Diffusion tubes are a cheap and easy way to measure nitrogen dioxide. They are supplied and analysed by laboratories. Councils use them to get an indication of air pollution levels across their borough by putting tubes in lots of different places. Residents and schools also use diffusion tubes for citizen science projects to measure their local air quality.
Diffusion tubes are small plastic tubes with a cap at each end one of which is coloured. Under the coloured cap is a steel mesh disc which is coated with triethanolamine (TEA) a chemical that absorbs nitrogen dioxide. When gases pass over this mesh the chemical changes. This chemical change tells us how much nitrogen dioxide was in the air during the monitoring period.
Tubes are attached in a vertical position with the coloured cap at the top to a stationary object such as a lamppost, road sign, railings or a drainpipe. The bottom white cap is removed so that the air can get into the tube (known as diffusion). Nitrogen dioxide in the air reacts with the chemical on the mesh at the top of the tube and changes into nitrite. The tube must be left in place with the bottom cap off for 2-4 weeks, after which time the bottom cap is replaced and the tube is returned to the laboratory for analysis.
In the laboratory, the steel mesh is removed and washed with distilled water which is then analysed. The concentration of nitrogen dioxide is found by shining ultra violet light (UV) through the water sample. The amount of light absorbed is equivalent to the concentration of nitrogen dioxide that was present in the air during the monitoring period.
Find your nearest diffusion tube on the Air Quality Map.
Citizen science project
A minimum period of three months should always be used. For assessment against the annual mean nitrogen dioxide objective six consecutive months of sampling is recommended; including three months of winter and three months of summer, preferably with monitoring commencing in January or July.
The location of the diffusion tubes should be carefully considered, so that there are not any other sources of pollution, such as extract vents, that will influence the results. In order to assess the effect of air pollution the potential exposure of individuals is an important factor and therefore this should be taken in to account in the choice of monitoring location.
The choice of laboratory is an important factor and it is recommended that an accredited laboratory is used that can demonstrate good performance in a Quality Assurance / Quality Control scheme.
For more information read LAQM Technical Guidance Note TG09