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.
Highlights
A considerable number of Degree of Linear Polarization (DoLP) curves exhibited large, highly intermittent spikes at 30°, 40°, 140°, 150°, and 160°. These isolated spikes at specific angles are generated by particulates such as large organisms and aggregates drifting into the sample volume, and isolated light paths for individual detectors
Spikes with magnitudes larger than 0.2 were flagged and these DoLPs were removed from the analysis
The HI data showed more variability in the backward direction due to the very low measured scattering signals in this clear water; this was from a higher contribution from Multi-Angle Scattering Optical Tool (MASCOT) instrument noise
Summary
Light scattering measurements of ocean waters have been used to infer the marine biological states and the microphysical properties of marine particulates (Brown and Gordon 1973; Gordon 1988; Zaneveld 1995; Subramaniam et al, 2001; Twardowski et al, 2001; Lee et al, 2002; Maritorena et al., 2002; Behrenfeld et al, 2005; Kostadinov et al, 2009). The VSF describes the angular distribution of unpolarized scattered radiation by a volume element of water. The VSF itself and its various derived quantities such as the scattering coefficient (b), backscattering coefficient (bb), and backscattering ratio (bb/b), have been increasingly studied and utilized in marine optical sensing (Twardowski et al, 2001; Lee et al, 2002; Maritorena et al, 2002; Boss et al, 2004; Sullivan et al, 2005; Loisel et al., 2007; Twardowski et al, 2007; Kostadinov et al, 2009; Zhang et al, 2011; Twardowski and Tonizzo 2018; Zhai et al, 2020)
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