Glossary of related terms

Terms beginning with 1

Index

1,3-Butadiene (chemical formula C₄H₆)

This is an organic compound emitted into the atmosphere mainly from fuel combustion e.g. petrol and diesel vehicles. It is not found in the fuels themselves, but is formed when they are burned. 1,3-butadiene is also an important chemical in certain industrial processes, particularly the manufacture of synthetic rubber. Apart from locations near to these processed, the main source of 1,3-butadiene in the atmosphere is motor vehicle emissions. 1,3-butadiene is known to cause cancer in humans.

Terms beginning with A

Index

Acid Deposition

Total atmospheric deposition of acidity is determined using both wet and dry deposition measurements. Wet deposition is the portion dissolved in cloud droplets and is deposited during rain or other forms of precipitation. Dry deposition is the portion deposited on dry surfaces during periods of no precipitation as particles or in a gaseous form. Although the term "acid rain" is widely recognized the dry deposition portion ranges from 20 to 60 percent of total deposition.

Acid Rain

When atmospheric pollutants such as sulphur dioxide and nitrogen oxides mix with water vapour in the air, they are converted to sulphuric and nitric acids. These acids make the rain acidic, hence the term 'acid rain'. Acid ran is defined as any rainfall that has an acidity level beyond what is expected in non-polluted rainfall. Acidity is measured using a pH scale, with the number 7 being neutral. Consequently, a substance with a pH value of less than 7 is acidic, while one of a value greater than 7 is basic. Generally, the pH of 5.6 has been used as the baseline in identifying acid rain. Thus any precipitation that has a pH value of less than 5.6 is considered to be acid precipitation.

Air Pollution Bandings

The Air Pollution Information Service uses four bands to describe levels of pollution. The bands are low, moderate, high and very high. Healthy people do not normally notice any effects from air pollution, except occasionally when air pollution is "very high".

Air Pollution Bulletins

Air Pollution Bulletins are issued daily for each zone of the UK. The bulletins show current air quality, and a forecast for the next 24 hours, classified into four bands and also into an index.

Air Pollution Index

A numerical index for air pollution from 1 to 10 related to the air quality bands of "Low" (index 1-3), "Moderate" (index 4-6), "High" (index 7-9) or "Very High" (index 10).

Air Quality Directive

The European Union's Directive 2008/50/EC of 21st May 2008, on Ambient Air Quality and Cleaner Air for Europe is often referred to as 'the Air Quality Directive'. These Directives came into operation while the UK was a member state of the European Union (EU). The provisions of the Directives were required to be incorporated (or ‘transposed’) into Northern Ireland’s own legislation: this was done by means of the Air Quality Standards Regulations (Northern Ireland) 2010. The full provisions of Directive 2008/50/EC therefore remain part of Northern Ireland’s own legislation, even after the UK’s departure from the EU in early 2020.

Air Quality Management Area (AQMA)

If a local authority finds any places where the objectives are not likely to be achieved, it must declare an Air Quality Management Area there. This area could be just one or two streets, or it could be much bigger. Then the local authority will put together a plan to improve the air quality - a Local Air Quality Action Plan

Air Quality Standards

Standards are the concentrations of pollutants in the atmosphere which can broadly be taken to achieve a certain level of environmental quality. The standards are based on assessment of the effects of each pollutant on human health including the effects on sensitive sub-groups.

Air Quality Standards Regulations (Northern Ireland) 2010

Ambient air quality in Northern Ireland is regulated by the Air Quality Standards Regulations (Northern Ireland) 2010 and their subsequent 2016 amendment. These Regulations cover the following pollutants: sulphur dioxide, oxides of nitrogen, particulate matter, lead, carbon monoxide, benzene, ozone, arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons (PAH) in ambient air. ( Prior to 31^st January 2020, the UK was a Member State of the European Union, and therefore required to incorporate - or ‘transpose’ - the provisions of EU Directives into its own national law by a specified date. The Air Quality Standards Regulations (2010) are the legislation by which the UK fulfilled this requirement for Directives relating to air quality.)

