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Aerosol: Solid particles or liquid droplets that are small enough to be suspended in the air. Aerosols cause most of the light extinction responsible for haze on the Colorado Plateau.

Area source: Many small sources of air pollution in which the contribution of each source is relatively small, but combined may be a significant source of air pollution. A city can be an area source (although large facilities within the city could release enough air pollution to warrant their analysis individually as a point source).

BART: Best Available Retrofit Technology, a process under the CAA to evaluate the need and, if warranted, install the most effective pollution controls on an already existing air pollution source.

BFS/Baseline Forecast Scenario: A computer model used by the Grand Canyon Visibility Transport Commission to estimate future haze-causing pollution and economics. The GCVTC based this scenario on current technologies, existing laws, and a variety of assumptions about how quickly various kinds of pollution sources will be retired and the type of facilities that will replace them. It is an extension of current policies, and allows comparisons with other air pollution management scenarios the GCVTC may propose.

CAA/CAAA: The Clean Air Act, and the Clean Air Act Amendments. National air pollution control is based on the Clean Air Act, passed in 1970. Congress amended the Clean Air Act in 1977 (adding many visibility sections that the GCVTC is addressing), and in 1990 (when it required creation of the GCVTC, assigned tribal governments power under the CAA similar to those of the states).

Class I site/area: In 1977, Congress identified 158 national parks, wilderness areas, international parks and other areas that were to receive the most stringent protection from increases in air pollution. It also set a visibility goal for these areas to protect them from future human-caused haze, and to eliminate existing human-caused haze, and required reasonable progress toward that goal.

Colorado Plateau: A high, semi-arid tableland in southeast Utah, northern Arizona, northwest New Mexico, and western Colorado. The unique erosional forms of the Plateau are world famous.

CNG: Compressed Natural Gas, a relatively clean-burning fossil fuel.

EC/OC: Elemental Carbon (such as soot, often the result of fire and diesel engines) and

Organic Carbon: carbon combined with other elements to form complex compounds, often given off by plants and most human activities.

Fine Mass Particulates: Aerosols that are smaller than 2.5 micrometers in diameter. (A micrometer is one millionth of a meter, a human hair is about 70 micrometers in diameter.)

GCVTC: The Grand Canyon Visibility Transport Commission, composed of the governors of eight western states (AZ, CA, CO, NM, NV, OR, UT, WY), five tribes (Acoma, Hopi, Hualapai, Navajo), four federal land managers (Bureau of Land Management, U.S. Fish and Wildlife Service, U.S. Forest Service, National Park Service), the Columbia River Inter-Tribal Fish Commission, and the Environmental Protection Agency. The states and tribes vote, the other GCVTC members do not. The Commission was established to recommend methods to preserve and improve visibility on the Colorado Plateau. Congress required establishment of the GCVTC through the Clean Air Act Amendments of 1990.

HC: Hydrocarbons. A group of chemicals containing hydrogen and carbon that often contribute to air pollution as OC's or VOC's. They are involved in forming ozone, and some hydrocarbons are toxic. Term often used interchangeably with VOCs.

Hopi Point: An important air quality monitoring site on the South Rim of the Grand Canyon in Grand Canyon National Park.

IAS: Integrated Assessment System, a computer model created by the GCVTC to generate information about future visibility and economic trends under a variety of pollution control scenarios.

IMPROVE: Interagency Monitoring of Protected Visual Environments, a group of federal agencies using a common set of standards to monitor visibility across the United States. Other nations have also adopted portions or all of the IMPROVE monitoring techniques.

Inverse megameters, Mm-1: A measurement unit used for light extinction, the higher the value, the hazier the air is.

LEV/ZEV: Low Emission Vehicle/Zero Emission Vehicle, motor vehicle classifications referring to their tailpipe release of air pollution. Today's ZEV's are generally battery powered, but may use hydrogen fuel cells and other energy sources in the future.

Light extinction: The "loss" of light as it travels through the air. Light can be truly lost by being absorbed by gases and aerosols in the air. Light can also be "lost" as it scatters off gases and aerosols.

LNG: Liquefied Natural Gas, a relatively clean-burning fossil fuel.

LPG: Liquefied Petroleum Gas, a relatively clean-burning fossil fuel such as propane.

Mm-1: Inverse megameters, a measurement unit used for light extinction, the higher the value, the hazier the air is.

MMA: Maximum Management Alternative, an computer model used to estimate the maximum visibility improvements possible regardless of the cost of the pollution controls used. The MMA was used for comparisons, rather than as a policy option.

Mobile source: A pollution source that moves. Mobile sources are often divided into road sources, including cars, trucks, buses, and motorcycles, and non-road sources like trains, planes, boats, lawnmowers, etc.

Modeling: The use of a computer to mimic reality and predict the future behavior of the subject under study. Models of complex subjects like visibility are often limited by the raw data available and the capacity of the computer itself. The GCVTC's IAS is a computer model of regional air quality for the Colorado Plateau and uses information from throughout western North America.

NAAQS: National Ambient Air Quality Standards, levels of air pollution set by the U.S. Environmental Protection Agency to protect public health and welfare. Standards are set for ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), lead (Pb), and particulates (solid aerosols).

Non-attainment area: A geographic region where concentrations of a particular air pollutant exceed the NAAQS. A particular location may be non-attainment for more than one pollutant.

NOx: A mixture of nitrogen dioxide and other nitrogen oxide gases. Nitrogen is the most common gas in the atmosphere. In high temperature and/or high pressure burning (as in an engine), the air's nitrogen is broken down and combined with oxygen, forming unstable or reactive NOx gases. Nitrogen dioxide (NO2) is yellowish brown, and thus contributes directly to haze. All the NOx gases react in the air to form haze-causing aerosols and smog.

