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Abstract
An instrument named as Volume Scattering and Attenuation Meter (VSAM) is presented. The VSAM can simultaneously measure the attenuation coefficient and the volume scattering function (VSF) from 10° to 170° with an interval of 10° at 659 nm. Using ultrapure water and NCRM-traceable polystyrene microsphere beads, the VSAM was calibrated, and the conversion factor χbθ for estimating the backscattering coefficient from the backward VSF was obtained based on Mie theory in the laboratory. For χbθ, the average relative deviation was no more than 7.77% in the range of 100°-160° between the modeled result based on VSAM and the theoretical result by Boss. Subsequently, the VSAM and ECO-VSF3 were deployed in situ in Zhanjiang Bay. The backscattering coefficient and VSF at the same angles measured by the two instruments were quite consistent. Some remarkable changes in the shape and magnitude of the VSF profile at different stations were found, with land-based pollutants composing an important suspicious source of these changes.
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1. Introduction
The propagation of underwater light depends on the absorption and scattering characteristics of substances in the water. The absorption and scattering characteristics of seawater and its components can be represented by the absorption coefficient $a$ (${\textrm{m}^{ - 1}}$) and volume scattering function (VSF, $\beta (\theta )$, ${\textrm{m}^{ - 1}}\textrm{s}{\textrm{r}^{ - 1}}$), respectively. Absorption only changes intensity, the VSF is the dominant parameter affecting the dynamics of radiance field distributions [1], and the shape of the upwelling radiance distribution, described with respect to the downwelling incident light field by the bidirectional reflectance distribution function (BRDF), has been shown to be largely governed by the shape of the VSF [2]. Therefore, a rigorous measurement and determination of VSF is critical for the upper-ocean heat balance [3,4], the photosynthetic productivity of the ocean [5–7], and aquatic environments monitor and pollution waring [8–10].
The VSF is one of the least-known optical properties of water, and there are few findings on its relationship with constituents of seawater relative to the absorption characteristics [9,11]. It is difficult to carry out general-angle VSF measurements due to complicated measurement geometries and the need for accurate calibration at forward and backward scattering angles [1,12]. The VSF measurement technique, particularly for the VSF over the full angular range, is still under exploration [1,3,13–16].
One typical measurement principle for VSF determination—advanced and used by investigators from the 1940s to the 2000s—is based on an optical sensor or a light projector rotating around the center of the scattering volume [3,17–23]. The latest advanced instrument, MVSM [3], can obtain VSFs at a general angle (from 0.6° to 177.9°) and high angular resolution (0.3°). However, due to the rotating element in these instruments, the measurements are typically quite time-consuming and power-wasting during step-by-step scattering over the entire range of angles. From the 2000s to the present, an improved method by mounting sensors at different discrete scattering angles has been developed [15,2], and the VSF from 10° to 170° (intervals 10°) can be acquired by the latest device (MASCOT) while synchronously measuring the attenuation coefficient by other meters, such as ac-s. This approach greatly improves the measurement speed, dynamic range, and calibration precision of the VSF. In the last few years, a novel image detection VSF meter (I-VSF) has been developed and used in the laboratory [14]. Based upon a combination of two reflectors and a CCD camera, without any moving optical parts, a determination of scattering over a wide range of angles (8°∼172°, 1° interval) can be obtained swiftly within a practical time period. Recently, starting with the LISST and a new eyeball component, a commercialized VSF instrument (LISST-VSF) was developed that can measure the VSF from 0.1° to 155° (intervals less than 1°) in several seconds. By combining attenuation coefficients, all the above methods and instruments have been gradually applied to bio-optical models, bubble traces, remote sensing of ocean color, radiative transfer, and upper-ocean heat balance. In the future, the quick and direct measurement technology of the general-angle VSF and other basic inherent optical properties, such as the attenuation coefficient and absorption coefficient, will play an important role in applying oceanic optics and its related fields with the increase in application requirements and progress of measurement technology and theoretical methods. The objective of this paper is to describe a new instrument, the so-called Volume Scattering and Attenuation Meter (VSAM), which can independently and synchronously obtain the attenuation coefficient, the VSF at discrete angles between 10° and 170° with 10° intervals, and the depth and temperature information in the profile of the seawater.
