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Definitions


Terms
Air Quality Related Value A scenic, cultural, physical, biological, ecological, or recreational resource which may be affected by a change in air quality as defined by the Federal Land Manager (FLM) ... for federal lands.
Class I Area Geographic area designated for the most stringent degree of protection from future degradation of air quality. The Clean Air Act designates as mandatory class I areas each national park over 6,000 acres and each national wilderness over 5,000 acres that existed as of the date of enactment (August 7, 1977).
Nonattainment Area In conventional use a term used to describe an area that does not meet one or more of the National Ambient Air Quality Standards.
Prevention of Significant Deterioration A process mandated by the Clean Air Act Amendments of 1977 which requires emission limitations for certain new or modified sources of air pollution. The Forest Service has input into this permitting process by providing comment to air regulatory agencies on potential source impacts to air quality related values.
Sensitive Receptor An attribute of an AQRV that is most responsive to, or first affected by, air pollution.
Sensitive Receptor Indicator A measurable, physical, chemical, biological or social characteristic of a sensitive receptor.
Wilderness Areas formally protected by the 1964 Wilderness Act, or its extension to eastern lands by the 1975 Eastern Wilderness Act and to the public domain by the Federal Land Policy and Management Act of 1976. The 1964 Wilderness Act defines a wilderness "... as an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain."
Visibility-Related Terms
Extinction The attenuation of light due to scattering and absorption as it passes through a medium. 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.
Deciview The unit of measurement of haze, or "haze index". Deciview is a measure of visibility derived from light extinction that is designed so that incremental 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)

The haze index will be less than 0 for bext values below 10.
Natural conditions Prehistoric and pristine atmospheric states, i. e., atmospheric conditions that are not affected by human activities.
Rayleigh scattering Light scattering by the natural gases in the atmosphere. At an elevation of 1500 meters (5000'), the light extinction from Rayleigh scattering is approximately 10 inverse megameters (Mm-1).
Current Conditions Refers to contemporary, or modern, atmospheric conditions that are affected by human activity.
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.
Acronyms
BLM Bureau of Land Management
EPA Environmental Protection Agency
FLAG Federal Land Managers Air Quality Related Values Workgroup
FLM Federal Land Manager
FS Forest Service
FWS Fish & Wildlife Service
IMPROVE Interagency Monitoring of Protected Visual Environments
LAC Limits of Acceptable Change
NAAQS National Ambient Air Quality Standards
NPS National Park Service
PSD Prevention of Significant Deterioration
USDA U.S. Department of Agriculture
USDI U.S. Department of Interior
USGS U.S. Geological Survey