Air Quality Strategy

The Air Quality Strategy for England, Scotland, Wales and Northern Ireland describes the plans drawn up by the Government and the Devolved Administrations to improve and protect ambient air quality in the UK in the medium-term. The Strategy sets Objectives for the main air pollutants to protect health. Performance against these Objectives is monitored where people regularly spend time and might be exposed to air pollution.

Air Quality Strategy Objective

The Air Quality Strategy sets objectives for the maximum concentrations of eight pollutants (These Regulations cover the following pollutants: sulphur dioxide, oxides of nitrogen, particulate matter, lead, carbon monoxide, benzene, 1-3 butadiene and ozone). These are at least as stringent as the limit values of the Air Quality Standards Regulations and the EU Directives on which they were based.

Ambient Air

Outdoor air.

Annual Mean

The average of the concentrations measured for each pollutant for one year. Usually this is for a calendar year, 1^st January to 31^st December.

Automatic Monitoring

Monitoring is usually termed "automatic" or "continuous" if it produces real-time measurements of pollutant concentrations. Automatic fixed point monitoring methods exist for a variety of different pollutants and these can provide high resolution data averaged over very short time periods.

Average Exposure Indicator (AEI)

The statistic on which the Air Quality Directive's national exposure reduction target is based, for PM_2.5 between 2010 and 2020. The AEI for the UK is calculated as follows: the arithmetic mean PM_2.5 concentration at appropriate UK urban background sites is calculated for three consecutive calendar years, and the mean of these values taken as the AEI.

Terms beginning with B

Index

Benzene (C₆H₆)

A chemical compound that is harmful to human health. As an air pollutant, benzene can be emitted from domestic and industrial combustion processes, and road vehicles.Benzene is an aromatic organic compound which is a minor constituent of petrol (about 2% by volume). The main sources of benzene in the atmosphere in Europe are the distribution and combustion of petrol. Benzene can be emitted in vehicle exhaust as unburnt fuel and also as a product of the decomposition of other aromatic compounds. Benzene is a known human carcinogen.

Benzo[a]pyrene

One of a group of compounds called polycyclic aromatic hydrocarbons (PAHs) that can be air pollutants. The main sources of B[a]P in the UK are domestic coal and wood burning, fires, and industrial processes such as coke production.

Beta Attenuation Monitor (BAM)

A type of instrument used for monitoring concentrations of particulate matter, which measures the attenuation of beta rays passing through a paper filter tape on which particulate matter from sampled air has been collected.

Black Carbon

Black Carbon is dark, soot-like airborne particulate material. It consists mostly of carbon and is formed from incomplete combustion of any carbon-containing material. It is measured by the optical absorption of specific wavelengths by particulates collected on a filter. Black carbon is one of the constituents of particulate matter; it is also known to contribute to climate change. Black carbon is similar to the 'black smoke' measurement described below.

Black Smoke

Black Smoke consists of fine particles of carbon-containing material. These particles can be hazardous to health especially in combination with other pollutants which can adhere to the particulate surfaces. Black smoke is emitted mainly from fuel combustion. Following the large reductions in domestic coal use the main source is diesel vehicles. Black smoke was measured for many years by its blackening effect on filters, but this has largely been superseded by measurements of black carbon (see above), using automated techniques.

Terms beginning with C

Index

Carbon Monoxide (CO)

A colorless, odorless gas resulting from the incomplete combustion of hydrocarbon fuels. CO interferes with the blood's ability to carry oxygen to the body's tissues and results in adverse health effects.

Climate Change

An increase in the temperature of the Earth's troposphere. The Earth's climate has changed in the past as a result of natural influences, but the term is most often used to refer to the 'anthropogenic' (i.e. caused by humans) warming, occurring as a result of increased emissions of greenhouse gases.

COMEAP

Committee on the Medical Effects of Air Pollutants. COMEAP is an Advisory Committee of independent experts that provides advice to Government Departments and Agencies on all matters concerning the potential toxicity and effects upon health of air pollutants.

