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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:
a mixture of wet and dry deposition (deposited material) from the atmosphere containing higher than normal amounts of nitric and sulfuric acids.

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 diminishment 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 resource:
anything that may be affected by a change in air quality. See air quality related value (AQRV).

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.

ambient monitoring:
the systematic assessment of pollutant levels that is accomplished by measuring the types and amounts of certain pollutants (O3, SO2, PM2.5, etc.) in outdoor air over time.

ammonium (NH4+):
a reduced form of nitrogen most often indicative of agricultural emissions.

produced by human activities.

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

attainment area:
an area considered to have air quality as good as or better than the national ambient air quality standards as defined in the Clean Air Act. An area may be an attainment area for one pollutant and a non-attainment area for others.

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Best Available Retrofit Technology (BART):
an emission limitation based on the degree of reduction achievable through the application of the best system of continuous emission reduction for each pollutant which is emitted by an existing stationary facility. The emission limitation must be established, on a case-by-case basis, taking into consideration the technology available, the costs of compliance, the energy and non air-quality environmental impacts of compliance, any pollution control equipment in use or in existence at the source, the remaining useful life of the source, and the degree of improvement in visibility which may reasonably be anticipated to result from the use of such technology.

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.

the process by which certain metals and organic pollutants accumulate in muscle or fatty tissue of organisms and dramatically increase in concentration as they move up the food chain.

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

Clean Air Status and Trends Network. A national program to monitor ozone and deposition.

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

the requirement that park operations be consistent with state regulatory plans to attain or maintain air quality standards.

Cooperative Ecosystem Study Unit (CESU):
a multi-agency and non-governmental organization (NGO) partnership with the nation's universities and other institutions. Participation in the CESUs enables the National Park Service to obtain high-quality science, usable knowledge for resource managers, responsive technical assistance, continuing education, and cost-effective research programs.

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. This level is unique for each ecosystem and resource, depending on its sensitivity to air pollution.

criteria pollutant:
a regulated air pollutant for which the EPA has established a national ambient air quality standard. The Clean Air Act identifies six air pollutants for regulation: ozone, particulate matter, nitrogen oxides, sulfur dioxide, carbon monoxide, and lead.

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.

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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):
the additional amount of nitrogen or sulfur deposition in a given 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.

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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:
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).

external threat:
a threat that orginates outside the park boundary but impacts park resources.

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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 (µ) in diameter.

gaseous uptake into plant tissue.

foliar injury:
injury or death of tissues in foliage. Ozone, sulfur dioxide and fluorides 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.

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Gaseous Pollutant Monitoring Network. A network of air quality monitoring stations operated by the NPS Air Resources Division to monitor ozone and sulfur.

greenhouse gas (GHG)
a gas such as water vapor (H2O), carbon dioxide (CO2), or methane (CH4) which absorbs energy, slowing or preventing the loss of heat from Earth to space.

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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).

heavy metal:
a metallic element with a high atomic weight (e.g. mercury, chromium, cadmium, arsenic, lead). Heavy metals can damage living things at low concentrations and tend to accumulate in the food chain.

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

readily absorbing moisture, as from the atmosphere.

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1) the degree to which a scenic view or distance of clear visibility is degraded by man-made pollutants;
2) 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.

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

lead (Pb):
a heavy metal that is hazardous to health if breathed or swallowed. Its use in gasoline, paints, and plumbing compounds has been sharply restricted or eliminated by federal laws and regulations.

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.

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mercury (Hg):
a naturally occurring element that is found in air, water, and soil. It exists in several forms: elemental or metallic mercury, inorganic mercury compounds, and organic mercury compounds. Exposures to mercury can affect both human health and wildlife health.

the process of adding a methyl (-CH3) group to a chemical compound. In the case of mercury, this process results in methylmercury.

methylmercury (CH3Hg):
the most toxic form of mercury. Methylmercury is bioavailable and accumulates in muscle tissue. Exposure to methylmercury is usually by ingestion, and it is absorbed more readily and excreted more slowly than other forms of mercury.

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

micron (µ):
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 stationary source.

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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.

an oxidized form of nitrogen most often indicative of mobile source or power plant emissions. Those gases and aerosols that have origins in the gas-to-aerosol conversion of nitrogen oxides, e.g., NO3- and NO2. Of primary interest are nitric acid (HNO3) and ammonium nitrate (NH4+). Ammonium nitrate is very hygroscopic so its contribution to visibility impairment is magnified in the presence of water vapor.

nitrogen (N):
an element that is an essential plant nutrient, but which, in excess, may contribute to shifts in plant species, disruption of ecosystem processes, changes in fire frequency, and possible increases in insect and disease outbreaks.

nitrogen dioxide (NO2):
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.

nitrogen oxides (NOx):
a group of gases made up of nitrogen and oxygen that cause acid rain and other environmental problems, such as smog. Burning fossil fuels, such as coal and gasoline, releases NOx into the atmosphere.

non-attainment 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 non-attainment at the same time. It has been estimated that 60% of Americans live in non-attainment areas.

