Relationship between particle size and imaginary refractive index in atmospheric dust

It has been pointed out that the imaginary indices for different materials commonly found in airborne dust can be radically different. The range of variability can be many orders of magnitude; and, therefore, it should be expected that since composition varies with particle size, the imaginary index might have a strong dependence upon particle size also. Since the Mie theory is nonlinear, it might also be expected that the use of an average or effective imaginary index which is the same for all particles (at one wavelength) could cause errors in the computations. This article attempts to show the results of a series of measurements that were made to illustrate this point and to quantify it to some extent. Measurements of imaginary refractive index and composition information for atmospheric dust collected from Feb. to March 1976, in south-central New Mexico at the White Sands Missile Range are presented. It is concluded that for visible and near IR wavelengths and for a sample of atmospheric dust from southern New Mexico, the imaginary index varies strongly with particle size.