Sensing profiles of the volume scattering function at 180° using a single-photon oceanic fluorescence lidar.

A novel oceanic fluorescence lidar technique has been proposed and demonstrated for remotely sensing the volume scattering function at 180° (ßf), which can be used to further retrieve the profiles of the absorption coefficient of phytoplankton (aph) at 532 nm and chlorophyll concentration (Chl). This scheme has these features. 1) The single-photon detection technology is employed to enhance the detection sensitivity to the single-photon level, enabling the oceanic lidar to obtain fluorescence backscatter profiles. 2) In terms of algorithms, the Raman backscattered signals of the water are utilized to normalize the backscattered signals of chlorophyll fluorescence, effectively minimizing the depth-dependent variation of the differential lidar attenuation coefficient (Δ K l i d a r f r). To reduce the contamination of fluorescence signals in the Raman backscatter signals, a Raman filter with a bandwidth of 6 nm was chosen. Subsequently, a perturbation method is utilized to invert the ßf of the fluorescence lidar. Finally, aph and Chl profiles can be inverted based on empirical models. 3) The value of Δ K l i d a r f r used in inversion is obtained through a semi-analytic Monte Carlo simulation. According to theoretical analysis, the maximum relative error of ßf for Chl ranging from 0.01 mg/m3 to 10 mg/m3 is less than 13 %. To validate this approach, a field experiment was conducted aboard the R/V Tan Kah Kee in the South China Sea from September 4th to September 5th, 2022, resulting in continuous subsurface profiles of ßf, aph, and Chl. These measurements confirm the robustness and reliability of the oceanic single-photon fluorescence lidar system and the inversion algorithm.

Sensing the profile of particulate beam attenuation coefficient through a single-photon oceanic Raman lidar.

A lidar technique has been proposed and demonstrated for remotely sensing particulate beam attenuation coefficient (cp) profiles using the Raman backscattered signal from water. In Raman lidar, the backscatter coefficient at 180° can be considered constant, allowing for the determination of the lidar attenuation coefficient (Klidar) from the Raman backscattered signal. This scheme has these features. 1) The bandwidth of the filter that used to extract the Raman component from the backscattered signal of the lidar was optimized to ensure sufficient lidar signal strength while minimizing the influence of chlorophyll fluorescence on inversion. 2) A receiving telescope with narrow field of view (FOV) and small aperture was utilized to suppress multi-scattering components in the backscattered signal. 3) A relationship between the beam attenuation coefficient (c) and Klidar was established after simulations via a semi-analytic Monto Carlo. 4) The value of cp was obtained by subtracting the attenuation coefficient of pure seawater (cw) from c. According to the theoretical analysis, the maximum relative error of cp is less than 15% for chlorophyll concentrations up to 10 mg/m3. Due to the water Raman backscattered signal being several orders of magnitude lower than the elastic backscattered signal, a single-photon detector is required to significantly improve the detection sensitivity to the single-photon level. To validate this approach, a field experiment was conducted aboard the R/V Tan Kah Kee in the South China Sea from September 4th to September 5th, 2022, and continuous subsurface profiles of cp were obtained. These measurements confirm the robustness and reliability of the oceanic single-photon Raman lidar system and the inversion method.

Lidar attenuation coefficient in the global oceans: insights from ICESat-2 mission.

The attenuation coefficient of natural waters plays a significant role in our understanding of hydrology from both the oceanographic and biological point of view. The advent of near-continuous observations by sophisticated space-based lidars now offers an unprecedented opportunity to characterize attenuation coefficients over open oceans on global and regional scales. At present, however, literature reports of lidar-derived attenuation coefficient estimates (klidar, m-1) in oceanic waters are very limited. In this study, we present a global survey of klidar derived from ATLAS/ICESat-2 nighttime measurements. Our results augment the existing passive sensor ocean color data set with a new diurnal component and extend the record to now include previously unavailable polar nighttime observations. The values of ATLAS measured klidar at 532 nm are between 0.045 and 0.39 m-1 with the higher values (>0.15 m-1) correlated with coastal waters and sea ice covered oceans. The average klidar in clearest oligotrophic ocean gyres is ∼0.058 ± 0.012 m-1 at 532 nm. The results reported here demonstrate the feasibility of using ATLAS/ICESat-2 lidar measurements for global klidar studies, which will in turn provide critical insights that enable climate models to correctly describe the amount of light present under sea ice, and for heat deposition studies in the upper ocean.

