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A diffusion tube for measuring NO2

Measuring nitrogen dioxide using diffusion tubes

Many local councils use diffusion tubes to measure nitrogen dioxide (NO2) as they are relatively cheap, easy to use, don’t require any power supply and can give a good indication of air pollution levels. The low cost allows many of them to be used, so a picture can be built up across a large area. Non-professionals such as environmental campaigners and schools are also using them because the tubes require only limited attention and are easy to operate.

The tubes are small and made of plastic with a cap at each end. Inside the top end of the tube is a metal disc coated with a substance that attracts nitrogen dioxide. The tube is placed vertically in a holder, usually attached to something like a lamppost or drainpipe, and the bottom cap is removed allowing air to travel into the tube. The diffusion tube is left at the site with the bottom cap removed for a month. After this time the tube has the cap replaced and is taken to a laboratory for analysis. Each tube then provides an average concentration of nitrogen dioxide for that month. The monthly results are then averaged over 12 months to give an annual mean.

In some areas, local communities are involved in positioning and changing the diffusion tubes every month. This gives local residents air quality data right where they live.

The Government sets out objectives for levels of NO2 in the Air Quality Strategy. The objective that is relevant to diffusion tubes is ‘An annual mean of 40 microgrammes per cubic metre’. The annual averages for Sheffield’s tubes, where available, are given on the ‘Diffusion Tube Data’ page.