Optical Properties Of Natural Waters Research Articles

Bio-geo-optical modelling of natural waters

The color of natural waters – oceanic, coastal, and inland – is determined by the spectral absorption and scattering properties of dissolved and particulate water constituents. Remote sensing of aquatic ecosystems requires a comprehensive understanding of these inherent optical properties (IOPs), their interdependencies, and their impact on ocean (water) color, i.e., remote-sensing reflectance. We introduce a bio-geo-optical model for natural waters that includes revised spectral absorption and scattering parameterizations, based on a comprehensive analysis of precisely measured IOPs and water constituents. In addition, specific IOPs of the most significant phytoplankton groups are modeled and a system is proposed to represent the optical variability of phytoplankton diversity and community structures. The model provides a more accurate representation of the relationship between bio-geo-optical properties and can better capture optical variability across different water types. Based on the evaluation both using the training and independent testing data, our model demonstrates an accuracy of within ±5% for most component IOPs throughout the visible spectrum. We also discuss the potential of this model for radiative transfer simulations and building a comprehensive synthetic dataset especially for optically complex waters. Such datasets are the crucial basis for the development of satellite-based ocean (water) color algorithms and atmospheric correction methods. Our model reduces uncertainties in ocean color remote sensing by enhancing the distinction of optically active water constituents and provides a valuable tool for predicting the optical properties of natural waters across different water types.

Transfer model to determine the above-water remote-sensing reflectance from the underwater remote-sensing ratio.

Remote-sensing reflectance, Rrs(λ, θ, Δϕ, θs), contains the spectral color information of the water body below the sea surface and is a fundamental parameter to derive satellite ocean color products such as chlorophyll-a, diffuse light attenuation, or inherent optical properties. Water reflectance, i.e., spectral upwelling radiance, normalized by the downwelling irradiance, can be measured under- or above-water. Several models to extrapolate this ratio from underwater "remote-sensing ratio", rrs(λ), to the above-water Rrs, have been proposed in previous studies, in which the spectral dependency of water refractive index and off-nadir viewing directions have not been considered in detail. Based on measured inherent optical properties of natural waters and radiative transfer simulations, this study proposes a new transfer model to spectrally determine Rrs from rrs for different sun-viewing geometries and environmental conditions. It is shown that, compared to previous models, ignoring spectral dependency leads to a bias of ∼2.4% at shorter wavelengths (∼400 nm), which is avoidable. If nadir-viewing models are used, the typical 40°-off nadir viewing geometry will introduce a difference of ∼5% in Rrs estimation. When the solar zenith angle is higher than 60°, these differences of Rrs have implications for the downstream retrievals of ocean color products, e.g., > 8% difference for phytoplankton absorption at 440 nm and >4% difference for backward particle scattering at 440 nm by the quasi-analytical algorithm (QAA). These findings demonstrate that the proposed rrs-to-Rrs model is applicable to a wide range of measurement conditions and provides more accurate estimates of Rrs than previous models.

The Two Faces of “Case-1” Water

Morel’s “Optical modeling of the upper ocean in relation to its biogenous matter content (Case I waters)” (J. Geophys. Res. - Oceans, Vol. 93, pp. 107,49-10,768, 1988) laid the groundwork to model the optical properties of natural waters based on the concentration of chlorophyll ([Chl], in mg/m 3 ). As stated in the abstract, it aims “tentatively to interpret the optical behavior of oceanic case-I waters,” where “Chlorophyll-like pigment concentration is used as the index to quantify the algal materials,” because [Chl] is routinely measured in marine/oceanic surveys. Specifically, Morel developed “statistical relationships between this index and the depth of euphotic layer, the spectral values of the attenuation coefficient for downwelling irradiance, or the scattering coefficient,” and further, “a pigment-dependent optical model is developed.” Thus, such a system allows many aspects of oceanographic applications when [Chl] (“this index”) is provided. In part, this system established [Chl] at the core of traditional ocean color remote sensing. To implement this system, however, it is necessary to have a complete understanding of the definition and evolution of this Case-1/Case-2 system, especially the qualitative definition of Case-1/Case-2 vs. the practical separation of Case-1/Case-2 as well as the quantitative modeling of the optical properties of Case-1 waters.

Comparison of Lake Optical Water Types Derived from Sentinel-2 and Sentinel-3

Inland waters play a critical role in our drinking water supply. Additionally, they areimportant providers of food and recreation possibilities. Inland waters are known to be opticallycomplex and more diverse than marine or ocean waters. The optical properties of natural waters areinfluenced by three different and independent sources: phytoplankton, suspended matter, andcolored dissolved organic matter. Thus, the remote sensing of these waters is more challenging.Different types of waters need different approaches to obtain correct water quality products;therefore, the first step in remote sensing of lakes should be the classification of the water types. Theclassification of optical water types (OWTs) is based on the differences in the reflectance spectra ofthe lake water. This classification groups lake and coastal waters into five optical classes: Clear,Moderate, Turbid, Very Turbid, and Brown. We studied the OWTs in three different Latvian lakes:Burtnieks, Lubans, and Razna, and in a large Estonian lake, Lake Võrtsjärv. The primary goal of thisstudy was a comparison of two different Copernicus optical instrument data for opticalclassification in lakes: Ocean and Land Color Instrument (OLCI) on Sentinel-3 and MultispectralInstrument (MSI) on Sentinel-2. We found that both satellite OWT classifications in lakes werecomparable (R2 = 0.74). We were also able to study the spatial and temporal changes in the OWTs ofthe study lakes during 2017. The comparison between two satellites was carried out to understandif the classification of the OWTs with both satellites is compatible. Our results could give us not onlya better overview of the changes in the lake water by studying the temporal and spatial variabilityof the OWTs, but also possibly better retrieval of Level 2 satellite products when using OWT guidedapproach.