Revised spectral optimization approach to remove surface-reflected radiance for the estimation of remote-sensing reflectance from the above-water method.

The effective sea-surface skylight reflectance (ρ) is an important parameter for removing the contribution of surface-reflected radiance when measuring water-leaving radiance (Lw) using the above-water approach (AWA). Radiative simulations and field measurements show that ρ varies spectrally. To improve the determination of Lw (and then remote sensing reflectance, Rrs) from the AWA, we further developed a wavelength-dependent model for ρ to remove surface-reflected radiance, which is applied with a spectral optimization approach for the determination of Rrs. Excellent agreement was achieved between the AWA-derived and skylight-blocked approach (SBA)-obtained Rrs (coefficient of determination > 0.92, mean absolute percentage deviation < ∼ 11% for Rrs > 0.0005 sr-1), even during high wave conditions. We found that the optimization approach with the new ρ model worked very well for a wide range of water types and observation geometries. For developing remote sensing algorithms and evaluating satellite products, it would be beneficial to apply this approach to current and historical above-water in situ measurements of Rrs to improve the quality of these data. In addition, this approach could also increase the number of useable spectra where previously rendered unusable when processed with a traditional scheme.

Comprehensive, Continuous, and Vertical Measurements of Seawater Constituents with Triple-Field-of-View High-Spectral-Resolution Lidar.

Measuring the characteristics of seawater constituent is in great demand for studies of marine ecosystems and biogeochemistry. However, existing techniques based on remote sensing or in situ samplings present various tradeoffs with regard to the diversity, synchronism, temporal-spatial resolution, and depth-resolved capacity of their data products. Here, we demonstrate a novel oceanic triple-field-of-view (FOV) high-spectral-resolution lidar (HSRL) with an iterative retrieval approach. This technique provides, for the first time, comprehensive, continuous, and vertical measurements of seawater absorption coefficient, scattering coefficient, and slope of particle size distribution, which are validated by simulations and field experiments. Furthermore, it depicts valuable application potentials in the accuracy improvement of seawater classification and the continuous estimation of depth-resolved particulate organic carbon export. The triple-FOV HSRL with high performance could greatly increase the knowledge of seawater constituents and promote the understanding of marine ecosystems and biogeochemistry.

Day and night continuous high-resolution shallow-water depth detection with single-photon underwater lidar.

Single-photon lidar has emerged as a strong technology for bathymetric measurements. However, its heightened sensitivity additionally makes it susceptible to solar radiation noise, particularly in the green light wavelength where solar radiation is strong, posing challenges for its daytime operation. To address this issue, a single-photon underwater lidar system is proposed and demonstrated. This scheme has these features. 1) Underwater applications not only mitigate the impact of the air-water interface on laser transmission but also significantly attenuate solar radiation reaching the lidar due to the absorption and scattering properties of water. 2) The telescope is designed with a small aperture and narrow field of view to significantly suppress solar radiation. 3) A combination of a narrowband laser and narrowband filter technique is effectively employed to minimize residual solar radiation, thus enabling continuous bathymetric observation capabilities during both day and night. 4) After acquiring the backscattered signal from the bottom, a water depth extraction algorithm utilizing bi-Gaussian fitting is proposed. To demonstrate the robustness of the lidar and the effectiveness of the algorithm, the underwater single-photon lidar system is deployed on a ship to conduct cruise surveys of two bays in the nearshore area, as well as a full-day stationary observation experiment. The lidar measurements are highly consistent with the synchronized sonar observations. The full-day stationary observation experiment showcased its capability to deliver continuous measurements throughout the day and night. These results demonstrate the potential of the system in various applications, including high-precision underwater terrain mapping, obstacle avoidance for underwater platforms, and underwater target imaging.

Direct measurement system of water-leaving albedo in the field by the skylight-blocked approach: Monte Carlo simulations.

