A Markov chain model for polarized radiative transfer in the thermal infrared and application in dust particle inversion

By accounting for emission, scattering and absorption within a coupled atmosphere-surface system, the polarized Markov chain (MarCh) radiative transfer (RT) model is extended to cover the entire spectral range from shortwave to thermal infrared. The MarCh model accounts for non-spherical aerosols with random or preferred orientations, inhomogeneity of aerosol vertical profile, surface bi-directional reflection/polarimetric distribution functions, and full sphericity of the atmosphere. A multi-stream scheme is adopted to resolve the angular dependence of the radiance distribution and to improve the modeling accuracy. The matrix multiplication basis of MarCh model makes it suitable for implementation on modern graphical processing units (GPUs) for high computational performance. Following a numerical verification against the Monte Carlo component of the libRadtran RT package for U.S. AFGL's six reference atmospheres (with the further addition of aerosols), we combine thermal infrared measurements with polarimetry to investigate their sensitivity to the loading, types, microphysical properties, and altitude range of mineral dust aerosols. Transported from continental areas to distant regions, mineral dust aerosols vary in their size from tenths of nanometers to hundreds of microns. For their remote sensing, the MarCh model is integrated with our recently developed correlation-based inversion approach. The retrieval's potential will be demonstrated using a combined set of observations from the POLarization and Directionality of the Earth's Reflectances (POLDER) and the Moderate Resolution Imaging Spectroradiometer (MODIS) over the Sahara Desert. This work forms a basis for enhancing dust retrievals using both polarimetric and thermal infrared measurements from NASA's upcoming Atmosphere Observing System (AOS) mission.