Using Diffusion Tubes
Many local councils use diffusion tubes to measure nitrogen dioxide 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 them to be used across a large area, so a picture of air pollution can be built up. Non-professionals such as environmental campaigners and schools are also using them, as 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 mesh disc coated with a substance that absorbs nitrogen dioxide. The absorbing substance used for nitrogen dioxide is triethanolamine (TEA). 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. When nitrogen dioxide in the atmosphere diffuses into the sampler it reacts with the TEA and is converted to nitrite. The nitrite remains in the TEA and more nitrogen dioxide diffuses into the sampler. 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. In the laboratory, the metal mesh is removed and washed with water. This water is collected in a small container and has special light shone through it called ultra violet light (UV). The amount of UV light the water absorbs is equivalent to the concentration of nitrogen dioxide measured in the air by the diffusion tube for that month.
In some areas, local communities are involved in positioning and changing the diffusion tubes every month. This allows the local residents access to air pollution measurements right where they live. In Sheffield, the information collected from the diffusion tubes that measure nitrogen dioxide can be found on the Sheffield Airmap website www.sheffieldairmap.org