This paper is organized as follows. First, the VSAM development is presented in detail, including the background and the design of the VSAM. Then, based on Mie scattering theory, the calibration procedure, including the calibration method, calibration process and estimation of the backscattering coefficient from backward VSF, is studied under controlled laboratory conditions. Finally, the in situ observation results measured by the VSAM and ECO-VSF3 are compared and discussed.
2. Development of the VSAM
2.1 Background
The volume scattering function (VSF, $\beta (\theta )$), which describes the angular distribution of light scattered from an incident beam, is radiometrically defined as the ratio of the second-order partial derivatives of the scattering flux ($\mathrm{\Phi }(\theta )$) with respect to the scattering volume (${V_\theta }$) and scattering solid angle ($\Omega $) under the given scattering angle ($\theta $) to the incident irradiance ($E$) [3,18,24]:
where $\beta (\theta )$ is also a function of the wavelength, although it does not contain the wavelength explicitly. Assuming azimuthal symmetry and integrating the VSF over all solid angles (from 0° to 180°) yields the total scattering coefficient, b in units of ${\textrm{m}^{ - 1}}$. Integrating the VSF in the backward direction (from 90° to 180°) yields the backscattering coefficient, ${b_b}$ in units of ${\textrm{m}^{ - 1}}$. The phase function ($\widetilde {\beta (\theta )}$), is the VSF normalized to total scattering.2.2 Design of the VSAM instrument
A general schematic diagram of the measurement principle of the attenuation coefficient and the VSF at discrete angles between 10° and 170° with 10° intervals by the VSAM is shown in Fig. 1(a). Figure 1(b) shows the appearance of the VSAM structure.
In addition to this measurement principle and structure, the parameters of the mathematical definition of the VSF in Eq. (1) can be described as follows:
Combining Eq. (1), Eq. (2), and Eq. (3), the VSF and attenuation coefficient can be rewritten as
As formulated in Eq. (1), the scattered intensity is proportional to the irradiance energy incident upon the scattering volume. For the VSAM, a 659 nm semiconductor laser (PGL-VI-650-50 mW, provided by Changchun New Industry Optoelectronics Technology Co.), which provides approximately 50 mW peak power output, is used as the light source. The beam divergence angle of the light source is less than 1.0 mrad, the light beam is circularly polarized, and the maximum diameter within the optical path required for measurement is just 3.6 mm. The laser is set to a modulation frequency of 1 kHz to distinguish between scattered light and the background optical field, such as the solar background light in the sea surface [25], and the transmission and scattering receivers are synchronized, demodulated and amplified to distinguish the primal signal from the stray background light to be suitable for the analog-digital converter. A semitransparent mirror reflects 5% intensity of the source beam to a reference detector while 95% of the beam is transmitted to the plane of the aperture and output through the optical glass to form the light source of the instrument (at ${\mathrm{\Phi }_0}({0,0} )$ in Fig. 1(a)). The reference detector is set up in front of the exit optical window to facilitate the compensation of light source variations and the calculation of the attenuation coefficient during the calibration and measuring process.
