Spatiotemporal variability in the diffuse attenuation coefficient of sea ice.

The diffuse attenuation coefficient (Kd) is known to be closely related to the light transmittance of sea ice, which plays a critical role in the energy balance and biological processes of the upper ocean. However, the commercial instruments cannot easily measure Kd in sea ice because sea ice is a solid. The authors of this study are developing an instrument with a high spectral solution to measure the irradiance profile of sea ice and the irradiance in the atmosphere. Three Kd experiments were carried out, including two in-situ experiments in the Liaodong Bay and one in the laboratory. The results showed that the Kd of the sea ice varied with depth, and the values in adjacent sea ice layers differed by up to 2 times. In addition, due to changes in the climate environment, the Kd of sea ice showed temporal variations. For example, there was a 1.38-fold difference in the Kd values of the surface layer of sea ice at different times in 2022. The values in different sea ice layers also showed different trends over time, and the coefficient of determination (R2) of Kd between adjacent layers over time was as low as 0.008. To explain the driving mechanism of spatio-temporal variability of Kd, an additional experiment focusing on the physical microstructure of sea ice was conducted in Liaodong Bay in 2022. The result shows that the change in air bubbles in the sea ice may be the main the reason for the change in Kd. For example, when the sea ice was exchanging brine and bubbles with the atmosphere above and the seawater below, the highly absorbent particles in it tend to remain in their original position. Considering that the total absorption coefficient changed slightly, the bubbles with the characteristic of intense scattering were found to be the main factor influencing the Kd changes. This conclusion is supported by the fact that the value of R2 between the bubbles and Kd was 0.52. If climatic changes have led to an increase in the volume of bubbles, the more bubbles will increase the scattering properties of sea ice and lead to an increase in Kd. Conversely, the reduced bubble volume would reduce the scattering properties of sea ice, which in turn would reduce Kd.

Approach for estimating the vertical distribution of the diffuse attenuation coefficient in the South China Sea.

The vertical distribution of the diffuse attenuation coefficient K(z, λ) is critical for studies in bio-optics, ocean color remote sensing, underwater photovoltaic power, etc. It is a key apparent optical property (AOP) and is sensitive to the volume scattering function ß(ψ, z, λ). Here, using three machine learning algorithms (MLAs) (categorical boosting (CatBoost), light gradient boosting machine (LightGBM), and random forest (RF)), we developed a new approach for estimating the vertical distribution of Kd(z, 650), KLu(z, 650), and Ku(z, 650) and applied it to the South China Sea (SCS). In this approach, based on in situ ß(ψ, z, 650), the absorption coefficient a(z, 650), the profile depths z, and Kd(z, 650), KLu(z, 650), and Ku(z, 650) calculated by Hydrolight 6.0 (HL6.0), three machine learning models (MLMs) without or with boundary conditions for estimating Kd(z, 650), KLu(z, 650), and Ku(z, 650) were established, evaluated, compared, and applied. It was found that (1) CatBoost models have superior performance with R2 ≥ 0.92, RMSE≤ 0.021 m-1, and MAPE≤ 4.3% and most significantly agree with HL6.0 simulations; (2) there is a more satisfactory consistency between HL6.0 simulations and MLMs estimations while incorporating the boundary conditions; (3) the estimations of Kd(z, 650), KLu(z, 650), and Ku(z, 650) derived from CatBoost models with and without boundary conditions have a good agreement with R2 ≥0.992, RMSE ≤0.007 m-1, and MAPE≤0.8%, respectively; (4) there is an overall decreasing trend with increasing depth and increasing offshore distance of Kd(z, 650), KLu(z, 650), and Ku(z, 650) in the SCS. The MLMs for estimating K(z, λ) could provide more accurate information for the study of underwater light field distribution, water quality assessment and the validation of remote sensing data products.

Evaluating semi-analytical algorithms for estimating inherent optical properties in the South China Sea.

