Diesel Particulate Morphology
Internal Report: Investigations into Diesel Particulate Morphology, Part I
- Brian P. Frank, Research Scientist II
- Particulate Group
- In-Use Programs Section
- Bureau of Mobile Sources and Technology Development
- Division of Air Resources
- New York State Department of Environmental Conservation
The objective of the experiments described here was to investigate a method to obtain information on the morphology (shape) of ultra fine diesel particles. These experiments were performed in collaboration with Synergetic Technologies, Inc. (One University Place, Suite D210, Rensselaer, NY 12144), which is developing a unique light scattering technology for measuring the morphological properties of particles (see full paper in right column under "Important Links"). These properties include the shape of aggregate particles as well as their "fractal dimension", i.e., how loosely or how tightly individual primary particles are packed together to form the aggregate.
Current methods for particle size measurement do not take into account the impact of particle morphology, nor do they take into account the potential difference in morphology of particles from different sources. The two primary technologies for ultra fine particle measurement - Scanning Mobility Particle Sizer (SMPS) and Electrical Low Pressure Impactor (ELPI) - both assume that all particles are spherical in nature. If this assumption is flawed, then there could be fundamental errors in our interpretation of particle sizing data, especially in the use of other metrics derived from particle sizing data such as surface area.
Currently, the only means of obtaining information about the morphology of particles is by means of electron or scanning force microscopy. Initial investigations using such instruments strongly suggests that diesel particulates are composed of spherical monomers that agglomerate and the morphology of these agglomerates are irregularly shaped. However, these methods are slow, expensive and labor intensive. Further, they can only examine particles that have been deposited on a substrate rather than in their native aerosol state, and are also sensitive to the method of sample collection.
The potential advantage of the developing Synergetics' technology is to provide relatively fast (compared to current methods) information on particle morphology in the native aerosol state, concurrently with particle sizing measurements. Such information could have a significant impact on our fundamental understanding of particle sizing results, especially for metrics such as particle surface area, which is central to evaluating the potential health effects of diesel particulates.