Abrasion mode:  A size range of particles, typically larger than about 3 micrometers in diameter, primarily generated by abrasion of solids.

absorption: a class of processes by which one material is taken up by another.

absorption coefficient: a measure of the ability of particles or gases to absorb photons; a number that is proportional to the number of photons removed from the sight path by absorption per unit length.

absorption cross section: the amount of light absorbed by a particle divided by its physical cross section.

Accumulation mode: A size range of particles, from about 0.1 to 3 micrometers, formed largely by accumulation of gases and particles upon smaller particles. They are very effective in scattering light.

Acid deposition: Wet and/or dry deposition of acidic materials to water or land surfaces. The chemicals found in acidic deposition include nitrate, sulfate, and ammonium.

acid precipitation: typically is rain with high concentrations of acids produced by the interaction of water with oxygenated compounds of sulfur and nitrogen which are the by-products of fossil fuel combustion.

Acid rain: (or acid The deposition of acid chemicals (incorporated into rain, snow, fog, or precipitation) mist) from the atmosphere to water or land surfaces. The pH of rain is considered acid when it is below about 5.2 pH.

Adverse impact: A determination that an air-quality related value is likely to be degraded within a Class I area.

Aerometric Information Retrieval System (AIRS): A computer-based repository of US air pollution information administered by the EPA Office of Air Quality Planning and Standards.

Aerosol: Suspensions of tiny liquid and/or solid particles in the air.

Aerosol extinction: See reconstructed light extinction.

Aethalometer: An aerosol monitoring instrument that continuously measures particle light absorption (aerosol black carbon) on a quartz fiber filter.

Agglomeration: The process of collisions of particles that stick together to become larger particles.

Air light: Light scattered by air (molecules or particles) toward an observer, reducing the contrast of observed images.

Air parcel: a volume of air that tends to be trans-ported as a single entity.

Air pollutant: An unwanted chemical or other material found in the air.

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 Values (AQRVs): including visibility, flora, fauna, cultural and historical resources, related values odor, soil, water, and virtually all resources that are dependent upon and affected by 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).

AIRS: Aerometric Information Retrieval System (of USEPA)

AIRWeb: Air Resources Web, an air quality information retrieval system for US parks and wildlife refuges developed by the Air Resources Division of the National Park Service and the Air Quality Branch of the US Fish and Wildlife Service.

Albedo: the fraction of total light incident on a reflecting surface that is reflected back omnidirectionally.

Ambient air: Air that is accessible to the public.

Anion: A negative ion, such as sulfate, nitrate, or chloride.

Anthropogenic: produced by human activities.

Apparent contrast: Contrast at the observer of a target with respect to some background, usually an element of horizon sky directly above the target.

Apparent spectral contrast: percent difference in radiant energy associated with an object and its background when the object is observed at some distance r.

Apportionment: to distribute or divide and assign proportionately.

Area Sources: Area sources collectively represent individual sources that have not been inventoried as specific point, mobile, or biogenic sources. These individual sources treated collectively as area sources are typically too small, numerous, or difficult to inventory using the methods for the other classes of sources. Area sources represent a collection of emission points for a specific geographic area, most commonly at the county level; however, any area can be defined as an area source. Facilities and emission points are grouped together with other like sources into area source categories. These area source categories are combined in such a way that emissions can be estimated for an entire category using one methodology.

Artifact: any component of a signal or measurement that is extraneous to the variable represented by the signal or measurement.

Atomic absorption spectroscopy: A method of chemical analysis based on the absorption of light of specific wavelengths of light by disassociated atoms in a flame or high temperature furnace. It is sensitive only to elements.

Atmospheric clarity: an optical property related to the visual quality of the landscape viewed from a distance.

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

Attenuation: the diminution of quantity. In the case of visibility, attenuation or extinction refers to the loss of image-forming light as it passes from an object to the observer.

Audit: An investigation of the ability of a system of procedures and activities to produce data of a specified quality.

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babs: Absorption coefficient. A measure of light absorption in the atmosphere by particles and gases. Standard reporting units are inverse megameters (Mm-1).

background luminance: a measure of light power reflected or emitted from the background of an object within a solid angle of one steradian per unit area projected in a given direction.

back trajectory: a trace backwards in time showing where an air mass has been.

Best Available Control Technology (BACT): A source emission limitation, based on the maximum degree of reduction for each pollutant, that must be applied by sources subject to the Prevention of Significant Deterioration program.

Best Available Retrofit Technology (BART): A source emission limitation, based on the maximum degree of reduction for each pollutant, that must be achieved by sources subject to the Prevention of Significant Deterioration program.

Bext:  see extinction.

Bias: An unfair influence, inclination, or partiality of opinion.

Bimodal distribution: a plot of the frequency of occurrence of a variable versus the variable. A bimodal distribution exists if there are two maxima of the frequency of occurrence separated by a mini-mum. See mode.

Biological effects: Ecological studies to determine the nature or extent of air pollution injury to biological systems.

Brightness: A measure of the light received from an object, adjusted for the wavelength response of the human eye, so as to correspond to the subjective sensation of brightness. For visually large objects, the brightness does not depend on the distance from the observer.

Brightness contrast:  The ratio of the difference in brightness between two objects to the brightness of the brighter of the two. It varies from 0 to -1.

bscat: Scattering coefficient. Measured directly by a nephelometer, the scattering coefficient includes scattering due to particles and atmospheric gases (Rayleigh scattering). Standard reporting units are inverse megameters (Mm-1).

Budget: See light extinction budget.

