Why we look at air quality

Air Quality It is well documented that air quality can affect human health. Therefore it is important to measure current levels and attempt to reduce public exposure to air pollution, where required. It is important to measure air pollution at a local level for the following reasons:
  • To establish whether the air quality limit values, target values and objectives set by European Union Directives and the UK’s own Air Quality Strategy are met in the area.
  • To provide information to the public, on air quality in their area.
  • So that air quality can be taken into account when making decisions on planning applications, for new developments or roads.
  • To fulfil Local Authorities’ obligations under Part IV of the Environment Act 1995, regarding Local Air Quality Management.
  • To find out whether changes intended to improve air quality are having an effect.
Air pollution levels have been monitored in the UK for over 100 years. In the early years, monitoring was based on simple sampler-based techniques. However, since the 1970s and 1980s, the technology has been available to measure many air pollutants on a real-time basis, and automatic air quality monitoring is now widely used.

What pollutants are monitored and what are their sources?

Nitrogen Dioxide (NO2)

All combustion processes in air produce a mixture of oxides of nitrogen. The most important are nitric oxide (NO) and nitrogen dioxide (NO2). Together, these are referred to as NOX.  Nitric oxide combines with oxygen in the air to produce NO2. It is NO2 that is of more concern as an air pollutant, and limit values and objectives are for NO2. Oxides of nitrogen are emitted from many sources including the electricity supply industry and other industrial and commercial sectors.  However, in urban areas the source which has the biggest effect on air quality is road transport.

Particulate Matter (PM): PM10 and PM2.5

Particulate matter (PM) is a mixture of different materials, from a variety of sources.  It includes “primary” particles – those emitted directly into the atmosphere from a source – and “secondary” particles – those formed by chemical reactions between other pollutants in the air. Primary PM comes from both human-made and natural sources. In the UK the biggest human-made sources are stationary fuel combustion (for example, industrial and domestic fuel burning) and transport. Road vehicles give rise to primary particles from engine emissions, tyre and brake wear and other non-exhaust emissions. Natural sources of primary PM include sea salt and wind-blown dust, in some cases transported over long distances (for example, Saharan dust can affect the UK). Secondary PM is formed from emissions of ammonia, sulphur dioxide and oxides of nitrogen as well as from emissions of organic compounds from both combustion sources and vegetation. Particulate matter is generally categorised on the basis of the size of the particles. The most commonly measured size fractions are PM10 (particles with a mean aerodynamic diameter of diameter of less than 10µm), and PM2.5 (particles with a mean aerodynamic diameter of less than 2.5µm).

Ozone (O3)

Ozone is a secondary pollutant. Although there are some human sources of ozone including water treatment, laser printers and welding the main source of ozone is not considered to be directly as a result of human activity. Ozone is formed naturally by chemical reactions between various air pollutants, mostly NOx and Volatile Organic Compounds (VOCs). The reactions are initiated by strong sunlight, and can take place over several hours or days. Ozone may affect air quality many hundreds of kilometres away from where the original pollutants were emitted. In the UK, ozone is the most common cause of poor air quality in rural areas.

Sulphur Dioxide (SO2)

In the UK, the main source of sulphur dioxide (SO2) is the combustion of fuels containing sulphur – for example the combustion of coal and heavy oils by power stations, refineries and industrial processes. In some parts of the UK (though not Kent and Medway) the use of coal for domestic heating is still a significant source. Ambient concentrations of SO2 have decreased greatly over the past 50 years, and it is now rare for air quality objectives to be exceeded for this pollutant.

Carbon Monoxide (CO)

Carbon monoxide (CO) is formed when fuels containing carbon are burned without enough oxygen to convert all the carbon into carbon dioxide. The largest source of CO is road transport, with residential and industrial combustion also making significant contributions. It is well-known that dangerously high concentrations of CO may occur indoors (for example due to faulty heating appliances); however, outdoor concentrations are much lower.

Where do we monitor?