Terms beginning with D

Index

Data Capture

'Data capture' is the percentage of all the possible measurements for a given period that were validly measured.

Days with Exceedences

The number of days in which at least one period has a concentration greater than, or equal to, the relevant air quality standard (the averaging period will be that defined by that standard). Since the national air quality standards cover different time periods (15 min average, 24 hour running mean etc.) this gives a useful way of comparing data for different pollutants.

Diffusion Tube Samplers

Diffusion tube samplers are small, single-use samplers that collect nitrogen dioxide and other pollutants by molecular diffusion along an inert tube to an efficient chemical absorbent. After exposure for a known time, the absorbent material is chemically analysed and the concentration calculated.

Dispersion model

A dispersion model is a means of calculating air pollution concentrations given information about the pollutant emissions and the nature of the atmosphere. In the action of operating a factory, driving a car, or heating a house, a variety of pollutants are released into the atmosphere. The amount of pollutant which is released can be determined from a knowledge of the process or actual measurements. Air quality objectives are set in terms of concentration values, not emission rates. In order to assess whether an emission is likely to result in an exceedence of a prescribed objective it is therefore necessary to know the ground level concentrations which may arise at distances from the source. This is the purpose of a dispersion model.

Terms beginning with E

Index

EMEP (Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe)

The EMEP programme consists on three main elements: (1) collection of emission data, (2) measurements of air and precipitation quality and (3) modelling of atmospheric transport and deposition of air pollution. EMEP regularly reports on emissions, concentrations and/or depositions of air pollutants, the quantity and significance of transboundary fluxes and related exceedences to critical loads and threshold levels. The EMEP programme is carried out in collaboration with a broad network of scientists and national experts that contribute to the systematic collection, analysis and reporting of emission data, measurement data and integrated assessment results.

Emission Factor

The relationship between the amount of pollution produced and the amount of raw material processed or burned. For mobile sources (such as cars), the emission factor is the amount of pollution produced and the distance travelled by the vehicle.

Emission Inventories

Emission inventories are estimates of the amount and the type of pollutants that are emitted to the air each year from all sources. There are many sources of air pollution, including traffic, household heating, agriculture and industrial processes.

EPAQS

The Expert Panel on Air Quality Standards was set up in 1991 to provide independent advice on air quality issues, in particular the levels of pollution at which no or minimal health effects are likely to occur. Members of the Panel are primarily drawn from those eminent in the fields of health research, practice and teaching. The Panel's recommendations were adopted as the benchmark standards in the National Air Quality Strategy.

Episode (Air Pollution Episode)

An 'air pollution episode' means a period of time (usually a day or several days) when air pollution is high (air quality is poor).

EU Directives

The European Union has been legislating to control emissions of air pollutants and to establish air quality objectives since the early 1970s. European Directives on ambient air quality require EU member states to undertake air quality assessment, and to report the findings to the European Commission on an annual basis. Historically this was under the Air Quality Framework Directive (1996/62/EC) and the Daughter Directives (DD) (1st DD -1999/30/EC, 2nd DD -2000/69/EC, 3rd DD 2002/3/EC and 4th DD- 2004/107/EC). In June 2008, a new Directive came into force: the Council Directive on ambient air quality and cleaner air for Europe (2008/50/EC), known as the "Air Quality Directive". This Directive consolidates the first three Daughter Directives. The 4th Daughter Directive remains in force. Although the UK is no longer a Member State of the EU, the provisions of the Air Quality Directive and 4th Daughter Directive were transposed into the Air Quality Standards Regulations of England, Scotland, Wales and Northern Ireland in June 2010. The provisions of these Directives therefore remain part of UK legislation even though the UK has left the EU.

Exceedence

A period of time where the concentration of a pollutant is greater than, or equal to, the appropriate air quality criteria. For air quality standards an exceedance is a concentration greater than the standard value. For air quality bands an exceedance is a concentration greater than, or equal to, the upper band threshold.