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CAA: Clean Air Act (including all of its amendments)

CAAA: Clean Air Act Amendments (generally refers to Clean Air Act Amendments of 1999)

Calibration: The process of submitting samples of known value to an instrument, in order to establish the relationship of value to instrumental output.

Camera: Device for recording visual range on film.

Carbon Monoxide: One of the six criteria pollutants. A colorless, odorless and poisonous gas produced by incomplete burning of carbon in fuels.

Cascade impactor: An instrument that samples particles by impacting on solid surfaces via jets of air. After passing the first surface, the air is accelerated toward the next surface by a higher speed jet, in order to capture smaller particles than could be captured by the previous one.

CENRAP: Central States Regional Air Partnership, one of five RPOs. Includes the states and tribal areas encompassed by Nebraska, Kansas, Oklahoma, Texas, Minnesota, iowa, Missouri, Arkansas, and Louisiana. Affiliated with CenSARA.

CenSARA: Central States Air Resource Agencies. Represents the states of Nebraska, Kansas, Oklahoma, Texas, Minnesota, Iowa, Missouri, Arkansas, and Louisiana.

Charge neutralization: A process of removing static electric charges. This is done to particle-

sampling filters in order to prevent electrostatic forces from distorting the apparent weight of the sample.

Clarity: Relative distinctness or sharpness of perceived scene elements.

Class I area: as defined by the Clean Air Act, include national parks greater than 6,000 acres, wilderness areas and national memorial parks greater than 5,000 acres, and international parks that existed as of August 1977.

Class II areas: Areas of the country protected under the Clean Air Act, but identified for somewhat less stringent protection from air pollution damage than Class I, except in specified cases.

Clean Air Act: Originally passed in 1963, the current national air pollution control program is based on the 1970 version of the law. Substantial revisions were made by the 1990 Clean Air Act Amendments.

Clean fuels: Low-pollution fuels that can replace ordinary gasoline, including gasohol, natural gas, and propane.

Cloud condensation nuclei: particles of liquids or solids upon which condensation of water vapor begins in the atmosphere.

CMAQ: Community Multiscale Air Quality modeling system

Coagulation: the process by which small particles collide with and adhere to one another to form larger particles.

Coarse mass: Mass of particulate matter with an aerodynamic diameter greater than 2.5 microns but less than 10 microns.

Coarse mode: A size range of particles between 2.5 microns and 10 microns. Coarse particles are mostly composed of soils. The sum of the masses of coarse and fine particles (all particles smaller than 10 microns) is called PM10.

Color: A qualitative sensation described by hue, brightness, and saturation

Color contrast or difference: Contrast between two adjacent scene element colors. Any difference in color hue, saturation, or brightness, between two perceived objects.

Colorimetric analysis: Chemical analysis based on the colors of dyes formed by the reaction of the analysis with reagents.

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

Condensation counternuclei: An instrument that counts nucleation mode particles by causing them to grow in a humid atmosphere, and observing light reflections from the individual enlarged particles.

Condensation nuclei: the small nuclei or particles with which gaseous constituents in the atmosphere (e.g., water vapor) collide and adhere.

Continuous sampling device: An air analyzer that measures air quality components continuously. (See also monitoring, integrated sampling device).

Contrast: Relative difference in light coming from a target compared to the surrounding background, usually the horizon sky. Any difference in the optical quality of two adjacent images.

Contrast change threshold: Minimum change in contrast perceptible to an observer.

Contrast threshold: Minimum apparent contrast at which a target is just perceptible.

Contrast transmittance: ratio between apparent and inherent spectral contrast. When the object is darker than its background, it has a value between 0 and -1. For objects brighter than their background, the value varies from 0 to infinity. When the contrast transmittance is equal to zero, the object cannot be seen.

Criteria Pollutant: EPA uses six "criteria pollutants" as indicators of air quality, and has established for each of them a maximum concentration above which adverse effects on human health may occur. These threshold concentrations are called National Ambient Air Quality Standards (NAAQS). The criteria pollutants are ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, particulate matter and lead.

Current conditions: refers to contemporary, or modern, atmospheric conditions that are affected by human activity.

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Datalogger: An electronic device for measuring analog or digital signals and recording the results on a storage media. Many of them can record inputs on a number of separate locations, reporting them as separate "channels."

Deciview: The unit of measurement of haze, as in the haze index (HI) defined below.

Deliquescence: the process that occurs when the vapor pressure of the saturated aqueous solution of a substance is less than the vapor pressure of the water in the ambient air. Water vapor is collected until the substance is dissolved and in equilibrium with its environment.

Dew point: The temperature at which humidity in the air will condense upon a solid surface.

Dichotomous: Any particle sampler that separately collects coarse and fine particles sampler from one atmosphere. Often refers to virtual impactor instruments.

Diffraction: modification of the behavior of a light wave resulting from limitations of its lateral extent by an obstacle. For example, the bending of light into the “shadow area” behind a particle.

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

Direct effects: the optical effects of aerosols on climate modification referring to absorption and scattering of solar radiation by airborne particles.

Discoloration: Any change in the apparent color of an image. Often refers to the loss of blue sky color due to air pollution.

Dose-response: The relationship between the dose of a pollutant and its effect on a biological system.

Dry deposition: Also known as dryfall, includes gases and particles deposited from the atmosphere to water and land surfaces. This dryfall can include acidifying compounds such as nitric acid vapor, nitrate and sulfate particles, and acidic gases.

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ECOS: Environmental Council of the States

Edge sharpness: characteristic of landscape features. Landscape features with sharp edges contain scenic features with abrupt changes in brightness.