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ozone (O3):
a molecule consisting of three oxygen atoms. High concentrations of ozone in the stratosphere shield the Earth against harmful rays from the sun, particularly ultraviolet B. Ground-level ozone, or tropospheric ozone, is a main component of smog. Ground-level ozone forms when NOx and VOCs react in the atmosphere in the presence of sunlight

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

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particulate matter (PM):
particles found in the air, including dust, dirt, soot, smoke, and liquid droplets. Some particles are large or dark enough to be seen as soot or smoke. Others are so small that individually they can only be detected with an electron microscope.

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

particulate matter less than 2.5 microns (µ) in diameter. Because of their small size (approximately 1/30th the average width of a human hair), the fine particles can lodge deeply into the lungs, and are believed to pose the greatest health risks.

particulate matter less than 10 microns (µ) in diameter. PM10 pose a health concern because they can be inhaled into and accumulate in the respiratory system.

Prevention of Significant Deterioration (PSD):
a program created in the Clean Air Act Amendments of 1977 to
  • protect public health and welfare,
  • preserve, protect, and enhance the air quality in national parks, national wilderness areas, national monuments, national seashores, and other areas of special national or regional natural, recreational, scenic, or historic value,
  • ensure that economic growth will occur in a manner consistent with the preservation of existing clean air resources, and
  • assure that any decision to permit increased air pollution in any area to which this section applies is made only after careful evaluation of all the consequences of such a decision and after adequate procedural opportunities for informed public participation in the decision making process.

primary particle:
a solid particle emitted directly into the atmosphere, for example smoke, dust, or soil.

primary standard:
an ambient air quality standard designed to protect human health with an adequate margin for safety.

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Reasonably Available Control Technology (RACT):
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.

Remote Automatic Weather Stations. A network of nearly 2,200 interagency stations that monitor the weather and provide weather data that assists land management agencies with a variety of projects such as monitoring air quality, rating fire danger, and providing information for research applications.

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 Rule:
a 1999 rule that requires states to coordinate with the EPA, the National Park Service, the US Fish and Wildlife Service, the US Forest Service and other interested parties to develop and implement air quality protection plans to reduce the pollution that causes visibility impairment in Class I areas.

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

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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.

scenic resource:
a location or area that is recognized and enjoyed for its visual and scenic qualities and whose features, views, patterns, and characteristics contribute to a distinct sense of appreciation of the natural and cultural environment.

screening level or screening level value (SLV):
the concentration or dose of air pollution below which estimated impacts from a 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 AQRVs like plants, visibility, etc. Secondary standards are set for criteria air pollutants. See also primary standard.

sensitive groups:
populations including asthmatics, individuals with bronchitis or emphysema, children, and the elderly, who are particularly susceptible to difficulty breathing and the aggravation of existing respiratory and cardiovascular disease as a result of air pollution.

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:
any building, structure, facility, or installation which emits or may emit any air pollutant.

sulfate (SO42-):
a type of particle formed in the atmosphere when gaseous sulfur dioxide (SO2) interacts with other oxidants.

sulfur dioxide (SO2):
a gas produced by burning coal, most notably in power plants, and by industrial processes, such as production of paper and smelting of metals. 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.

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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 gets worse.

toxic compound:
one of a suite of contaminants that includes mercury and pesticides. These contaminants can accumulate in organisms (e.g., in lipids, muscles, organs) to levels that are harmful to ecosystem and human health.

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

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ultraviolet B (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.

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the total landscape seen or potentially seen from a point, or from all or a logical part of a travel route, use area, or water body.

the ability to see colors and details in distant views.

visual range:
the distance at which a large dark object can just be discerned against the horizon. Visual range is estimated by dividing a the Koschmieder Coefficient, K, by the extinction coefficient, bext:
Visual range=K/bext

Where visual range is in kilometers (km), bext is in inverse megameters [Mm-1], and K is the natural log of the contrast threshold of the human eye and bext is the sum of Rayleigh scattering and particle extinction. A K of 3912 is commonly used, which is derived assuming a 2% contrast detection threshold.

Rayleigh scattering is site specific and varies with altitude. To compare visibility data across different monitoring sites, visual range estimates can be standardized by assuming a constant altitude of 5000 feet, which corresponds to Rayleigh scattering of 10 Mm-1. This normalized visual range is called standard visual range.

When interpreting visual range values, note that these values are estimates based on a large black object against a cloud-free horizon. These estimates do not account for changes in haze across the landscape, nor do they account for changes to the scene, such as loss of textures or discoloration as haze increases. Fog or rain is not accounted for in above visual range calculation.

vital signs monitoring:
long-term monitoring conducted by the NPS Inventory and Monitoring (I & M) program to track a subset of physical, chemical, and biological elements and processes of park ecosystems that are selected to represent the overall health or condition of park resources, known or hypothesized effects of stressors, or elements that have important human values.

volatile organic compound (VOC):
an organic chemical compound whose composition and physical properties make it possible for it to evaporate or sublimate under normal atmospheric conditions of temperature and pressure. VOCs in the atmosphere result from both human caused and natural processes. Human-caused VOCs can result from a number of sources including paint strippers, pesticides, glues, and adhesives. There may be a relationship between elevated levels of methane and VOCs in areas with expanding natural gas development.

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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.

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Last updated: August 31, 2020


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