What are the disadvantages of the CATI PM system?
The disadvantage of this measurement system is that the response is dependent on the size and composition of the particles. As a result, there may be a systematic measurement bias associated with each individual vehicle. This bias has been reported for other light scattering instruments (i.e. DustTrak). CATI attempts to minimize this bias by the use of virtual impactors which eliminate course mode particles (over 1-2 microns), which are almost entirely composed of road dust, engine wear particles, and soot particles previously deposited within the exhaust system. Also, the sampling system is designed to allow for the very small nuclei-mode particles (less than 100mm) to conglomerate or to condense onto soot particles. This narrows the particle size distribution, allowing for a more accurate conversion of the reading to particulate mass concentration and PM mass emissions.
