Air Quality Monitoring

Air Pollution

Use of nitrogen dioxide passive diffusion tubes

Passive diffusion tubes are not as accurate as real time monitoring, however they do give a good indication of what is happening with air quality.

West Wiltshire District currently has 11 sites within the district to monitor for nitrogen dioxide. Five of these sites are outside Air Quality Management Areas (AQMAs) and six are within an AQMA. View map of location sites (PDF file, opens in new window).

A nitrogen dioxide passive diffusion tube is a clear plastic tube open at one end and at the closed end a mesh is impregnated with a pollutant absorbing chemical. The diffusion tube collects the pollutant during the exposure period and then is resealed and returned to a laboratory for analysis. Each tube is exposed for a month period. The laboratory then assesses the quantity of the pollutant absorbed by calculating the average ambient NO2 concentration over the exposure period.

Each tube is mounted on a lamp-post or similar structure ensuring that the open end is at the bottom to prevent rainwater collection.

P

hoto of diffusion tube

West Wiltshire District Council utilises diffusion tubes prepared and analysed by Bristol City Scientific Services. The laboratory participate in the Workplace Analysis Scheme for Proficiency (WASP) for nitrogen dioxide tubes.

They analyse a solution supplied by Netcen as part of the QA/QC scheme that they run.

The laboratory also participate in a field inter-comparison scheme which is controlled by Netcen and organised by the Health and Safety Laboratory. Three tubes are co-located with a continuous analyser which provides a reference value.

The tubes are prepared by pipetting 30l of solution of 20% triethanolamine in water onto the metal grids in the end of the cap, then assembling the tube components. A fresh batch of tubes is prepared each month ready to dispatch in time for the required exposure date.

Laboratory blanks are retained so that at least one is run alongside each batch of samples. Travel blanks are supplied three-monthly as required by the UK survey procedure.

Please note that due to the creation of Wiltshire Council on the 1st April 2009 the information on this page may not be up to date. Please contact the Environmental Protection Team at Bradley Road, Trowbridge on 01225 776655 regarding any queries you may have.