E-GRID: Emissions & Generation Resource Integrated Database

EI: Emission Inventory

Electrical aerosol: A particle sampler that puts electrical charges on particles and sorts analyze them by their different drift rates in an electric field.

Elevated layer: A pollution distribution that is not in contact with the ground.

Emissions: Release of pollutants into the air from a source.

EPA: Environmental Protection Agency

Equilibration: a balancing or counter balancing to create stability, often with a standard measure or constant.

Equivalent contrast: any scene can be fourier decomposed into light and dark bars of various frequencies and intensities modulated in accordance with a sine wave function. Equivalent contrast is the average contrast of those sine waves within a specified range of spatial frequencies.

Externally mixed: particulate species that co-exist as separate particles without co-mingling or combining.

Extinction: the attenuation of light due to scattering and absorption as it passes through a medium.

Extinction budget: Apportioning the extinction coefficient to atmospheric constituents to analysis estimate the change in visibility caused by a change in constituent concentrations.

Extinction coefficient: a measure of the ability of particles or gases to absorb and scatter photons from a beam of light; a number that is proportional to the number of photons removed from the sight path per unit length. See absorption.

Extinction cross section: the amount of light scattered and absorbed by a particle divided by its physical cross section.

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Fine particles: Particulate matter with an aerodynamic diameter of 2.5 microns or less (PM2.5). Fine particles are responsible for most atmospheric particle-induced extinction. Ambient fine particulate matter consists basically of five species:sulfates, ammonium nitrate, organics, elemental carbon, and soil dust.

Fine particulate matter: particulate matter with an aerodynamic diameter less than 2.5 microns(PM2.5).

Fine soil: Particulate matter composed of pollutants from the Earth’s soil, with an aerodynamic diameter less than 2.5 microns. The soil mass is calculated from chemical mass measurements of fine aluminum, fine silicon, fine calcium, fine iron, and fine titanium as well as their associated oxides.

FIP: Federal Implementation Plan

FLM: Federal Land Manager

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Hazardous air pollutants (HAP): Airborne chemicals that cause serious health and environmental effects.

Haze: an atmospheric aerosol of sufficient concentration to be visible. The particles are so small that they cannot be seen individually, but are still effective in scene distortion and visual range restriction. See an example of uniform and Layered Hazes.

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)

High volume: A simple particle sampler consisting of a filter holder and a vacuum sampler cleaner blower, in a simple rain shelter. Some units have flow measuring or controlling features.

Homogenous nucleation: process by which gases interact and combine with droplets made up of their own kind. For instance, the collision and subsequent adherence of water vapor to a water droplet is homogenous nucleation. See nucleation.

Hue: Attribute of color that determines whether it is red, yellow, green, blue, or other color. It is most strongly related to wavelength of light.

Humidity: Water in air, as a gas. Often measured as a percentage, compared to the maximum amount of water vapor the air can contain at that temperature.

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

Hydrophobic: lacking affinity for water, or failing to adsorb or absorb water.

Hygroscopic: readily absorbing moisture, as from the atmosphere.

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Illumination: Application of visible radiation to an object.

Impairment: The degree to which a scenic view or distance of clear visibility is degraded by man-made pollutants.

IMPROVE: Interagency Monitoring of PROtected Visual Environments; a collaborative monitoring program established in the mid-1980s as past of the Federal Implementation Plans. IMPROVE objectives are to provide data needed to assess the impacts of new emission sources, identify existing man-made visibility impairment, and assess progress toward the national visibility goals that define protection of the 156 Class I areas.

IMPROVE Protocol Site: Monitoring sites that operate using standard, approved IMPROVE Program methods. Data from such sites are collected, analyzed, and reported consistently with all other IMPROVE and IMPROVE Protocol sites. Four factors (operation, duration, location, and data availability) define IMPROVE and IMPROVE Protocol sites:

Operation -

 Aerosol samplers must be sited following standard IMPROVE criteria.
 Aerosol samplers must contain all four filter modules (PM2.5 Teflon, nylon, quartz, and PM10 Teflon).
 Aerosol sampler operation must follow the same sampling, handling, analytical, and quality assurance procedures used for IMPROVE sites.
 Aerosol samplers must follow the standard IMPROVE sampling frequency, without seasonal breaks. Daily sampling would be permitted, since it would include the standard IMPROVE sampling days as a subset.
 Optical sites must operate with an Optec LPV-2 transmissometer or an Optec NGN-2 nephelometer.
 The IMPROVE Program encourages camera monitoring systems at all sites.

Duration - The site must operate for at least one year.

Location - Any area may be considered IMPROVE Protocol, however, the IMPROVE aerosol sampler was designed to provide maximum sensitivity in comparatively pristine environments. IMPROVE is a non-urban network and monitors only in Class I areas. The appropriate network for urban sites is the EPA Speciation Network.

Data Availability - Data from IMPROVE and IMPROVE Protocol sites are available to the public.  The sole difference between IMPROVE and IMPROVE Protocol sites is the managing agency. While IMPROVE sites are the direct responsibility of the IMPROVE Steering Committee, IMPROVE Protocol sites may be operated by a Federal Land Manager, a state, or other entity.

Indirect effects: non-optical atmospheric effects of aerosols on cloud albedo and formation, e. g., as condensation nuclei for cloud droplets.

Inhalable particulate matter: Particles smaller than about 12 micrometers in diameter, capable of being drawn into the human bronchial system. Larger particles tend to be filtered out in the upper respiratory tract.

Inherent contrast: Contrast of the target against the horizon sky background when viewed at the target. Same as intrinsic contrast. The contrast that would be seen between two adjacent scenic elements if there were no intervening atmosphere.