Water-leaving albedo (αw(λ)) is an important component of the ocean surface albedo. Direct measurement of αw(λ) in the field is not yet available due to difficulties in removing the contribution of surface-reflected solar radiation. Following the concept of the skylight-blocked approach (SBA), a novel system, termed αwSBA, is proposed in this study to directly measure Ew(λ), where a wide-angle black cone is used to block the surface-reflected radiance. The shading errors associated with the cone and the measuring system are examined via Monte-Carlo (MC) simulations for a wide range of water inherent optical properties (IOPs), solar zenith angle, and different configurations of the αwSBA system (i.e., half cone angle, and the length of supporting arm). Based on sensitive analysis using MC simulations, an optimal configuration of αwSBA is recommended. We further propose a mathematical expression to parameterize the shading error (ɛ), along with an error correction scheme (αwOPT). It is found that, with the optimal configuration and αwOPT, the uncertainties of obtained αw(λ) by αwSBA are generally less than 7% for a wide range of waters with different IOPs and particulate scattering phase functions.

On the measurement of remote sensing reflectance by a traditional above-water approach in small water bodies.

Small water bodies are an important part of the Earth's freshwater system, protecting biodiversity and providing ecosystem services. Because of various surrounding features, it is unknown to what extent we can obtain accurate remote-sensing reflectance (Rrs) of such an environment by the conventional above-water approach (AWA). In this study, we used both AWA and the skylight-blocked approach (SBA) side-by-side to measure Rrs in a typical small water body. It was found that the variation of Rrs in the UV-blue domain from AWA is around 50% and is inconsistent with the variation of the total absorption coefficient (at) obtained from water samples; on the contrary, the variation of Rrs obtained from SBA is highly consistent, with a coefficient of variation under ∼5%. These results highlight the large uncertainties in the measured Rrs from AWA due to the complexity of such an environment and further echo the robustness of SBA to measure Rrs in the field, even in such challenge environments.

Scheme to estimate water-leaving albedo from remotely sensed chlorophyll-a concentration.

Water-leaving albedo (αw(λ)) is an important component of the ocean surface albedo and is conventionally estimated based on remotely sensed chlorophyll-a concentration (Chl) (termed Chl-αw). We show that estimated αw(λ) by Chl-αw could be significantly biased in global oceans, because there is no guarantee of closure between the modeled remote sensing reflectance (Rrs(λ)) from Chl-inferred inherent optical properties (IOPs) and the input Rrs(λ) that is used to derive Chl. We thus propose a simple and improved scheme, termed Chl-αw_new, and show that the step to infer IOPs from Chl is not necessary, where αw(λ) can be accurately estimated from satellite-measured Rrs(λ) and a Chl-based look-up-table (LUT) for the bidirectional relationships of angular Rrs(λ). Evaluations with both HydroLight simulations and satellite measurements show that Chl-αw_new is equivalent to the recently developed αw scheme based on IOPs (IOPs-αw, [Remote Sens. Environ. 269, 112807]), where both schemes could significantly improve the estimation of αw(λ) compared to Chl-αw. Comparisons among Chl-αw, Chl-αw_new, and IOPs-αw highlight that optical closure of Rrs(λ) is essential for accurate remote sensing of αw(λ), while the model for Rrs(λ) bidirectionality has rather minor impacts. The impact of improved αw(λ) estimations on the solar flux exchanges at the air-sea interface is preliminarily evaluated in this effort, where the use of Chl-αw_new could increase the estimation of reflected solar radiation by over 68.7% in turbid waters compared to that using Chl-αw, highlighting the necessity of incorporating accurate αw schemes into the coupled ocean-atmosphere models, especially for regional models in coastal oceans.

An evaluation of remote sensing algorithms for the estimation of diffuse attenuation coefficients in the ultraviolet bands.

In this study, six algorithms (both empirical and semi-analytical) developed for the estimation of Kd in the ultraviolet (UV) domain (specifically 360, 380, and 400 nm) were evaluated from a dataset of 316 stations covering oligotrophic ocean and coastal waters. In particular, the semi-analytical algorithm (Lee et al. 2013) used remote sensing reflectance in these near-blue UV bands estimated from a recently developed deep learning system as the input. For Kd(380) in a range of 0.018 - 2.34 m-1, it is found that the semi-analytical algorithm has the best performance, where the mean absolute relative difference (MARD) is 0.19, and the coefficient of determination (R2) is 0.94. For the empirical algorithms, the MARD values are 0.23-0.90, with R2 as 0.70-0.92, for this evaluation dataset. For a VIIRS and in situ matchup dataset (N = 62), the MARD of Kd(380) is 0.21 (R2 as 0.94) by the semi-analytical algorithm. These results indicate that a combination of deep learning system and semi-analytical algorithms can provide reliable Kd(UV) for past and present satellite ocean color missions that have no spectral bands in the UV, where global Kd(UV) products are required for comprehensive studies of UV radiation on marine primary productivity and biogeochemical processes in the ocean.

