Air Quality Glossary

Abbreviation/Acronym List (PDF)



Acid Deposition

Air pollution produced when acid chemicals are incorporated into rain, snow, fog or mist.


The decrease of acid neutralizing capacity in water or base saturation in soil caused by natural or anthropogenic processes.


A mixture of microscopic solid or liquid particles in a gaseous medium. Smoke, haze, and fog are examples of aerosols.

Air Pollution

The introduction of unwanted chemicals, particulates, or other materials into the Earth’s atmosphere.

Air Quality

The measurement of properties, composition and degree of air resource.

Air Quality Related Value (AQRV)

A resource, as identified by the Federal Land Manager for one or more Federal areas, which may be adversely affected by a change in air quality. The resource may include visibility or an ecological response to air quality. "These values include visibility and those scenic, cultural, biological, and recreation resources of an area that are affected by air quality" (43 Fed. Reg. 15016).


A geographic area that, because of topography, meteorology, and/or climate is frequently affected by the same air mass.

Ambient Monitoring

The systematic, long-term assessment of pollutant levels by measuring the quantity and types of certain pollutants in the surrounding, outdoor air.


Produced by human activities.


The official EPA Air Quality System database application used to house and store ambient monitored data.


Air Quality System code identifying a monitoring station at which air pollution data is collected by EPA, state, local and tribal air pollution control agencies.

Atmospheric Pressure

Pressure caused by the weight of the air.


Any area that meets the National Ambient Air Quality Standards (NAAQS) for a pollutant.

Attainment Area

A geographic area in which levels of a criteria air pollutant meet the health-based National Ambient Air Quality Standard (NAAQS) for that specific pollutant.


Calibration Gases

The known traceable concentration of a gas used to confirm the ambient air monitor’s response of a specific reference point.


A type of air monitoring system that uses a vessel to collect a sample for further analysis.

Carbon Monoxide (CO)

A colorless, odorless, poisonous gas produced by incomplete combustion of carbon-based fuels. Vehicular exhaust is a source of carbon monoxide.

Class I Area

As defined in the Clean Air Act, the following areas that were in existence as of August 7, 1977: national parks over 6,000 acres, national wilderness areas and national memorial parks over 5,000 acres, and international parks.

Class II Area

Areas of the country protected under the Clean Air Act, but identified for somewhat less stringent protection from air pollution damage than a Class I Area for visibility protection.

Clean Air Act

The original Clean Air Act was passed in 1963, but our national air pollution control program is actually based on the 1970 version of the law. (See also Clean Air Act pages of EPA website).

Criteria Pollutants

Six (6) pollutants that EPA uses as indicators of air quality; Particulate Matter, Ground-level Ozone, Nitrogen Dioxide, Carbon Monoxide, Sulfur Dioxide and Lead.

Critical Load

The quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.



A programmable instrument typically used in ambient monitoring stations to record, process, store, and transmit data from the station to a specified known location.

Delta Temperature

The difference in ambient temperature between measurements taken at two (2) different heights.

Design Value

A statistic that describes the air quality status of a given location relative to the level of the National Ambient Air Quality Standards (NAAQS).


A monitoring location towards or on the side away from the prominent wind direction.



Release of pollutants into the air from a source.


One (1) occurrence of a measured concentration that exceeds the National Ambient Air Quality Standard (NAAQS).

Exceptional Event

A monitored event, that exceeds the National Ambient Air Quality Standard (NAAQS), and affects air quality, not reasonably controllable or preventable, is an event caused by human activity that is unlikely to recur or a natural event.


Federal Reference or Equivalent Methods

EPA approved methods/instrumentation used for measuring ambient concentrations of specified air pollutants.

Fugitive Emission

Emissions that do not pass through a stack, chimney, vent, or other functionally equivalent opening.


Gaseous Inlet

The location at which the ambient air is drawn into the ambient monitoring system. Inlets are specially designed to meet all EPA requirements with very specific products.


Hazardous Air Pollutants (HAP)

Airborne chemicals that cause serious health and environmental effects. Hazardous air pollutants are released by sources such as factories, refineries, power plants, and motor vehicles.


An atmospheric aerosol of sufficient concentration to be visible. The particles are so small that they cannot be seen individually, but are still effective attenuating light and reducing visual range.


An organic compound containing only hydrogen and carbon, these often occur in petroleum, natural gas and coal. Examples: methane, benzene, and decane.


Ideal Conditions (image)

An image with very little visibility impairment. Visibility impairment occurs as a result of the scattering and absorption of light by particles and gases in the atmosphere.


Interagency Monitoring of Protected Visual Environments, a collaborative monitoring program to establish present visibility levels and trends, and to identify sources of man-made impairment. See IMPROVE website.

Incoming UV

Ultraviolet rays hitting the Earth’s surface.



A colorless odorless flammable hydrocarbon gas.

Micrograms per cubic meter (µg/m3)

A unit of measurement used to determine the concentration of particulate in the air. A microgram is one millionth of a gram.


A unit of length equal to one millionth of a meter; the unit of measure for particle size.

Mixing Layer

The atmospheric boundary layer characterized by vigorous turbulence tending to stir and uniformly mix the air.

Mobile Sources

Moving objects that release regulated air pollutants; mobile sources include cars, trucks, buses, planes, trains, motorcycles, and gas-powered lawn mowers.

Multigas Calibrator

The Multigas Calibrator and the Zero Air Supply work together to provide the monitoring station’s gaseous analyzers specific levels of known gases to calibrate and verify the analyzers function properly.


