Abatement of pollutants - the reduction in degree or intensity, or elimination, of pollution; this includes emission reductions by developing clean systems as well as reduction through altered source sector activity.

Acid deposition - Atmospheric input to ecosystems of pollutants which may acidify soils and freshwaters; this includes species derived from SO2, NOx and NH3 emissions, as well as a number of other minor pollutants (e.g. HCl, HF). Acid deposition is more general than "acid rain", since it includes both wet deposition and dry deposition.

Acid Neutralising Capacity (ANC) - Measure of ability of water or soil to resist changes in pH. An important chemical parameter used in critical load calculations.

Acid rain - Precipitation which has been rendered (made) acidic by airborne pollutants; Also see Acid deposition.

Acidification - a decrease in pH of surface waters and soils. As soils naturally become acidified over time; acidification generally refers to the enhanced changes due to anthropogenic deposition of sulphur and nitrogen species.

Acidification Strategy - 1997 EU strategy that aims to significantly reduce the areas under threat from critical loads exceedance of acidity in the European Union by 2010. This includes the establishment of national emission ceilings for acid rain pollutants (sulphur dioxide, nitrogen oxides, ammonia and Volatile Organic Compounds) to reduce emissions beyond existing commitments, a directive limiting the sulphur content of heavy fuel oils, and an amendment of the large combustion plant directive (more information about the Acidification Strategy).

Acute Effect - An adverse effect on any living organism in which severe symptoms develop rapidly and often subside after the exposure stops; occurring over a short period of time; used to describe brief exposures and effects which appear promptly after exposure. Also see: Chronic effect

Aerosol - a suspension of small size (generally under one micron) liquid or solid particles in air gas.

Air monitoring - sampling for and measuring of pollutants present in the atmosphere; monitoring systems range from continuous and hourly sampling to weekly or monthly sampling.

Air pollutant - Any substance in air that could, in high enough concentrations, harm man, other animals, vegetation, or material. Pollutants may include almost any natural or artificial composition of airborne matter. They may be in the form of solid particles, liquid droplets, gases, or any combination thereof. Generally, they fall into two main groups: (1) those emitted directly from identifiable sources ("primary pollutants") and (2) those produced in the air by interaction between two or more primary pollutants, or by reaction with normal atmospheric constituents ("secondary pollutants")

Air pollution episode - A period of abnormally high concentration of air pollutants, often due to low winds and temperature inversion, that can cause illness and death. Also see: episode pollution.

Air quality standards - The level of pollutants prescribed by regulations that may not be exceeded during a given time in a defined area.

Airborne particulates - Total suspended particulate matter found in the atmosphere as solid particles or liquid droplets. Chemical composition of particulates varies widely, depending on location and time of year. Airborne particulates include: windblown dust, emissions from industrial processes, smoke from the burning of wood and coal, and motor vehicle or non-road engine exhausts.

Ammonia (NH3) - a pungent, colourless, gaseous pollutants formed mainly from volatilisation of decomposing excreta or fertilisers. NH3 is alkaline, but may be acidifying if oxidised to nitrate in soils.

AOT - unit to estimate critical levels for effects of phytotoxic pollutants, such as ozone: Accumulated exposure Over a Threshold of e.g. 40 ppb ("AOT40")

Area Source - Any small source of non-natural air pollution that is released over a relatively small area but which cannot be classified as a point source. Such sources may include vehicles and other small engines, small businesses and household activities.

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Background Level - In air pollution control, the concentration of air pollutants in a definite area during a fixed period of time prior to the starting up or on the stoppage of a source of emission under control; also the average level of pollutants present in ambient air, originally referring to natural sources.

BAT - Best Available Techniques

BATNEEC - Best Available Techniques Not Entailing Excessive Cost.

Bioaccumulants - Substances that increase in concentration in living organisms as they take in contaminated air, water, or food because the substances are very slowly metabolised or excreted.

Biodiversity - the variability among living organisms from all sources including, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems.

