Theoretical Analysis of PM2.5 Mass Measurements by Nephelometry

With the recently promulgated PM2.5 mass standards, a need has appeared for continuous PM2.5 mass measuring instrumentation to complement standard filter based aerosol samplers. It is generally accepted that in most cases, the PM2.5 mass distribution and light scattering is dominated by particles with diameters in the size range 0.1–1.0μm. Early field studies indicated a reasonable correlation between gravimetric aerosol mass and integrating nephelometer measurements of aerosol scattering coefficient. Nephelometry is a mature science dating back 50 years with well understood design philosophies and inherent limitations. Nephelometers have proven to be capable of making highly accurate, precise continuous measurements of the aerosol scattering coefficient. In addition, nephelometers are very portable, rugged, requiring low maintenance and of moderate cost when compared to filter based aerosol samplers. These factors have lead to the reconsideration of employing size-cut nephelometers or light scattering photometers as surrogate continuous PM2.5 monitors. The main uncertainty is due to the fact that the measured aerosol scattering coefficient is not linearly proportional to aerosol mass, but rather a complex function of the ambient aerosol chemistry, shape, density, size distribution, and index of refraction as well as the optical properties and geometry of the nephelometer used. This paper uses Lorenz-Mie theory, reasonable estimates of the variation of ambient aerosol properties, and the optical characteristics of currently available nephelometers and light scattering photometers to investigate the theoretical limits of the accuracy and precision of PM2.5 mass measurements estimated by nephelometry.