Since the advent of the particulate matter (PM) standards by the United States Environmental Protection Agency (US EPA), particle sampling has become a primary goal of both scientists and lawmakers. The addition of the PM2.5 (fine particle) and the soon to be developed PM10-PM2.5 (coarse particle) standards to the PM10 standard has created a need for reliable continuous coarse and fine PM measurement devices.
An essential component of a modern PM monitoring device is a size pre-selective inlet. This is even more important when the size range to be removed prior to sampling consists of large particles. If the inlet allows even a small fraction of the undesirable PM into the measuring device, the error could be large. The reason for this is that large particles have large masses, which may heavily influence the measurement of a mass-based monitor.
Because of their large mass, hence inertia, coarse particles are difficult to sample and collect. When these heavy particles are accelerated in an impactor jet, their substantial inertia causes them to hit the impactor collection plate. Many times the particle will bounce off this plate and become re-entrained in the air stream. This may cause overestimation of the mass downstream of the inlet. Re-entrainment of particles may also be related to the flow rate of the aerosol flowing into the inlet. As the flow rate increases, re-entrainment increases.
Another problem that occurs because of the inertia of these particles is the underestimation of particle mass that results from anisokinetic sampling. Anisokinetic sampling is a condition in which the mean velocity of the flowing air differs from the mean velocity of the air entering the inlet of the sampling probe. In addition, unlike the PM in smaller size ranges, coarse PM is not as uniformly dispersed in the atmosphere. It settles and becomes resuspended due to localized events (i.e. high wind episodes).