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Air Quality Glossary


The list below contains definitions for some of the terms related to protecting air quality and air resources. The definitions of complex legal terms contain references to the associated Code of Federal Regulations (CFR) section or other sources of additional information.

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the process by which incident light is removed from the atmosphere and retained by a particle.

absorption coefficient:
a number that is proportional to the "amount" of light removed from a sight path by absorption per unit distance.

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

acid deposition (wet):
air pollution produced when acid chemicals are incorporated into rain, snow, fog, or mist.

adverse impact on an air quality related value (AQRV):
an unacceptable effect, as identified by an FLM, that results from current, or would result from predicted, deterioration of air quality in a Federal Class I or Class II area (See also Clean Air Act). It should be based on a demonstration that the current or predicted deterioration of air quality will cause or contribute to a diminshment of the area's AQRV's, such as national significance, impairment of the structure and functioning of the area's ecosystem, or impairment of the quality of the visitor experience in the area.

adverse impact on visibility:
Visibility impairment which interferes with the management, protection, preservation, or enjoyment of a visitor's visual experience of a Federal Class I or Class II area. This determination must be made on a case-by-case basis taking into account the geographic extent, intensity, duration, frequency and time of visibility impairments, and how these factors correlate with (1) times of visitor use of the Class I area, and (2) the frequency and timing of natural conditions that reduce visibility.

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

air pollution:
degradation of air quality resulting from unwanted chemicals or other materials occurring in the air.

air quality:
(in context of the national parks:) the properties and degree of purity of air to which people and natural and heritage resources are exposed.

air quality related value (AQRV):
a resource, as identified by the FLM for one or more Federal areas, that may be adversely affected by a change in air quality. the resource may include visibility or a specific scenic, cultural, physical, biological, ecological, or recreational resource identified by the FLM for a particular area. "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).

Air Resources Web, an air quality information web site for US parks, wildlife refuges and the public, developed by the Air Resources Division of the National Park Service and the Air Quality Branch of the Fish and Wildlife Service.

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

produced by human activities.

atmospheric deposition:
pollutants deposited from the air including nitrogen and sulfur compounds.

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.



certain metals and organic pollutants tend to bioaccumulate, meaning they accumulate in muscle or fatty tissue of organisms and dramatically increase in concentration as they move up the food chain.



carbon monoxide:
a colorless, odorless, poisonous gas produced by incomplete burning of carbon-based fuels; e.g. gasoline, oil, and wood.

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, except in specified cases.

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. The 1990 Clean Air Act Amendments are the most far-reaching revisions of the 1970 law. See also Clean Air Act pages.

clean fuels:
low-pollution fuels that can replace ordinary gasoline. These are alternative fuels, including gasohol (gasoline-alcohol mixtures), natural gas and LPG (Liquefied petroleum gas).

the process by which molecules in the atmosphere collide and adhere to small particles.

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.

cumulative effect:
the impact on an AQRV resulting from total pollutant loading from all sources including the contributing effects of new and modified sources of air pollution. A single source may cause individual minor, but cumulatively significant, effects on AQRVs.



any reduction in the intended use or value of a biological or physical resource. For example, economic production, ecological structure or function, aesthetic value, or biological or genetic diversity that may be altered by a pollutant.

deposition analysis threshold (DAT):
a DAT is the additional amount of nitrogen or sulfur deposition within a Class I area, below which estimated impacts from a proposed new or modified source are considered insignificant.

a process by which substances, heat, or other properties of a medium are transferred from regions of higher concentrations to regions of lower concentration.

dry deposition:
delivery of air pollutants in the gaseous or particle phase to surfaces.



ecological effects:
long-term or short-lived changes in the normal functioning of an ecosystem, resulting in biological, economic, social, and aesthetic losses. Studies are conducted to determine the nature or extent of air pollution and acid deposition to ecosystems. See also our ecological effects web page.

ecosystem services:
goods and services provided by ecosystems, such as air purification, nutrient cycling, and recreational experiences, that are vital to human health and livelihood.

release of pollutants into the air from a source.

endocrine-disrupting compounds:
Endocrine-disrupting compounds are chemical substances that are thought to mimic hormones. These chemicals affect hormone balance and interfere with normal processes regulated by hormones including cell metabolism, reproduction, development, and/or behavior.

legal methods used to make polluters obey the Clean Air Act. Enforcement methods include citations of polluters for violations of the law, fines and even jail terms. The EPA and the state and local governments are responsible for enforcement of the Clean Air Act. Citizens may also file lawsuits if they believe anyone is violating the law, including a polluter or govermental agency.

Environmental Protection Agency (EPA):
the federal agency responsible for regulating air quality. See also EPA web site.

the process of a lake, pond, or slow-moving stream, in which organic material accumulates and slowly replaces oxygen. Eventually, the body of water fills in and becomes dry land. In recent years, this process has been accelerated by plant or algae growth in many bodies of water, encouraged by environmental pollution from such sources as detergents containing phosposrus, the leaching of fertilizers, sewage and toxic dumping, and heated water from the cooling systems of power plants and other industries.(Source: Mintzer, 1992).



