Spectral properties of ice‐particulate mixtures and implications for remote sensing: 1. Intimate mixtures

Abstract

The spectral properties of water ice‐particulate mixtures are studied for the purpose of deriving the ice and particulate abundances from remotely obtained spectra (particulates referring to nonicy materials in the form of grains). Reflectance levels and ice absorption band depths are a complex function of the single scattering albedo of the particulates embedded in the ice. The ice absorption band depths are related to the mean optical path length of photons in ice through Beers law, Fresnel reflection from the ice‐crystal faces on the surface, and ice absorption coefficient as a function of wavelength. Laboratory spectra of many ice‐particulate mixtures are studied with high‐, medium‐, and low‐albedo particulates. From the laboratory spectra the band depth, the continuum reflectance, and the ratio of band depth to continuum reflectance are derived for each particulate albedo as a function of the logarithm of the particulate weight fraction in the sample. Ice band depths are dependent on the particulate albedo and increase with smaller weight fractions of particulates until the bands saturate and their depths decrease. The continuum reflectance is a complex function of the particulate albedo and wavelength of light, while the band depth to continuum reflectance ratio appear independent of particulate albedo such that, for a given grain size of particulate, a calibration curve to abundance of ice and particulates is derived. The derived abundance calibrations are accurate if the source of scattering is dominated by particulates and not ice‐vacuum (air) interfaces. If ice‐vacuum interfaces are dominant, then the derived particulate abundance is an upper limit. Theoretically produced frost spectra show similar curves of growth of band depth, continuum reflectance, and corresponding ratio versus the photon mean optical path length similar to that for intimate mixtures of ice and high‐albedo particulates. In all scattering cases the ice absorption coefficients determine the path length needed to produce a given band depth; thus different absorptions are sensitive to different amounts of particulates, with the 3‐μm fundamental being the most sensitive to the presence of ice (very low ice abundance) and the 1.04‐μm overtone being the most sensitive to the presence of particulates (very high ice abundance).