Polarized infrared radiative transfer in an optically anisotropic medium: A Markov chain solution for remote sensing of atmospheric suspended particles

• A Markov chain formalism is developed for polarized infrared radiative transfer in an optically anisotropic medium. • Our model accounts for emission, scattering, and absorption of the medium, as well as directional and polarized surface emission and reflection. • Numerical calculation shows multi-angle infrared polarimetry provides remote sensing sensitivity to type, orientation, and morphology of the non-spherical particles. A Markov chain approach is developed to model polarized mid- to long-wave infrared radiative transfer in an optically anisotropic medium. Such a medium contains oriented non-spherical particles with azimuthal randomness. Our model considers variations of temperature, pressure, gas concentration, and particle size distribution in the vertical dimension of the medium and resolves the total and polarized radiation in two-dimensional angular space. It also accounts for emission, scattering, and absorption of the medium, as well as directional and polarized surface emission and reflection. Illustrative simulations are performed using MODIS infrared bands centered at 9.73, 11.03, and 12.02 μm and several standard model atmospheres containing oriented dust or ice spheroids. The results are compared to those obtained for spherical droplets. Our preliminary numerical results demonstrate that while infrared brightness temperature contains information about particle amount, size, and layer height, adding multi-angle and spectro-polarimetry provides further remote sensing sensitivity to type, orientation, and morphology of the non-spherical particles. When reliable a priori information on the atmospheric physical temperature and absorbing gas profiles is available, infrared multi-angle polarimetry is a promising tool to fill the gap of shortwave and microwave remote sensing by resolving micro-meter scale particle properties to which shortwave and microwave frequencies have less sensitivity. Our simulation also reveals that solar radiation exerts a pronounced influence on the top-of-atmosphere brightness temperature when the wavelength approaches the short-wavelength end of the mid-wave infrared (≈3–4 μm).