Federal Land Manager (FLM):
the Secretary of the Department with authority over such lands. [40 CFR 51.166(b)(24)] The FLM role for the Department of the Interior has been delegated the Assistant Secretary for Fish and Wildlife and Parks; the FLM role for the Department of Agriculture has been delegated to the Forest Service, and has been redelegated to the Regional Forester or individual Forest Supervisor.

enrichment of soils and vegetation by plant nutrients, including atmospheric deposition of nitrogen, resulting in increased productivity, sometimes at the expense of plant diversity.

fine particle:
particulate matter less than 2.5 microns in diameter.

gaseous uptake into plant tissue.

foliar injury:
injury or death of tissues in foliage. Ozone, sulfur dioxide and florides are capable of causing foliar injury. These pollutants are taken into the leaf through the stomata. Once inside the leaf, the pollutant or its breakdown products react with cellular components, mainly cellular membranes, causing injury or death to tissues.

fugitive emission:
emissions, which do not pass through a stack, chimney, vent, or other functionally equivalent opening.



hazardous air pollutants (HAP):
airborne chemicals that cause serious health and environmental effects. Hazardous air pollutants are released by sources such as chemical plants, dry cleaners, printing 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.

haze index (HI):
A measure of visibility derived from calculated light extinction measurements that is designed so that uniform changes in the haze index correspond to uniform incremental changes in visual perception, across the entire range of conditions from pristine to highly impaired. The haze index, in units of deciviews (dv), is calculated directly from the total light extinction, bext expressed in inverse megameters (Mm-1), as follows: HI = 10 ln(bext /10).

compounds containing only hydrogen and carbon. Examples: methane, benzene, and decane.

readily absorbing moisture, as from the atmosphere.



•the degree to which a scenic view or distance of clear visibility is degraded by man-made pollutants.
•with respect to park resources and values generally an impact that would harm the integrity of park resources or values.

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 also IMPROVE Newsletter.

any physical or biological response to pollutants, such as a change in metabolism, reduced photosynthesis, leaf necrosis, premature leaf drop, or chlorosis.



LAER: (Lowest Achievable Emissions Rate)
The control level required of a source subject to non-attainment review.

light extinction:
a measure of how much light is absorbed or scattered as it passes through a medium, such as the atmosphere. The aerosol light extinction refers to the absorption and scattering by aerosols, and the total light extinction refers to the sum of the aerosol light extinction, the absorption of gases (such as NO2), and the atmospheric light extinction (Rayleigh scattering).

light extinction budget:
the percent of total atmospheric extinction attributed to each aerosol and gaseous component of the atmosphere.



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

a unit of length equal to one millionth of a meter; the unit of measure for wavelength.

mobile sources:
moving objects that release regulated air pollutants; mobile sources include cars, trucks, buses, planes, trains, motorcycles, and gas-powered lawn mowers. See also source; stationary source.



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.

natural visibility conditions:
visibility conditions attributable to Rayleigh scattering and aerosol associated with natural processes. Natural conditions include naturally occurring phenomena that reduce visibility as measured in terms of light extinction, visual range, contrast, or coloration.

an instrument that measures the amount of light scattered.

those gases and aerosols that have origins in the gas-to-aerosol conversion of nitrogen oxides, e.g., NO2; of primary interest are nitric acid and ammonium nitrate. Ammonium nitrate is very hygroscopic so its contribution to visibility impairment is magnified in the presence of water vapor.

nitrogen dioxide:
a gas (NO2) consisting of one nitrogen and two oxygen atoms. It absorbs blue light and therefore has a reddish-brown color associated with it.

nonattainment area:
a geographic area in which the level of a criteria air pollutant is higher than the level allowed by the federal standards. 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 at the same time. It has been estimated that 60% of Americans live in nonattainment areas.



ozone (O3):
a gas which is a variety of oxygen. The oxygen gas found in the air consists of two oxygen atoms stuck together; this is molecular oxygen. Ozone consists of three oxygen atoms stuck together into an ozone molecule. Ozone occurs in nature. High concentrations of ozone gas are found in a layer of the atmosphere, the stratosphere, high above the Earth. Stratosphere ozone shields the Earth against harmful rays from the sun, particularly ultraviolet B. Smog's main component is ozone; this ground-level ozone is a product of reactions among chemicals produced by burning coal, gasoline and other fuels, and chemicals found in products including solvents, paints, hairsprays, etc.

oxidant stipple:
small brown or black interveinal necrotic lesions on the adaxial surface of leaf tissue that can be attributed to exposure to ozone.



particulate matter:
a criteria air pollutant. Particulate matter includes dust, soot, and other tiny bits of solid materials that are released into and move around in the air. See also fine particle, PM10, Visibility Research Program pages.

instrumental methods, including analytical methods, employing measurement of light intensity. See telephotometer.

a criteria air pollutant that is particulate matter in ambient air exceeding 10 microns in diameter.