Australian fire nourishes ocean phytoplankton bloom.

An unprecedented devastating forest fire occurred in Australia from September 2019 to March 2020. Satellite observations revealed that this rare fire event in Australia destroyed a record amount of more than 202,387 km2 of forest, including 56,471 km2 in eastern Australia, which is mostly composed of evergreen forest. The released aerosols contained essential nutrients for the growth of marine phytoplankton and were transported by westerly winds over the Southern Ocean, with rainfall-induced deposition to the ocean beneath. Here, we show that a prominent oceanic bloom, indicated by the rapid growth of phytoplankton, took place in the Southern Ocean along the trajectory of fire-born aerosols in response to atmospheric deposition. Calculations of carbon released during the fire versus carbon absorbed by the oceanic phytoplankton bloom suggest that they were nearly equal. This finding illustrates the critical role of the oceans in mitigating natural and anthropogenic carbon dioxide releases to the atmosphere, which are a primary driver of climate change.

Experimental analysis of the measurement precision of spectral water-leaving radiance in different water types: reply.

Reliable in situ water-leaving radiance (Lw) measurements are critical for calibrating and validating the ocean color products from remote platforms (e.g., satellite). In an experimental effort, Wei et al. [Opt. Express29, 2780 (2021)10.1364/OE.413784] reported that the on-water radiometry allows for high-precision radiance determination. Zibordi [Opt. Express29, 19214 (2021)10.1364/OE.421786] questioned the use of the "1% radiometry" term in the former and commented on the data collection with the sensor's optical window submerged in water. This reply responds to the comments and discusses the on-water data processing protocol, which shows the obtained Lw is not affected by the questions raised therein.

Experimental analysis of the measurement precision of spectral water-leaving radiance in different water types.

The on-water radiometric approach employs a unique provision to obtain water-leaving radiance from nadir (Lw(λ)) which can be used for the calibration of ocean color satellites. In this effort, we address the measurement precision associated with Lw(λ) from a single on-water instrument, which is an important aspect of measurement uncertainty. First, we estimated the precision as the ratio of the standard deviation of the means of repeated measurements to the mean of these measurements. We show that the measurement precision for Lw(λ) is within 2.7-3.7% over 360-700 nm. The corresponding remote sensing reflectance spectra (Rrs(λ)) from the same instrument also exhibit a high precision of 1.9-2.8% in the same spectral domain. These measured precisions of radiance and reflectance over the 360-700 nm range are independent of the optical water type. Second, we quantified the consistency of on-water Lw(λ) and Rrs(λ) from two collocated systems for further insight into their measurement repeatability. The comparison reveals that Lw(λ) measurements in the 360-700 nm agree with each other with an absolute percentage difference of less than 3.5%. The corresponding Rrs(λ) data pairs are subjected to increased differences of up to 8.5%, partly due to variable irradiance measurements (Es(λ)). The evaluation of measurement precision corroborates the reliability of the on-water acquisition of radiometric data for supporting satellite calibration and validation.

Evaluation of forward reflectance models and empirical algorithms for chlorophyll concentration of stratified waters.

