Method to measure refractive indices of small nonspherical particles: Application to black carbon particles

We demonstrate a new method to measure refractive indices m of small nonspherical particles by simultaneous measurement of particle volume and light-scattering cross section. In contrast to traditional methods, this method can suppress uncertainties due to unknown particle shape. As demonstration of the method, laboratory experiments using an aerosol particle mass analyzer (APM) and single-particle soot photometer (SP2) have been conducted to determine the refractive indices of seven types of commercial black carbon particles and ambient soot at 1064 nm wavelength. Difference in measured |(m2−1)/(m2+2)| values among the seven commercial black carbon samples is consistent with the difference in their crystalline structures identified by a transmission electron microscope. The complex refractive index m=(n, k) of ambient soot particles in Tokyo urban area was determined to be m=(2.26±0.13, 1.26±0.13).