Inherent spectral contrast: percent difference in radiant energy associated with an object and its background at an observer distance equal to zero.

Integral vistas: Scenic views which extend beyond Class I boundaries, that are critical to the enjoyment of the area.

Integrated Planning Model (IPM): . An electric utility planning model that EPA uses to estimate air emission changes, incremental electric power system costs, changes in fuel use and prices, and other impacts of various approaches to air pollution control.

Integrated sampling: An air sampling device that allows estimation of air quality components device over a period of time (e.g., 24 hours to two weeks) through laboratory analysis of the sampler's medium.

Integrating nephelometer: an instrument that measures the amount of light scattered (scattering coefficient).

Internally mixed: refers to the situation where individual particles contain one or more species. For example, water is internally mixed with its hygroscopic hosts.

Inversion: See temperature inversion.

Ion: A charged molecular group or atom.

Ion chromatography: A method of separating ions by their different speeds of passage through an ion-exchange resin. The ions are usually detected by their conductivity.

Isopleth: a line drawn on a map through all points having the same numerical value.

Isotropic: a situation where a quantity (or its spatial derivatives) are independent of position or direction.

Isotropic scattering: the process of scattering light equally in all directions.

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Just noticeable change: a variation of just noticeable difference that relates directly to human visual perception. A JNC corresponds to the amount of optical change in the atmosphere required to evoke human recognition of a change in a given landscape (scenic) appearance. The change in atmospheric optical properties may be expressed as the number of JNC's between views of a given scene at different intervals of time.

Just noticeable difference: measure of change in image appearance that affects image sharpness. Counting the number of JND's (detectable changes) in scene appearance is regarded as an alternative method of quantifying visibility reduction (light extinction).

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Koschmeider constant: the constant in the reciprocal relationship between standard visual range and the extinction coefficient.

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LAC: See Light-Absorbing Carbon.

LADCO: Lake Michigan Air Directors Consortium. Represents states of Illinois, Indiana, Michigan, and Wisconsin.

Layered haze: Haze that obscures a horizontal layer of a vista.

Least-impaired days: Data representing a subset of the annual measurements that correspond to the clearest, or least hazy, days of the year.

Light-absorbing carbon: Carbon particles in the atmosphere that absorb light; also reported as elemental carbon.

Light: radiant energy that is capable of exciting the retina and producing a visual sensation. This definition is the one most meaningful for display professionals, although it differs from the definition frequently used by physicists. Our definition excludes ultraviolet (UV) and infrared (IR) wavelengths. UV is shorter in wavelength than light as we've defined it, and IR is longer. The visible wavelengths of the electromagnetic spectrum extend from about 380 to 770 nm. The unit of light energy is the lumen second.

Light energy: electromagnetic energy in the visibility spectrum, i.e. wave lengths between 0.4 and 0.7 micrometers.

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.

Liquid water: the water present within a cloud expressed as a percent of total cloud constitutents, or liquid phase water in an aerosol.

Long path measurement: an atmospheric measurement process that is made over distances in excess of a few hundred meters.

Luminance: a measure of light power refected or emitted from an object within a solid angle of one steradian per unit area area projected in a given direction. The SI unit is the candela per square meter, which is sometimes called a nit. See Brightness, Luminance, and Confusion from Information Display, March 1993 by Charles P. Halsted at

Luminous flux: visible power, or light energy per unit of time. It is measured in lumens. One watt of radiant power at 555 nm--the wavelength at which the typical human eye is most sensitive--is equivalent to a luminous flux of 680 lumens. See brightness, luminance, and confusion from Information Display, March 1993 by Charles P. Halsted at

Luminous intensity: the luminous flux per solid angle emitted or reflected from a point. The unit of measure is the lumen per steradian, or candela (cd). (The steradian is the unit of measurement of a solid angle.

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Magnehelic gauge: A differential pressure gauge suitable for measuring pressure differences as small as 0.1 inches of water.

Major source: A stationary facility that emits a regulated pollutant in an amount exceeding the threshold level (100 or 250 tons per year, depending on the type of facility).

Mandatory Federal Class I areas: Certain national parks (over 6,000 acres), wilderness areas (over 5,000 acres), national memorial parks (over 5,000 acres), and international parks that were in existence as of August 1977. Appendix A lists the mandatory Federal areas.

MARAMA: Mid-Atlantic Regional Air Management Association. Represents the states of Delaware, Maryland, New Jersey, North Carolina, Pennsylvania, Virginia, West Virginia, the city of Philadelphia and the District of Columbia.

Matrix filter: A filter that is formed of a mat or matrix of fibers. It is physically thick, and particles are trapped deep in its structure.

Membrane filter: A thin filter, usually made of a synthetic polymer, with microscopic holes in

  1. Particles are collected only on the surface facing the air flow.

met.: Meteorology (data)

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

Midwest RPO: One of the five RPOs. Affiliated with LADCO. Includes the states and tribal areas encompassed by Illinois, Indiana, Michigan, Ohio, and Wisconsin.

Mie scattering: the attenuation of light in the atmosphere by scattering due to particles of a size comparable to the wavelength of the incident light. This is the phenomenon largely responsible for the reduction of atmospheric visibility. Visible solar radiation falls into the range from 0.4 to 0.8 µm, roughly with a maximum intensity around 0.52 µm.

Mixing layer: An unstable layer of air that has turbulent mixing, usually due to solar heating of the ground. It is often capped by a stable layer of air.

MM5: Mesoscale Meteorological Model. A numerical model for weather prediction on scales from continental to one km.