Eighteen receivers for simultaneously measuring the beam transmission flux and volume scattering flux at eighteen discrete angles are tightly mounted on a semicircle with a diameter of 30 cm (see Fig. 1(b)). The detectors and light source are coplanar, and the angles range from 0° to 170° with 10° intervals, while the detector at 0° is used to measure the beam transmission (i.e., the attenuation coefficient). Each volume scattering flux receiver has an interference filter to reduce ambient light. Since the amount of scattering varies drastically depending on the angle, the amplifier gains and FOVs for different detectors are customized and adopted to improve the sensitivity and dynamic range of the VSF at different angles. The FOV are set as 0.8° (0°, 10°), 2.5° (20°∼70°), and 5° (80°∼170°) with different diameter stops to minimize smearing of the VSF forward signal and maintain effective detection of the scattering flux at different angles. For the 0° transmission detector, a part of the incident light that reaches the incident window is reflected by the window glass, and the scattering signal is contaminated by this reflected light. To minimize the primary light beam reflected from the 0° transmission detector, a custom-made light trap, i.e., a 5° sloped quartz glass (determined by the optical-mechanical structure of the VSAM), was used as the 0° transmission detector incident glass window. Considering the immersion factor, to obtain the minimum reflection contamination, the installation direction of the 0° transmission detector should be adjusted and fixed underwater.
For the transmission and scattering flux photoelectric signals, in addition to common frequency demodulation, the gain of each detector is optimized for the dynamic range specifically, and a 20-channel self-contained data acquisition system based on two ADS1262 chips (a 10-channel 32-bit analog-digital converter), an STM32 MCU and a TF card is designed and developed. In addition, a depth probe was integrated to acquire the depth and temperature of water and the sample process of the VSAM can be automatically controlled by its real-time profile depth. The raw transmitted and scattered photoelectric signals and their corresponding underwater depth and temperature are stored in real time in the TF card.
3. Calibration in the laboratory
3.1 Calibration method
The calibration method of the VSAM includes the VSF calibration method and the attenuation coefficient calibration method. The VSF calibration method based on Lorenz–Mie scattering theory and standard polystyrene beads was built, and the attenuation coefficient calibration method based on ultrapure water was studied. To carry out the calibration experiment in the laboratory, an anodal oxidized tin aluminum alloy plate was fixed to the bottom of the VSAM and formed a watertight calibration sample cell for liquid and beads (see Fig. 3(a)).
3.1.1 Calibration of the attenuation coefficient
The calibration of the attenuation coefficient was finished by importing a calibration constant ${C_t}$, which can be determined by the following equations. Equation (6) shows the general mathematical relationship between the signals of the transmission receiver and light source reference receiver after the introduction of ${C_t}$, and Eq. (7) shows the formula for calculating ${C_t}$ obtained by measuring ultrapure water.
Once the calibration constant ${C_t}$ is obtained, the attenuation coefficient of the sample can be calculated:
3.1.2 Calibration of $\boldsymbol{\beta }(\boldsymbol{\theta } )$
The calibration of the VSF is performed to acquire an accurate quantitative relationship between the raw scattering flux signals of each of the 17 scattering angle detectors and the volume scattering functions $\beta (\theta )$. The mathematical relationship in Eq. (4) can be simply represented as
Based on Lorenz-Mie scattering theory and beads, the key challenge for VSF calibration is to obtain the calibration factor ${f_\theta }$. Since the scattering phase function $\widetilde {{\beta _p}(\theta )}$ of the beads (the refractive index and size distribution are known) can be calculated by Lorenz-Mie scattering theory, Eq. (9) can be rewritten as
In Eqs. (9) and (10), to obtain $\beta (\theta )$, a synchronized absorption coefficient measuring instrument must be incorporated within the VSAM. To simplify the measurement and calibration process without an additional instrument configuration, the scattering correction part in Eq. (9) is approximated, and Eq. (9) can be rewritten as
Because of the closure of the inherent optical properties of the water ($c - {a_w} - {a_{pg}} = b$), it is clear that ${a_{pg}}$ is overcorrected in Eq. (11); to investigate the effect this overcorrection can have on the calibration result, a simple computational model is constructed as in Eq. (12). The simplification results are shown in Eq. (13).
