Diffusion tubes used for monitoring air pollution

What are they how?

They are “diffusion tubes” used for monitoring air pollution.

Why Are They Used?

Air quality is an important environmental issue that affects our health, well-being, and climate. One of the main pollutants that we need to monitor is nitrogen dioxide (NO2), which is emitted from vehicles, power plants, and other combustion sources. NO2 can cause respiratory problems, aggravate asthma, and contribute to acid rain and smog.

But how can we measure the levels of NO2 in the air? One of the simplest and cheapest methods is using diffusion tubes. In this blog post, we will explain what diffusion tubes are, how they work, how they are analysed, and how the results are used.

What is a diffusion tube?

A diffusion tube is a small plastic tube that contains a chemical reagent that reacts with NO2. The tube has a cap at one end and a mesh at the other end. The mesh allows air to enter the tube, but prevents insects and dust from entering. The tube is exposed to the air for a prescribed period of time (usually a month) and then sent to a laboratory for analysis.

How does a diffusion tube work?

The principle behind a diffusion tube is simple: the NO2 molecules in the air diffuse into the tube and react with the reagent, forming a compound that can be measured. The amount of NO2 that enters the tube depends on the concentration of NO2 in the air, the temperature, the humidity, and the length of exposure. The diffusion rate of NO2 is constant under standard conditions, so by knowing the exposure time and the volume of the tube, we can calculate the average NO2concentration over the exposure period.

How are diffusion tubes analysed?

The most common reagent used in diffusion tubes is triethanolamine (TEA), which reacts with NO2 to form nitrite ions. The nitrite ions can be measured by a colorimetric method, which involves adding a dye that changes colour in the presence of nitrite. The intensity of the colour is proportional to the amount of nitrite, and hence to the amount of NO2. The colour can be measured by a spectrophotometer, which gives a numerical value of the absorbance. The absorbance can be converted to the mass of NO2 using a calibration curve, and then divided by the exposure time and the volume of the tube to get the average NO2 concentration in micrograms per cubic metre (µg/m3).

Given that the process is not a “reference” method of measurement, diffusion tube results may also have to be bias corrected utilising other local or national co-location studies.

How are diffusion tube results used?

Diffusion tube results are used to monitor the spatial distribution and temporal trends of NO2 in the air. They can help identify areas of high NO2 concentration, such as near busy roads or industrial sites, and assess the compliance with the air quality standards and objectives set by the government. They can also help evaluate the effectiveness of air quality management plans and policies, such as traffic restrictions, low emission zones, or public transport schemes.

For prospective development sites that may be exposed to elevated levels of pollution, diffusion tubes allow a site specific statement of baseline pollutant concentrations to be obtained – albeit that several months of monitoring are normally required to obtain a reasonable estimate  of long term (e.g. annual mean) concentrations.

Diffusion tubes are not the only method of measuring NO2, but they are a useful and cost-effective tool for indicative monitoring of ambient air quality. They are widely used by local authorities, environmental agencies, researchers, and citizens to collect data and raise awareness about air pollution and also for the determination of baseline levels of pollution affecting prospective new development.

By using diffusion tubes, we can better understand the state of our air, take actions to improve it and help minimise the potential impacts of poor air quality on new development.