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Monitoring air pollution

Air monitoring methodologies can be divided into five main types, covering a wide range of costs and performance levels. Different measurement techniques may be appropriate in different situations. It is also important to choose the most appropriate monitoring location for investigating a specific air pollution source or problem.

The table below describes the five main types of air quality monitoring method, and gives examples of where and how they are used in Northern Ireland’s air quality monitoring networks. This information is taken from Appendix A of Defra’s Local Air Quality Management Technical Guidance LAQM.TG(09) (PDF).

Advantages and Disadvantages of Monitoring Methods

Please click the method links to view detailed discription of the method.

Method Advantages Disadvantages
Passive Sampling Low cost - simple. Useful for screening and base-line studies and in support of automatic monitoring for Detailed Assessments.

Indicative measurements only – inferior precision and accuracy to automatic methods. Laboratory analysis required. In general, only provide weekly or longer averages.

Photochemical and optical sensor systems Can be used portably. Sensitivity low.  May only provide spot measurements.
Active (semi-automatic sampling) Low cost - relatively easy to operate (although care must be taken with filter handling and conditioning).
Usually only provide daily (or longer) averages. Some methods are labour intensive. Filter conditioning may be required. Laboratory analysis may be required.
Automatic point monitoring Provide high resolution data. On-line data collection possible.
Relatively expensive. Trained operator required.
Regular calibration required. Regular service and maintenance costs.
Remote optical/long-path monitoring Provide path or range-resolved data. Useful near sources. Multi-component measurements possible. Relatively expensive. Trained operator required. Regular calibration required. Data not readily comparable with point measurements.

Since monitoring instrumentation covers a wide range in capital and running costs, it is usually advisable to choose the simplest method available to meet the specified monitoring objectives. Many baseline monitoring, spatial screening and indicative surveys can be served perfectly well by inexpensive active or passive sampling methods. Only proven and generally accepted measurement methods should be considered.

Monitoring Methods

Passive sampling

Passive sampling methods

This is the simplest and least expensive method of air quality monitoring. The best-known example is the nitrogen dioxide diffusion tube, used by many District Councils in Northern Ireland. These small samplers are fixed to street furniture such as lamp posts etc., and collect a sample over a period of typically several weeks. They are therefore useful for assessment against annual mean objectives, and for identifying 'hot spots' of high pollutant concentration, such as alongside major roads. They are less useful for monitoring where greater accuracy is required, or where there may be short-term peaks in pollutant concentration (such as around industrial sources). The tubes must be analysed by laboratories that can offer suitable quality assurance and quality control measures to ensure the results meet the necessary data quality objectives.
Diffusion tubes are available for the following pollutants in the Air Quality Strategy:

  • Nitrogen dioxide
  • Benzene (so-called BTEX diffusion tubes are available which measure a suite of four hydrocarbons: (benzene, toluene, ethylbenzene and xylene)
  • 1,3-butadiene (though monitoring of this pollutant is rarely needed).

Diffusion tubes are also available for sulphur dioxide, but are rarely used by District Councils. This is because the limit values and objectives for SO2 are based on short periods, such as the hourly and daily mean, while diffusion tubes (when used outdoors) require longer exposure.

Photochemical and optical sensor systems

Photochemical and optical sensor systems

A range of relatively low-cost automatic analysers has been developed specifically for portable and personal exposure monitoring applications. These are battery or mains operated electrochemical or solid-state sensor based systems which can continuously monitor a range of pollutants (carbon monoxide, nitrogen dioxide, sulphur dioxide) with a time average of 15-minutes or less.

These sensors are of low sensitivity and mostly suitable for identifying hotspots at roadsides and near point sources etc. Portable sensors using the light scattering principle are available for PM 10 monitoring.

None of the monitoring sites featured on this website currently use techniques of this type.

Active sampling systems

Active (semi-automatic) sampling

These methods collect pollutant samples either by physical or chemical means for subsequent analysis in a laboratory. They include the gravimetric method for PM 10 and PM2.5 monitoring; a known volume of air is pumped through a filter for a known period of time, and the mass of particulate matter collected on the filter is later weighed accurately in the laboratory, so that the ambient concentration of PM10 or PM2.5 can be calculated. The material collected can also be analysed for metals or other pollutants. Active sampling techniques are used in Urban and Industrial Metals Network, and the PAH Network, both of which have sites in Northern Ireland.  Most samplers of this type are able to hold several days’ or weeks’ worth of filters, and to change them automatically at a set time. 

Automatic point monitoring systems

Automatic point monitoring

These are the most sophisticated (and usually the most expensive) air quality monitoring systems. Automatic analysers draw in ambient (outdoor) air, and measure the concentration of the pollutant in the sampled air. Automatic monitoring techniques are used in Northern Ireland for oxides of nitrogen, sulphur dioxide, carbon monoxide, ozone and particulate matter (PM 10 and PM2.5). They are also used elsewhere for other pollutants such as hydrocarbons. Automatic air quality monitoring systems produce high-resolution measurements (typically hourly or shorter period averages), which can be communicated to the public in near-real time, using telemetry. In order to ensure that the data produced are accurate and reliable, a high standard of operation is required, including regular maintenance, calibration, and detailed quality assurance/quality control procedures.

The automatic monitoring sites in Northern Ireland are shown on the map on the home page of this website; to see current pollutant levels and details of the site, click on the site’s marker.

Remote optical/long-path monitoring

Remote optical/long-path monitoring

These are instruments that use a long-path spectroscopic technique to make real-time measurements of the concentration of a range of pollutants integrated along a path between a light source and a detector. Instruments using the Differential Optical Absorption Spectroscopy (DOAS) system can be used to monitor data for nitrogen dioxide, sulphur dioxide and benzene. In order to ensure that the data produced are accurate and reliable, a high standard of maintenance, calibration, operational and quality assurance and quality control procedures is required.

None of the monitoring sites featured on this website currently use techniques of this type.

Monitoring Locations

Monitoring sites are classified according to the type of environment in which they are located, and the types of pollution sources which are likely to influence them.

The classifications used for the automatic monitoring sites in Northern Ireland are described in the table below. These are based on those in the 2009 Technical Guidance for Local Air Quality Management LAQM.TG(09) (PDF).  (A different set of monitoring site classifications is used throughout the European Union to describe sites used for monitoring compliance with EU air quality legislation.)

Type Description
Urban Centre

An urban location representative of typical population exposure in towns or city centres e.g. 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.

A site sampling between one to five metres of the kerb of a busy road (distance can be up to 10m from the kerb).

Kerbside A site sampling within 1 m of the kerb of a busy road.
Industrial An area where industrial sources make an important contribution to the total pollution burden. 

An open countryside location, in an area of low population 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 or tunnels.