Polarized lidar and ocean particles: insights from a mesoscale coccolithophore bloom

Brian L. Collister; Richard C. Zimmerman; Victoria J. Hill; Charles I. Sukenik; William M. Balch

doi:10.1364/AO.389845

Applied Optics
Vol. 59,
Issue 15,
pp. 4650-4662
(2020)
•https://doi.org/10.1364/AO.389845

Polarized lidar and ocean particles: insights from a mesoscale coccolithophore bloom

Brian L. Collister, Richard C. Zimmerman, Victoria J. Hill, Charles I. Sukenik, and William M. Balch

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Brian L. Collister, Richard C. Zimmerman, Victoria J. Hill, Charles I. Sukenik, and William M. Balch, "Polarized lidar and ocean particles: insights from a mesoscale coccolithophore bloom," Appl. Opt. 59, 4650-4662 (2020)

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- Atmospheric and Oceanic Optics
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About this Article
History
- Original Manuscript: February 20, 2020
- Revised Manuscript: April 13, 2020
- Manuscript Accepted: April 13, 2020
- Published: May 14, 2020

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Abstract

Oceanographic lidar can provide remote estimates of the vertical distribution of suspended particles in natural waters, potentially revolutionizing our ability to characterize marine ecosystems and properly represent them in models of upper ocean biogeochemistry. However, lidar signals exhibit complex dependencies on water column inherent optical properties (IOPs) and instrument characteristics, which complicate efforts to derive meaningful biogeochemical properties from lidar return signals. In this study, we used a ship-based system to measure the lidar attenuation coefficient ($ \alpha $) and linear depolarization ratio ($ \delta $) across a variety of optically and biogeochemically distinct water masses, including turbid coastal waters, clear oligotrophic waters, and calcite rich waters associated with a mesoscale coccolithophore bloom. Sea surface IOPs were measured continuously while underway to characterize the response of $ \alpha $ and $ \delta $ to changes in particle abundance and composition. The magnitude of $ \alpha $ was consistent with the diffuse attenuation coefficient (${K_{\text{d}}}$), though the $ \alpha $ versus ${K_{\text{d}}}$ relationship was nonlinear. $ \delta $ was positively related to the scattering optical depth and the calcite fraction of backscattering. A statistical fit to these data suggests that the polarized scattering properties of calcified particles are distinct and contribute to measurable differences in the lidar depolarization ratio. A better understanding of the polarized scattering properties of coccolithophores and other marine particles will further our ability to interpret polarized oceanographic lidar measurements and may lead to new techniques for measuring the material properties of marine particles remotely.

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Symbol	Definition	Dimensions
Lidar Parameters
$α$	Lidar attenuation coefficient	$m^{- 1}$
$δ$	Lidar depolarization ratio	Dimensionless
$r_{ss}$	Range from the lidar face to the sea surface; along the path of the lidar beam	m
$r$	Range from the sea surface; along the path of the lidar beam	m
$S_{c}$	Co-polarized PMT voltage	V
$S_{x}$	Cross-polarized PMT voltage	V
$t_{0}$	Time zero of the lidar return signal	ns
$t_{ss}$	Timing of the lidar return from the sea surface	ns
$Δ t_{ss}$	$t_{0} - t_{ss}$	ns
$t$	Time since $t_{0}$	ns
Inherent Optical Properties (IOPs) of the Water Column
$a_{g}$	Absorption by dissolved ( $< 0.2 µ m$ ) material	$m^{- 1}$
$a_{p}$	Absorption by particulate material	$m^{- 1}$
$a_{gp}$	Absorption by particulate and dissolved ( $< 0.2 µ m$ ) material	$m^{- 1}$
$a_{w}$	Absorption by pure water	$m^{- 1}$
$a$	Total absorption ( $= a_{gp} + a_{w}$ )	$m^{- 1}$
$b_{p}$	Scattering by particulate material	$m^{- 1}$
$b_{sw}$	Scattering by pure seawater	$m^{- 1}$
$b$	Total scattering ( $= b_{p} + b_{sw}$ )	$m^{- 1}$
$b_{bp}$	Backscattering by particulate material	$m^{- 1}$
$b_{bsw}$	Backscattering by pure seawater	$m^{- 1}$
$b_{b}$	Total backscattering ( $= b_{bp} + b_{bsw}$ )	$m^{- 1}$
${\tilde{b}}_{b}$	Backscattering ratio ( $= b_{b} / b$ )	Dimensionless
$b_{b}^{'}$	Acid-labile backscattering	$m^{- 1}$
${b_{b}}^{'} / b_{b}$	Acid-labile backscattering ratio	Dimensionless
$c_{g}$	Beam attenuation by dissolved ( $< 0.2 µ m$ ) material	$m^{- 1}$
$c_{p}$	Beam attenuation by particulate material	$m^{- 1}$
$c_{gp}$	Beam attenuation by dissolved ( $< 0.2 µ m$ ) and particulate material	$m^{- 1}$
$c$	Total beam attenuation coefficient ( $= a + b$ )	$m^{- 1}$
$ℓ_{b}$	Scattering optical depth ( $= b r$ )	Dimensionless
$M_{xy} (θ)$	Normalized Mueller matrix element in row $x$ , column $y$ ; scattering angle of $θ$	Dimensionless
$ω_{o}$	Single scattering albedo ( $= b / c$ )	Dimensionless
Apparent Optical Properties (AOPs) of the Water Column
$K_{d}$	Diffuse attenuation coefficient measured using Satlantic HyperPro	$m^{- 1}$
$K_{d,s}$	Diffuse attenuation coefficient modeled from surface IOP measurements using the model of Lee, et al. [35]; solar zenith angle was set to 0 (directly overhead)	$m^{- 1}$
Biogeochemical Properties
[PIC]	Particulate inorganic carbon concentration	$mg m^{- 3}$
[POC]	Particulate organic carbon concentration	$mg m^{- 3}$