Mobile sources: Moving objects that release regulated air pollutants, (e.g., cars, trucks, buses, airplanes, trains, motorcycles, and gas-powered lawn mowers). See also source; stationary source.

Mode: the maximum point in a plot of the frequency of occurrence of a variable versus the variable.

Models-3/CMAQ: Community Multiscale Air Quality model is a unique numerical grid model capable of operating as part of the Models-3 framework for the purpose of estimating pollutant concentrations for multiple pollutants (including ozone, particulate matter, precursor and component species, regional haze, air toxins, etc.) in "one-atmosphere" model applications.

Modulation transfer function (MTF): mathematical function which describes contrast transmittance in spatial-frequency space. It is the ratio between scene equivalent contrast at the observer and equivalent contrast at the object. When the object of interest is small compared to its surroundings, the modulation transfer function and contrast transmittance reduce to the same value.

Monitoring: Measurement of air pollution and related atmospheric parameters. See also continuous sampling device, integrated sampling device.

Most impaired days: Data representing a subset of the annual measurements that correspond to the dirtiest, or haziest, days of the year.

MOU: Memorandum of Understanding.

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NACAA: National Association of Clean Air Agencies. The national association representing air pollution control agencies in the 54 states and territories and over 150 major metropolitan areas across the United States.

National Ambient Air Quality Standards: Permissible levels of criteria air pollutants established to protect public health and welfare. Established and maintained by EPA under authority of the Clean Air Act.

National Acid Precipitation Assessment (NAPAP): The 10-year (1980-1990) interagency research program designed to investigate acid deposition and its effects nationwide. The products of this program are the series of State of the Science and Technology Program documents that summarize what we know about the severity of acid deposition and the resources it affects.

National Atmospheric Program: A national network of about 200 sites where wet deposition is collected weekly and sent to the Central Analytical Laboratory in Illinois for Deposition chemical analysis. This network has operated since 1977 and is funded (NADP) by seven federal agencies, and numerous cooperators in agencies, universities, and industry. This network of predominately rural sites is designed to represent broad, regional patterns of deposition.

NAS: National Academy of Sciences

Natural conditions: prehistoric and pristine atmospheric states, i. e., atmospheric conditions that are not affected by human activities.

Nephelometer: an instrument used to measure the light scattering component of light extinction.

Neutron activation: A method of chemical analysis in which the sample is bombarded with analysis neutrons in a nuclear reactor. The nuclei of various elements in the sample are modified to radioactive forms, and the concentrations of the elements are then determined by the intensities and wavelengths of the radiation emitted.

NGM: Nested Grid Model, a regional atmospheric model.

Nitrate: Solid or liquid particulate matter containing ammonium nitrate [NH4NO3] or other nitrate salts. Atmospheric nitrate aerosols are often formed from the atmospheric oxidation of oxides of nitrogen (NOx).

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.

NO2: See nitrogen dioxide.

NOx: Nitrogen oxides. One of the six criteria pollutants. The term used to describe the sum of nitric oxide (NO), nitric dioxide (NO2), and other oxides of nitrogen, which plays a major role in the formation of ozone. The major sources of man-made NOx emissions are high temperature combustion processes, such as those occurring in automobiles and power plants.

Nonattainment area: A geographic area in which the level of a criteria air pollutant is higher than the level allowed by the federal standards. For NAAQS, where the pattern of "violations of standard" is sufficient to require remedial action; a boundary is determined around the location of the violations. The area within that boundary is designated to be in non-attainment of the particular NAAQS standard and an enforceable plan is developed to prevent additional violations.

NSPS: New Source Performance Standard. A standard for emissions from new stationary sources. These sources are divided into several categories.

NSR: New Source Review. Federal air program that establishes control technologies and emission limits for new major sources and for major modifications at existing sources.

Nucleation: process by which a gas interacts and combines with droplets. See homogenous nucleation.

Nuclei mode: A size range of particles below about 0.1 micrometer in diameter. These particles are the nuclei around which larger particles grow.

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OAQPS: Office of Air Quality Planning and Standards (of USEPA)

OAR: Office of Air and Radiation

Object luminance: a measure of light power reflected or emitted from an object itself within a solid angle of one steradian per unit area projected in a given direction.

Optical depth: the degree to which a cloud or haze prevents light from passing through it. It is a function of physical composition, size distribution, and particle concentration. Often used interchangeably with "turbidity."

Optical monitoring: Optical monitoring refers to directly measuring the behavior of light in the ambient atmosphere.

Optical particle: An instrument which measures the size of individual particles by the counter amount of reflected light from a microscopic illuminated volume.

Organic carbon: Aerosols composed of organic compounds, which may result from emissions from incomplete combustion processes, solvent evaporation followed by atmospheric condensation, or the oxidation of some vegetative emissions.

Organic compounds: Chemicals that contain the element carbon.

Orifice audit device: A device which measures air flow based on the known relationship of air flow through and orifice to the pressure drop across it.

Origins: Particle origins can be anthropogenic (man-made) or natural. Another origin classification is primary (particles that are emitted into the atmosphere as particles, such as organic and soot particles in smoke plumes or soil dust particles), and secondary (those formed from gas-to-particle conversion in the atmosphere, such as sulfates, nitrates, and secondary organics).

QTAG: Ozone transport Assessment Group. A national workgroup that addressed the problem of ground level ozone (smog) and the long-range transport of air pollution across the Eastern United States.