Using large amounts of bio-optical data collected in the South China Sea (SCS) from 2003 to 2016, this study checks the consistency between well-known semi-analytical algorithms (SAAs)-the quasi-analytical algorithm (QAA) and the default generalized inherent optical property (GIOP-DC)-in retrieving the non-water absorption coefficient (anw(λ)), phytoplankton absorption coefficient (aph(λ)) and particulate backscattering coefficient (bbp(λ)) from remote-sensing reflectance (Rrs(λ)) data at 412, 443, 490, 531, and 555 nm. The samples from the SCS are further separated into oligotrophic and mesotrophic water types for the comparison of the SAAs. Several findings are made: First, the values of anw(λ) derived from the two SAAs deliver similar performance, with R2 values ranging from 0.74 to 0.85 and 0.74 to 0.87, implying absolute percent error differences (APDs) from 37.93% to 74.88% and from 32.32% to 71.75% for the QAA and GIOP-DC, respectively. The QAA shows a value of R2 between 0.64 and 0.91 and APDs between 43.57% to 83.53%, while the GIOP-DC yields R2 between 0.76 to 0.89 and APDs between 44.65% to 79.46% when estimating aph(λ). The values of bbp(λ) derived from the QAA are closer to the in-situ bbp(λ) values, as indicated by the low values of the normalized centered root-mean-square deviation and normalized standard deviation, which are close to one. Second, a regionally tuned estimation of aph(λ) is proposed and recommended for the SCS. This consistency check of inherent optical properties products from SAAs can serve as reference for algorithm selection for further applications, including primary production, carbon, and biogeochemical models of the SCS, and can provide guidance for improving aph(λ) estimation.

Estimation of suspended particulate matter in turbid coastal waters: application to hyperspectral satellite imagery.

An empirical algorithm is proposed to estimate suspended particulate matter (SPM) ranging from 0.675 to 25.7 mg L-1 in the turbid Pearl River estuary (PRE). Comparisons between model predicted and in situ measured SPM resulted in R2s of 0.97 and 0.88 and mean absolute percentage errors (MAPEs) of 23.96% and 29.69% by using the calibration and validation data sets, respectively. The developed algorithm demonstrated the highest accuracy when compared with existing ones for turbid coastal waters. The diurnal dynamics of SPM was revealed by applying the proposed algorithm to reflectance data collected by a moored buoy in the PRE. The established algorithm was implemented to Hyperspectral Imager for the Coastal Ocean (HICO) data and the distribution pattern of SPM in the PRE was elucidated. Validation of HICO-derived reflectance data by using concurrent MODIS/Aqua data as a benchmark indicated their reliability. Factors influencing variability of SPM in the PRE were analyzed, which implicated the combined effects of wind, tide, rainfall, and circulation as the cause.

An instrument for in situ measuring the volume scattering function of water: design, calibration and primary experiments.

The optical volume scattering function (VSF) of seawater is a fundamental property used in the calculation of radiative transfer for applications in the study of the upper-ocean heat balance, the photosynthetic productivity of the ocean, and the chemical transformation of photoreactive compounds. A new instrument to simultaneously measure the VSF in seven directions between 20° to 160°, the attenuation coefficient, and the depth of water is presented. The instrument is self-contained and can be automatically controlled by the depth under water. The self-contained data can be easily downloaded by an ultra-short-wave communication system. A calibration test was performed in the laboratory based on precise estimation of the scattering volume and optical radiometric calibration of the detectors. The measurement error of the VSF measurement instrument has been estimated in the laboratory based on the Mie theory, and the average error is less than 12%. The instrument was used to measure and analyze the variation characteristics of the VSF with angle, depth and water quality in Daya Bay for the first time. From these in situ data, we have found that the phase functions proposed by Fournier-Forand, measured by Petzold in San Diego Harbor and Sokolov in Black Sea do not fit with our measurements in Daya. These discrepancies could manly due to high proportion of suspended calcium carbonate mineral-like particles with high refractive index in Daya Bay.

[Progress in Teflon AF LWCC/LCW applications].

Teflon AF is chemically very inert, quite physically and optically stable, a highly vapor-permeable polymer with optical transparency through much of the UV-Vis region and with an RI lower than that of water, so Teflon AF LWCC/LCW (Long path-length liquid waveguide capillary cell/liquid core waveguides) has been used with a range of different detection techniques, including absorbance spectroscopy, fluorescence spectroscopy, Raman spectroscopy, and gas sensor. The present article describes the properties and the aspects of Teflon AF LWCC/LCW instrumentation and applications. And finally,the future prospect and outlook of Teflon AF LWCC/LCW is also discussed.

[Design and application of hyperspectral radiation system for sea ice observation].

Sea ice plays an important role in the global climate systems. In the present article, a hyperspectral radiation system for the observation of optical properties of sea ice was designed. The system consists of three optical channels, which can operate simultaneously. Two kinds of optical detectors were designed, and the problem relevant to the water-tightness was resolved. The system can be used to measure the solar radiation beneath the sea ice by an "L" bracket. Another bracket for detecting bidirectional reflectance was designed, which can fix the optical detector at any angle ranging over 0-180 measured with an angle detector. In order to make a most suitable and automatic integrated time, the system can adjust the integrated time intelligently by itself. The system can work stably under extremely low temperature. Furthermore, the system was equipped with four thermistors and one GPS. The system was validated showing a good stability and veracity in situ in the Liaodong Bay.