Biodiversity Action Plan (BAP) - The UK Biodiversity Action Plan (published in 1994) sets out a 20 year strategy, a vital part of which includes schemes to conserve certain endangered wildlife habitats and plant/animal species. The overall aim is to preserve, and where possible enhance, the range and biodiversity of naturally occurring wildlife in the UK. The Habitats Directive is part of this strategy.

Biodiversity Convention - The Convention on Biological Diversity agreed at the Earth Summit in Rio de Janeiro in 1992 is a framework for maintaining and enhancing global biodiversity.

Biological Indicator - selected organism(s) or population sensitive to environmental changes. Good choices for biological indicators are species with well understood biology and that are relatively sedentary.

Bog - A type of wetland that accumulates appreciable peat deposits. Bogs depend primarily on precipitation for their water and nutrient source (i.e. they are ombrotrophic) and are usually acidic and rich in plant residue with a conspicuous mat of living green moss.

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Calcicolous - growing or living in soil rich in lime

Calcifuge - plant growing or living in acid soil

Carbon sequestration - The removal of carbon dioxide from the atmosphere; generally through photosynthesis to fix it as organic compounds in plants and soils; increasing the potential for carbon sequestration is a goal of the Kyoto Protocol.

Catalytic Converter - an air pollution abatement device used primarily on motor vehicles; removes organic contaminants by oxidising them into carbon dioxide and water through chemical reaction. May convert NOx to nitrogen and oxygen, or promote other similar reactions. Inefficiencies in catalytic converters lead to increased emissions of N2O and NH3.

CBED (Concentration Based Estimated Deposition) - a model, used in APIS, which calculates the concentration and deposition of pollutants in the UK.    For detailed information see  

Chlorinated hydrocarbons - a class of persistent, broad-spectrum insecticides, that linger in the environment and accumulate in the food chain. Examples are DDT, aldrin, dieldrin, heptachlor, chlordane, lindane, endrin, mirex, benzene, hexachloride, and toxaphene.

Chlorofluorocarbons (CFCs) - A family of inert, nontoxic, and easily liquefied chemicals used in refrigeration, air conditioning, packaging, insulation, or as solvents and aerosol propellants. Because CFCs are not destroyed in the lower atmosphere they drift into the upper atmosphere where their chlorine components destroy ozone.

Chlorosis - deficiency of chlorophyll in a normally green part of a plant so that it appears yellow-green yellow or white, as a result of mineral deficiency, inadequate light or infection.

Chronic Effect - long-lasting or frequently recurring; e.g. chronic health effect - an adverse effect with symptoms that develop slowly over a long period of time or that recur frequently. Also see: Acute Effect.

Climate change - This term is commonly used interchangeably with "global warming" and "the greenhouse effect," but is a more descriptive term. Climate change refers to the build-up of man-made gases in the atmosphere that trap the sun's heat (radiative forcing), causing changes in weather patterns on a global scale. The effects include changes in rainfall patterns, sea level rise, potential droughts, habitat loss, and heat stress. The greenhouse gases of most concern are carbon dioxide, methane, and nitrous oxides. If these gases double, the earth could warm up by 1.5° C to 4.5° C by the year 2050, with changes in global precipitation having the greatest consequences.

CLRTAP (Convention on Long-range Transboundary Air Pollution) - Between 1972 and 1977 several studies confirmed the hypothesis that air pollutants could travel several thousands of kilometres before deposition and damage occurred, which implied that cooperation at the international level was necessary to solve problems such as acidification. In response to these acute problems, the Convention on Long-range Transboundary Air Pollution by 34 Governments and the European Community (EC) was created in 1983, as the first international legally binding instrument to deal with problems of air pollution on a broad regional basis. Besides laying down the general principles of international cooperation for air pollution abatement, the Convention sets up an institutional framework bringing together research and policy (more details on CLRTAP).

Critical Chemical Value - the highest value of a critical chemical parameter or combination of parameters (e.g. pH, Al/Ca ratio) that does not cause a significant harmful response in a biological indicator.

Critical level - threshold for direct effects of pollutant concentrations according to current knowledge (Also see Critical load). The exceedance of a critical level is defined as the atmospheric concentration of the pollutant above the critical level. (Also see exceedance).