QTAQ: Office of Transportation and Air Quality (of USEPA)

OTC: Ozone Transport Commission. One of the five RPOs, affiliated with the Northeast States for Coordinated Air Use Management Association (NESCAUM) and the Mid-Atlantic Regional Air Management Association (MARAMA). Includes the states and tribal areas encompassed by Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Northern Virginia, the District of Columbia, and suburbs of Washington, D.C.

Ozone: One of the six criteria pollutants. Ozone (O3) is a photochemical oxidant and the major component of smog.

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Particle sampler: An instrument to measure particulate matter in ambient air.

Particle scattering coefficient: Proportion of incident light scattered by particles per unit distance (Mm-1).

Particulate matter:  Material that is carried by liquid or solid aerosol particles with aerodynamic diameters less than 10 microns (in the discussions of this report). The term is used for both the in situatmospheric suspension and the sample collected by filtration or other means.

Path function: Radiance per unit path length from a specified point along the path radiated towards the observer.

Path radiance: or "airlight," a radiometric property of the air resulting from light scattering processes along the sight line, or path, between a viewer and the object (target).

Perceived Visual Air Quality (PVAQ): an index that relates directly to how human observers perceive changes in visual air quality.

Perceptible: Capable of being seen.

Phase function: Relationship of scattered to incident light as a function of scattering angle; volume scattering function.

Phase shift: a change in the periodicity of a wave-form such as light.

Photochemical: Any chemical reaction which is initiated by light. Such processes are process important in the production of ozone and sulfates in smog.

Photometer: Instrument for measuring photometric quantities such as luminance, illuminance, luminous intensity, and luminous flux. An instrument for measuring the brightness of an object. It has been suggested that this name be reserved for those instruments which have been adjusted to match the wavelength response of the human eye, but established usage is not yet this consistent, and radiometers are sometimes called photometers.

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

Photon: a bundle of electromagnetic energy that exhibits both wave-like and particle-like characteristics.

Photopic: Vision or wavelength response of the cones of a normal eye when exposed to a luminance of at least 3.4 candelas per square meter.

Plume: Airborne emissions from a specified source and the path through the atmosphere of these emissions.

Plume blight: visual impairment of air quality that manifests itself as a coherent plume.  See  an example of plume bight.

PM2.5: Measure of particulate matter (pollutants from combustion and natural sources); denotes particles smaller than 2.5 micrometers in diameter.

PM10: Measure of particulate matter (pollutants from combustion and natural sources); denotes particles with a nominal size less than 10 micrometers in diameter.

Point source: a source of pollution that is point-like in nature. An example is the smoke stack of a coal-fired power plant or smelter. See source.

Polarization: A property of light. Light can be linearly polarized in any direction perpendicular to the direction of travel, circularly polarized (clockwise or counterclockwise), unpolarized, or mixtures of the above.

Polar nephelometer: an instrument that measures the amount of light scattered in a specific direction. See integrating nephelometer.

ppb: Parts per billion (1 in 10^9).

ppm: Parts per million (1 in 10^6).

ppm: Parts per trillion (1 in 10^12).

Precursor: A substance or condition whose presence generally precedes the formation of another, more notable, condition or substance.

Precursor emissions: emissions from point or regional sources that transform into pollutants with varied chemical properties.

Prescribed burn: A wildland fire whose progress has been controlled by a combination of strategies, including: construction of artificial fire breaks, selection of natural firebreaks and burnout of vulnerable fuels within the fire control line. A wildfire may be declared a controlled burn if ignition occurs within an area for which an approved burning plan exists and weather conditions fall within the acceptable range. While a forest management burn is referred to as a prescribed burn in the planning stage, the same project may be referred to as a controlled burn in the implementation stage.

Prevention of Significant Deterioration: 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 particles: primary particles are suspended in the atmosphere as particles from the time of emission, e. g., dust and soot.

PSD: Prevention of Significant Deterioration; 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.

Psychophysical: the branch of psychology that deals with the relationships between physical stimuli and resulting sensations and mental states.

Psychrometer: An instrument for measuring humidity based on the temperature drop of a thermometer with a wet wick on the bulb.

PVAQ: See Perceived Visual Air Quality.

Pyranometer: instrument that measures directly the loss of total solar radiance under clear sky conditions.

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Quadratic detection model: model used to predict the amount of change in equivalent contrast or perceived landscape structure required to evoke a single noticeable change in landscape appearance.

Quality assurance: An overall plan undertaken to quantify, control, and perhaps improve the quality of data acquired by a system.

Quality control: Actions routinely taken to maintain a specified level of quality of acquired data.

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RACT: Reasonably Available Control Technology

RAVI: Reasonably Attributable Visibility Impairment; visibility impairment caused by a single or small number of sources.

Radiometer: A name for light-measuring instruments which do not match the wavelength response of the human eye.

Rayleigh scattering: Light scattering of the natural gases in the atmosphere. At an elevation of 1.8 kilometers, the light extinction from Rayleigh scattering is approximately 10 inverse megameters (Mm-1).

Reconstructed light extinction: The relationship between atmospheric aerosols and the light extinction coefficient. Can usually be approximated as the sum of the products of the concentrations of individual species and their respective light extinction efficiencies.

Reflectance: Ratio of reflected to incident light.

Reflection: Return of radiation by a surface without a change of frequency.

Refraction: the change of direction of a ray of light in passing obliquely from one medium into another in which the speed of propagation differs.

Regional haze: A cloud of aerosols extending up to hundreds of miles across a region and promoting noticeably hazy conditions. Condition of the atmosphere in which uniformly distributed aerosol obscures the entire vista irrespective of direction or point of observation. Is not easily traced visually to a single source.

