Diesel Particulate Matter

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Abstract: Diesel particulate matter (DPM) is the most complex of diesel emissions. Diesel particulates, as defined by most emission standards, are 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 DPM are elemental carbon, heavy hydrocarbons derived from the fuel and lubricating oil, and hydrated sulfuric acid derived from the fuel sulfur. DPM contains a large portion of the polynuclear aromatic hydrocarbons (PAH) found in diesel exhaust. Diesel particulates include small nuclei mode particles of diameters below 0.04 µm and their agglomerates of diameters up to 1 µm.

What Are Diesel Particulates

Particulate matter—perhaps the most characteristic of diesel emissions—is responsible for the black smoke traditionally associated with diesel powered vehicles. The diesel particulate matter emission is usually abbreviated as PM or DPM, the latter acronym being more common in occupational health applications. Diesel particulates form a very complex aerosol system. Despite considerable amount of basic research, neither the formation of PM in the engine cylinder, nor its physical and chemical properties or human health effects are fully understood. Nevertheless, on the basis of what is already known, PM is perceived as one of the major harmful emissions produced by diesel engines. Diesel particulates are subject to diesel emission regulations worldwide and, along with NOx, have become the focus in diesel emission control technology.

Contrary to gaseous diesel emissions, PM is not a well defined chemical species. The definition of particulate matter is in fact determined by its sampling method, the detailed specification of which is an important part of all diesel emission regulations. PM sampling involves drawing an exhaust gas sample from the vehicle’s exhaust system, diluting it with air, and filtering 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. Whenever reference to “particulate matter” is made throughout these papers, it is quietly assumed that the sampling was performed from diluted and cooled exhaust, in accordance with these standards (unless explicitly stated otherwise). However, it should be realized that other sampling procedures and corresponding definitions of PM are perfectly possible. Ongoing research in Europe is aimed at developing standardized measuring methods based on particle number emissions, rather than mass, for the inclusion in future emission standards in addition to the PM mass metric [823]. So far no common standard has been reached in the area of diesel occupational health regulations, where a number of different measuring methods—and corresponding DPM definitions—exist in parallel.

Diesel particulate matter, as specified by the US EPA procedures and most other standards and regulations worldwide, is sampled by filtering diluted diesel exhaust at a temperature of 47°C ± 5°C. This 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 diesel exhaust gas are known as dilution tunnels. The intention of this procedure is to simulate conditions at which diesel particulates are released from vehicles into the atmosphere. The substance which is sampled and regulated is supposed to correspond to diesel soot which is suspended in the ambient air. Fiberglas 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”—all solid and liquid material (condensate)—present in the diluted and cooled diesel exhaust. It should be emphasized that the above definition of diesel particulates is to a large degree arbitrary. Since the atmospheric dilution ratios of PM (about 500-1000) are much higher than those used in laboratory dilution tunnels, the simulation of the atmospheric dilution is far from perfect [430].