Laser In-Situ Scattering and Transmissometry (LISST-100X)		Sequoia Scientific, Inc.
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To assist in the interpretation of the data, it can be displayed using the Hawaii Ocean Time-series Data Organization & Graphical System (HOT-DOGS©).

Sampling Procedure

Forward light scattering can be used for rapid determination of in situ particle size distribution and particle concentration based on an inversion of the volume scattering function at small forward angles. One advantage of this technique is that it can capture continuous (1-Hz) in situ data. The LISST-100X measures the near-forward angular scattering distribution between 0.0017 to 0.34 radians (0.097-19.48^o; Type-B), at 670 nm, which is a region where scattering is strongly influenced by particle size. Using Mie scattering theory, the LISST estimates a volumetric particle size distribution (V(D_i) in units of mL L^-1) for 32 logarithmically spaced size classes with geometric mean diameters (D_i) ranging from 1.36 - 230.14 mm (for spherical particles). The shape of the PSD is based on an inversion of the angular pattern of forward scattering, and the concentration of particles is derived by the magnitude of scattering that reaches the detector.

Before the measured light scattering distribution is inverted to obtain the particulate volume distribution, the signal must be corrected for background scattering due to pure water. After the inversion the data are corrected for the difference in laser power between the factory calibration and the in situ data, and an instrument-specific correction factor is applied to obtain the calibrated particle volume concentration, in volume particles per volume of water. The areal size distribution (A(D_i)) is then calculated from the volume size distribution (µL L^-1) by assuming spherical geometry: A(D_i) = ³/₂ V(D_i)D_i^-1. The mean particle size (D_AVG), the slope of the particle size distribution, and the total particle number (Σ 1.36mm - 230.14mm size classes) can then be calculated.

Results

On July 24th 2020, the manufacturer of the LISST-100X (Sequoia Scientific) announced that the LISST-100X would be discontinued and no longer serviceable. The replacement instrument (the LISST-200X) has a shorter pathlength and requires evaluation for limit of detection in the relatively clear waters of Station ALOHA. For this reason, we are reprocessing the full LISST-100X dataset for final release and conducting comparisons between LISST 100-X and LISST 200-X detection limits for 2020-2021. No additional LISST data will be released prior to this final re-processing.