Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction

Abstract

[1] We present new observational data on small-angle light scattering properties of natural, random shaped particles, as contrasted with spherical particles. The interest in this ''shape effect'' on scattering arises from the need for a suitable kernel matrix for use in the laser diffraction method (LD) of particle sizing. LD is now used broadly for measuring size distribution of suspended marine particles. LD involves the measurement of small-angle forward scattering at multiple angles. This data is inverted using the kernel matrix to produce size distribution. In the absence of a suitable matrix for random shaped particles, past practice has been to use a model based on Mie theory, applicable strictly only to homogeneous spheres. The present work replaces Mie theory with empirical data. The work was motivated in part by anomalous field observations of size distribution and settling velocity distributions reported in literature. We show that a kernel matrix for random shaped particles results in improved interpretation of field multiangle scattering observations. In particular, a rising edge at the fine particle end of the size spectrum is shown to be associated with shape effects.

A regional comparison of particle size distributions and the power law approximation in oceanic and estuarine surface waters

“…[12] Field data obtained with the LISST-100X often shows a "tail" at the small end of the size range where the slope is appreciably different from the preceding particle concentrations [Agrawal et al, 2008]. This tail was evident for the majority of stations sampled here and can be produced from assumptions inherent in the scattering matrix used to invert the raw LISST measurements into particle size data.…”

“…This tail was evident for the majority of stations sampled here and can be produced from assumptions inherent in the scattering matrix used to invert the raw LISST measurements into particle size data. When high concentrations of nonspherical particles or submicron particles (below 2 mm) are present [Agrawal et al, 2008], more laser energy can be diffracted onto the outer rings of the LISST detector than would normally be attributed to spherical particles of 1-2 mm diameter. In addition, ambient red light can be "forward" scattered into the bottom lobe of the outer ring, particularly in near surface waters with sufficient concentrations of red light.…”

“…Known inaccuracies with the LISST instruments can result from schlieren in pycnoclines and particles with nonspherical geometry [Pedocchi and Garcia, 2006;Karp-Boss et al, 2007;Agrawal et al, 2008]. For comparison purposes, discrete surface water samples from the LIS0505 cruise were also analyzed with an electrical sensing zone method (Elzone particle size analyzer) on the ship within hours after collection.…”

Optical techniques for remote and in-situ characterization of particles pertinent to GEOTRACES

“…This allows calculating a calibration equation converting pixel area to real surface area of particles. When looking at smaller particles (<100 lm) inserting and tracking them in the light field becomes impossible and calibrated beads or culture of phytoplankton cells are used to replace natural targets (Agrawal et al 2008;Karp-Boss et al, 2007). Finally, cross-calibration of sensor output with a well characterized and calibrated sensor ('gold standard') can also be applied to insure inter-comparability of measurements.…”

In situ variability of mass‐specific beam attenuation and backscattering of marine particles with respect to particle size, density, and composition

“…Also, unlike the Coulter Counter and the FlowCAM, there is no need for discrete water sampling or sample handling, which may disrupt fragile aggregates. LISST instruments have been shown to provide PSDs that are comparable with other sizing instruments over a wide range of environmental particle types (Agrawal et al 2008;Andrews et al 2010;Reynolds et al 2010). However, to obtain particle size information, an optical model needs to be assumed for inversion of the angular pattern of near-forward scattered light recorded by the LISST.…”