Particulate matter mass measurements for low emitting diesel powered vehicles: whatʹs next?

Legislation word-wide imposes stringent emission standards for particulate matter emitted by diesel engines, as an outcome of evidence associating emitted particulate with a series of adverse health effects. Such emission standards call for significant reductions in the particulate matter emission levels for small and large on-road diesel vehicles. In fact, emission levels adopted for the near future are so low that the existing, regulated procedures might not be sensitive enough to accurately determine the absolute level of emissions. In this regard, extensive research is conducted in the direction of both improving existing procedures and developing alternative methods for more sensitive particulate matter measurements. This review paper summarizes the developments in the area and provides an outlook to the future. At first, the character of diesel particulate matter (DPM) is shortly discussed, mainly to show that the complex nature of emitted particulate is responsible for a series of implications when its quantitative determination is considered. Secondly, the regulated procedure for sampling and measurement of DPM is reviewed and its main qualities and shortcomings are presented. The recent technical advances aimed at improving the characteristics of the reference procedure in the measurement of ultra-low emission levels are then presented. Next the presentation of alternative methods follows which are gradually incorporated in regulations for particulate matter sampling. The strength of these techniques is that only a small fraction of exhaust is sampled and conditioned before measurement, hence significantly reducing the cost compared to the reference procedure. In addition to sampling, the determination of the actual particulate mass emitted is an area which has attracted significant developments. The most relevant instrumentation for DPM determination is also described and their potential for application in emissions certification is examined based on published evidence. Finally, the cost of the different sampling and instrumentation techniques is compared to demonstrate the cost-effectiveness of each option.