There are many types of particle counters dependent upon the depth of particle analysis required:
- To measure ultrafine particles (size in nanometer range) one may utilize a condensation nucleus particle counter. Ultrafine airborne particle smaller than 0.1µm (micron), which generally cannot be detected by ordinary light-scattering particle counters, can be optically measured by condensation growth of particle through supersaturated conditions formed by compulsion mixing of sampled air and high temperature alcohol vapor. The condensation nuclei counter is an optical method for counting atmospheric aerosol particles. A pulse counter is used to count the number of particles passing through the laser beam. One type of instrument known as a Scanning Mobility Particle Sizer employs a combination of methods providing size distribution in the lower nanometer range. The Scanning Mobility Particle Sizer is based on the principle of the mobility of a charged particle in an electric field. Particles entering the system are neutralized (using a radioactive source) such that they have a Fuchs equilibrium charge distribution. They then enter a Differential Mobility Analyser (DMA) where the aerosol is classified according to electrical mobility, with only particles of a narrow range of mobility exiting through the output slit. This monodisperse distribution then goes to a Condensation Particle Counter which determines the particle concentration at that size.
- More commonly used for particle counting, are devices based on standard nephelometry. A nephelometer is an instrument for measuring suspended particulates in a liquid or gas. It does so by employing a light beam (source beam) and a light detector set to one side (usually 90°) of the source beam. Particle density is then a function of the light reflected into the detector from the particles. The more common term for these instruments are optical particle counters. The Optical Particle Counter is an instrument based on the principle of light scattering from particles. It is a real time instrument that is typically used to measure particles above 0.1 μm in diameter. The scattered light intensity is a function of the wavelength of incident radiation, the refractive index of the particle, the size of the particle and the angle made with respect to the incident beam. Light scattering is a non-intrusive method of measurement, and instantaneous information can be obtained to allow for real time or continuous measurement.
Some manufacturers of light scattering optical particle counters (OPC) calibrate their instruments for well-defined particle material, to convert optical equivalent size both into aerodynamic and volume equivalent diameters. For specific test dusts, the calibrated OPC provides a direct read-out dust mass vs. aerodynamic diameter (assuming a standard density for the particle) and data is then provided as continuous particle mass measurements such as PM2.5 mass concentration.
A helpful analysis is studying particles generated by resuspension or cleaning efforts in a room. Much of the dust that becomes airborne is resuspended dust which is typically not submicron but more in the range of 2 microns and above. Thus for this work the use of standard optical particle light scattering equipment, with size discrimination may be well suited for the work. It would be best to look at real time instrumentation that can provide several bin size range outputs beginning around 0.1 to 0.5 microns.
When and where to measure
This is very much a function of what the study objectives will be. If researchers will be attempting to capture temporal variation in airborne particle concentrations as a function of some activity, then clearly they will need to establish a baseline/background concentration of the concentration to be able to gauge changes as a function of the activity of interest. Depending on occupant activity, there must be careful attention applied to identifying sites that have consistent occupancy and use for the duration of the work. If comparing sites such as one with vacuuming twice per week as compared to once per month, there is a need to ensure sites are similar in nature during the study. In addition, extended real time measurements may be necessary to gauge differences under these circumstances. Settled dust measurements may be another good indicator to measure over prescribed periods during tests to gauge impact of cleaning regimes on the environment.
It is important to conduct a power analysis of a number of measurement sites/tests necessary to achieve a given degree of statistical significance to ensure the data generated will be representative commensurate to the goals of the project.
Will any of this work be conducted within a chamber environment with set parameters or will it all come from field data of occupied settings? These are key questions that also need answering.
It is important to formulate a study plan that will produce sound and defensible results.