Passive Diffusion Tubes
Diffusion tubes are a relatively cheap, convenient and pollutant specific method of monitoring many different types of air pollution. The tubes are small, and can be located exactly where we need to monitor. They are useful for providing longer term measurements and observing trends in pollution concentrations.
Tubes can be placed on the outside of buildings, or suitably located street furniture (road signs, or lamp posts) around the Borough. It is not possible to measure inside as there are too many internal sources of nitrogen dioxide (gas combustion, cigarette smoke etc).
The convenience of diffusion tubes comes with two main disadvantages. The main disadvantage is accuracy (the results are considered to be +/- 20% accurate), and of course we only get one number representing a whole month, meaning it is not possible to examine daily or weekly fluctuations.
We currently monitor nitrogen dioxide and (at a small number of sites) benzene diffusion tubes.
Diffusion tubes passively absorb the pollutant to which they are exposed in a given place over a given time. There are no pumps or complicated technologies involved; tubes are exposed, normally for a month, resealed and sent to a laboratory. The laboratory then completes the analysis of the tubes using nationally agreed procedures.
The monthly results are then adjusted at the end of the year to take account of laboratory and other inaccuracies, and a single annual mean figure is produced. This figure is used for the purposes of Review and Assessment and comparison with the Air Quality Objective.