Technologies - Particle and Aerosol Measurement

Particle and Aerosol Measurement

Detection and sizing of small particlesSome typical particles sizes

There are many different interactions of small particles with other matter or physical phenomina, which are used to characterise their size and properties.

One of the most common currently in use is the interaction of particles with electromagnetic radiation, usually visible light.

Light Scattering Instruments

Light is scattered in quite unexpected ways when it hits a particle, due to complex relationships between the reflection, refraction and electromagnetic coupling with the electronic structure in the particles.

If these relationships are understood and analysed, then detectors can be placed in different angular positions relative to the source of the incident light and the scattering point and the different intensity seen in the scattered light, also at different wavelengths, can be related to particle size.

Modern optical particle characterisation instruments use monochromatic lasers or high intensity white light sources, such as the Xenon lamps used the the welas® range of sensors, to illuminate a measuring zone where the particles are flowed through.

Where information of size distribution of an aerosol is required, the scattering is monitored for individual particles, with numbers of particles measured in each of a series of defined size channels collated and a histogram presented. These instruments are often refered to as aerosol spectrophotometers.

However, where total concentration only is required, then a totalised scattered light figure can be measured and compared with a calibrated level to give a comparative value. This can be referenced as using an instrument in nephlometric mode or photometric mode. Aerosol photometers have been used for years to assess efficiencies of filter devices, where an aerosol of known distribution is generated and the total amount penetrating the filter compared with the total amount upstream, to give an efficiency value.

Condensation Particle Counters

Molecules in gases also scatter light, and for particles of less than 0.1um in diameter, it starts to become difficult to resolve the scattering from that of surrounding gas molecues.

Instruments used for these particles generally use a condensation process, with the very small particle forming a condensation core onto which saturated vapour can condense to form a liquid coating as it is passed through a chamber containing a saturated vapour. This grows the size of the particle to again allow it to be detected by light scattering. These instruments are known as Condensation Particle Counters (CPC's), such as the UF-CPC, or Condensation Nucleus Counters (CNC's). However, as these instruments can only count particles, for sizing information they need to be combined with classification devices which separate a monodisperse aerosol flow of a known size from a polydisperse aerosol. These instruments are typically based on differential mobility classifiers, which select a specific particle size range based on its mobility in an applied electric field which depends on the charge / mass ratio of the particle.

Diluters and Charge Neutralisers

When the conecntrations of aerosols get too high for individual particles to be isolated in a measurement zone in and instrument, then the only option is to reduce the concentration before measurement. Dilution requires the mixing of a clean gas with the sample gas at a known ratio. Diluters, such as the VKL, KHG and DC10000, must achieve this without impacting on the size distribution, just reducing the number concentration.

Aerosols contain particles which become highly charged.

The level of charge can impact on the characteristics of an aerosol, resulting for example, in electrostatic losses of smaller particles, which can distort the true picture of an aerosol distribution. Classifier instruments, identified earlier, absolutely rely for their operation on a known charge to mass ratio being present on an aerosol, for correct selection to occur.

Charge Neutralisers provide a highly ionised environment for an aerosol to be flowed through so that the charge level can be brought to a stable equilibrium known as the Boltzmann equilibrium where the size of the particle determined how much charge it can carry. The highly ionised environment can be achieved by bipolar discharge equipment, such as the CD2000, or by using a small radioactive alpha source to bombard the chamber through which the aerosol flows.

A bit of Scientific History - Light Scattering by particles

The science behind the interaction of small particles in gases with light goes back to the 19th century. Some of the early developers of electomagnetic theory, Rayleigh and Maxwell for example, were interested in the interaction of light with particles.

In 1861, James Clerk Maxwell published 4 partial differential equations (Maxwell's Equations) which related electric charge and currents to electric and magnetic fields. These formed the basis of understanding for scientists trying to model and make sense of all electro-magnetic interations.

Rayleigh identified in 1881 that the air is full of particles (e.g. dust, gas molecules, etc.) all of which scatter light. The kind of scattering was identified to depend on the particle size and the wavelength of the light. If particles are larger than the wavelength, the light is scattered uniformly in all directions independent of the wavelength. If the particles are about the size of the wavelength of light or smaller, then the amount of scattering depends on the wavelength of the light.