3.2 Calibration in the laboratory
3.2.1 Collection of ${f_\theta }$
According to Eq. (10), ${f_\theta }$ is the key factor in calibrating the raw scattered flux signal of each scattering angle $\theta $ to $\beta (\theta )$, and the dark current (${D_\theta }$), the raw scattered flux signals to ultrapure water ($D{N_{w\theta }}$), and the raw scattered flux signals to NCRM-traceable polystyrene microsphere beads ($D{N_\theta }$) should be obtained to obtain ${f_\theta }$. Beads with a modal diameter of 0.2 $\mathrm{\mu} \textrm{m}$ and a variation coefficient of 2.3% as shown in Table 1 (size parameters were provided by the manufacturer, Beijing Coastal Hongmeng Standard Material Co.) are used. The parameter $\widetilde {{\beta _p}(\theta )}$ of the beads can be calculated by the MieSimulatorGUI program (https://github.com/VirtualPhotonics/MieSimulatorGUI/wiki/Downloads), which is provided by the Virtual Photonics Program, Beckman Institute for Laser Research, University of California. Figure 2 shows the $\widetilde {{\beta _p}(\theta )}$ of 0.2 $\mathrm{\mu} \textrm{m}$ beads in Table 1.
Fig. 2. Parameter $\widetilde {{\beta _p}(\theta )}$ of 0.2 $\mathrm{\mu} m$ standard polystyrene microsphere beads calculated by MieSimulatorGUI.
Table 1. Details of the NCRM-traceable polystyrene microsphere beads
For VSAM calibration, the first step is pumping in approximately 2 L ultrapure water (resistivity: 18.2$\mathrm{\;\ M\Omega }$), which has been filtered by a 5 µm filter to prevent large bubbles from entering the sample cell (see Fig. 3(b)), and the raw digital signals of transmitted and scattered and their relative standard deviations (RSD) are measured, calculated and recorded. Then, the standard polystyrene beads, which have been adequately mixed by ultrasonic oscillation, are dropped into the center of the scattering volume (see Fig. 3(c)), and the raw digital signals of transmitted and scattered light and their relative standard deviations (RSD) are also calculated and recorded.
Fig. 3. (a) Structure diagram of the calibration sample cell; (b) pumping of ultrapure water into the sample cell; (c) mixed solutions with standard beads.
Based on all the above signals and Eq. (10), to obtain ${f_\theta }$, another key parameter, the scattering coefficient ${b_p}$ of the beads, needs to be determined. This issue was considered when designing the experiment since the absorption characteristics of standard polystyrene beads can be negligible in comparison to the scattering characteristics of the beads (${c_p} = {b_p}$); in addition, the attenuation coefficient c of the samples, which is measured in synchrony by the 0° detector of the VSAM and calibrated by Eq. (8), includes the attenuation of the ultrapure water (${c_w}$) and beads (${c_p}$), where ${b_p}$ is obtained by $c - {c_w}$ while ${c_w}$ is known. Then, ${\beta _p}(\theta )$ is calculated by multiplying the theoretical phase function with the measured ${b_p}$ by the VSAM.
Once all the above parameters are acquired, the calibration factors ${f_\theta }$ can be obtained according to Eq. (10).
3.2.2 Result of calibration
Figure 4(a) shows the RSD of dark offset (${D_\theta }$) for all detectors of the VSAM, including the reference light (REF), transmitted angle (0°), and scattering angles (10°∼170°), and the average is 3.21%. Figure 4(b) shows the RSD of the response raw signals of the VSAM to ultrapure water for all detectors, in which the minimum and maximum variation are at 0° and 110°, reaching 0.05% and 8.53%, respectively, and the average is 2.56%. Similarly, Fig. 4(c) shows the RSD of the measurement values by VSAM for standard beads, and the average is merely 1.35%. This is a perfect illustration of the stability of the VSAM, and the above-detailed data can be found in Table 2.
Fig. 4. (a) RSD of the dark offset; (b) RSD of the response signals to ultrapure water; (c) RSD of the response signals to standard beads, d = 0.2 µm.
