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Characterizing Diesel Particle Exhaust

Recent concern about the risks to human health from airborne particulates such as those in diesel exhaust has motivated a group at the Environmental Energy Technologies Division to investigate the use of scattered polarized light. The goals are to characterize these particles and develop an instrument to measure these characteristics in real time. Having such an instrument can help regulatory authorities develop standards and monitor air quality. Airborne particulates, especially those less than 2.5 micrometers (µm) in diameter, are known to be a major human health risk. Diesel exhaust particles are principally in this size range; they are also a major source of reduced visibility in populated areas. California recently declared particulates in diesel exhaust a hazardous material.

Schematic of light scattering from diesel particle exhaust using a well-established polarization-sensitive nephelometer.

Current and proposed emission standards for diesel particles are based on measurements of the mass of particles emitted by a vehicle per kilometer. New particulate standards limit the total mass of particle sizes under 2.5 µm without considering size and composition. Conventional methods of measuring diesel particles have various shortcomings that can result in inaccurate data. Standard particle-collection techniques, for example, can influence the size distribution of collected particles. Instruments used to determine size distribution based on the motion of particles with respect to gases are biased toward measuring the total mass of particles collected, not the number of particles. The potential consequence of this is that a few large particles foreign to the diesel exhaust source may bias the results, obscuring the more important small-particle concentration.

Light Scattering is the Key

To overcome the limitations associated with these techniques, we have measured light scattering from diesel particle exhaust using a well-established polarization-sensitive nephelometer, a light-scattering instrument that provides rapid, in situ, non-intrusive monitoring of these emissions. The technique of angle-scanning, polarization-modulation nephelometry used in our research has been demonstrated to be sensitive and diagnostic of a number of the scatterers of physical properties.

The figure shows a schematic of the nephelometer. The instrument is based on the principle that the angular dependence of the linear and circular polarization of light scattered by diesel particles depends on the size, distribution, complex refractive index, and shape of the particles in the exhaust plume. Measuring the polarization properties of light scattered at all angles from the exhaust stream and analyzing these measurements provide information about the size distribution, complex refractive index, and shape of the particles. This information is determined from the data by simultaneously fitting the measured angular dependence of three or four scattering transformation matrix elements with Mie scattering calculations. Details of this method are available from the author.

Measurements from a Simple Diesel Engine

Using the nephelometer, we measured the polarized light scattering of the undiluted exhaust stream from a one-cylinder, 3-kW diesel engine. Analysis of these measurements suggests that diesel particles are randomly oriented and appear spherically symmetric to light. Under no-load conditions, the engine generated small particles with a mean diameter of 0.04 µm. At full-load conditions, the particles were larger, 0.12 µm, and more absorbing. The scattering in the full-load case was characteristic of particles that are a composite of materials—probably water and soot. In the no-load case, the particles contained very little soot. The results suggest that a real-time measurement of diesel particle size distribution and shape is possible, given a minimum set of measurements taken at a number of fixed angles.

Future Work

We are now working with a fully instrumented Cummins diesel engine at the University of California, Berkeley, and plan to measure exhaust from a wider variety of engine types. We are building and calibrating two real-time scatterometers dedicated to diesel measurements, one for use at Berkeley Lab/eetd and one at Oak Ridge National Laboratory. These instruments, with their rapid response times, can perform particle characterization for engine manufacturers, service facilities, and possibly emissions compliance.

— Arlon Hunt

For more information, contact:

  • Arlon Hunt
  • (510) 486-5370; fax (510) 486-7303

 

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