Visibility Intro






























Glossary of Terms 


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.


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.