Complex Index of Refraction of Airborne Soil Particles

Abstract

The angular variation of the intensity of light scattered from a collimated beam by airborne soil particles and the size distribution of the particles were measured simultaneously 1.5 m above the ground. These measurements gave an estimate of the complex index of refraction m=nRE?nIMi of airborne soil particles, where nRE is the real part and nIM the imaginary part of the refractive index. Standard microscopic analysis procedures were employed to determine nRE. Although a wide range of values was observed, the value 1.525 was taken as representative. By applying Mie scattering theory to each of the observed distributions of particle size, the expected angular variation of the intensity of the scattered light was calculated for a fixed value of nRE and a wide range of values of nIM. For each set of simultaneous measurements, the value of nIM was taken to be that value which provided the best fit to the experimental data. The upper limit of the value of nIM for the airborne particles studied in the experiment was determined to be 0.005 with an uncertainty factor of about 2. The estimate of nIM was found to be fairly insensitive to the assumed value of nRE.