Rural monitoring site The Kent and Medway Air Quality Monitoring network covers the whole county of Kent. For more information on the locations of the monitoring sites, please see the Latest pollution levels page. The automatic monitoring stations and sampling sites are categorised according to the descriptions in Defra’s 2009 Technical Guidance document for Local Air Quality Management (LAQM.TG(09)). These are as follows:
  • Urban centre: an urban location representative of typical population exposure in towns or city centres. For example pedestrian precincts and shopping areas.
  • Urban background: an urban location distanced from sources and therefore broadly representative of city-wide background conditions, e.g. urban residential areas.
  • Suburban: a location type situated in a residential area on the outskirts of a town or city.
  • Roadside: A site sampling typically within one to five metres of the kerb of a busy road (although distance can be up to 15m from the kerb in some cases).
  • Kerbside: a site sampling within one metre of the kerb of a busy road.
  • Industrial: an area where industrial sources make an important contribution to the total pollution burden.
  • Rural: an open countryside location in an area of low population density distanced as far as possible from roads, populated and industrial areas.
  • Other: any special source-orientated or location category covering monitoring undertaken in relation to specific emission sources such as power stations, car parks airports or tunnels.

How do we monitor?

Continuous Analysers (automatic)

The Kent and Medway Air Quality Monitoring Network uses the same automatic air quality monitoring techniques as those used in the UK’s national air quality monitoring network (the Automatic Urban and Rural Network, or AURN).  The table below shows the methods used.

Monitoring Methods used in Kent and Medway Network

Pollutant Principle of Operation
NO/NO2/NOx Chemiluminescence:  Nitric oxide (NO) molecules react with ozone (O3), to form nitrogen dioxide (NO2). The reaction gives off energy in the form of light, a process called “chemiluminescence”. In the chemiluminescence analyser, a sample of ambient air is drawn into a small evacuated reaction vessel and mixed with ozone (generated within the instrument). The energy emitted from the reaction is used to calculate the concentration of NO in the sampled air. Total NOx is also measured in the same instrument and by the same principle, but in this sample, all the NOx in the sample is first converted to NO before the reaction with O3. The ambient concentration of NO2 is calculated from the difference between the total NOx and NO measurements.
PM10 and PM2.5 The Kent and Medway Air Quality Monitoring Network uses three methods to measure particulate matter:


  • the Beta-Attenuation Monitor (BAM) which measures the attenuation of beta rays passing through a paper filter on which particulate matter from sampled air has been collected.
  • The Tapered Element Oscillating Microbalance (TEOM), which determines particulate concentration by continuously weighing particles deposited on a filter.
  • The Filter Dynamics Measurement System (FDMS)-TEOM, a modified form of the TEOM, which is able to measure both the non-volatile and volatile components of PM. The FDMS modification was developed in response to a known artefact with the unmodified TEOM: its elevated operating temperature (necessary to prevent condensation of moisture inside) can result in the loss of volatile PM.
O3 UV absorption: ozone absorbs ultraviolet (UV) light. The absorption of UV by sampled air is used to calculate the ozone concentration. Sampled air is drawn through a small cell inside the instrument. UV light passes from a lamp on one side of the cell, to a detector on the other. The reduction in intensity is measured, and is used to calculate the concentration of O3 in the sample.  A sample of air from which all the O3 has been removed provides a zero measurement for reference.
SO2 UV fluorescence: SO2 molecules are excited to higher energy states by UV radiation. These excited molecules then release this energy as light (fluorescent radiation). The intensity of this can be used to calculate the concentration of SO2 in sampled air.
CO Infra-Red absorption: CO strongly absorbs infrared (IR) radiation, and this is used as the basis of the measuring instrument. IR passes through sampled air, to an infrared detector which gives a signal proportional to the CO concentration. This measurement is alternated with a “reference” measurement based on a high-concentration pure CO sample.

Passive Sampling (non-automatic)

In addition to the automatic monitoring discussed in the previous sections, most of the Kent and Medway Local Authorities supplement their automatic monitoring of nitrogen dioxide (NO2) with indicative measurements made using diffusion tubes. Diffusion tubes of the type used in this network are small plastic tubes about 7 cm long; one end contains a compound that absorbs the pollutant of concern (in this case nitrogen dioxide) directly from the ambient air. Diffusion tubes are fixed place at the sampling site for a period of typically one month. They are then analysed in a laboratory, and the ambient concentration of NO2 (averaged over their exposure period) is calculated. Diffusion tubes do not need electricity, can be fixed unobtrusively to street furniture or buildings, and are low in price. They are a useful tool for investigating spatial variation in NO2 concentration.