Secret Scotland

If it's secret, and in Scotland…

Diffusion tubes – revealed!

It’s some time since I made some posts in various places trying to identify the items shown below:

I had tried to find something similar online, but just couldn’t format any sort of search string that trawled up anything even remotely similar.

Apart from a few suggestions, nobody else came up with anything that more of an educated guess, and the only agreement was that it was related to environmental monitoring.

Now that I have identified the devices, I’m rather disappointed that we didn’t get an answer, as they are pretty common, used around the country by councils, and easy to find… once you know their proper name, at least.

These are passive diffusion tubes, probably being used to measure the level of nitrogen dioxide (NO2) in the surrounding area, although they can be used to sample other substances such as sulphur dioxide, ozone, and benzene. The tubes are often placed on the façades of buildings, and usually near busy roads since road traffic is the main source of NO2.

A nitrogen dioxide passive diffusion tube is a clear plastic tube open at one end, and fitted at the closed end with a mesh is impregnated with a pollutant absorbing chemical. The diffusion tube collects the pollutant during the exposure period, usually one month, and is then sealed and returned to a laboratory for analysis.

Diffusion tube

Diffusion tube parts

The mesh (shown just below the cap at the top of the tube) is soaked in a special fluid called ‘triethanolamine’ or TEA which functions as an emulsifier and surfactant, neutralizes fatty acids, adjusts and buffers the pH, and solubilises oils and other ingredients that are not completely soluble in water.

This passive sampling system needs no pumps or similar to take its sample, and as its name suggests is dependent on diffusion. At its simplest, diffusion caused the air molecules plus whatever gasses they may be mixed with, to be transported to the top of the top as a result of the random motion of the molecules as they bump into one another. Wind and other air movements play no part in this process, although they may (or may not) assist,

NO2 (and other gasses in the air) are then collected by the TEA fluid on the mesh, which is then collected after the exposure period and sent for analysis.

If you’d like a real introduction:

Microsoft Word – NO2_WG_PracticalGuidance_Issue1a.doc – 0802141004_NO2_WG_PracticalGuidance_Issue1a.pdf

The tubes I spotted are just a small contribution to a much larger UK wide survey:

Compilation of Diffusion Tube Collocation Studies … – UK-Air – Defra

The fancy version

Although these diffusion tubes have been around (and puzzling me) for years, Glasgow City Council upped their game a little while ago, and installed something a little more complicate at Parkhead Cross, just a few metres from one of the diffusion tubes pictured above:

Air sampler

Parkhead Cross air sampler

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June 24, 2013 - Posted by | council | , , , , , ,

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