Using a Gaussian decomposition approach to model absorption spectra of chromophoric dissolved organic matter

The chromophoric dissolved organic matter (CDOM) is a significant water constituent influencing inherent and apparent optical properties of natural waters and plays a key role in ecosystem functioning. The spectral slope (S) describing the approximate exponential decline in CDOM absorption with increasing wavelength is widely used for tracing changes in the chemical composition of CDOM. The currently accepted method of characterizing CDOM absorption (i.e., fitting a simple exponential model) can lead to loss of information and large errors. We propose a better method for modeling CDOM absorption spectra based on a Gaussian decomposition approach that removes the errors associated with the choice of the spectral range used to estimate S. Using artificially generated spectra with known parameters (n=1000), we show that our method provides robust estimates of S closely resembling the original values. On average, the error on S estimations was 0.16% for the proposed method compared to 27% and 11% for the traditional modeling approaches fitted over 300–700nm and 240–700nm respectively. We further demonstrate the ability of the method to decompose and model chromophores present in complex spectra from oceanic water samples from around the world. The proposed method opens avenues for long-term or cross-site comparison studies of the dynamics of the CDOM pool and constitutes a promising supplement to techniques based on CDOM fluorescence.

Estimating the Bulk Refractive Index and Related Particulate Properties of Natural Waters From Remote-Sensing Data

A proof-of-concept method is developed to estimate bulk refractive index ( ${{{\eta }}_{{\bm {bp}}}}$ ) of particles for deriving information on their composition and optical properties of natural waters using satellite ocean color data. It estimates ${{{\eta }}_{{\bm {bp}}}}$ as a function of the backscattering ratio ( ${{\bm {B}}_{\bm {p}}}$ ), hyperbolic slope of the particle size distribution ( ${{\xi }}$ derived from the beam attenuation slope ${{\beta }}$ ), and apparent density of particles ( ${{{\rho }}_{\bm {a}}}$ ) as described in our previous inversion model. For deriving these parameters from remote-sensing data, large in situ measurement data of the remote-sensing reflectance ( ${{\bm {R}}_{{\bm {rs}}}}$ ), particulate scattering ( ${{\bm {b}}_{\bm {p}}}$ ), backscattering ( ${{\bm {b}}_{{\bm {bp}}}}$ ) beam attenuation ( ${{\bm {c}}_{\bm {p}}}$ ), and seawater constituents’ concentrations are used to establish the required relationships and derive the model parameters (i.e., ${{{\eta }}_{{\bm {bp}}}} \leftarrow {{\bm {B}}_{\bm {p}}}$ , ${{\xi }}$ , ${{{\rho }}_{\bm {a}}}$ ; ${{\bm {B}}_{\bm {p}}} \leftarrow {{\bm {R}}_{{\bm {rs}}}}$ ; ${{\xi }} \leftarrow {{\beta }} \leftarrow {{\bm {c}}_{\bm {p}}} \leftarrow {\bf Turbidity}$ ; ${\bf Turbidity} \leftarrow {{\bm {R}}_{{\bm {rs}}}}$ ; and ${{{\rho }}_{\bm {a}}} \leftarrow {{\bm {c}}_{\bm {p}}}$ ). When validated with those derived from in situ data spanning the large dynamic range in water optical properties and relevant water constituents impacting the remote-sensing signal, the derived model parameters and bulk refractive index had errors of a few percent which falls well within the benchmark for a validated uncertainty of ${\rm \pm }{\bf 35}\% $ endorsed for the remote-sensing retrieval of chlorophyll- $a$ in oceanic waters. The model was applied to regional and global images of satellite ocean color observations provided by the MODIS-Aqua sensor. Global images of seasonal climatology over a decade of MODIS-Aqua ocean color observations provided interesting insights into the seasonal variability of ${{{\eta }}_{{\bm {bp}}}}$ in coastal and open ocean waters. To further investigate the bulk particulate assemblages in some regional waters, several MODIS-Aqua imageries from different regional waters dominated by sediment-laden river plumes and phytoplankton blooms were analyzed. Satellite retrievals of the ${{{\eta }}_{{\bm {bp}}}}$ varied from 1.035 to 1.05 for clear oceanic waters, 1.045–1.07 for phytoplankton-dominated waters (higher water content), 1.05–1.15 for floating algal blooms and Cocolithophore blooms, 1.07–1.10 for coastal waters dominated by detrital particles, and 1.10–1.28 for river plume and sediment-laden coastal waters (lower water content). These results were consistent with those expected for the waters examined, being in excellent agreement with the previous reports and estimates based on the laboratory/ in situ measurements and modeling studies. Given the fact that there is currently no satellite-based method to differentiate between bulk particulate compositions, the new method provides a valuable tool for estimating ${{{\eta }}_{{\bm {bp}}}}$ values as a proxy for the apparent density and composition of particles and such information when derived from satellite ocean color data provides new tools for improving our current understanding of particle dynamics of pelagic ecosystems, advancing biogeochemical modeling, assessing the global ocean ecosystem, and elucidating the coastal processes.

