Influence of Particle Refractive Index on the Lower Detection Limit of Light Scattering Aerosol Counters

ABSTRACT

Light scattering particle counters are widely used for aerosol research. They are also important tools for monitoring airborne particles in the semiconductor and pharmaceutical industries. For the latter application, it is important to know the influence of particle material properties on the counter response, particularly the effect of particle refractive index on the lower detection limit of the counter. In this paper, the effect of particle refractive index on the lower detection limit of aerosol particle counters has been studied using the Mie theory. Counting efficiencies have also been measured to verify the theoretical results. The measurements were made with PSL (polystyrene latex), silicon, silicon nitride, and silicon dioxide particles. Two commercially available aerosol counters and a condensation nucleus counter were used in the study. The theoretical study show that both the real and the imaginary parts of the particle refractive index affect the lower detection limit of a light scattering particle counter. For transparent particles, an increase in the particle refractive index causes a decrease in the lower detection limit. And the absorptive component in the refractive index of the particle causes a further drop in the lower detection limit for the specific counters studied. Experimental measurements show good agreement with the theoretical results. Among the test particles used, silicon had the largest refractive index, followed by silicon nitride, PSL, and silicon dioxide. The lower detection limit of the counters studied also shows a corresponding decreasing trend with silicon dioxide giving the highest lower detection limit, followed by PSL, silicon nitride, and silicon as the refractive index of the particle is increased and the lower detection limit of the counter is decreased. The difference between the theoretical and experimental lower detection limits was found to be less than 10% in most cases.