Particulate Measurement: Collecting Methods

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Abstract: In collecting methods, particles are first deposited on a sampling filter and then analyzed. Under most emission regulations, PM emissions are determined through gravimetric analysis of the collected particulates. A number of changes and refinements have been introduced to the regulatory measurement protocols to improve the accuracy of the gravimetric analysis and enable its application to low emission diesel engines. For research and non-regulatory purposes, the sample of collected particulates can be also analyzed using thermal mass analysis (e.g., coulometric analysis) and a number of other methods to determine particulate composition, surface area and other parameters.

Sample Collection

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. With very few exceptions, most test protocols require gravimetric diesel particulate matter determination from diluted exhaust. The dilution can be done in a CVS dilution tunnel or a partial flow dilution system. An important parameter of the sampling process is the gas temperature. For example, the US EPA protocols require that the temperature of the diluted exhaust gas at the filter face be at 47°C ± 5°C. Earlier protocols, before 2007, specified a filter face temperature of less or equal to 125°F = 51.7°C.

A sample of gas from the dilution tunnel is drawn through a high efficiency (99%+) filter element. Commonly used filter media include polytetrafluoroethylene (PTFE or Teflon) coated borosilicate fiberglass filters or polytetrafluoroethylene membrane filters. 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.

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.

Arguably the main advantage of collecting methods is 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 47°C. It should be remembered that, because of this definition, diesel particulate matter includes not only solids but also liquid material that would condense in the form of mist or droplets at the above temperature, such as sulfuric acid or high-boiling hydrocarbons (see also Exhaust 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 [1030]. 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.

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.

To help overcome the latter drawback, filter holders have been developed that incorporate an in-situ particle sensor to add real-time detection to the gravimetric measurement, Figure 4. The real-time detection module includes a diffusion charger (DC) that allows monitoring PM accumulation on the filter during sampling. The sensitivity of the DC sensor is about 1 μg/m³ for 70 nm particles [3398]. The filter holder is compatible with conventional gravimetric sampling systems such as those used for regulatory emission testing.