Critical load - a quantitative estimate of exposure to deposition of one or more pollutants, below which significant harmful effects on sensitive elements of the environment do not occur, according to present knowledge (a detailed description is given here; also see: Critical level). The exceedance of a critical load is defined as the atmospheric deposition of the pollutant above the critical load. (Also see�exceedance).

Concentration - includes high or low concentration of air pollution which may occur over short periods of time (< 24 h) or continuously over longer periods (> 24h).

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Deposition - The main pathways for removing pollutants from the atmosphere are through dry deposition and wet deposition.

Direct Impacts - occur through the toxic effects of air pollutants. These are normally scaled according to the expose to air pollution concentrations in the air. A critical level represents the threshold for direct toxicity of air pollutants. Direct impacts and critical levels are particularly relevant to short term pollutant exposure events of several hours or days. (see also indirect impacts).

Dry deposition - direct input of atmospheric pollutants onto surfaces or uptake by plants upon contact with the ground; this pathway is important for uptake of gases and particles; for other deposition pathways, also see: wet deposition and Occult deposition.

Dystrophic - used to describe unproductive waterbodies, which are deficient in calcium, very poor in plants nutrients, especially nitrates (typical for acid peat areas).

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Ecological Indicator - A characteristic of the environment that, when measured, quantifies magnitude of stress, habitat characteristics, degree of exposure to a stressor, or ecological response to exposure. The term is a collective term for response, exposure, habitat and stressor indicators.

Ecosystem - A community of different organisms that are interdependent on each other and their non-living environment, which is relatively self contained (i.e. in terms of energy flows, food chains, etc.) and is distinct from neighbouring communities. An action taken at any level in the food chain, use of a pesticide for example, has a potential domino effect on every other occupant of that system.

Emission - pollution discharged into the atmosphere from smoke- stacks, other vents, and surface areas of commercial or industrial facilities; from residential chimneys; and from motor vehicle, locomotive or aircraft exhausts.

Emission - the direct or indirect release of substances, vibrations, heat or noise from individual or diffuse sources into air, water or onto land.

Episode (pollution) - An air pollution incident in a given area caused by a concentration of atmospheric pollutants under meteorological conditions that may result in a significant increase in effects on people and ecosystems.

ETS - The Effective Temperature Sum is the annual sum of daily temperatures exceeding a threshold temperature, expressed as degree days per year. A threshold of ETS > 5 °C of 1000 degree days is used in the setting of the SO2 critical level.

Eutrophic - used to describe highly productive ecosystems, which are rich in plant nutrients; Eutrophic waterbodies may become depleted of oxygen in warm conditions which favour algal blooms. Eutrophic terrestrial ecosystems are often of reduced conservation value.

Exceedance - violation of environmental protection standards by exceeding allowable limits or concentration levels.

Exceedance of critical loads/levels - see Critical load/Critical level.

Exposure - means deposition experienced on an area basis e.g. m-2 yr-1; kg ha-1 yr-1.

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Fluorocarbons (FCs) - organic compounds analogous to hydrocarbons in which one or more hydrogen atoms are replaced by fluorine; was used as a propellant for domestic aerosols, now found mainly in coolants and some industrial processes. FCs containing chlorine are called chlorofluorocarbons (CFCs). They are believed to be modifying the ozone layer in the stratosphere, thereby allowing more harmful solar radiation to reach the Earth's surface.

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Habitat - place where an organism (e.g. human, animal, plant, micro-organism) or population of organisms live, characterised by its surroundings, both living and non-living.

Habitats Directive - EC Directive aiming to achieve the conservation of natural habitats and species, as well as the protection and where possible improvement of biodiversity (Also see: Biodiversity Action Plan). The main aim is to promote the maintenance of biodiversity, taking account of economic, social, cultural and regional requirements, and create a network of protected areas across the European Union known as "Natura 2000".

Hydrogen Sulphide (H2S)- a gas characterised by a "rotten egg" smell that is emitted during organic decomposition; often produced by and found in the vicinity of oil refineries, chemical plants and sewage treatment plants; can cause illness or kill at high concentrations.

