Aerosol particle counters based on light-scattering have a broad range of applications including cleanroom monitoring, air pollution research, and pharmaceutical studies. Each application may deal with particle materials having various refractive indices. The effect of the particle refractive index on the lower detection limit of aerosol particle counters was investigated using the Mie theory. Counting efficiency measurements were made to verify the theoretical results. Measurements were performed with PSL (polystyrene latex), silicon, silicon nitride, and silicon dioxide particles. Two commercial aerosol counters and a condensation nucleus counter were used in the study.
The theoretical study showed that both the real and the imaginary parts of the particle refractive index play an important role in the lower detection limit of an aerosol counter. For transparent particles, as the absolute difference between the particle and medium refractive index increases, the lower detection limit of a counter is decreased. Light-absorbing particles generally showed a smaller lower detection limit than transparent particles. Experimental measurements agree well 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 showed a decreasing trend with an increasing real part of the refractive index as the particle material is changed from silicon dioxide to PSL, silicon nitride, and silicon. The difference between the theoretically calculated and experimentally determined lower detection limits were found to be less than 10 percent.