Exhaust Particulate Matter

W. Addy Majewski, Hannu Jääskeläinen

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Abstract: Exhaust particulate matter (PM) is the most complex of exhaust emissions. Particulate matter, as defined by most emission standards, is filterable material sampled from diluted and cooled exhaust gases. This definition includes both solids, as well as liquid material which condenses during the dilution process. The basic fractions of PM are carbonaceous solids and heavy hydrocarbons derived from the fuel and lubricating oil. In cases where the fuel contains significant sulfur, hydrated sulfuric acid can also be a major component. PM contains a large portion of the polynuclear aromatic hydrocarbons (PAH) found in engine exhaust. PM includes small solid primary soot particles of diameters below 40 nm and their agglomerates of diameters up to 1 µm as well a nucleation mode particles consisting almost entirely of condensed liquid.

What Are Exhaust Particulates

Exhaust particulates, whether from diesel or gasoline fueled engines, form a very complex aerosol system. Despite a considerable amount of basic research, a full understanding of exhaust particulate matter (PM), its physical and chemical properties, and its effect on human health and the environment is still lacking.

Particulate matter—most commonly associated with diesel engines—is responsible for the black smoke traditionally associated with diesel powered vehicles. Diesel particulate matter emissions are usually abbreviated as PM or DPM, the latter acronym being more common in occupational health applications. The existing medical research suggests that PM is one of the major harmful emissions produced by diesel engines. Diesel particulate matter is subject to diesel emission regulations worldwide. Diesel PM and NOx are the primary focus of diesel emission control technology.

Interest in particulate emissions from gasoline engines is relatively recent and arises primarily from the widespread shift in gasoline engines to fuel injection technology that injects fuel directly into the cylinder—gasoline direct injection (GDI). While gasoline engine particulate emissions have been less studied than those for diesel engines, many of the harmful effects of diesel engine PM are assumed to be associated with gasoline PM as well. Regulations limiting PM emissions from gasoline engines, especially GDI engines, have been implemented in many jurisdictions including the US, Europe, Japan, China and India.

Unlike gaseous emissions, PM is not a well defined chemical species. The definition of particulate matter is in fact determined by its sampling method (i.e., it is operationally defined), the detailed specification of which is an important part of all diesel emission regulations. PM sampling involves drawing a sample of exhaust gas that has been diluted with air and filtering it through sampling filters. The mass of particulate emissions is determined based on the weight of PM collected on the sampling filter. It is quite obvious that any changes in the procedure, for example using a different type of sampling filter or different dilution parameters, may produce different results. Standardization of sampling methods is of utmost importance if results from different laboratories are to be comparable. Such standards have been developed for the measurement of PM mass in the area of public health regulations (i.e., emission standards for diesel engines and vehicles) worldwide.

A number of sampling filters with PM deposits from diesel and gasoline engines are shown in Figure 1 [2914]. The appearance of the filters suggests that diesel engines (top row) can produce significant amounts of carbonaceous particulate emissions. It is also evident from the figure (bottom row) that PM emissions can be effectively controlled using diesel particulate filters (DPF). Among gasoline engine technologies, gasoline direct injection (GDI) engines typically produce higher PM emissions than port fuel injected (PFI) engines. In the example in Figure 1, the GDI PM emissions were especially high during the cold start phase of the test (FTP bag 1).

Figure 1. Visual comparison of PM deposits from various LDV technologies

Light-duty vehicles, FTP-75 test (bag 1 = cold start)

Figure 2 further illustrates the differences shown in Figure 1 by comparing the emission rates of particulate mass and particulate number for different light-duty engine technologies [2914]. Diesel engines have traditionally had the highest emission rates of PM and for many years, were the focus of efforts to reduce vehicle related PM emissions. In comparison, particulate emissions from PFI gasoline engines were very low. With the introduction of diesel particulate filters (DPF), particulate emissions from diesel engines were reduced to levels comparable to PFI gasoline engines. However, as efforts to reduce fuel consumption in light-duty vehicles resulted in the widespread shift away from PFI to GDI technology for gasoline engines, PM emissions from gasoline vehicles have increased significantly compared to diesels with DPFs.

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Figure 2. Comparison of the emission rate of particulates from different light-duty engine technologies

Total PN includes solid and liquid particles

Whenever reference to “particulate matter” is made in this and other papers, it is assumed—unless explicitly stated otherwise—that the sampling was performed from diluted and cooled exhaust (i.e., exhaust gas that has exited the tailpipe), in accordance with the common PM mass measurement procedures for mobile sources. However, a variety of alternative metrics for particulate matter emissions, along with the respective sampling and measurement procedures are possible. For example, in European emission standards based on Euro 6 limits, particle number (PN) emission limits must be observed in addition to the mass-based limits. In the area of diesel occupational health regulations, a common particulate matter standard has never been agreed upon and a number of different measuring methods and corresponding DPM definitions exist in parallel.

Particulate matter, as specified by US EPA procedures, is determined by gravimetric analysis from a PM sample obtained by filtering diluted engine exhaust at a temperature of 47°C ± 5°C. Similar sampling temperatures are specified by a number of other standards and regulations worldwide. The required cooling effect is typically achieved with laboratory dilution ratios in the range 3:1 - 20:1. Devices which are used in the laboratory to produce the mixture of air with engine exhaust gas are known as dilution tunnels. The intention of this procedure is to simulate conditions at which exhaust particulates are released from vehicles into the atmosphere and to correspond to the PM contribution from engines and vehicles to the ambient PM load. The high-efficiency sample collection filters which are used for laboratory PM sampling capture solid particles, as well as liquid droplets, or mist, which condense from exhaust gases during the dilution process. In effect, the definition of PM extends to “any matter”—solid and liquid (condensate)—present in the diluted and cooled diesel exhaust. It should be emphasized that this definition of particulate matter is to a large degree arbitrary. Since the atmospheric dilution ratios of PM—often in the range of 500 to 1000—are much higher than those used in laboratory dilution tunnels, the simulation of atmospheric dilution is far from perfect [430].