Table 2. VSAM measurement signals and their RSD for dark offset, ultrapure water, and 0.2 ${\mathrm{\mu} \mathrm{m}}$ standard polystyrene beads
The average of the calibration factors ${f_\theta }$ can subsequently be computed from multiple experimental measurements of the 0.2 $\mathrm{\mu} \textrm{m}$ bead scattering coefficient and scattered raw signals at different angles; ${f_\theta }$ and its RSD are shown in Fig. 5, and the average RSD of ${f_\theta }\; $ is less than 1.35%.
3.2.3 Estimation of the backscattering coefficient and validation of the calibration factor
The linear relationship between the backscattering coefficient and the VSF at certain angles in the backscattering direction has been studied by many scholars. In Oishi’s study [29], 120° is the most appropriate angle for linear inversion of the backscattering coefficient of particle form VSF. After reformulating Oishi’s analysis, Maffione and Dana [24] found that 140° is a more appropriate angle for BBC-4 (a four-wavelength backscattering sensor), and the linear inversion factor in 140$^\circ $ is 1.08. Subsequently, Boss et al. [30] proposed a more suitable value of approximately 120° ($\mathrm{\chi }({117^\circ } )= 1.1$), resulting in an error of less than 4%. They also inferred that 120° to 160° is a possible range for inversion backscattering coefficients from the VSF, and based on the in situ measured data by LISST-VSF, Zhang [31] verified this inference by estimating ${\chi _p}$ from the VSF.
For the VSAM, we obtain the suitable conversion factor ${\chi _{b\theta }}$. The normalized backscattering coefficient ($\widetilde {{b_b}}$) is introduced to calculate ${b_b}$ of the standard polystyrene beads (0.2 $\mathrm{\mu} \textrm{m}$) and ultrapure water solution sample as in Eq. (14), and $\widetilde {{b_b}}$ can be obtained by integrating $\widetilde {\beta (\theta )}$ as in Eq. (15), where b and $\beta (\theta )$ for the beads and ultrapure water solution sample can be measured by the VSAM. Then, ${\chi _{b\theta }}$ can be calculated as in Eq. (16).
To obtain the linear conversion factor ${\chi _{b\theta }}$, a series of attenuation coefficients c and the VSF $\beta (\theta )$ of the standard polystyrene beads (0.2 $\mathrm{\mu} \textrm{m}$) and ultrapure water solution sample are measured in the lab. Based on the measured b (the measured $c = \; {b_p} + {b_w} + {a_w}$ and $b = {b_p} + {b_w}$ for calibration) and $\widetilde {{b_{bp}}}$ and $\widetilde {{b_{bw}}}$ obtained according to the theoretical volume scattering phase function of ultrapure water and beads, respectively, ${b_b}$ of the solution sample was obtained by the additivity of IOPs. Then, combined with the measured $\beta (\theta )$, Eq. (16), ${\chi _{b\theta }}$ at different backward angles is calculated. Figure 6 shows a similar comparison of ${\chi _{b\theta }}$ (in 100°∼160°) among the VSAM and the theoretical estimation result based on Lorenz–Mie scattering theory by Boss et al. [30]. The maximum deviation is no more than 14% (at 140°), the minimum deviation is less than 0.5% (at 110°), and the average relative deviation is no more than 7.77% in the range of 100°-160° between the VSAM and Boss’s conclusion. This not only verifies the feasibility and reliability of the model for estimating the backscattering coefficient based on the VSAM backward single-angle volume scattering function but also confirms the calibration accuracy of the VSAM.