National Ambient Air Quality Standards (NAAQS)

Permissible levels of criteria air pollutants established to protect public health and welfare. See also EPA's NAAQS webpage.


An instrument that measures the amount of light scattered.

Net UV

The difference between incoming and outgoing ultraviolet rays (W/m2)

Network Assessment

Monitoring agencies are required to conduct a network assessment once every five (5) years and submit this assessment to EPA. This process assesses if the network meets the monitoring objectives.

Network Plan

Monitoring agencies are required to submit an annual network plan, to EPA, to provide information on current ambient monitoring and any proposed network changes for the upcoming year.

Nitrogen Dioxide (NO2)

A criteria pollutant. A gaseous chemical compound that is emitted during high temperature combustion. Nitrogen Dioxide presents itself as a reddish-brown haze or plume downwind of industrial areas or facilities.


Reactive nitrogen compounds which are precursors for both ozone (O3) and fine particulate matter (PM2.5). NOy includes NO, NO2, and other reactive nitrogen compounds.

Non-methane Hydrocarbons

Total organic hydrocarbons excluding methane. A significant precursor to ozone formation.

Nonattainment Area

Any area that does not meet the National Ambient Air Quality Standard (NAAQS) for a pollutant. A single geographic area may have acceptable levels of one criteria air pollutant but unacceptable levels of one or more other criteria air pollutants; thus, an area can be both attainment and nonattainment, for different pollutants, at the same time.


Outgoing UV

Ultra violet rays reflected off the Earth’s surface.

Ozone (O3)

A criteria pollutant. Ground level ozone is formed through a complex chemical reaction involving precursor emissions (VOCs and NOx) and sunlight when weather conditions are optimal.


Particulate Matter

Particulate matter includes dust, soot, and other tiny bits of solid materials that are released into and move around in the air.


A criteria air pollutant that is particulate matter 10 µm (micrometers) or less in diameter.


A criteria air pollutant that is particulate matter 2.5 µm (micrometers) or less in diameter. Also called fine particulate.


Particulate matter with a diameter of less than 10 µm (micrometers) and greater than 2.5 µm (micrometers). Also called coarse particulate.

Pollution Rose

A graphic tool used to give a view of how wind direction and a pollutant are typically distributed at a particular location.


The liquid and solid water particles that fall from clouds and reach the ground.

Primary Standard

A pollution limit based on human health effects. Primary standards are set for criteria air pollutants.


Quality Assurance

The systematic process of checking the ambient monitors to ensure the data to be of sufficient quality.

Quality Control

A series of specific activities performed to provide a reproducible quality product.


Relative Humidity

The amount of water vapor in the air, expressed as a percentage of the maximum amount of water vapor that the air can hold at a given temperature.


Scalar Wind Speed

A measure of the magnitude of the wind speed but not the direction.


An interaction of light with an object (e.g., a fine particle) that causes the light to be redirected in its path.

Secondary Standard

An air pollution limit based on environmental effects such as damage to property, plants, visibility, etc. Secondary standards are set for criteria air pollutants.


State and Local Air Monitoring Stations are used for supplying general monitoring data for criteria pollutants and determining compliance with the National Ambient Air Quality Standard (NAAQS).


A mixture of air pollutants, principally ground-level ozone, produced by chemical reactions.

Solar Radiation

Radiant energy emitted by the sun.


Any place or object from which air pollutants are released.

Special Purpose Monitor (SPM)

Monitoring stations used to provide information needed by the State and local agencies to support air program activities and fulfill the objectives of the air monitoring network.

Speciated PM2.5

Chemical species composition of PM2.5.

Speciated VOCs

Chemical species composition of VOCs.

Stagnation Periods

Lengths of time during which little atmospheric mixing occurs over a geographical area.

Standard Deviation Wind Direction

A measure of the variability of the direction from which the wind is blowing. Zero standard deviation means that there is no variability in the wind direction, over a specified amount of time.

State Implementation Plan (SIP)

A detailed description of the programs a State will use to carry out its responsibilities under the Clean Air Act.

Station Temperature

The temperature inside a monitoring station.

Stationary Source

A fixed source of regulated air pollutants (e.g. industrial facility).

Sulfur Dioxide (SO2)

A criteria air pollutant. Sulfur dioxide is a chemical compound produced by volcanoes and in various industrial processes. Coal and petroleum often contain sulfur compounds, and their combustion generates sulfur dioxide.


Temperature Inversion

A thin layer of the atmosphere where the normal decrease in the temperature with height switches to the temperature increasing with height. An inversion acts like a lid, keeping rising air from penetrating through the thin layer.



An area that cannot be classified on the basis of available information as meeting or not meeting the National Ambient Air Quality Standard (NAAQS) for a pollutant.


A monitoring location towards or on the side of the prominent wind direction.


Visual Impairment

The degree to which a scenic view or distance of clear visibility is degraded by man-made pollutants. Visibility impairment occurs as a result of the scattering and absorption of light by particles and gases in the atmosphere.

Volatile Organic Compounds (VOC)

Any compound of carbon (excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides, and ammonium carbonate) which participates in atmospheric photochemical reactions.



An ozone index that multiplies each specific concentration by the sigmoidal weighted function, then sums all values.

Wind Direction

The direction from which the wind is blowing. Can be expressed in scalar or vector wind direction.

Wind Rose

A graphic tool used to give a view of how wind speed and direction are typically distributed at a particular location.

Wind Speed

The rate of the motion of the air on a unit of time.


Zero Air Supply

An instrument that can deliver pure filtered air for use with the multigas calibrator within the monitoring station.