[Reflectance of sea ice in Liaodong Bay].

In the present study, the relationships between sea ice albedo and the bidirectional reflectance distribution in Liaodong Bay were investigated. The results indicate that: (1) sea ice albedo alpha(lambda) is closely related to the components of sea ice, the higher the particulate concentration in sea ice surface is, the lower the sea ice albedo alpha(lambda) is. On the contrary, the higher the bubble concentration in sea ice is, the higher sea ice albedo alpha(lambda) is. (2) Sea ice albedo alpha(lambda) is similar to the bidirectional reflectance factor R(f) when the probe locates at nadir. The R(f) would increase with the increase in detector zenith theta, and the correlation between R(f) and the detector azimuth would gradually increase. When the theta is located at solar zenith 63 degrees, the R(f) would reach the maximum, and the strongest correlation is also shown between the R(f) and the detector azimuth. (3) Different types of sea ice would have the different anisotropic reflectance factors.

[Design and experimentation of marine optical buoy].

Marine optical buoy is of important value in terms of calibration and validation of ocean color remote sensing, scientific observation, coastal environment monitoring, etc. A marine optical buoy system was designed which consists of a main and a slave buoy. The system can measure the distribution of irradiance and radiance over the sea surface, in the layer near sea surface and in the euphotic zone synchronously, during which some other parameters are also acquired such as spectral absorption and scattering coefficients of the water column, the velocity and direction of the wind, and so on. The buoy was positioned by GPS. The low-power integrated PC104 computer was used as the control core to collect data automatically. The data and commands were real-timely transmitted by CDMA/GPRS wireless networks or by the maritime satellite. The coastal marine experimentation demonstrated that the buoy has small pitch and roll rates in high sea state conditions and thus can meet the needs of underwater radiometric measurements, the data collection and remote transmission are reliable, and the auto-operated anti-biofouling devices can ensure that the optical sensors work effectively for a period of several months.

Measuring natural phytoplankton fluorescence and biomass: a case study of algal bloom in the Pearl River estuary.

A moored optical buoy was deployed in the Pearl River estuarine waters for a 15-day period. A four-day algal bloom event occurred during this study period. Both chlorophyll a concentration and algal cell density (a proxy for biomass) changed dramatically before and after the event. The chlorophyll concentration at a 2.3m depth rose from 5.15 mg/m(-3) at 15:00 h on August 19 to 23.62 mg/m(-3) at 9:00 h on August 21, and then decreased to 3.24 mg/m(-3) at 15:00 h on August 24. The corresponding cell density ranged from 1.57 x 10(5) to 1.76 x 10(6)cells/L. We used normalized fluorescence line height (NFLH) and normalized fluorescence intensity (NFI) in order to determine fluorescence activity. Combined with the in situ sampling dataset, we were able to correlate natural fluorescence (NFLH and NFI) with chlorophyll a concentrations, and found correlation coefficients of 0.72 and 0.75, respectively. We also found correlations between natural fluorescence and cell density, with correlation coefficients of 0.71 and 0.65, respectively. These results indicate that applying continuous time series of natural fluorescence can reflect changes in biomass. This technique will prove extremely useful for in situ and real-time observations using an optical buoy. Although there are still problems to solve in the real-time observation of natural fluorescence in algal bloom events, we discuss the primary factors affecting fluorescence signals and suggest possible methods for mitigating these issues.

Variation of particulate organic carbon and its relationship with bio-optical properties during a phytoplankton bloom in the Pearl River estuary.

In this study, variations in the particulate organic carbon (POC) were monitored during a phytoplankton bloom event, and the corresponding changes in bio-optical properties were tracked at one station (114.29°E, 22.06°N) located in the Pearl River estuary. A greater than 10-fold increase in POC (112.29-1173.36 mg m⁻³) was observed during the bloom, with the chlorophyll a concentration (Chl-a) varying from 0.984 to 25.941 mg m⁻³. A power law function is used to describe the relationship between POC and Chl-a, and the POC:Chl-a ratio tends to change inversely with Chl-a. Phytoplankton carbon concentration is indirectly estimated using the conceptual model proposed by Sathyendranath et al. (2009), and this carbon is found to contribute 47.21% (±10.65%) to total POC. The estimated carbon-to-chlorophyll ratio of phytoplankton in diatom-dominated waters is found to be comparable with results reported in the literature. Empirical algorithms for determining the concentrations of Chl-a and POC were developed based on the relationships of these variables with the blue-to-green reflectance ratio. With these bio-optical models, the levels of particulate organic carbon and Chl-a could be predicted from the radiometric data measured by a marine optical buoy, which showed much more detailed information about the variability in biogeochemical parameters during this bloom event.