Diffusion tube

[1] Diffusion tubes are widely used by local authorities for monitoring air quality in urban areas,[2] in citizen science pollution-monitoring projects carried out by community groups and schools,[3][4] and in indoor environments such as mines[5] and museums.

One of the caps (coloured white) is either completely removed to activate the tube (in the case of nitrogen dioxide sampling) or contains a filter allowing in just the gas being studied.

The other cap (a different colour) contains metal mesh discs coated with a chemical reagent that absorbs the gas being studied as it enters the tube.

Diffusion tubes are reasonably accurate, relatively cheap, easy to use, extremely compact, passive (they need no power source), and have a fairly long shelf life; with careful positioning, they can be deployed more or less anywhere, indoors or outdoors.

It's also common for local authorities to mount a number of tubes in different places over the same time period so pollution hotspots in towns and cities can be identified.

A typical diffusion tube attached, by a cable tie, to a drainpipe. The bottom end is open to the atmosphere. Pollution is captured by a chemical inside the red cap at the top.