Light scattering by nonspherical particles: Application to coccoliths detached from Emiliania huxleyi

Computation of the light scattering properties of marine particles has typi cally been effected using Mie theory (i.e., modeling the particles as homog eneous or layered spheres). Because scattering by irregularly shaped partic les is significantly different from that of spheres, particularly in backsc attering directions, it is of interest to examine the efficacy of using mor e complex formulations of light scattering that are not limited to spherica lly symmetric particles. We applied the discrete dipole approximation (DDA) to the computation of the scattering properties of detached calcium carbon ate coccoliths from the coccolithophorid Emiliania huxleyi. Three distinct models of E. huxleyi coccoliths were studied: thin disks with a diameter of approximately 2.75 mum, washers with a 1.38-mum hole in the disk, and two parallel disks joined by a hollow tube (a "fishing reel"). The model coccol iths all had the same volume (mass approximate to 0.19 pg carbon) and disk diameter and a refractive index of 1.20 relative to water. DDA computations for randomly oriented model coccoliths showed that the total scattering cr oss section and its spectral variation are similar for each of the three pa rticle shapes and agree well with measurements made in natural E. huxleyi b looms both on a per-coccolith and per-calcite concentration basis. The back scattering cross section and its spectral variation was found to be strongl y dependent on particle morphology. This dependence was shown to be due to multiple reflections within the particle. Scattering and back-scattering co efficients for volume-equivalent spheres were within a factor of two of tho se for the disklike models.