LOWER DETECTION LIMIT OF AEROSOL-PARTICLE COUNTERS

Aerosol particle counters based on light-scattering have a broad range of applications including cleanroom monitoring, air pollution researc h, and pharmaceutical studies. Each application may deal with particle materials having various refractive indices. The effect of the partic le refractive index on the lower detection limit of aerosol particle c ounters was investigated using the Mie theory. Counting efficiency mea surements were made to verify the theoretical results. Measurements we re performed with PSL (polystyrene latex), silicon, silicon nitride, a nd silicon dioxide particles. Two commercial aerosol counters and a co ndensation nucleus counter were used in the study. The theoretical stu dy showed that both the real and the imaginary parts of the particle r efractive index play an important role in the lower detection limit of an aerosol counter. For transparent particles, as the absolute differ ence between the particle and medium refractive index increases, the l ower detection limit of a counter is decreased. Light-absorbing partic les generally showed a smaller lower detection limit than transparent particles. Experimental measurements agree well with the theoretical r esults. Among the test particles used silicon had the largest refracti ve index, followed by silicon nitride, PSL, and silicon dioxide. The l ower 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 exper imentally determined lower detection limits were found to be less than 10 percent.