Particle counting and sizing (nanoparticles).

Although legislation for airborne particles is currently based on the total mass of particles below a certain size (ie PM₁₀ and PM_2.5), many other parameters are important for health or scientific studies. One important measurement is the total number of particles in a given volume of air. Unlike mass concentration, which is dominated by particles larger than 1 μm, airborne particle numbers are dominated by the 20 – 200 nanometre size range. These are loosely called either nanoparticles or ultrafine particles. The number of particles within different size bands can also be measured to form size distributions, as in the results below, which show the difference between a site dominated by traffic emissions and a site away from traffic.

Roadside particle size distributions.

Rural particle size distributions.

Part of our work at NPL is to help maintain the network of instruments funded by Defra, and to carry out research into future methods of monitoring particles in the air. Further information is available on the Air Quality Networks page.

Laboratory research focuses on the following techniques:

Condensation Particle Counter
Scanning Mobility Particle Sizer

Condensation Particle Counters (CPC)

This instrument is used to count the total number of particles in the air. It works by swelling particles too small to detect optically using a saturated vapour of butanol or water, so that they can then be counted using a laser and detector assembly. Many instruments of this type are deployed around the world both as stand-alone instruments and as a part of a SMPS (see below).

Scanning Mobility Particle Spectrometers (SMPS)

With this instrument, it is possible to measure the particle size distribution. The instrument is essentially two parts - an electrostatic classifier and a particle detector. The electrostatic classifier is used to separate particles of different sizes by using the fact that particles in the air can be given a known electrical charge distribution. The charged particles are passed between two electrodes which have a high voltage between them. By varying this voltage, particles with different mass to charge ratios pass through to the second part of the system, the detector. The detector used is usually a CPC (see above). The results of particle numbers measured as the voltage is changed leads via complicated data processing to a particle size distribution.

Particle Generators

Particle generators allow us to produce particles of various types and compositions, with a size range from around 10 to 500 nanometres. Various other equipment allows us to control the concentration of the aerosol, and various aspects of the atmosphere it resides in, to challenge and calibrate detectors.