The Degree of Linear Polarization for Suspended Particle Fields from Diverse Natural Waters

Abstract

The Degree of Linear Polarization (DoLP) for unperturbed particle fields in waters from six diverse regions around the globe was measured with the custom Multi-Angle Scattering Optical Tool (MASCOT). DoLP here is defined as the ratio of two elements of Mueller scattering matrix, i.e., -M12/M11. Field sites covered inland waters, coastal oceans and open oceans, including both ocean color Case I and II water types. The angular shape of the measured particulate DoLP was analyzed in detail for each field site and for the ensemble average. Three parameters used to quantitatively characterize DoLP shape were the symmetry with respect to 90°, peak magnitude, and peak angle of measured DoLP angular curve. Vertical profiles of particulate DoLP were analyzed with maximum recorded depth of 111 m. Converse to Rayleigh scatterers, we found measured particulate DoLPs were not symmetric with respect to 90°. On average, DoLP peaks were shifted slightly toward larger angles, with most falling between estimated values of 90° and 95°. All particulate DoLP peak magnitudes generally varied within [0.6, 0.9]. Lorenz-Mie (homogeneous sphere) light scattering theory was used to construct a new inversion for bulk particulate refractive index from a lookup table based on DoLP and spectral attenuation measurements. We compared the Mie-DoLP-based particulate refractive index retrieval with the backscattering-based model from (Twardowski et al., J. Geophys. Res., 2001, 106(C7), 14,129–14,142). Particulate refractive index retrieved with the two models were in some cases comparable. At two of the six field sites we saw good agreement between the two models, whereas at another two field sites we observed large discrepancies between the two models. Further investigation on the choice of the modeled particle shapes and compositions may improve this retrieval approach. Results are compatible with previous studies on DoLPs in natural waters and comprehensive observations are provided on the particulate DoLP angular shape, vertical profile and global distributions that are important for future vector radiative transfer simulations. This study is relevant to future PACE polarimeters and associated remote retrieval of oceanic particle composition using polarimetry.

Contribution of Submicron Particles to the Unpolarized and Linearly Polarized Angular Scattering

“…The values of θ max were 95 °and 91 °during NPO-18 for bulk and <0.7 μm particles, respectively, whereas the corresponding values during NAO-21 were 95 °and 93 °. Overall, θ max varied between 90 °and 95 °, shifting toward larger angles with increasing particle sizes, a pattern which is also observed in the previous studies (Koestner et al, 2020;Zhai and Twardowski, 2021). DoLP max for bulk particles was similar between the two sites with a median value of 0.73.…”

“…P 11,sw was estimated by the theoretical model developed by Zhang et al (2009) with concurrently measured temperature and salinity while P 12,sw and P 22,sw were calculated from P 11,sw by assuming a seawater depolarization ratio of 0.039 (Zhang et al, 2019). By definition, the degree of linear polarization is calculated as (Koestner et al, 2018;Zhai and Twardowski, 2021)…”

“…Again, DoLP by particles <0.2 μm was noisy and not shown. The peak magnitude (DoLP max ) and peak angle (θ max ) of DoLP were determined by fitting a skewed Rayleigh DoLP into the measured angular DoLP following the method proposed by Zhai and Twardowski (2021). DoLP max and θ max differed slightly between bulk and submicron particles in each site.…”

“…The Mueller matrix elements are determined by the particle size distribution (PSD), refractive index, shape, and internal structure of the particles. Through inversion, the measurements of Mueller matrix have also been used to characterize particles (Gordon and Brown, 1972;Brown and Gordon, 1973;Zaneveld et al, 1974;Kopilevich, 1983;Agrawal et al, 2008;Agrawal and Mikkelsen, 2009;Zhang et al, 2011;Twardowski et al, 2012;Zhang et al, 2012;Hu et al, 2020b;Zhai and Twardowski, 2021). For example, commercial instruments (e.g., LISST-100X) have been developed to infer concentration of particles of sizes from 1 to 200 μm from P 11 measured at angles from 0 to 15 ° (Agrawal and Pottsmith, 2000;Agrawal et al, 2008).…”

Hydrosol Scattering Matrix Inversion Across a Fresnel Boundary

“…All such instruments are complex and labor intensive to build, making hydrosol Mueller matrix measurements sparse. The MASCOT instrument (Twardowski et al, 2012) has been providing some notable datasets and analyses (Twardowski and Tonizzo, 2018;Zhai and Twardowski, 2021) for several years, and the POLVSM (Chami et al, 2014) is a recent entry to the field. A commercial instrument capable of in situ polarized light scattering measurements is now available (LISST-VSF; Sequoia Scientific), however it provides only 3 Mueller matrix elements.…”