Relative humidity: Partial pressure of water vapor at the atmospheric temperature divided by the vapor pressure of water at that temperature, expressed as a percentage.

REMSAD: Regulatory Modeling System for Aerosols and Deposition; a numerical grid model for rapid scoping and strategy assessments for particulate matter, regional haze, PM species, and deposition of air toxins.

RFP: Request for Proposal

RPO: Regional Planning Organization

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Saturation: One part of the description of color, it qualitatively corresponds to the purity of color: the lack of mixed black or white.

Scattering (light): an interaction of a light wave with an object that causes the light to be redirected in its path. In elastic scattering, no energy is lost to the object.

Scattering angle: the angle between the direction of propagation of the scattered and incident light (or transmitted light)

Scattering coefficient: a measure of the ability of particles or gases to scatter photons out of a beam of light; a number that is proportional to the amount of photons scattered per unit length.

Scattering cross section: the amount of light scattered by a particle divided by its physical cross section.

Scattering efficiency: The relative ability of aerosols and gases to scatter light. A higher scattering efficiency means more light scattering per unit mass or number of particles, this in turn means poorer visibility. In general, fine particles (diameter less than 2.5 microns) are efficient scatterers of visible light.

Scene element: Discrete segment of a landscape scene.

Scene monitoring: Scene monitoring is the monitoring of a specific vista or target. Optical and aerosol monitoring measure an abstract, but easily quantifiable parameter of the atmosphere. Scene monitoring captures the effects of all atmospheric parameters simultaneously, but in an inherently difficult manner to quantify. It is, for example, difficult to determine quantitatively which of two photographs represent "better" visibility conditions. Scene monitoring is generally done to help relate quantitative data in a "user-friendly" format.

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

Secondary particles: form in the atmosphere by a gas-to-particle conversion process.

SESARM: Southeast States Air Resource Managers. Affiliated with Southern Appalachian Mountain Initiative (SAMI). Represents the states of Alabama, Georgia, Kentucky, North Carolina, South Carolina, and Tennessee.

SGA: Southern Governors Association.

Sight path: The straight line between the observation point and the target.

SIP: State Implementation Plan; a detailed description of the measures a state will use to carry out its responsibilities under the Clean Air Act.

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

SMOKE: Sparse Matrix Object Kernel Emission-EPA processor for preparation of emission data.

SO2: See sulfur dioxide.

Soot: Black particles with high concentrations of carbon in graphitic and amorphous elemental forms. It is a product of incomplete combustion of organic compounds.

SOP: Standard Operating Procedure

Source: in atmospheric chemistry, the place, places, group of sites, or areas where a substance is injected into the atmosphere. Can include point sources, elevated sources, area sources, regional sources, multiple sources, etc.

Southern Appalachian Mountain Initiative (SAMI): A consortium of government agencies, industry, and environmental groups, formed to investigate the status of air quality and its effects in the highland regions of the southeastern United States. The objective of this regional cooperative is to determine the current and future impacts of regional air pollutants, such as ozone and acid deposition, and to recommend regional air management strategies to control the formation of these pollutants.

Spatial frequency: the reciprocal of the distance between sine wave crests (or troughs) measured in degrees of angular subtense of a sine wave grating. Spatial frequency is a general term for the frequencies associated with the image radiance in a scene along the path of radiance (path of sight). Landscape features contain multiple landscape scenic elements. Each element generates its own image radiance with its own frequency and intensity.

Spectral: an adjective implying a separation of wavelengths of light or other waves into a spectrum or separated series of wavelengths.

Stable air mass: an air mass which has little vertical mixing. See temperature inversion.

Stagnant: referring to meteorological conditions that are not conducive to atmospheric mixing.

Stagnation episodes: See stagnation periods.

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.

standard visual range: reciprocal of the extinction coefficient. The distance under daylight and uniform lighting conditions at which the apparent contrast between a specified target and its background becomes just equal to the threshold contrast of an observer, assumed to be 0.02.

State Implementation Plan: A collection of regulations used by the state to carry out its Implementation responsibilities under the Clean Air Act.

Stationary source: A fixed source of regulated air pollutants (e.g., industrial facility). See also source; mobile sources.

Stratification: The process of separating a database into different groups according to (of data) some detail of their origin, for the purposes of improving statistical sensitivity.

Strip chart recorder: A device for making a time record of some signal, usually an applied voltage. The signal drives a pen in one direction, while paper is moved under the pen in the perpendicular direction at a uniform rate.

Sulfate: Solid or liquid particulate matter composed of sulfuric acid [H2SO4], ammonium bisulfate [NH4HSO4], or ammonium sulfate [(NH4)2SO4]. Atmospheric sulfate aerosols are often formed from the atmospheric oxidation of sulfur dioxide.

Sulfur dioxide: a gas (SO2) consisting of one sulfur and two oxygen atoms. Of interest because sulfur dioxide converts to an aerosol that is a very efficient light scatterer. Also, it can convert into acid droplets consisting primarily of sulfuric acid.

Sun angle: refers to the angle of the sun above the horizon of the earth.

Sun radiometer: A device for measuring the intensity of sunlight falling on the ground. If the sky is cloudless and the angle of the sun is known, then a measure of the clarity of the air can be had by this measurement.

Surface layer: A concentration of air pollution that extends from the ground to an elevation where the top edge of a pollution layer is visible.

Super-VHS: A high definition video format which is capable of achieving horizontal resolution of over 400 lines. A tape recorded in S-VHS format cannot be played on a recorder which is designed to accommodate only the VHS format. See also VHS.