4. Observations
4.1 Field test overview
In situ experiments were performed in Zhanjiang Bay (ZJB) from January 6th to 7th, 2021. A total of 15 volume scattering function and attenuation coefficient profiles were acquired with the VSAM, normally in conjunction with depth-temperature profiles. To better illustrate the profile measurement capability of the VSAM, ECO-VSF3 [32] was simultaneously deployed while obtaining several profiles. All in situ profile measurements were carried out on an 80-ton fishing boat. Since there was no winch on the boat, the underwater profile lower and raising of the VSAM and ECO-VSF3 were controlled manually. To investigate the variation in the VSF with the angle, depth, and space in ZJB, the experiments were performed in areas with various water qualities, including an industrial sewage outlet, a pier, and the mouth of the river. Subject to the length of the cable and the operability and safety of manual lowering and lifting, the VSAM profile depths were relatively shallow to protect the VSAM and the operators. The maximum depth of measurements was 14 $\textrm{m}$, and the minimum depth was 3.5 $\textrm{m}$ during the entire measurement process. The profile self-contained measurements of the VSAM were automatically controlled by the depth underwater, and the detailed VSF, attenuation coefficient, depth, and temperature data were continuously recorded in situ. Figure 7(a) provides a map of the voyage operation locations, and a photograph of the in situ experiments is shown in Fig. 7(b).
Fig. 7. (a) Station map of the Zhanjiang Bay field test (Drawn by Ocean Data view); (b) Instrument being lowered.
4.2 Analysis of the in situ data
ZJB is located on the southern coast of Guangdong and is surrounded by the Leizhou Peninsula, Donghai Island, and Nansan Island [33]. With 241 km of coastline, including 97 km of deep-water shoreline, ZJB is an excellent natural harbor, but it also is a semi-closed bay with poor hydrodynamic conditions, and there are more than ten canals of industrial and domestic sewage discharge wastewater into the bay, which makes ZJB turbid and eutrophic.
The variation characteristics of the VSF with angle and depth in the observation profiles are presented in Fig. 8, and to describe a more realistic feature of the VSF in angular and spatial distribution, all data in Fig. 8 are original and unsmoothed. Due to the manual lowering, control of the depth resolutions of some stations, such as S15, S17, and S18, is not uniform. Fortunately, the sampling frequency of the VSAM is higher (10 Hz in situ), and satisfactory profile sampling results are obtained at most of the observation stations despite some adverse conditions that cannot be overcome.
At all 15 stations, three stations (S1, S2, and S3 in Fig. 7(a)) near the industrial sewage outlet from a steel mill located southwest of the S1 station, have larger VSFs than the other stations. In addition, a general decreasing trend of VSFs and attenuation coefficients can be detected from S1 to S20 (Fig. 9), and the VSF profiles of inshore stations are higher than those of offshore stations. For example, the VSF of 10° in the S18 profile is an order of magnitude higher than the neighboring station profiles (S17, S19). This may be caused by the discharge from the land into the sea pollutants since ZJB is a typical land pollution bay, and the pollution from land and river brings sediment, minerals, and nutrients into the bay and makes ZJB turbid and eutrophic. This speculation is borne out by several other similar sites, such as the mouth of the Nanliu River, where it empties into ZJB (station S8, S9), and the bifurcation of the main channel and the diversion in ZJB (station S13, S18). Moreover, in the S12 and S13 profiles, there is an apparent difference in the VSF order for the inshore locations, and the main reason for the difference is also caused by the kinds of discharged land-based pollutants, while the inshore locations near S12 and S13 are dominated by sediment and mangrove forests, respectively.
Fig. 9. Surface distribution of attenuation coefficients and VSFs in Zhanjiang Bay (Drawn by Ocean Data view).
4.3 Comparison of the VSAM and ECO-VSF3
Due to the poor water quality in Zhanjiang Bay caused by land-based pollution, ECO-VSF3, which is less sensitive to high scattering and absorbing environment and can provide reasonable estimates of the backward scattering coefficient, was selected for comparison with VSAM [34]. The work wavelengths and scattering angles of ECO-VSF3 are 470 nm, 532 nm, and 660 nm and 100°, 125°, and 150°, respectively. The work wavelengths and scattering angles of ECO-VSF3 are 470 $\textrm{nm}$, 532 $\textrm{nm}$, and 660 $\textrm{nm}$ and 100°, 125°, and 150°, respectively. In this comparison, the 100° and 125° angles at 660 $\textrm{nm}$ were selected to match the work wavelength (659 nm) of the VSAM. Figure 10 shows a comparison of the VSF measured by the VSAM and ECO-VSF3 at S15 and S18 in Fig. 7(a), while Fig. 11 shows a comparison of ${b_b}$ calculated by the VSAM and ECO-VSF3 at S15 and S18.
