Monitoring air pollution

Monitoring Methods

The following air pollutants are currently monitored in Northern Ireland:

• Oxides of nitrogen (NO_X), comprising nitric oxide (NO) and nitrogen dioxide (NO₂);
• Particles (as PM₁₀, PM_2.5, and black carbon);
• Ozone (O₃);
• Carbon monoxide (CO);
• Sulphur dioxide (SO₂);
• Benzene;
• Polluting elements – including lead, arsenic, cadmium, nickel and mercury;
  
• Polycyclic Aromatic Hydrocarbons (PAHs); and
• Toxic Organic Micropollutants (TOMPs).

A range of monitoring techniques are used, depending on the pollutant being monitored, the reason for the monitoring, and the location where the monitoring is taking place. Below, we provide information on the main types of monitoring used in Northern Ireland.

Automatic Point Monitoring

These are the most sophisticated air quality monitoring systems. Automatic analysers continuously draw in ambient (outdoor) air and measure the concentration of the pollutant in the sampled air.

Automatic air quality monitoring techniques are currently used in Northern Ireland for; oxides of nitrogen, sulphur dioxide, carbon monoxide, ozone, particulate matter (PM₁₀ and PM_2.5) and black carbon. The actual measurement technique is different for each pollutant.  These methods analyse the sample online, producing measurements with a high time resolution (typically, they can produce hourly or shorter period averages). To find out more about the measurement techniques used for each pollutant, please refer to the most recent report in the series “Air Pollution in the UK” which can be found in the UK-AIR reports library.

The data can be communicated to the public in near-real time via telemetry, via websites such as this one.

In order to ensure that the data produced are accurate and reliable, a high standard of operation is required, including comprehensive operator training, regular maintenance, calibration using standard gas mixtures, and detailed quality assurance/quality control procedures. As a result, automatic monitoring is a relatively expensive option. Also, automatic monitoring stations tend to be large, and are not suitable for installation in every location.

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.

Active (Semi-Automatic) Sampling

These methods collect pollutant samples either by physical or chemical means for subsequent analysis in a laboratory. Typically, a known volume of air is pumped through a collector such as a filter for a known period of time, for subsequent laboratory analysis. ‘Sequential’ samplers (which automatically expose a series of filters for set periods of time (e.g. one per day or per week) are used in some cases. Active sampling techniques are used in Northern Ireland for; benzene (pumped-tube samplers), a suite of polluting elements (including lead, arsenic, cadmium and nickel), and PAHs.

Active samplers are relatively low-cost compared to automatic techniques; however, they usually provide lower time resolution (e.g. daily, weekly or two-weekly averages), may be labour-intensive and usually require laboratory analysis.

Sensor-Based Methods

Air quality sensors are the newest of the monitoring techniques described here. They are typically small, low power devices, ranging in price from a few hundred pounds for a single pollutant, to several thousand pounds for a complete multi pollutant system. They are typically battery powered and can be used at fixed locations (e.g. attached to a lamppost) or handheld for mobile pollution studies.

They can provide very highly time resolved data – 1-minute average or less – and because of the lower costs, they can be deployed in larger numbers than conventional automatic analysers. This means they can be usefully deployed to identify and map pollution “hot spots”, and also to investigate how pollution concentrations vary over the course of a day.

This is a rapidly developing area, but at the time of writing none of the systems currently available on the market have undergone any formal type testing process. Depending on the system used, they will require ongoing maintenance and regular tests to ensure reasonable data quality. However, with suitable quality control procedures, sensors of this type can be used very successfully.

Data from sensor-based monitoring systems are not currently published on this website, however an increasing number of local authorities are using these techniques as part of their air quality management.

Passive Sampling Methods (e.g. diffusion tubes)

Several of Northern Ireland’s district councils use diffusion tubes for indicative monitoring of NO₂. These small, low-cost samplers absorb the pollutant directly from the air and need no power supply.  Diffusion tubes of the type used for NO₂ are approximately 7 cm long and can be fixed unobtrusively to lampposts and other street furniture. After a specified exposure period (typically one month) they are sent for laboratory analysis. The quantity of pollutant they have absorbed is used to calculate an average concentration over the sampling period.

Diffusion tubes represent a simple and cost-effective method of monitoring air quality in an area, giving a good general indication of average pollution concentrations. They provide a useful and economical supplement to more expensive automatic monitoring. The low cost per tube permits sampling at a number of points in the area of interest; this is useful in highlighting hot spots of high concentrations, such as alongside major roads. Diffusion tubes are easy to use, and very little operator training is required. However, while they are very useful for assessment against the annual mean objective, their long sampling period makes them less useful for measurement around (for example) industrial pollution sources where greater time resolution is required.

Diffusion tubes are available for several pollutants including ammonia, benzene, toluene, xylene and 1,3-butadiene: however, in Local Air Quality Management their use is largely confined to monitoring NO₂. Several district councils have made their NO₂ diffusion tube data available via this website.

Passive samplers (of a different type) are also used for rural monitoring of ammonia; these data are also available via this website.

Please note: the above information is intended only to provide a short, non-technical description of passive sampling methods and how they work. Anyone planning an air quality monitoring study using diffusion tubes will need more detailed technical guidance on how the samplers should be handled, exposed, analysed and their data reported. Please refer to the following sources for more information:

• The Local Air Quality Management Technical Guidance for UK regions (excluding London), available at https://laqm.defra.gov.uk/technical-guidance/. Chapter 7 deals with air quality monitoring, and specifically sections 7.197 – 7.234 deal with diffusion tubes.
• Defra’s Practical Guidance on using diffusion tubes for Local Air Quality Management: https://laqm.defra.gov.uk/diffusion-tubes/practical-guidance.html

Monitoring Locations

Monitoring stations (or sites) are usually classified according to the type of environment in which they are located. This allows more meaningful evaluation of the data, and allows the data user to compare data from sites of a similar type. The monitoring site classifications used by District Councils for Local Air Quality Management purposes, are described in the table below. (This is taken from the Local Air Quality Management Technical Guidance document, LAQM.TG22).These site descriptions typically reflect the general surroundings (e.g. urban or rural), and/or indicate important sources of pollution (e.g. ‘roadside’ or ‘industrial’).

Monitoring Site Classifications