Award: OCE-1459180

Summary of Work and Accomplishments This is a collaborative project based on the collaboration between the University of North Dakota (UND) and Texas A&M University (TAMU). The overarching goal is to derive quantitative particle size and composition information from in situ measurements of the volume scattering function (VSF) to quantify marine biogeochemical stocks. The major objectives are to: 1) develop an inversion method to infer particle properties from polarized VSF measurements of the VSF, and (2) validate the inversion method to quantify the size distribution and composition of particle subpopulations. The effort of the TAMU team focused on the simulation of the optical properties of nonspherical particles, particularly, the theoretical computation of Mueller matrix for oceanic particles of various shapes and structures using a suite of state-of-the-art light-scattering computational techniques. In addition, the TAMU team assisted the UND team to incorporate the simulated single-scattering properties into the inversion algorithms. To model the inherent optical properties (IOPs) of aquatic particles, the TAMU team has developed a statistical approach in defining particle shapes in terms of an ensemble of distorted hexahedra is employed. This approach is inspired by the vast variability in shape of aquatic particle that cannot be described by one particular shape. The average single-scattering properties of an ensemble of randomly distorted hexahedra with an assigned aspect ratio and roughness parameter is used to represent the counterparts of aquatic particles. In the light-scattering computations, we considered numerous values the index of refraction. For particles with equivalent-volume spherical radius larger than 2 microns, the physical geometric optics method (PGOM) is used. In the PGOM, geometric ray tracing is employed for the near-field computations while a volume integral equation is used to map the near field to the far field. For particles with equivalent-volume spherical radius smaller than 2 microns, we use the invariant imbedding T-matrix (II-TM) method that is a numerically exact method. A combination of the II-TM and PGOM cover the entire particle size spectrum from 0.001 to 300 microns in the optical range. The data sets of the simulated optical properties of aquatic particles have been delivered to the UND team. Through this project, we also made significant progress in improving the light-scattering computational capabilities. In particular, we developed a new physical-geometric optics method to compute the single-scattering properties of faceted particles, which incorporates a general absorption vector to accurately account for inhomogeneous wave effects. In the new model, we also implemented a new beam-tracing technique to significantly enhance the computational efficiency. The new method has been extended to an inhomogeneous particle (e.g., a particle with a core). Furthermore, the TAMU team studied the propagation of a lidar beam in a coupled atmosphere-ocean model consisting of multiple atmospheric and upper ocean layers and a rough ocean surface, using a vectorized Monte Carlo radiative transfer solver optimized specifically for application to lidar. In particular, we studied the effects of assumed phytoplankton morphology variations and its vertical distribution on the lidar attenuated backscatter and depolarization ratio. Our sensitivity study indicates that both particle morphology and phytoplankton vertical density variations have strong impacts on the lidar measurement. Lidar polarization is identified as a suitable tool to investigate phytoplankton morphology, however, it is not expected to be strongly influenced by morphological details, such as inclusions. Peer-reviewed publications Xu, G., B. Sun, S. D. Brooks, P. Yang, G. W. Kattawar, X. Zhang, 2017: Modeling the inherent optical properties of aquatic particles using an irregular hexahedral ensemble, J. Quant. Spectrosc. Radiat. Transfer, 191, 30-39. Poulin, C., X. Zhang, P. Yang, and Y. Huot, 2018: Diel variations of the attenuation, backscattering and absorption coefficients of four phytoplankton species and comparison with spherical, coated spherical and hexahedral particle optical models. J. Quant. Spectrosc. Radiat. Transfer, 217, 288-304. Sun, B., P. Yang, G. W. Kattawar, and X. Zhang, 2017: Physical-geometric optics method for large size faceted particles, Optics Express, 25, 24044-24060. Sun, B., G. W. Kattawar, P. Yang, and X. Zhang, 2018: A brief review of Mueller matrix calculations associated with oceanic particles, Special Issue ?Outstanding Topics in Ocean Optics? in Applied Sciences (accepted and in Press) Stegmann, P. G., B. Sun, J. Ding, P. Yang, and X. Zhang, 2018: Study of the Effects of Phytoplankton Morphology and Vertical Profile on Lidar Attenuated Backscatter and Depolarization Ratio, J. Quant. Spectrosc. Radiat. Transfer, (submitted). In addition, we gave 6 presentations at international conferences. Last Modified: 11/02/2018 Submitted by: Ping Yang