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Target: Object in the distance observed by a person or instrument for visibility measurements.

Telephotometer: an instrument that measures the brightness of a specific point in either the sky or vista.

Temperature: Weather condition in which warm air sits atop cooler air, promoting inversion stagnation and increased concentrations of air pollutants. A condition of a layer of atmosphere in which temperature increases with altitude. Such a layer is stable, and pollutants migrate through it very slowly. Also known as an inversion layer.

Temperature inversion: in meteorology, a departure from the normal decrease of temperature with increasing altitude such that the temperature is higher at a given height in the inversion layer than would be expected from the temperature below the layer. This warmer layer leads to increased stability and limited vertical mixing of air.

Texture: Roughness of the landscape.

Threshold contrast: a measure of human eye sensitivity to contrast. It is the smallest increment of contrast perceptible by the human eye.

TIP: Tribal Implementation Plan; a detailed description of the measures a tribe will use to carry out its responsibilities under the Clean Air Act.

Total carbon: Sum of the light absorbing carbon and organic carbon.

Total light extinction: The sum of scattering (including Rayleigh scattering) and absorption coefficients. See also extinction coefficient.

Total suspended particulates: Total particulate matter in a sample of ambient air.

Toxic air pollutants: See hazardous air pollutants.

Tracer elements: An element which is emitted most strongly by a specific source or class of sources, and can therefore be used as evidence for an impact by such a source when the element is detected in an air pollution sample.

Transmission gauge: A device for determining the amount of particles collected on a filter by the attenuation of light passing through the filter. Beta rays are sometimes used in place of visible light, and the resulting instrument is called a beta gauge.

Transmissometer: an instrument that measures the amount of light attenuation over a specified path length.

Transmittance: the fraction of initial light from a light source that is transmitted through the atmosphere. Light is attenuated by scattering and absorption from gases and particles.

Tribal Implementation Plan (TIP): A collection of regulations used by the Indian tribes to carry out its responsibilities under the Clean Air Act.

Turbidity: a condition that reduces atmospheric transparency to radiation, especially light. The degree of cloudiness, or haziness, caused by the presence of aerosols, gases, and dust.

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Uniform haze: Pollutants that are uniformly distributed both horizontally and vertically from the ground to a height well above the highest terrain.

Unstable air mass: an air mass that is vertically well mixed. See also stable air mass, temperature inversion.

USEPA: United States Environmental Protection Agency.

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VIEW: Visibility Intensive Experiment in the West, a project of the US EPA, with cooperation of the National Park Service, to measure visibility at many stations throughout the western United States to document current visibility and examine trends.

Violation of standard: A regulatory situation, (i.e., NAAQS), where the pattern of "exceedences of standard" is greater than the frequency allowable under that standard.

Virtual impactor: A type of dichotomous sampler which separates large particles from an air stream by impacting them on the "virtual surface" of a slowly moving column of air.

Visibility impairment:  Any humanly perceptible change in visibility (light extinction, visual range, contrast, coloration) from that which would have existed under natural conditions. This change in atmospheric transparency results from added particulate matter or trace gases.

Visibility: refers to the visual quality of the view, or scene, in daylight with respect to color rendition and contrast definition. The ability to perceive form, color, and texture.

Visibility indexes: have been formalized for aerosol, optical, and scenic attributes. Aerosol indexes include mass concentrations, particle concentrations, physical characteristics, and size distributions. The optical indexes include coefficients for scattering, extinction, and absorption. Scenic indexes comprise visual range, contrast, radiance, color, and just noticeable changes.

Visibility Metric: A statistical summary of a set of visibility data including the median (or mean) of the cleanest 20% of the samples, the median (or mean) of all samples, and the median (or mean) or the dirtiest 20% of the samples.

Vista contrast: see Contrast.

VISTAS: Visibility Improvement States and Tribal Association of the Southeast, one of the five RPOs. Includes the states and tribal areas encompassed by Alabama, Tennessee, Virginia, West Virginia, Georgia, Kentucky, Florida, Mississippi, North Carolina, and South Carolina. Affiliated with SASARM.

Visual air quality: Air quality evaluated in terms of pollutant particles and gases that affect how well one can see through the atmosphere.

Visual image processing: the digitizing, calibration, modeling, and display of the effects of atmospheric optical parameters on a scene. The process starts with a photograph of landscape features viewed in clean atmospheric conditions and models the effects of changes in atmospheric composition.

Visual range: the distance at which a large black object just disappears from view.

Visual reduction: is the impairment or degradation of atmospheric clarity. Becomes significant when the color and contrast values of a scene to the horizon are altered or distorted by airborne impurities.

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Washout: The process by which particles are removed from air by capture by raindrops.

Wavelength: 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.

WESTAR: Western States Air Resources Council. Represents the states of Alaska, California, Oregon, Washington, Hawaii, Arizona, New Mexico, Montana, Nevada, Colorado, Idaho, North Dakota, Oregon, South Dakota, Utah, and Wyoming.

Wet deposition: The deposit of atmospheric gases and particles (incorporated into rain, snow, fog, or mist) to water or land surfaces.

Wildfire: Any wildland fire that requires a suppression response. A controlled burn may be declared a wildfire if part of it escapes from the control line or if weather conditions deteriorate and become unacceptable, as described in the burning plan.

WRAP: The Western Regional Air Partnership is a voluntary partnership of states, tribes, federal land managers, local air agencies and the US EPA whose purpose is to understand current and evolving regional air quality issues in the West.  The WRAP covers a 15-state region with more than 100 Class I areas, including tribal areas, of Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming. Affiliated with WESTAR.

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