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Collecting techniques of PM analysis are defined as those where particles are first deposited in or on a filter—a task performed by the sampling system—and then analyzed. A sample of gas, typically from the dilution tunnel, is drawn through a high efficiency (99%+) filter element. Teflon-coated fiberglass is an example of commonly used filter media. The filter element is placed in a filter cartridge which is then fitted into a filter holder, Figure 1. The holder with the filter cartridge is installed in the sampling line using quick-connect fittings.
Figure 1. Filter Cartridges (left) and Holders (right)
(Source: Sierra Instruments)
During the test cycle, a controlled volume of gas is drawn through the filter and the particles (solid and liquid) are deposited on the filter element. Two filters with PM samples are shown in Figure 2. The filter on the left contains an engine-out PM sample (taken from upstream of a diesel particulate filter, DPF), which contains a high fraction of solid, black carbon. The sample on the right—taken after the DPF—contains less PM mass and is composed mainly of liquid, transparent or light-colored PM fractions.
Figure 2. Filters with PM Samples
Perhaps the main advantage of collecting methods is related to the flexibility to subject the collected sample to a large number of analysis methods that are available. The PM mass emission can be determined by gravimetry, and the sample can be further analyzed for its composition, other physical properties, or subjected to biological assays.
On the other hand, one of the principal difficulties and a source of error in all collecting techniques is the unstable character of the collected PM sample. By definition, “diesel particulates” are measured as any material deposited on the filter from the dilute exhaust gases sampled at a temperature of approximately 52°C. It should be remembered that, because of this definition, diesel particulate matter includes not only solids but also liquid material which would condense in the form of mist or droplets at the above temperature, such as sulfuric acid or high-boiling hydrocarbons (see also the paper on particulate matter).
During deposition, as well as during analysis, particles may considerably change and their properties may become very different from those of the airborne (or tailpipe) particulates. Phase transitions may lead to condensation and adsorption of initially gaseous material, or material may evaporate from the filter, as illustrated in Figure 3 . In that experiment, exhaust gas was sampled from the engine, which led to a linear increase in mass on the filter (phase I). Then (phase II) particle free air was drawn through the filter. A significant reduction in mass—on the order of 20%—was observed, which was ascribed to volatilization of the collected material.
Figure 3. Diesel Particle Mass on Filter
Measurement by beta-absorption; Engine at a constant operating condition
The presence of highly reactive species on the sampling filter may also lead to chemical reactions, changing the nature of the particles. An example of such chemical change is the degradation of polycyclic aromatic hydrocarbons.
Another disadvantage of collecting methods is their inability to quantify PM emissions during transient conditions—such as during accelerations—which are characterized by high PM emission rates. Rather, collecting methods can only provide an average result over a given test cycle.