The optical volume scattering function in a surf zone inverted to derive sediment and bubble particle subpopulations

The effects of particle fields including bubbles on the optical volume scattering function (VSF) were investigated in the surf zone off Scripps Pier as part of an ongoing effort to better understand the underlying dynamics in the VSF in the subsurface ocean. VSFs were measured at 20 Hz at angles spanning 10°–170° in 10° increments with a device called the Multiangle Scattering Optical Tool (MASCOT). Modification of the phase function was observed in passing suspended sediment plumes, wave‐injected bubble plumes, and combinations of these particle populations relative to the background. Phase function enhancement in the 60°–80° range was observed in association with bubble plumes, consistent with theoretical predictions. VSFs were inverted to infer size distributions and composition using a least squares minimization fitting procedure applied to a library of phase functions, each representing a lognormally distributed subpopulation with refractive index and coating, where applicable. Phase functions representative of nonspherical mineral particle subpopulations were computed using discrete dipole approximation (DDA) and improved geometric optics method (IGOM) techniques for randomly oriented, asymmetric hexahedra. Phase functions for coated bubbles were computed with the Lorenz‐Mie theory. Inversion results exhibited stable solutions that qualitatively agreed with concurrent acoustical measurements of bubbles, aggregate particle size distribution expectations, and anecdotal videography evidence from the field. Although a comparable inversion with a library that assumed spherical shaped particles alone provided less stable results with some incorrectly assigned subpopulations, several dominant subpopulation trends were consistent with the results obtained using nonspherical representations of mineral particles. Key Points Volume scattering functions are highly dynamic in the surf zone VSF variability is due to changes in the composition/magnitude of particles Optically relevant particle fields consist of sediments and bubbles