Terms beginning with F

Index

FDMS

This stands for 'Filter Dynamic Measurement System' and refers to a type of instrument for monitoring concentrations of particulate matter. The FDMS is a modified form of TEOM.

Fidas^TM

A type of instrument which uses an optical technique for monitoring concentrations of particulate matter.

Fourth Daughter Directive

The European Union's Directive 2004/107/EC, which covers the four metallic elements cadmium, arsenic, nickel and mercury together with polycyclic aromatic hydrocarbons (PAH). (Its name comes from its origin as one of four so-called Daughter Directives set up under an overarching 'framework Directive'.) Although the UK is no longer a Member State of the EU, the provisions of the Air Quality Directive and 4th Daughter Directive were transposed into the Air Quality Standards Regulations of England, Scotland, Wales and Northern Ireland in June 2010. The provisions of these Directives therefore remain part of UK legislation even though the UK has left the EU.

Terms beginning with G

Index

Global Warming

An increase in the temperature of the Earth's troposphere. Global warming has occurred in the past as a result of natural influences, but the term is most often used to refer to the warming predicted by computer models to occur as a result of increased emissions of greenhouse gases.

Gravimetric Equivalent PM₁₀ or PM_2.5 Data

Measurements of PM₁₀ or PM_2.5 carried out using methods that have demonstrated equivalence to the gravimetric method. (The gravimetric method is the reference technique for particulate matter, but as it cannot produce near-real time data, most PM₁₀ and PM_2.5 monitoring for compliance purposes is carried out using automatic methods that have demonstrated equivalence to the gravimetric method.)

Gravimetric Sampler

A type of instrument used to measure ambient concentrations of particulate matter. It draws a measued volume of air through a filter, on which the particulate matter is collected. The filter is subsequently weighed, and the ambient concentration of particulate matter calculated. However, because the filters must be weighed after exposure, this method cannot produce real time measurements. Therefore most monitoring in the UK is carried out using automatic methods that have been demonstrated to give equivalent results to the gravimetric method.

Greenhouse Gases

Atmospheric gases such as carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, ozone, and water vapour that slow the passage of re-radiated heat through the Earth's atmosphere.

Terms beginning with H

Index

Hydrocarbons

Compounds containing various combinations of hydrogen and carbon atoms. They may be emitted into the air by natural sources (e.g., trees) and as a result of fossil and vegetative fuel combustion, fuel volatilization, and solvent use. Hydrocarbons are a major contributor to smog.

Terms beginning with L

Index

Limit Value

The Air Quality Standards Regulations set 'limit values' for ambient concentrations of pollutants. Limit values (in this context) are legally binding and must not be exceeded.

Local Air Quality Action Plan

Where a local authority has set up an AQMA, it must produce an action plan setting out the measures it intends to take in pursuit of the air quality objectives in the designated area. The plan should be in place, wherever possible, within 12-18 months of designation and should include a timetable for implementation.

Local Air Quality Management (LAQM)

LAQM requires local authorities periodically to review and assess the current and future quality of air in their areas. A local authority must designate an air quality management area (AQMA) if any of the objectives set out in regulation is not likely to be met in its area in the relevant period.

Long-Term Objectives

As well as limit values and target values, the Air Quality Directive sets 'long-term objectives' for ozone concentration. These are similar to limit values but are not legally mandatory. Member States must take all necessary measures not entailing disproportionate costs to meet the target values and long-term objectives.

Terms beginning with M

Index

Maximum hourly average

The highest hourly reading of air pollution obtained during the time period under study.

Member States

Countries that are part of the European Union.

Microgramme per cubic metre (µg/m³ or µg m^-3)

Unit often used to express concentration of a pollutant in air. 1 µg = 1 millionth of a gramme or 1 x 10^-6g.

Micrometre (µm)

Unit of length often used for the size of particulate pollutants. 1 µm = 1 millionth of a metre (1 x 10^-6m) or one thousandth of a millimetre.

