R.A. Fletcher (CSTL) and R.A. Dobbins (Brown University)
Objective: To analyze and compare diesel soot formed from an engine operating at various controlled conditions to soot collected from an environmental site and to the NIST SRM material. The ultimate goal is to determine the source of polycyclic aromatic hydrocarbons (PAHs) in engine exhaust products.
Problem: Diesel engines are widely used in trucks, buses and to a lesser extent in automobiles. Diesel soot, a product of the combustion system, is a known environmental pollutant. Diesel soot is a complex mixture of particulate materials primarily composed of black carbon, often referred to as elemental carbon, that is a soiling agent. It also contains many organic compounds including PAHs that are potential carcinogens. Human health and property damage are potential impacts of exposure to diesel particles. To abate soot formation, it is vital to understand the particle formation mechanism during combustion and the source of PAHs, whether from the combustion process or from the fuel. Brown University and NIST formed a collaboration to study diesel particles emitted from diesel engines operating at controlled conditions.
Approach: The diesel particles were collected on pre-cleaned, uncoated transmission electron microscope (TEM) grids by thermophoresis. Analysis was accomplished by directly inserting the samples, without further preparation, into a laser microprobe mass spectrometer (LAMMA 500). The material was irradiated with 266 nm radiation to produce ions that were mass analyzed using a time-of-flight mass spectrometer. Approximately thirty mass spectra were taken from each sample. TEM micrographs of the diesel soot were taken at Brown University. The environmental samples were collected from the Fort McHenry tunnel. The tunnel is divided into two tubes -- one for trucks (diesel soot producers) and one for automobiles. The particle samples were collected from the air using a low-pressure impactor that deposited the aerosol on the central region of a quartz coverslip slide that mounted directly in the LAMMA. The SRM 1650 diesel particulate material was analyzed by dispersing a small amount on a quartz coverslip and also on a TEM grid.
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Figure 1. Laser microprobe mass spectrum of particles collected from the Fort McHenry tunnel. Peaks corresponding to PAHs are identified. (Sample collected by S. Hoeft) |
Results: Spectra like the one shown in Figure 1, taken from a sample of the tunnel air, indicated the presence of PAHs in the m/z range of 165 to 300. These PAH peaks match closely with peaks found in an ethene gas diffusion flame and are common to a wide variety of combustion processes. |
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Last Updated
March 5, 2002
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