NPS: National Park Service, a federal agency charged with protecting the natural and cultural resources and the processes that create and sustain them, in the National Park System.

New Source Review: A review of a new facility that has the potential to emit air pollutants in amounts specified by law. The review is done to establish the impact of the pollution, and the options available to control that pollution.

OC: Organic Carbon, complex carbon-containing compounds often emitted by plants and many human activities. OC2.5 is organic carbon of 2.5 micrometers of less.

PAC: Public Advisory Committee, established by the GCVTC to represent a broad range of public interests. Members are drawn from all levels of government, business, industry, environmental organizations, academia, and private citizens. The GCVTC Commissioners charged the PAC with developing consensus recommendations for managing visibility.

Particulates: Solid material small enough to remain suspended in the air.

PM2.5: Aerosols with a diameter smaller than 2.5 micrometers, the most effective size range to create haze (a micrometer, or micron, is one millionth of a meter, an inch is 25,400 micrometers long).

PM10: Aerosols with a diameter smaller than 10 micrometers, on which the EPA has based current NAAQS. Larger aerosols in this size range (larger than 2.5 micrometers) are less effective in creating haze than the smaller ones. In addition to creating haze, higher concentrations of PM10 can also cause irritation of the throat and lungs, cancers, and early death.

Point source: A specific source of air pollution.

Prescribed fire: Fires in wildland areas that are allowed to burn under prescribed conditions. The "prescription" reflects ecosystem management goals, ability to control the fire, and air quality concerns.

Prescribed natural fire: A fire started by natural processes (usually lightning) and allowed to burn as long as it meets prescribed fire conditions.

PSD: Prevention of Significant Deterioration. A program established under the Clean Air Act Amendments of 1977, whose goal is to prevent major increases in air pollution in areas with cleaner air. The program sets the tightest limits on pollution increases from large point sources in Class I areas.

Rayleigh or Rayleigh Scattering: The natural scattering of light caused by nitrogen and oxygen in the atmosphere which makes the sky look blue. Also called "blue sky."

Reasonable progress: Reasonable progress refers to progress in reducing human-caused haze in Class I areas under the national visibility goal. The Clean Air Act indicates that "reasonable" should consider the cost of reducing air pollution emissions, the time necessary, the energy and non-air quality environmental impacts of reducing emissions, and the remaining useful life of any existing air pollution source considered for these reductions. The GCVTC Public Advisory Committee has developed the following definition: "Reasonable progress towards the national visibility goal is achieving continuous emission reductions necessary to reduce existing impairment and attain steady improvement of visibility in mandatory Class I areas, and managing emissions growth so as to prevent perceptible degradation of clean air days."

Re-entrained dust/road dust: Fine and coarse dust stirred up from paved or dirt surfaces by the passage of vehicles. The dust may include soil particles, tire rubber, soot, and other materials.

Regional cap: A limit on the amount of specific air pollutants that can be released in a defined geographic area, or a limit on the amount of a specific air pollutant that is allowed to be in the air in a defined geographic area.

ROG: Reactive Organic Gases, typically hydrocarbons (HC), but include oxygenated hydrocarbons.

SIP/TIP: State Implementation Plan/Tribal Implementation Plan, plans devised by states and tribes to carry out their responsibilities under the Clean Air Act. SIP's and TIP's must be approved by the U.S. Environmental Protection Agency and include public review.

SO2, SOx, sulfates: Compounds composed of oxygen and sulfur. Burning fuels, manufacturing paper, or smelting rock containing sulfur produces sulfur dioxide gas (SO2) which is converted in the air to other sulfur oxides (SOx) or haze-causing aerosols (sulfates).

Source: Where air pollutants are released. Sources are usually classified as point, mobile, or area sources.

Source attribution: Determining how much a single source contributes to air pollution.

Stationary source: An air pollution source that remains in one place (generally a business or industrial facility).

Species: A term used to refer to types of pollutants.

TIP/SIP: Tribal Implementation Plan/State Implementation Plan, plans devised by states and tribes to meet requirements of the Clean Air Act as defined by the U.S. Environmental Protection Agency.

Trading program: In air quality management, a plan under which some limit is set on the amount of an air pollutant that can be released into the air. If a facility releases less than its limit, it may trade or sell the ability to release "unused" amount of air pollutant to another facility, so the second facility can release more than the limit.

Transfer coefficient: In computer modeling of air quality, a geographic area is divided into "cells." Transfer coefficients are mathematical formulas that tell the computer how much air pollution to "move" from one cell to another. Determining a transfer coefficient requires the computer model designer to consider wind directions, chemical changes to the air pollutants as they travel, loss of pollutants from the air, and other factors.

Transmissometer: A device that measures light extinction by shining a light beam of known brightness through the air and measuring how much is lost when the beam reaches a receiver, usually about 4 miles away.

Urban plume/plume blight: An urban plume is the "cloud" (either visible or invisible) of air pollution blown downwind of an urban area. Plume blight is a distinct band or layer of visible air pollution, often from a single pollution source.

Visibility impairment: The loss of clarity in the air that results when gases or aerosols scatter and absorb light. We usually see visibility impairment as a general haze or a distinct plume.

VMT: Vehicle Miles Traveled. This number is a measure of vehicle usage and is used to calculate the air pollution produced by mobile sources, such as passenger cars, tailpipe emissions and or road dust.

VOC: Volatile Organic Compound. A carbon-containing material that evaporates, such as gasoline, some paints, solvents, dry cleaning fluids, and the like. VOC's contribute to ozone formation and may form OC aerosols.

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