What is a diffusion tube?
This is a simple and relatively inexpensive device consisting of a plastic tube (about the size of a finger) which can be attached to street furniture (e.g. a street light) allowing a sample of air to be collected (over a recorded exposure period e.g. month) by diffusion into the sampler. The sampler absorbs nitrogen dioxide and can be used to give estimates of the annual average levels. Therefore these results can only be compared to the annual mean objectives for the pollutants. The tubes are sealed and then sent off for quantitative analysis. They are not as accurate and precise as automatic monitors but give a good general indication of average pollutant levels. As passive diffusion tubes have been used since 1993 they also help demonstrate trends in NO2 levels over time.