For waters with stratified chlorophyll concentration (Chl), numerical simulations were carried out to gain insight into the forward models of subsurface reflectance and empirical algorithms for Chl from the ocean color. It is found that the Gordon and Clark (1980) forward model for reflectance using an equivalent homogeneous water with a weighted average Chl (⟨Chl⟩) as the input works well, but depending on the contribution of gelbstoff, the difference in reflectance between stratified and the equivalent homogeneous water can be more than 10%. Further, the attenuation of upward light is better approximated as ∼1.5times that of the diffuse attenuation coefficient of downwelling irradiance. On the other hand, although the forward model for reflectance developed in Zaneveld et al. [Opt. Express13, 9052 (2005)] using equivalent homogeneous water with a weighted average of the backscattering to absorption ratio as the input also works well, this model cannot be used to obtain equivalent ⟨Chl⟩ for reflectance. Further, for empirical Chl algorithms designed for "Case 1" waters, it has been discovered that, for surface Chl in a range of ∼0.06-22.0mg/m3, the predictability of surface Chl is basically the same as that of ⟨Chl⟩ from the blue-green band ratio or the band difference of reflectance. Because ⟨Chl⟩ is wavelength and weighting-formula dependent, and it is required to have profiles of both Chl and the optical properties, these results emphasize that for empirical Chl algorithms, it is easier, less ambiguous, and certainly more straightforward and simple to use surface Chl for algorithm development and then its evaluation, rather than to use ⟨Chl⟩, regardless of whether or not the water is stratified.

Atmospheric correction in coastal region using same-day observations of different sun-sensor geometries with a revised POLYMER model.

In this paper, with a revised POLYMER (POLYnomial based approach applied to MERIS data) atmospheric correction model, we present a novel scheme (two-angle atmospheric correction algorithm, termed as TAACA) to remove atmospheric contributions in satellite ocean color measurements for coastal environments, especially when there are absorbing aerosols. TAACA essentially uses the same water properties as a constraint to determine oceanic and atmospheric properties simultaneously using two same-day consecutive satellite images having different sun-sensor geometries. The performance of TAACA is first evaluated with a synthetic dataset, where the retrieved remote-sensing reflectance (Rrs) by TAACA matches very well (the coefficient of determination (R2) ≥ 0.98) with the simulated Rrs for each wavelength, and the unbiased root mean square error (uRMSE) is ∼12.2% for cases of both non-absorbing and strongly absorbing aerosols. When this dataset is handled by POLYMER, for non-absorbing aerosol cases, the R2 and uRMSE values are ∼0.99 and ∼7.5%, respectively, but they are ∼0.92 and ∼39.5% for strongly absorbing aerosols. TAACA is further assessed using co-located VIIRS measurements for waters in Boston Harbor and Massachusetts Bay, and the retrieved Rrs from VIIRS agrees with in situ measurements within ∼27.3% at the visible wavelengths. By contrast, a traditional algorithm resulted in uRMSE as 3890.4% and 58.9% at 410 and 443 nm, respectively, for these measurements. The Rrs products derived from POLYMER also show large deviations from in situ measurements. It is envisioned that more reliable Rrs products in coastal waters could be obtained from satellite ocean color measurements with a scheme like TAACA, especially when there are strongly absorbing aerosols.

Experimental evaluation of the self-shadow and its correction for on-water measurements of water-leaving radiance.

Accurate determination of the water-leaving radiance (Lw) is key to correctly interpret in-water optical properties and to validate the atmospheric correction schemes in ocean color studies. Among the various approaches adopted to measure Lw in the field, the skylight-blocked approach (SBA) is the only scheme that can potentially measure Lw directly. However, the apparatus associated with an SBA system will introduce self-shading effects to the measured Lw, which is required to be corrected for an accurate Lw determination. In this study, we experimentally evaluate several factors that could contribute to the self-shading effects of the SBA-measured Lw, including solar zenith angle (∼18∘-64∘), water's optical properties, and cone size (radius of 22 mm and 45 mm). For waters with the total absorption coefficient at 440 nm as high as ∼6.0m-1, the normalized root-mean-square difference between the SBA-measured Lw after shade correction and the "true" Lw is generally between ∼5% and ∼10% for wavelengths in the range of 400-750 nm. These results suggest that SBA can obtain highly accurate and precise Lw in nearly all natural aquatic environments.

Hyperspectral polarimetric imaging of the water surface and retrieval of water optical parameters from multi-angular polarimetric data.

