Two-term Reynolds-McCormick phase function parameterization better describes light scattering by microalgae and mineral hydrosols.

The presence of hydrosols, taken as suspension of micro- or macroscopic material in water, strongly alters light propagation and thus the radiance distribution within a natural or artificial water volume. Understanding of hydrosols' impacts on light propagation is limited by our ability to accurately handle the angular scattering phase function inherent to complex material such as suspended sediments or living cells. Based on actual quality-controlled measurements of sediments and microalgae, this Letter demonstrates the superiority of a two-term five-parameter empirical phase function as recently proposed for scattering by nanoparticle layers [Nanoscale11, 7404 (2019)NANOHL2040-336410.1039/C9NR01707K]. The use of such phase function parameterizations presents new potentialities for various radiative transfer and remote sensing applications related to an aquatic environment.

Forward modeling of inherent optical properties from flow cytometry estimates of particle size and refractive index.

A Mie-based forward modeling procedure was developed to reconstruct bulk inherent optical properties (IOPs) from particle size distributions (PSDs) and real refractive index distributions (PRIDs) obtained using a previously developed flow cytometric (FC) method [Appl. Opt.57, 1705 (2018)APOPAI0003-693510.1364/AO.57.001705]. Given the available PSDs, extrapolations for the particle fraction outside the detection limits of the method and a complex refractive index input (with real part nr directly estimated and imaginary part ni adapted from the literature separately for organic and inorganic components), the model produces volume scattering functions that are integrated to produce scattering and backscattering coefficients, and absorption efficiencies that are used to calculate absorption coefficients. The procedure was applied to PSDs and PRIDs derived from natural samples retrieved in UK coastal waters and analyzed using a CytoSense flow cytometer (CytoBuoy b.v., The Netherlands). Optical closure analysis was carried out between reconstructed IOPs and in situ IOPs measured using an ac-9 spectrophotometer and a BB9 backscattering meter (WET Labs Inc., OR) in the same waters. The procedure is shown to achieve broad agreement with particulate scattering (bp) and backscattering (bbp) [root mean square percentage error (RMS%E): 35.3% and 44.5%, respectively) and to a lesser degree with backscattering ratio (b˜bp) (RMS%E: 77%). The procedure, however, generally overestimated particulate absorption (ap) (RMS%E: 202.3%). This degree of closure was dependent on applying recently developed scattering error corrections to both absorption and attenuation in situ measurements. Not only do these results indirectly validate the FC method as a useful tool for PSD and PRID determination in natural particle populations, they also suggest that Mie theory may be a sufficient model for bulk IOP determination, with previously reported difficulties potentially being caused by inadequately corrected IOP measurements. Finally, in a feature unique to the FC method, the concurrent size and refractive index retrieval enabled assessment of the relative contributions that organic versus inorganic, fluorescent versus non-fluorescent fractions of the particle populations had on the IOPs, and identified which size classes had the largest influence on each of these properties.

Evaluation of a flow cytometry method to determine size and real refractive index distributions in natural marine particle populations.

A flow cytometric (FC) method was developed to retrieve particle size distributions (PSDs) and real refractive index (nr) information in natural waters. Geometry and signal response of the sensors within the flow cytometer (CytoSense, CytoBuoy b.v., Netherlands) were characterized to form a scattering inversion model based on Mie theory. The procedure produced a mesh of diameter and nr isolines where each particle is assigned the diameter and nr values of the closest node, producing PSDs and particle real refractive index distributions. The method was validated using polystyrene bead standards of known diameter and polydisperse suspensions of oil with known nr, and subsequently applied to natural samples collected across a broad range of UK shelf seas. FC PSDs were compared with independent PSDs produced from data of two LISST-100X instruments (type B and type C). PSD slopes and features were found to be consistent between the FC and the two LISST-100X instruments, but LISST concentrations were found in disagreement with FC concentrations and with each other. FC nr values were found to agree with expected refractive index values of typical marine particle components across all samples considered. The determination of particle size and refractive index distributions enabled by the FC method has potential to facilitate identification of the contribution of individual subpopulations to the bulk inherent optical properties and biogeochemical properties of the particle population.