This understanding could now explain why clouds, consisting of water droplets, were white. Also, the effect of atmospheric dust is to let the long wavelength (around 600nm = red) light pass through, while scattering the short wavelength (around 450nm = blue), hence the sky appears blue and the sun appears orange / red. The term nephlometer, still used today in aerosol measurement, is derived from the greek nephos - meaning cloud, and so a nephlometer is a cloud 'measurer'.

In 1908, Gustav Mie and colleagues produced a more general analytical solution of Maxwell's equations for the scattering of electromagnetic radiation by spherical particles.

The Technical Outcomes of the theory

Formal light scattering theory is now generally categorised in terms of two main theoretical frameworks.

  • Rayleigh scattering - it will not be expanded on here, but Rayleigh scattering can only be applied to very small (< 0.3um) non-adsorptive particles and the intensity of the scattered light is found to be proportional to the (particle diameter)6 and inversely proportional to the (wavelength)4.
  • Mie scattering - which encompasses the general spherical scattering solution (absorbing or non-absorbing) without a limit on particle size. Mie scattering theory therefore has no size limitations and may be used for describing most spherical particle scattering systems, including Rayleigh scattering. However, due to the complexity of the Mie scattering calculations, Rayleigh scattering theory is generally preferred, if it is applicable. As diameter increases from 0.3um the scattered intensity of found to be proportional to (particle diameter)2 and so more difficult to resolve particles into sizes and additionally the impact of an e/m adsorption component becomes significant.

Polar Intensity Light Scattering PlotLight scattered from a particle or particles will have a 3-dimensional intensity distribution and also vary in polarisation. The selection of the incident light source, the angles at which the scattered light is detected and detection systems used vary from design to design. A sample intensity plot for scattering angle v intensity is shown for a 10µm water droplet illuminated by 650nm vertically polarised red light.

Increases in computing power have made the simulation and modeling of the scattering characteristics of particle fluid suspensions much easier to predict.

Particle Analysis Systems

Particle analysis systems can be operated in different modes, single particle counting mode or 'nephlometric' / photometric mode.

Photometers and nephlometers generally look at a concentration and compare the transmission or scattering characteristics of the total aerosol mass with a calibrated value.

Single particle counting systems rely on an optical definition of a small measurement volume within a sampling chamber with the scattering characteristics of individual particles passing through this volume being assessed. If the flowrate is known, then combining the size distribution with the flowrate allows a volumetric concentration to be calculated and this can be related to mass.

Accuracy and Concentration Sensitivity Limits

Uniformity of illumination

The ideal situation is for the intensity of the incident light to be high and uniform across the measurement zone, so that a particle anywhere in the measurement zone scatters a lot of light uniformly as it travels through the zone.

With a focused monochromatic source such as a laser, the intensity is certainly high but generally varies with a peak in the centre of the beam. High intensity white light sources, such as xenon arc lights, have a broad flat intensity curve which gives uniform scattering as the particle passes through the measurement zone and a smooth response curve over a range of particle sizes.

Concentration Limits

The maximum and minimum concentration limits that a particular counter can accomodate are defined by several parameters, intensity, wavelength, optics, photomultiplyer sensitivity etc. but one significant influence is the measuring volume itself.

For the maximum limit as an example, if a flowing gas sample is passed through a measuring volume where the measurement zone is optically defined as a 100µm sided cube, then the approximate maximum concentration that can be measured without coincidence (i.e. only one particle in the measurement zone) is approximately 1 particle per 0.000001cc or 106 particles per cc. It will be less than this in reality, as the aerosol will not be uniformly distributed. If a larger measurement zone is used, for example a 1mm sided cube, then the maximum concentration drops to 1 particle per 0.001cc or 103 particles per cc.

For the minimum limit, if the aerosol contains only 1 particle per cc, then to measure sufficient particles to be statistically sure that you have the correct concentration, you need the measurement zone to be as large as possible. This proves to be an issue with filter testing where upstream concentrations are high and downstream low. The ideal situation is to have two sensors with different measurement zone characteristics, as can be achieved with the welas digital 3000, where a single controller contains a source and detector and the sensors are connected with fibre optic cable allowing different sensors to be used in the two positions.

This gets around having to validate one sensor for both upstream and downstream use.

Please contact us for more detailed information.