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Indirect impacts - occur as a consequence of the deposition of air pollutants. In situations where pollutant concentrations may be less than the thresholds for direct impacts, effects on species and habitats may still occur as a result of the long term input of pollutants from the atmosphere. A critical load represent the threshold for indirect effects of air pollutants though long term atmospheric deposition. Indirect effects normally occur as a result of accumulated pollutant deposition over several years (see also direct impacts).

Inversion - an atmospheric condition caused by a layer of warm air preventing the rise of cool air trapped beneath it. This traps pollutants that might otherwise be dispersed and diluted, and can cause an air pollution episode.

IPC - The UK Integrated Pollution Control Directive.

IPPC - The EC Integrated Pollution Prevention and Control Directive aims to achieve a high level protection of the environment through measures to prevent or, where that is not practicable, to reduce emissions to air, water and land from activities listed in Annex I (e.g. energy industries, production and processing of metals, mineral industry, chemical industry, intensive pig and poultry farming, waste management, some food and drink industries etc.). Member States are to implement measures to ensure that no new installation is operated without a permit issued in accordance with IPPC. (more information about IPPC)

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Mesotrophic - used to describe moderately productive ecosystems containing moderate amounts of plant nutrients.

Methane - a colourless, non-poisonous, flammable gas emitted by marshes and dumps undergoing anaerobic decomposition.

Mineralisation - decomposition or oxidization of the chemical compounds in organic matter into plant-accessible forms

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NAQS - National Air Quality Strategy 1997

Nitric Oxide (NO) - precursor of ozone, nitrogen dioxide (NO2), and nitrate; usually emitted from combustion processes. Converted to NO2 in the atmosphere, it then becomes involved in the photochemical process and/or particulate formation.

Nitrification - is the biological oxidation of ammonia with oxygen into nitrite, often followed by oxidation of nitrites into nitrates.

Nitrogen oxide (NOx) - product of combustion of fossil fuels (transport, stationary sources); a major contributor to the formation of ozone in the troposphere and acid deposition.

Nitrophilous - plants favouring conditions with high available nitrogen supply.

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Occult deposition - deposition of acid compounds and other pollutants onto vegetation, buildings etc. by direct contact with mist or cloud containing droplets of contaminated water. The mass of pollutants deposited in this way is much less than in wet deposition from precipitation, but concentrations in cloud/mist droplets can be very high, leading to direct impacts. Also see: Dry deposition and deposition.

Oligotrophic - used to describe unproductive ecosystems, which are poor in plant nutrients; such waterbodies are clear water due to lack of plankton.

Ombrotrophic - An ombrotrophic bog/mire receives its water supply and all nutrients from the atmosphere alone, i.e. precipitation and deposition from the atmosphere (from Greek: ombros = rainstorm, trophos = feeder). Other peatlands, e.g. fens, receive at least some of their water supply and nutrients from the groundwater ("geotrophic" (=earth-fed)). Ombrotrophic bogs/mires (e.g. raised bogs and blanket bogs) are some of the most nutrient-poor and acidic habitats in the UK, and are therefore very sensitive to air pollutant deposition.

Ozone - a pungent, colourless, toxic gas. Close to the earth's surface ("tropospheric ozone") it is produced photochemically from hydrocarbons, NOx and sunlight, and is a major component of smog. In the stratosphere, it protects the earth from harmful ultraviolet radiation.

Ozone depletion - Destruction of the stratospheric ozone layer which shields the earth from ultraviolet radiation. This destruction of ozone is caused by certain chlorine and/or-bromine containing compounds (chlorofluorocarbons or halons), which break down when they reach the stratosphere and then catalytically destroy ozone molecules.

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Parts Per Billion (ppb)/Parts Per Million (ppm) - Units commonly used to express contamination ratios, as in establishing the maximum permissible amount of a contaminant in water, land or air; the number of parts of a given pollutant in a million/billion parts of air.

pH - a unit for measuring hydrogen ion concentrations. A pH of 7 indicates a "neutral" water or solution. At pH lower than 7, a solution is acidic. At pH higher than 7, a solution is alkaline.

