Inherent optical properties of non-spherical marine-like particles - From theory to observation

In situ measurements of inherent optical properties (IOPs) of aquatic particles show great promise in studies of particle dynamics. Successful application of such methods requires an understanding of the optical properties of particles. Most models of IOPs of marine particles assume that particles are spheres, yet most of the particles that contribute significantly to the IOPs are non-spherical. Only a few studies have examined optical properties of non-spherical aquatic particles. The state-of-the-art knowledge regarding IOPs of non-spherical particles is reviewed here and exact and approximate solutions are applied to model IOPs of marine-like particles. A comparison of model results for monodispersions of randomly oriented spheroids to results obtained for equal-volume spheres shows a strong dependence of the biases in the IOPs on particle size and shape, with the greater deviation occurring for particles much larger than the wavelength. Similarly, biases in the IOPs of polydispersions of spheroids are greater, and can be higher than a factor of two, when populations of particles are enriched with large particles. These results suggest that shape plays a significant role in determining the IOPs of marine particles, encouraging further laboratory and modelling studies on the effects of particle shape on their optical properties.