxic. See also TAC and HAP.

Title V

Title V of the Clean Air Act requires comprehensive operating permits for major air pollution sources. See Types of Permits


An English unit of weight (see mass), also called a "short ton". 1 (short) ton = 2000 lb = the weight of 909 kg. A metric ton (or tonne) = 1000 kg.

total suspended particulates (TSP):

Particles of solid and liquid matter suspended in air. TSP is collected on filtration media and analyzed by weight only. Particle sizes represented by the method are up to 100 µm in aerodynamic diameter.

tpd, TPD:

Tons per day, a measure of pollutant emissions.

tpsd, TPSD:

Tons per summer day, a measure of pollutant emissions.

tpy, TPY:

Tons per year, a measure of pollutant emissions.


The federal Toxic Substances Control Act, which regulates asbestos among other substances.


Total suspended particulates

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Urban Airshed Model.


A plain ASCII text approximation of µg/m3.

ultra fine particles:

Particles with aerodynamic diameters less than 0.08 µm. They are emitted directly from combustion sources or condense from cooled gases soon after emission.

ultraviolet light, ultraviolet radiation (UV):

Electromagnetic radiation of higher energy (shorter wavelengths) than visible light but lower energy than most X-rays, with wavelengths from about 400 nm down to about 10 nm.

Urban Airshed Model (UAM):

A three-dimensional grid-based photochemical computer model of the production of ozone from precursors (VOCs and NOx) and the dispersion of air pollution in a specific geographic area over a period of one or two days.


ultraviolet (light)

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vehicle miles traveled (VMT):

The number of vehicles that travel in a particular geographic region in a specified period of time (usually a year) multiplied by the number of miles each vehicle traveled. A measure of the total distance covered by vehicles in that area during that period. Normally the phrase refers only to travel by on-road motor vehicles.


Vehicle emissions testing, specifically the 1984-2003 Vehicle Emissions Testing program of the APCD.


vehicle miles traveled.


Volatile organic compound.

volatile organic compound:

An organic compound (meaning a chemical combination of carbon and other elements, whether natural or manmade) which is volatile, meaning it readily produces vapors at room temperature. Many VOCs react in the atmosphere with nitrogen oxides in the presence of heat and sunlight to form ozone. Methane and other compounds determined by EPA to have negligible photochemical reactivity are excluded from the EPA definition of VOCs. Examples of VOCs include gasoline fumes and solvents from oil-based paints.

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zero air:

Pure air, used for calibrating air monitoring instruments. The US EPA requires zero air to have less than 0.1 ppm of hydrocarbons.

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