In Fig. 10, a similar VSF profile trend can be found for the VSAM and ECO-VSF3, and the VSF measured by the VSAM is slightly higher than that measured by ECO-VSF3. For the tiny discrepancy between the two instrument measurements, the possible reasons are that ECO-VSF3 was not calibrated before the field measurements and that the two instruments were not deployed at the same time. Unfortunately, due to the time limit and the equipment support of the experimental vessel, comparative measurements have been made at only two stations with depths that do not vary satisfactorily.
In Fig. 11, a comparison is made between the estimated backscatter coefficients based on the VSAM profile measurements and those estimated by ECO-VSF3 combined with Boss theory. The profile shape of the backscattering coefficients obtained by the VSAM are in more significant agreement with those obtained by ECO-VSF3, while the magnitude of ${b_b}$ obtained from the VSAM are slightly higher than from ECO-VSF. This is consistent with the VSF in the backward direction in Fig. 10.
5. Conclusions
The VSF is an important inherent optical property of seawater. A new instrument (VSAM) to simultaneously measure the VSF from 10° to 170° with 10° intervals, the attenuation coefficient (0°), the depth and the temperature of the seawater has been developed and implemented. The path length for all scattering flux measurements is 30 cm (namely, the distance from the center of the source window to the center of the sample volume to the center of the detector window). An alterable FOV and amplifier gain for the scattering detector have been designed to meet the different dynamic range scattering flux. The maximum frequency for all simultaneous samples in all 18 channels (including 17 VSFs and attenuation coefficient) and the theoretical deepest profile depth are as high as 20 Hz and 200 $\textrm{m}$, respectively. Sloped quartz glass is used as the 0° transmission detector window to minimize the possibility of reflection from 0° transmission. The attenuation coefficient c (transmission flux in 0°) and the VSF at angles from 10° to 170° with an interval of 10° can be simultaneously measured easily and immediately by VSAM without an additional meter (such as ac-s, C-Star, etc.), which surely helps with the routine work of collecting VSF values for seawater and its constituents.
Using ultrapure water and NCRM-traceable polystyrene microsphere beads, the VSAM was calibrated in the laboratory, and the conversion factor ${\chi _{b\theta }}$ for estimating the backscattering coefficient from the backward volume scattering function was built and evaluated based on Mie theory. The RSD of the VSAM for the bead response signal was only 1.35%, and the difference between the VSAM-based backscattering coefficient estimation model and the Boss Mie-based backscattering coefficient estimation model was merely 7.77%, which also illustrates the calibration and measurement performance of the VSAM.
The VSAM was deployed in ZJB at different stations and made some of the profile measurements of the VSF and attenuation coefficient of natural bay waters. A high degree of environmental variability (including spatial and vertical variations) was found in the VSF and attenuation coefficient, including in all directions. Based on the in situ data, the backscattering coefficient estimation model at the deployment station was evaluated. The above in situ measurements clearly show that the use of a volume scattering function(s) for the modeling and study of radiative transfer and underwater optical field distribution is necessary and important.
Funding
National Natural Science Foundation of China (41976170, 41976172, 41976181); Scientific and Technological Planning Project of Guangzhou City (201707020023); State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences (LTOZZ2003).
Acknowledgments
We thank Dr. Jibiao Zhang from the Guangdong Ocean University for helping carry out field test for this work. And we thank the ODV software (https://odv.awi.de) for the convenience in drawing.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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