Automatic monitoring

Measuring tool

Ideally, the measurement of pollutant concentrations outdoors (ambient air) should reflect public exposure but this is not always possible as air quality monitoring stations sample from a single fixed point. It is important that monitoring stations are placed in areas where high pollution is likely such as close to busy main roads and large industrial sources to record the ‘worst case scenario’.

The National Air Quality objectives are expressed in a variety of ways depending on the pollutant being measured; 1 hour, 8 hour, daily and annual averages. To calculate the various averages data is collected by automatic air quality monitoring stations which sample and analyse air 24 hours a day, this is known as ‘continuous monitoring’. Continuous monitoring equipment is both expensive to install and maintain but provide invaluable information. Find your nearest continuous monitoring station on the London Air website.

Each pollutant is measured in a different way:

Types of Particulate
  • Nitrogen dioxide – Nitric oxide in the air sample reacts with ozone generated inside the monitor which emits light (chemiluminescence). The resulting light is measured and the concentration of nitrogen dioxide calculated.
  • Ozone – Ultraviolet light is passed through the air sample. The amount of light absorbed is compared to a reference sample which has no ozone, the ratio between the two readings allows the calculation of the ozone concentration.
  • Particulates – the two main monitors used are Tapered Element Oscillating Microbalance (TEOM) and Beta Attenuation Monitors (BAM), both rely on the direct measurement of particles of a specific size range. Particulate matter is collected and the mass used to calculate concentration.