Milligramme per cubic metre (mg/m³ or mg m^-3)

Unit often used to express concentration of carbon monoxide in air. 1 mg = 1 thousandth of a gramme or 1 x 10^-3g.

Terms beginning with N

Index

Nanogramme per cubic metre (ng/m³ or ng m^-3)

Unit often used to express concentrations of pollutants such as metallic elements and PAH, which are usually found at low concentrations in air. 1 ng = 1 billionth of a gramme or 1 x 10-9g.

National Atmospheric Emissions Inventory (NAEI).

The NAEI compiles estimates of emissions to the atmosphere from UK sources such as cars, trucks, power stations and industrial plant. These emissions are estimated to help to find ways of reducing the impact of human activities on the environment and our health. The NAEI is funded by Defra, the Scottish Government, the Welsh Government and the Department for Agriculture, Environment and Rural Affairs in Northern Ireland.

National Statistics

Emissions and concentration statistics shown in the air quality database are National Statistics. National Statistics are produced to high professional standards set out in the National Statistics Code of Practice. They undergo regular quality assurance reviews to ensure that they meet customer needs. They are produced free from any political interference.

Terms beginning with O

Index

Oxides of Nitrogen (NO_x)

Combustion processes emit a mixture of oxides of nitrogen, primarily nitric oxide (NO) and nitrogen dioxide (NO₂), collectively termed NOX. Nitrogen dioxide has a variety of environmental and health impacts. It is a respiratory irritant which may exacerbate asthma and possibly increase susceptibility to infections. In the presence of sunlight, it reacts with hydrocarbons to produce photochemical pollutants such as ozone. Nitrogen dioxide emissions can also be further oxidised in air to acid gases, which contribute to the production of acid rain.

Ozone (O₃)

A pollutant gas which is not emitted directly from any source in significant quantities, but is produced by reactions between other pollutants in the presence of sunlight. (This is what is known as a 'secondary pollutant'.) Ozone concentrations are greatest in the summer. O3†can travel long distances and reach high concentrations far away from the original pollutant sources. Ozone is an irritant to the airways of the lungs, throat and eyes: it can also harm vegetation.

Terms beginning with P

Index

Particulate Matter (PM)

Small airborne particles. PM may contain many different materials such as soot, wind-blown dust or secondary components, which are formed within the atmosphere as a result of chemical reactions. Some PM is natural and some is man-made. Particulate matter can be harmful to human health when inhaled, and research shows a range of health effects associated with PM. In general, the smaller the particle the deeper it can be inhaled into the lung.

Percentile

A value that is the rank at a particular point in a collection of data. For instance, a 98^th percentile of values for a year is the value that 98% of all the data in the year fall below, or equal.

PM₁₀

Particles which pass through a size-selective inlet with a 50 % efficiency cut-off at 10 µm aerodynamic diameter, as defined in ISO 7708:1995, Clause 6. This size fraction is important in the context of human health, as these particles are small enough to be inhaled into the airways of the lung - described as the 'thoracic convention' in the above ISO standard. PM₁₀ is often described as 'particles of less than 10 micrometres in diameter' though this is not strictly correct.

PM_2.5

Particles which pass through a size-selective inlet with a 50 % efficiency cut-off at 2.5 µm aerodynamic diameter, as defined in ISO 7708:1995, Clause 7.1. This size fraction is important in the context of human health, as these particles are small enough to be inhaled very deep into the lung and described as the 'high risk respirable convention' in the above ISO standard. PM2.5 is often described as 'particles of less than 2.5 micrometres in diameter' though this is not strictly correct.

Polycyclic Aromatic Hydrocarbons (PAH)

PAHs are a large group of chemical compounds that are toxic and carcinogenic. Once formed, they can remain in the environment for a long time, and can be passed up the food chain. The main sources are domestic coal and wood burning, outdoor fires, and some industrial processes. The pollutant benzo[a]pyrene is a PAH, and because it is one of the more toxic PAH compounds it is measured as a 'marker' for this group of pollutants.

