Interpreting LISST‐100X measurements of particles with complex shape using digital in‐line holography

Abstract

[1] Measurements of suspended sediment volume concentrations, particle size and number density are routinely collected in marine and fresh-water environments with LISST-100X instruments to understand sediment transport, biological processes and fundamental opto-acoustic problems. A LISST-100X was simultaneously deployed with a novel holographic camera (holocam) in UK coastal waters to assess the performance of the laser diffraction technique when measuring natural suspensions. Volume distributions from the LISST-100X, truncated to exclude non-overlapping size bins with the holocam, exhibit an increase in small particles and median particle size is elevated in comparison to the holocam by 20-40%. We observe positive offsets between LISST-100X and holocam number distributions of up to 2 orders of magnitude for particle sizes between 58-218 mm, with discrepancies rising to 4 orders of magnitude for finer and coarser sizes. To explain these differences, a novel multiscale representation of particle size is used. The method quantifies individual dimensions that make up any two-dimensional geometrical structure, it can be used as a metric for particle complexity, and offers a plausible explanation for an apparent increase in small particles (<58 mm) reported by the LISST-100X. The results suggest that for non-spherical natural suspensions the LISST-100X may be sensitive to optical scattering from sub-scales within larger particles, reporting them as individual particles regardless of the way in which they may be packaged into particles of larger overall size. We urge caution in over interpretation of LISST size distributions obtained in natural suspensions without verification with independent particle imaging.

Spatial and Seasonal Variation of Biomineral Suspended Particulate Matter Properties in High-Turbid Nearshore and Low-Turbid Offshore Zones

“…Particles smaller than the size range affect the entire PSD, with an increase in the volume concentration of the smallest two size classes, a decrease in the next size classes and, an increase in the largest size classes [40]. Similar remarks have been formulated by Graham and coworkers [41], who observed an overestimation of one or two orders of magnitude in the number of fine particles measured by the LISST. A rising tail in the lowest size classes of the LISST occurs regularly in the data during highly turbulent conditions and is interpreted as an indication of the presence of very fine particles and thus a break-up of the aggregates.…”

“…The occurrence of rising tail in the largest size classes indicates the occurrence of large particles rather than an absolute value. Other uncertainties of the LISST-100C are related to the often non-spherical shape of the particles occurring in nature [40,41,44]. A hull mounted, acoustic Doppler current profiler (ADCP) type, Workhorse Mariner 300 kHz (RD Instruments, Poway, CA, USA), was used to determine the velocity profiles.…”

Estuarine Turbidity Maxima and Variations of Aggregate Parameters in the Cam-Nam Trieu Estuary, North Vietnam, in Early Wet Season

“…However, particles less than the smallest size class or bigger than the largest size class affect the measurements in the spectrum [42][43][44][45][46][47][48][49][50][51][52]. In this study, we followed the recommendation of former authors to remove the first and last classes for calculating the general slope of the particle size distribution and the mean apparent diameter D 50 [42][43][44][45][46][47][48][49][50][51][52]. Based on data between class #2 and class #31, D 50 was thus calculated as the diameter corresponding to 50% of the cumulative volume concentration of aggregates between 1.48 and 212 µm.…”

Why and How Do We Study Sediment Transport? Focus on Coastal Zones and Ongoing Methods

“…Drones are also equipped with multi-or hyperspectral sensors to map water colour and water quality parameters at high definition [289]. The potential of an instrument such as the LISST 100-X since its commercialization (about fifteen years) has not yet been encompassed that a second generation of instrument is available using holograms (LISST-HOLO, [290]), then a third generation (LISST-200X) to better measure larger particles and benefit from more robust optics. New sensors are regularly introduced, applied and compared to older ones: infrared laser turbidimeter adapted to very turbid water [291], comparison of acoustic and optical backscatter sensors in cohesive suspension [292,293], etc.…”