Prevention of Significant Deterioration (PSD):
a program established by the Clean Air Act that limits the amount of additional air pollution that is allowed in Class I and Class II areas.

primary standard:
a pollution limit based on human health effects. Primary standards are set for criteria air pollutants. See also secondary standard.



RACT (Reasonably Available Control Technology):
the lowest emissions limit that a particular source can meet by the application of control technology that is reasonably available considering technological and economic feasibility.

rayleigh scattering:
the scattering of light by particles much smaller than the wavelength of the light, e.g., molecular scattering in the natural atmosphere.

reformulated gasoline:
specially refined gasoline with low levels of smog-forming volatile organic compounds (VOCs) and low levels of hazardous air pollutants. The 1990 Clean Air Act requires sale of reformulated gasoline in the smoggiest areas. Reformulated gasoline was sold in several smoggy areas even before the 1990 Clean Air Act was passed.

regional haze visibility impairment:
any humanly perceptible change in visibility (light extinction, visual range, contrast,coloration) from that which would have existed under natural conditions, caused predominantly by a combination of many sources from, and occurring over, a wide geographic area.



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

scattering coefficient:
measure of the ability of particles to scatter light; measured in number proportional to the "amount" of light scattered per unit distance.

screening level or screening level value (SLV):
the concentration or dose of air pollution below which estimated impacts from proposed new or modified source are considered insignificant. The SLV is dependent on existing air quality and on the condition of the AQRV of concern.

secondary aerosols:
aerosols formed by the interaction of two or more gas molecules and/or primary aerosols.

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. See also primary standard.

sensitive receptor:
the AQRV, or part thereof, that is the most responsive to, or the most easily affected by the type of air pollution in question. For example, at Great Smoky Mountains National Park, spruce-fir forest is a sensitive receptor indicator.

a mixture of air pollutants, principally ground-level ozone, produced by chemical reactions involving smog-forming chemicals. See also haze.

any place or object from which air pollutants are released.

State Implementation Plan (SIP):
A detailed description of the programs a state will use to carry out its responsibilities under the Clean Air Act. State implementation plans are collections of the regulations used by a state to reduce air pollution. The Clean Air Act requires that EPA approve each state implementation plan. Members of the public are given opportunities to participate in review and approval of state implementation plans.

stagnation periods:
lengths of time during which little atmospheric mixing occurs over a geographical area, making the presence of layered hazes more likely. See temperature inversion.

stationary source:
a fixed source of regulated air pollutants (e.g. industrial facility). See also source; mobile sources.

those aerosols that have origins in the gas-to-aerosol conversion of sulfur dioxide; of primary interest are sulfuric acid and ammonium sulfate. Sulfuric acid and ammonium sulfate are very hygroscopic so their contribution to visibility impairment is magnified in the presence of water vapor.

sulfurdioxide (SO2):
a criteria air pollutant. Sulfur dioxide is a gas produced by burning coal, most notably in power plants. Some industrial processes, such as production of paper and smelting of metals, produce sulfur dioxide. Sulfur dioxide is closely related to sulfuric acid, a strong acid. Sulfur dioxide plays an important role in the production of acid rain.

the sum of all hourly average concentrations above 0.00 ppb.

the sum of all hourly average concentrations at or above 60 ppb.



target load:
the level of exposure to one or more pollutants that results in an acceptable level of resource protection; may be based on political, economic, or temporal considerations.

temperature inversion:
one of the weather conditions that are often associated with serious smog episodes in some portions of the country. In a temperature inversion, air doesn't rise because it is trapped near the ground by a layer of warmer air above it. Pollutants, especially smog and smog-forming chemicals, including volatile organic compounds, are trapped close to the ground. As people continue driving, and sources other than motor vehicles continue to release smog-forming pollutants into the air, the smog level keeps getting worse.

an instrument that measures the amount of light extinction over a fixed, specified path length.



UltravioletB (UVB):
a type of sunlight. The ozone in the stratosphere, high above the Earth, filters out ultraviolet B rays and keeps them from reaching the Earth. Ultraviolet B exposure has been associated with skin cancer, eye cataracts and damage to the environment. Thinning of the ozone layer in the stratosphere results in increased amounts of ultraviolet B reaching the Earth.



visual range:
the distance at which a large black object would just disappear from view.

Volatile Organic Compounds (VOC):
Organic chemicals all contain the element carbon (C); organic chemicals are the basic chemicals found in living things and in products derived from living things, such as coal, petroleum and refined petroleum products. Many of the organic chemicals we use do not occur in Nature, but were synthesized by chemists in laboratories.



the distance, measured in the direction of propagation of a wave, between two successive points in the wave that are characterized by the same phase of oscillation.

wet deposition:
delivery of air pollutants in the aqueous phase to surfaces (via rain, snow, clouds, or fog).

an ozone index that multiplies each specific concentration by a sigmoidal weighted function, then sums all values. Wi=1/[1+Me-(A x Ci)], where M and A are constants 4403 and 126 ppm-1, respectively, Wi is the weighting factor for Ci, and Ci is concentration in ppm.


Last Updated: January 10, 2013