Air Quality Monitoring

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The monitoring techniques Wiltshire Council employs to monitor nitrogen dioxide and small particulates are in accordance with statutory guidance produced by DEFRA: Local Air Quality Management Technical Guidance LAQM.TG(09)

The guidance details the monitoring techniques that should be employed, where to site monitoring, the duration and quality assurance and quality control.

Monitoring for Nitrogen Dioxide (NO₂)

Diffusion Tubes

The measurement of nitrogen dioxide is initially undertaken using a diffusion tube. These are simple acrylic tubes, closed at one end that contains a chemical that can absorb nitrogen dioxide from the atmosphere.

They are widely used for indicative monitoring and are particularly useful for longer term monitoring usually for a minimum period of a year. The tubes are relatively cheap and for this reason can be used at a large number of locations for a period of many years if necessary. The tubes are used to give an indication of the levels of nitrogen dioxide and to help determine if other more accurate forms of measurement are required.

Tubes are generally mounted on lampposts or downpipes on buildings at a height of around three metres, and exposed to the air for a period of one month before being swapped with a replacement tube.

The tubes are then analysed at an accredited laboratory and the amount of nitrogen dioxide they have been exposed to over the month is measured. The results of the analysis are then provided on a monthly basis. At the end of the year the twelve monthly averages are added together and divided by twelve to produce a mean annual average.

Co-location studies and adjustments to diffusion tubes

Diffusion tubes have an overall uncertainty of about +/- 20% so an appropriate ‘bias adjustment ‘factor is applied to the annual mean. Bias represents the overall tendency of the diffusion tubes to depart from the true value, i.e. to over read relative to the automatic analyser results. The bias adjustment factor may be determined from a local study that has co-located diffusion tubes with an automatic analyser, or from the national database of co-location studies.

When an automatic monitoring station is sited, three diffusion tubes are also placed in the immediate vicinity. This is known as a co-location study and allows the readings from the diffusion tubes to be compared with the more accurate automatic monitoring results. This in turn allows an adjustment factor to be calculated to standardise the tube results at the end of the year.

These bias adjustments are calculated using the combined information from a range of monitoring sites across the country in accordance with the guidance issued by Defra. The adjustment factor is calculated once a year and issued to local authorities to allow them to adjust all tube results for the previous year.

Automatic Monitoring Stations (nitrogen dioxide and small particulates)

Nitrogen dioxide

If the second stage assessment and use of diffusion tubes has indicated that further monitoring is necessary an automatic monitoring station is usually located at or as near as possible to the area of concern.

These automatic monitoring stations (AMS) are much more accurate than the diffusion tubes and measure the nitrogen dioxide levels on an hourly average basis. This monitoring continues over one or more years and allows the 200 µg/m³ hourly target to be assessed as well as the annual mean average to be accurately measured.

The monitoring stations require regular site visits to carry out equipment checks, manual calibrations and routine maintenance such as changing filters. These checks are all part of the quality assurance and control programme to ensure data is accurate and valid.

Only if acceptable data quality and high capture rates are achieved can the performance of the analyser be regarded as fully satisfactory. (A data capture of 90% for ratified (i.e. usable) data is recommended as a target for automatic monitoring)

The hourly data is downloaded from the machine onto a computer so that it can be converted from raw values to more useful pollutant concentrations. Once a calibration factor has been applied, the data is screened by visual examination for any unusual measurements (possibly due to equipment failure, power failures, human error). The data is then ratified to ensure the data has been scaled correctly amongst other factors.

Once the data has been through this process the hourly readings are added up over a month period then divided by the number of hours in that month to provide a monthly average. At the end of twelve months the twelve monthly averages are used to calculate the mean annual average.

Particulates (PM₁₀)

Wiltshire Council currently measures particulates in three locations using fixed monitoring locations similar to the AMS above. The particulate monitors have to be located adjacent to the location being assessed and produce data which is downloaded and used to calculate the annual and 24 hour means listed in the air quality standards.

Last updated: 9 January 2012