Photochemical Oxidants - air pollutants formed by the action of sunlight on oxides of nitrogen and hydrocarbons. The primary photochemical oxidant pollutant of concern is tropospheric ozone. Another photochemical oxidant is peroxyacetyl nitrate (PAN).

Photochemical Process - the process by which sunlight acts upon various compounds, causing a chemical reaction to occur.

Photochemical Smog - produced when hydrocarbons and NOx combine in the presence of sunlight to form ozone.

Phytotoxic - harmful to plants.

PM10 - Particulate Matter less than 10 microns, tiny solid or liquid particles of soot, dust, smoke, fumes, and aerosols. The size of the particles (10 microns or smaller, about 0.0004 inches or less) allows them to easily enter the air sacs in the lungs where they may be deposited, resulting in adverse health effects. PM10 also reduces visibility.

PM2.5 - Particulate Matter less than 2.5 microns, tiny solid or liquid particules, generally soot and aerosols. The size of the particles (2.5 microns or smaller, about 0.0001 inches or less) allows them to easily enter the air sacs deep in the lungs where they may cause adverse health effects; PM2.5 also causes visibility reduction.

Pollutants (Air) - substances which, when present in the atmosphere under certain conditions, may become injurious to human, animal, plant or microbial life, or to property, or which may interfere with the use and enjoyment of life or property.

Primary Pollutants - pollutants emitted directly from identifiable sources. Also see: Secondary pollutants.

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Radiative forcing - Global-mean radiative forcing is a valuable concept for giving at least a first-order estimate of the potential climatic importance of various forcing mechanisms. It is a change imposed on the planetary energy balance that has the potential to alter global temperature. Greenhouse gases, for example, intercept some of the outgoing radiation and thereby act to force the Earth's surface to come to a higher equilibrium temperature. In contrast to greenhouse gases, which act only on outgoing infrared radiation, aerosol particles can influence both sides of the energy balance.

Receptors - refers to living organisms or materials which are affected and include interrelated collections of living organisms - i.e. ecosystems. A receptor may or may not be the most sensitive component in a given region.

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Saturation - The condition of a liquid (water) or air when it has taken the maximum possible quantity of a given substance at a given temperature and pressure.

Secondary Pollutants - pollutants produced in the air by interaction between two or more primary pollutants, or by reaction with normal atmospheric constituents. Also see: Primary pollutants.

Seeder-feeder effect - process which enhances the concentration of pollutants in precipitation over hilltops. Hill (orographic) cloud is formed above uplands as the air is forced to rise by the orography. The orographic cloud forms largely within boundary layer air and so contains larger concentrations of pollutants. Precipitation from higher level cloud washes out the hill cloud, increasing rainfall amount and the concentrations of the pollutants reaching the ground

Significant harmful effects - could be the consequences of a short or long term deposition. Several types of harmful effects may be difined including:

  • chemical changes in soils and water which might cause direct or indirect effects on organisms
  • changes in individual organisms, in populations and ecosystems.

Sink - Place in the environment where a compound or material collects.

Stratosphere - the portion of the atmosphere that is 10 to 25 miles above the earth's surface.

Sulphur Dioxide (SO2) - a pungent, colourless, gaseous pollutant formed primarily by the combustion of fossil fuels, especially coal and oil.

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Transboundary Pollutants - air pollution that travels from one jurisdiction to another, often crossing international boundaries.

Troposphere - the layer of the atmosphere nearest the earth's surface. The troposphere extends outward about 5 miles at the poles and 10 miles at the equator.

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UNECE - United Nations Economic Commission for Europe.

VOC - Volatile Organic Compounds; family of highly evaporative organic materials used in a variety of industrial applications, such as paints and solvents; VOCs emissions are major pre-cursors of ground-level ozone and smog.

Wetdeposition - removal of airborne pollutants from the atmosphere to the earth's surface (plants, soil, materials) through precipitation, e.g. "acid rain". For other deposition pathways Also see: deposition, dry deposition and occult deposition.

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