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Validation of Particle Size Distribution Obtained by Laser In-Situ Scattering and Transmission (LISST) in Flow-Through Mode
- Authors
- Name
- Universal Lab
- @universallab
The particle size distribution (PSD) in aquatic environments exhibits key biogeochemical and ecosystem dynamics characteristics, such as particle surface processes, ascent and settling rates, and predator-prey interactions. Therefore, obtaining large-scale particle size information in the surface ocean is of significant importance.
Several algorithms have been proposed to retrieve particle size information from satellite ocean color remote sensing data, though their validation has been limited due to the low coverage of in-situ measurements. Traditional benchtop particle analyzers, such as the Coulter Counter, are not equipped with continuous automated sampling devices, and measuring discrete water samples requires significant time.
In-situ observation techniques, such as the Laser In-Situ Scattering and Transmission (LISST) instrument (LISST-100X, Sequoia Scientific), allow for rapid automatic particle counting and sizing. Over the past two decades, LISST instruments have been successfully used to obtain high-resolution vertical profiles of PSD, yet there remains limited surface water spatial coverage to match satellite ocean color remote sensing data.
Although LISST was originally designed for sediment transport studies in particle-dense bottom boundary layers, the incorporation of dynamic gain has extended its applicability to clearer oceanic waters. Here, we tested the potential of LISST in a shipboard flow-through system, in which water was continuously pumped onto a research vessel, enabling large-scale particle size measurements across oceanic regions. Measurements were conducted during a North Atlantic cruise, and the particle size spectra obtained by LISST were evaluated and validated using optical theory and independent size measurements collected with the Coulter Counter (CC) and Imaging FlowCytoBot (IFCB).
We also compared the power-law fit exponent of the LISST-derived PSD spectra with the exponent obtained from spectral particle beam attenuation and particle backscattering measurements. The latter properties have been shown to contain information related to the steepness of PSD, both theoretically and through in-situ observations in coastal environments and bottom boundary layers. However, they have yet to be linked to size spectra in the surface ocean.
Flow-Through System
In September 2017, during a North Atlantic expedition, we deployed a flow-through system aboard the R/V Atlantis to measure the optical properties of surface ocean waters (Fig. 1). This system was similar to those described in previous studies. A unique aspect of these systems is the transition between unfiltered seawater sampling and 0.2-micron filtered water sampling for 10 minutes every hour. If stable between consecutive cycles, measurements of filtered water provide a baseline calibration for particle optical measurements (via subtraction). The stability of this baseline calibration was also monitored using an online CDOM fluorometer.
LISST Measurements and Inferred Particle Size Distributions
Standard procedures were followed for LISST data processing, including attenuation correction along the laser path, source output variation (laser reference), detector area correction (referred to as dcal), and background subtraction (zscat). Particle beam attenuation cp(670) was computed by measuring transmittance through the beam (0.0269° acceptance angle) and the laser reference detector via a pinhole detector. For most applications, Sequoia provides a standard detector area correction (detector array calibration value of 32), which is applied to raw detector array scattering measurements.
However, in this study, the LISST was post-cruise calibrated by the manufacturer using NIST-traceable submicron beads, where it was determined that the dcal of the two outer detector rings should be adjusted. This step significantly improved the consistency between CC and LISST at the smaller size edge of the derived PSD. Using this factor, the calibrated light intensity (cscat) for each detector was obtained. For attenuation and scattering measurements, background measurements were based on a linear interpolation of filtered seawater measurements conducted for 10 minutes every hour.
We applied the manufacturer-provided spherical particle (Mie scattering) inversion to obtain the volume distribution (VD, μL L⁻¹ or ppm), from which the differential volume distribution (VSD, μL L⁻¹ μm⁻¹) was computed by dividing each size bin value by the bin width. Assuming particles were spherical, the differential area size distribution (ASD, m⁻¹ μm⁻¹) was further derived from VSD. The instrument output consisted of 32 size bins ranging from 1.25 μm to 250 μm.
It has been observed that inversion results may be problematic for the first and last bins due to scattering contributions from particles outside this size range [10,15]. Thus, we focused on particles >2.03 μm (the third smallest LISST size bin). Due to the presence of large rare particles, significant uncertainty exists at the higher end of the size range. LISST optical measurements are most sensitive to particle cross-sectional area, meaning that for non-spherical particles, the inferred size distribution is broader than that obtained from instruments that rely on particle volume measurements.
Results and Discussion
Particle beam attenuation estimated from LISST and ac-s showed good correlation, with LISST values being slightly higher (by an average of 0.04 m⁻¹, Fig. 2(a), Table 1). Given the differences in acceptance angle, this discrepancy was expected. The strong numerical consistency suggests that, compared to coastal environments, oceanic particles have a lower enrichment in large particles.
LISST beam attenuation also correlated well with cross-sectional area inverted from angular scattering measurements (Fig. 2(b)), as well as their ratio. The attenuation efficiency factor (Qc) fell within an acceptable theoretical range (Fig. 2, median Qc = 2.7, mean Qc = 3.1 ± 0.05), indicating consistency between LISST attenuation and scattering measurements. This also highlights the fact that most attenuation-related particles were captured by LISST scattering measurements (if a significant fraction of particles were missed, Qc would be much higher due to underestimation of cross-sectional area). Ignoring the first three and last three inverted bins altered results by approximately 15% (median Qc = 3, mean Qc = 3.5 ± 0.07).
We found that LISST provided a size distribution that generally aligned in shape with the CC-derived size range. However, CC overestimated the LISST size distribution by a factor of 2.5. The post-cruise calibration of LISST validated the amplitude scaling factor used in our analysis. Furthermore, when comparing LISST particle volume estimates with chlorophyll-containing particle volumes from LISST, IFCB (covering the 2.9–20.8 μm range), and discrete POC samples, we found that LISST PSD estimates were consistent with reported phytoplankton contributions to POC as well as published POC-to-microbial volume ratios.
[1].Emmanuel Boss, Nils Haëntjens, Toby K. Westberry, Lee Karp-Boss, and Wayne H. Slade, "Validation of the particle size distribution obtained with the laser in-situ scattering and transmission (LISST) meter in flow-through mode," Opt. Express 26, 11125-11136 (2018)