Total and polarized radiances from above the ocean surface are measured by a state-of-the-art snapshot hyperspectral imager. A computer-controlled filter wheel is installed in front of the imager allowing for recording of division-of-time Stokes vector images from the ocean surface. This system, to the best of our knowledge, for the first time provided a capability of hyperspectral polarimetric multi-angular measurements of radiances from above the water surface. Several sets of measurements used in the analysis were acquired from ocean platforms and from shipborne observations. Measurements made by the imager are compared with simulations using a vector radiative transfer (VRT) code showing reasonable agreement. Analysis of pixel-to-pixel variability of the total and polarized above-water radiance for the viewing angles of 20°-60° in different wind conditions enable the estimation of uncertainties in measurements of these radiances in the polarized mode for the spectral range of 450-750 nm, thus setting requirements for the quality of polarized measurements. It is shown that there is a noticeable increase of above-water degree of linear polarization (DoLP) as a function of the viewing angle, which is due both to the larger DoLP of the light from the water body and the light reflected from the ocean surface. Results of measurements and VRT simulations are applied for the multi-angular retrieval of the ratio of beam attenuation coefficient (ctot) to absorption coefficient (atot) in addition to the other parameters such as absorption and backscattering coefficients retrieved from traditional unpolarized methods.

Active and passive optical remote sensing of the aquatic environment: introduction to the feature issue.

Through decades of efforts and practices, we have achieved great progress in understanding ocean biology and biogeochemistry through satellite measurements of ocean (water) color, or passive remote sensing. These include detailed global maps of the distribution of surface phytoplankton, the production of newly formed particulate organic matter through photosynthesis (i.e., primary production), as well as the change and feedback of phytoplankton in a changing climate, to name a few. However, these results are still far from a full account of ocean biology and biogeochemistry, where we want more detailed information of phytoplankton (e.g., types and sizes), as well as information in the vertical dimension. For such, we are happy to see new developments in ocean optics and ocean color remote sensing. These include, but certainly are not limited to, hyperspectral sensors, measurements via polarized setups, as well as ocean lidar systems. In particular, through pumping laser light into deeper ocean, lidar has demonstrated great potential to fill the gap of passive ocean color remote sensing. These developments in technology are providing exciting new findings where breakthroughs in ocean biogeochemistry are on the horizon. Thus, we organized this feature issue in Applied Optics to summarize a few recent developments and achievements, where readers and the community can easily capture progress on both fronts, as well as the potential and advantages of the fusion of passive and active optical sensing. Specifically, this issue contains 12 papers describing research in both active and passive optical remote sensing of aquatic environment. They are still limited in number and subject, but are expected to stimulate the ocean color community with findings relevant for satellite applications.

Evaluation of glint correction approaches for fine-scale ocean color measurements by lightweight hyperspectral imaging spectrometers.

Low-power, lightweight, off-the-shelf imaging spectrometers, deployed on above-water fixed platforms or on low-altitude aerial drones, have significant potential for enabling fine-scale assessment of radiometrically derived water quality properties (WQPs) in oceans, lakes, and reservoirs. In such applications, it is essential that the measured water-leaving spectral radiances be corrected for surface-reflected light, i.e., glint. However, noise and spectral characteristics of these imagers, and environmental sources of fine-scale radiometric variability such as capillary waves, complicate the glint correction problem. Despite having a low signal-to-noise ratio, a representative lightweight imaging spectrometer provided accurate radiometric estimates of chlorophyll concentration-an informative WQP-from glint-corrected hyperspectral radiances in a fixed-platform application in a coastal ocean region. Optimal glint correction was provided by a spectral optimization algorithm, which outperformed both a hardware solution utilizing a polarizer and a subtractive algorithm incorporating the reflectance measured in the near infrared. In the same coastal region, this spectral optimization approach also provided the best glint correction for radiometric estimates of backscatter at 650 nm, a WQP indicative of suspended particle load.

Impact of ship on radiometric measurements in the field: a reappraisal via Monte Carlo simulations.

The presence of a ship in water disturbs the ambient light field and propagates errors to radiometric measurements. This study investigated the ship perturbation via Monte Carlo simulations with a reflective 3D ship. It is found that the height of ship could cause significant perturbation. However, these perturbations could be compensated by the reflection of the ship's hull, where such compensations vary from sun angle to hull's reflectance. Further, as a rule of thumb, to keep the perturbation on water-leaving radiance under ∼3% from an operating ship, a look-up table is generated with the requirements of viewing angle for the radiometers operated at the deck and for the deployment distance of floating and profiling instruments.