Development of a scanning nephelometer for rapid particle size analysis
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A concept for a new nephelometer, an instrument used to measure particulate matter (PM) size distribution and mass concentration, was investigated. The nephelometer characterizes particle size based upon the angular dependence of light scattered by a particulate-laden sample. More specifically, the size distribution is found by comparing an experimental light scattering pattern, measured via the angular light scattering detection capabilities of the nephelometer, to a database of theoretical scattering patterns generated by Mie theory calculations. The novel aspect of the device, called a scanning nephelometer, is the use of a 5.3 kHz scanning mirror to change the angle of the laser beam incident on the particulate sample relative to a fixed detector. The high scan frequency offers the possibility of very high temporal resolution measurements of aerosol properties. The design and performance characteristics of the scanning nephelometer system evolved through a development process that compared its behavior against fundamental measurements and expected theoretical behavior. The experiments studied the scattering characteristics of both water droplet aerosol flows and static measurements using calibrated polystyrene latex microspheres of different sizes. Poor correlations between preliminary experimental data and theoretical predictions from an existing computer code spurred an investigation of available Mie scattering computer codes. It was discovered that the different codes produced different Mie scattering patterns for the same input parameters. The inconsistencies seen among the codes motivated the development of a new computer code for calculating Mie scattering, starting from fundamental equations. This exercise uncovered subtle intricacies involved with the computation procedure. A comparison of the scattering patterns found by the scanning nephelometer system to benchmark patterns found by a simple, non-scanning nephelometer indicated the relative performance of the scanning system. While a good comparison was seen when examining latex spheres, the same was not observed when measuring aerosol droplets. The cause of the discrepancy is not understood. Scattering pattern comparison techniques and other complexities of light scattering particle sizing were also explored.