POPS

Persistent Organic Pollutants (POPs) are chemical substances that persist in the environment, bioaccumulate through the food web, and pose a risk of causing adverse effects to human health and the environment. These include dioxins and furans (see TOMPS)

ppb

Parts per billion. The concentration of a pollutant in air in terms of volume ratio. A concentration of 1 ppb means that for every billion (10⁹) units of air, there is one unit of pollutant present.

ppm

Parts per million. The concentration of a pollutant in air in terms of volume ratio. A concentration of 1 ppm means that for every million (10⁶) units of air, there is one unit of pollutant present.

Terms beginning with R

Index

Running mean

This is a mean - or series of means - calculated for overlapping time periods, and is used in the calculation of several of the national air quality standards. For instance, an 8 hour running mean is calculated every hour, and averages the values for eight hours. The period of averaging is stepped forward by one hour for each value, so running mean values are given for the periods 00:00 - 07:59, 01:00 - 08:59 etc. By contrast a non-overlapping mean is calculated for consecutive time periods, giving values for the periods 00:00 - 07:59, 08:00 - 15:59 and so on. There are, therefore, 24 possible 8-hour means in a day (calculated from hourly data) and 3 non-overlapping means.

Terms beginning with S

Index

Secondary pollutant

A pollutant which is formed by chemical reactions from other pollutants in the atmosphere. Ozone, for example, is a secondary pollutant.

Sulphur Dioxide (SO₂)

An acid gas formed when fuels containing sulphur impurities are burned. SO₂ irritates the airways of the lung and is also harmful to the environment, contributing to acid precipitation.

Terms beginning with T

Index

Target Value

As well as limit values, the Air Quality Standards Regulations (2010) set target values for some pollutants. These are similar to limit values, but while the UK must take all necessary measures not entailing disproportionate costs to meet the target values by the specified date, achievement is not legally mandatory.

TEOM

The tapered element oscillating microbalance (TEOM) was widely used to continuously measure particulate concentrations during the 1990s and into recent decades, but is less widely used now. It measures the mass collected on an exchangeable filter cartridge by monitoring the corresponding frequency changes of a tapered element. The sample flow passes through the filter, where particulate matter collects, and then continues through the hollow tapered element on its way to an electronic flow control system and vacuum pump. A principal concern with the TEOM instrument is that the filter is held at an elevated temperature (50°C) in order to minimise errors associated with the evaporation and condensation of water vapour. This can lead to the loss of the more volatile species and has led to the identification of differences between TEOM and gravimetric measurements at co-located sites. A factor of 1.3 was therefore applied to all TEOM-measured concentrations to estimate the gravimetric equivalent. In more recent years, the Volatile Correction Method (VCM) uses measurements of volatile particulate matter made by nearby FDMS instruments to correct TEOM measurements for the loss of such volatile material. Measurements corrected in this way were demonstrated to be equivalent to the gravimetric reference equivalent: however, as the FDMS has also become less widely used, the areas of the UK in which the VCM can be used have reduced.

TOMPs

This stands for 'Toxic Organic Micropollutants'. These are compounds that are present in the environment at very low concentrations, but are highly toxic and persistent. They include dioxins and dibenzofurans.

Terms beginning with V

Index

Volatile organic compounds (VOCs)

Carbon-containing compounds that evaporate into the air. Examples include the solvents used in paints. VOCs are involved in the formation of other pollutants, such as ozone.

Terms beginning with Z

Index

Zones and Agglomerations

The UK has been divided into 43 zones and agglomerations for the purposes of air pollution monitoring, in accordance with the Air Quality Standards Regulations (2010). There are 28 agglomerations in the UK: an agglomeration is defined as any urban area with a population greater than 250,000. The rest of the UK outside of the agglomerations is divided into 16 zones, whose boundaries were were originally set up to match: 1. The boundaries of England's Government Offices for the Regions 2. The boundaries agreed by the Devolved Administrations for Scotland, Wales and Northern Ireland.