Optical Properties of Airborne Silica Microspheres: Implications for Nephelometer Calibration

The use of aerosolised silica microspheres holds considerable significance in the calibration of scientific instrumentation i.e. Nephelometers. It is common to assume that the real component of the refractive index for silica microspheres is identical to that of bulk silica which is approximately 1.45 at 589 nm. In the use of nephelometers it is assumed calibration aerosols, such as silica, are perfect smooth spheres, with homogenous optical properties. Manufactured silica microspheres typically do not fit these assumptions. They have a degree of surface roughness and/or density variation stemming from variations in manufacturing via the St¨ ober process. The refractive index of airborne silica microspheres (~1 μ m radii) from four different suppliers were investigated using a combination of laser tweezers and Mie spectroscopy. The refractive index at 589 nm was lower than that of bulk silica. A range of refractive index values between 1.37 and 1.44 was determined in this study. The radii of all microspheres were within the manufactures reported ranges. The assumption of a bulk silica refractive index may not be accurate for the calibration of nephelometers. The refractive index of commercial silica microspheres can vary between manufacturers significantly, which may lead to substantial calibration errors in the measurement of size using a nephelometer. For a micron-sized aerosol, assuming the bulk silica refractive index in calibration may result in a 14% error from the nephelometer measurements.