Regression Model
$f (x, y) = a + b x + c y + d x y$ ; $x = {b_{b}}^{'} / b_{b}$ ; $y = ℓ_{b}$
Model Parameter	$Value \pm 95 % CI$
$a$	$0.130 \pm 0.0060$
$b$	$- 0.123 \pm 0.049$
$c$	$0.00859 \pm 0.0020$
$d$	$0.0967 \pm 0.012$
Regression Statistic	Value
$r^{2}$	0.87
df	947
RMSE	0.0234
SSE	0.518

Symbol	Definition	Dimensions
Lidar Parameters
$α$	Lidar attenuation coefficient	$m^{- 1}$
$δ$	Lidar depolarization ratio	Dimensionless
$r_{ss}$	Range from the lidar face to the sea surface; along the path of the lidar beam	m
$r$	Range from the sea surface; along the path of the lidar beam	m
$S_{c}$	Co-polarized PMT voltage	V
$S_{x}$	Cross-polarized PMT voltage	V
$t_{0}$	Time zero of the lidar return signal	ns
$t_{ss}$	Timing of the lidar return from the sea surface	ns
$Δ t_{ss}$	$t_{0} - t_{ss}$	ns
$t$	Time since $t_{0}$	ns
Inherent Optical Properties (IOPs) of the Water Column
$a_{g}$	Absorption by dissolved ( $< 0.2 µ m$ ) material	$m^{- 1}$
$a_{p}$	Absorption by particulate material	$m^{- 1}$
$a_{gp}$	Absorption by particulate and dissolved ( $< 0.2 µ m$ ) material	$m^{- 1}$
$a_{w}$	Absorption by pure water	$m^{- 1}$
$a$	Total absorption ( $= a_{gp} + a_{w}$ )	$m^{- 1}$
$b_{p}$	Scattering by particulate material	$m^{- 1}$
$b_{sw}$	Scattering by pure seawater	$m^{- 1}$
$b$	Total scattering ( $= b_{p} + b_{sw}$ )	$m^{- 1}$
$b_{bp}$	Backscattering by particulate material	$m^{- 1}$
$b_{bsw}$	Backscattering by pure seawater	$m^{- 1}$
$b_{b}$	Total backscattering ( $= b_{bp} + b_{bsw}$ )	$m^{- 1}$
${\tilde{b}}_{b}$	Backscattering ratio ( $= b_{b} / b$ )	Dimensionless
$b_{b}^{'}$	Acid-labile backscattering	$m^{- 1}$
${b_{b}}^{'} / b_{b}$	Acid-labile backscattering ratio	Dimensionless
$c_{g}$	Beam attenuation by dissolved ( $< 0.2 µ m$ ) material	$m^{- 1}$
$c_{p}$	Beam attenuation by particulate material	$m^{- 1}$
$c_{gp}$	Beam attenuation by dissolved ( $< 0.2 µ m$ ) and particulate material	$m^{- 1}$
$c$	Total beam attenuation coefficient ( $= a + b$ )	$m^{- 1}$
$ℓ_{b}$	Scattering optical depth ( $= b r$ )	Dimensionless
$M_{xy} (θ)$	Normalized Mueller matrix element in row $x$ , column $y$ ; scattering angle of $θ$	Dimensionless
$ω_{o}$	Single scattering albedo ( $= b / c$ )	Dimensionless
Apparent Optical Properties (AOPs) of the Water Column
$K_{d}$	Diffuse attenuation coefficient measured using Satlantic HyperPro	$m^{- 1}$
$K_{d,s}$	Diffuse attenuation coefficient modeled from surface IOP measurements using the model of Lee, et al. [35]; solar zenith angle was set to 0 (directly overhead)	$m^{- 1}$
Biogeochemical Properties
[PIC]	Particulate inorganic carbon concentration	$mg m^{- 3}$
[POC]	Particulate organic carbon concentration	$mg m^{- 3}$

Regression Model
$f (x, y) = a + b x + c y + d x y$ ; $x = {b_{b}}^{'} / b_{b}$ ; $y = ℓ_{b}$
Model Parameter	$Value \pm 95 % CI$
$a$	$0.130 \pm 0.0060$
$b$	$- 0.123 \pm 0.049$
$c$	$0.00859 \pm 0.0020$
$d$	$0.0967 \pm 0.012$
Regression Statistic	Value
$r^{2}$	0.87
df	947
RMSE	0.0234
SSE	0.518

Abstract

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Applied Optics