10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
A Novel Mie Theory Inversion Technique for Retrieving the Complex Refractive Index from Optical Measurements
BENJAMIN SUMLIN, William Heinson, Rajan K. Chakrabarty, Washington University in St. Louis
Abstract Number: 402 Working Group: Aerosol Physics
Abstract Mie theory is a powerful tool that can accurately calculate the scattering and absorption behavior of a particle with any radial symmetry, such as spheres and long cylinders, and whose diameter is on the order of the wavelength of incident light. However, the complex refractive index m=n+ik is often the unknown parameter in aerosol optics studies and is a vital parameter for climate modeling and satellite retrieval algorithms. Mie theory equations take m as an input, therefore recovering it from measurements presents an inverse problem which is confounded by the multidimensional parameter space the equations require. Typically, Mie theory inversions are often constrained to the particle size and real m, but the specific inverse problem of recovering complex m from experimental measurements is the focus of this work.
Since m cannot be measured directly, a method to determine the refractive index from experimental data is needed. We present a novel, visual method to determine m from optical measurements by identifying intersections of scattering and absorption contour maps in n-k space. Our method allows arbitrary curves to be treated geometrically and algebraically, eliminating the need to develop complicated analytic expressions for the contours. The minimum inputs are scattering, absorption, particle size, and wavelength, however, this can lead to an improperly constrained problem, with multiple valid values of refractive index that will reproduce the measured parameters. This can be fully constrained by measuring an additional independent parameter, such as the backscattering efficiency.
Our method is presented in the context of The Python Mie Scattering package, or PyMieScatt, which was developed to be a complete tool for both forward and inverse Mie theory calculations. This package is open-source, fully documented, and available to the community, allowing research groups to bypass the step of developing their own Mie theory software.