Evaluation of scatter corrections for ac-9 absorption measurements in coastal waters

Determinations of inherent optical properties of natural waters are fundamental in marine optical research. In situ measurements of light absorption are mostly obtained with an instrument that uses a reflective tube design to reduce concomitant errors induced by light scattering (ac-9, WETLabs Inc.). The remaining, generally still substantial, error is commonly corrected using one of a number of different approaches, each of which is based on a set of assumptions. Until now, the errors in these measurements have only been theoretically examined using Monte Carlo modeling Leymarie et al. (2010). The study presented here used a lab-based point source integrating cavity absorption meter (PSICAM) which avoids scattering errors. The PSICAM data were used to evaluate the absorption determination with an ac-9 in coastal waters for each of the scattering correction approaches. The results showed that the assumption of negligible absorption at wavelengths >700 nm is not valid in coastal waters and that, as a result, ac-9 measurements strongly underestimate absorption at longer wavelengths (>600 nm). An empirical relationship between uncorrected (for scattering) ac-9 measurements and the true absorption at 715 nm was included in the correction scheme; this improved the quality of ac-9 data at longer wavelengths but showed overestimation at shorter wavelengths. However, additional inclusion of a scatter correction for the ac-9 attenuation measurement resulted in a significant improvement of the proportional scatter error correction across the spectrum. Despite these innovations, variations in scattering properties can, combined with low absorption at specific wavelengths, result in relatively large percentage errors for individual measurements.

Studies on some of the inherent optical properties of natural waters [Invited

The author's experimental activities in ocean optics that are related to the inherent optical properties of natural waters are discussed. The specific subjects discussed are (1) measurements of Mueller matrices for several ocean water regions; (2) measurements of the spectral absorption of pure water throughout the visible spectrum; (3) the development of an in situ absorption meter that provides higher accuracy than is available for any other inherent optical property; (4) a relatively simple expression for the refractive index of water as a function of temperature, salinity, and wavelength; and (5) development of the first instrument to directly determine the backscattering coefficient b(b).

Spectral variations in the near-infrared ocean reflectance

The optical properties of natural waters beyond the visible range, in the near-infrared (NIR, 700–900 nm), have received little attention because they are often assumed to be mostly determined by the large absorption coefficient of pure water, and because of methodological difficulties. It is now growingly admitted that the NIR represents a potential optical source of unambiguous information about suspended sediments in turbid waters, thence the need for better understanding the NIR optical behaviour of such waters. It has recently been proposed (Ruddick et al., Limnology and Oceanography. 51, 1167–1179, 2006) that the variability in the shape of the surface ocean reflectance spectrum in the NIR is negligible in turbid waters. In the present study, we show, based on both in situ and remote sensing data, that the shape of the ocean reflectance spectrum in the NIR does vary in turbid to extremely turbid waters. Space-borne ocean reflectance data were collected using 3 different sensors (SeaWiFS, MODIS/Aqua and MERIS) over the Amazon, Mackenzie and Rio de la Plata turbid river plumes during extremely clear atmospheric conditions so that reliable removal of gas and aerosol effects on reflectance could be achieved. In situ NIR reflectance data were collected in different European estuaries where extremely turbid waters were found. In both data sets, a flattening of the NIR reflectance spectrum with increasing turbidity was observed. The ratio of reflectances at 765 nm and 865 nm, for instance, varied from ca. 2 down to 1 in our in situ data set, while a constant value of 1.61 had been proposed based on theory in a previous study. Radiative transfer calculations were performed using a range of realistic values for the seawater inherent optical properties, to determine the possible causes of variations in the shape of the NIR reflectance spectrum. Based on these simulations, we found that the most significant one was the gradual increase in the contribution of suspended sediments to the color of surface waters, which often leads to the flattening of the reflectance spectrum. Changes in the scattering and absorption properties of particles also contributed to variations in the shape of the NIR surface ocean reflectance spectrum. The impact of such variations on the interpretation of ocean color data is discussed.

Comparison of the molecular mass and optical properties of colored dissolved organic material in two rivers and coastal waters by flow field-flow fractionation.

Colored dissolved organic material (CDOM) is an important sunlight absorbing substance affecting the optical properties of natural waters. However, little is known about its structural and optical properties mainly due to its complex matrix and the limitation of the techniques available. A comparison of two southwestern Florida rivers [the Caloosahatchee River (CR) and the Shark River (SR)] was done in terms of molecular mass (MM) and diffusion coefficients (D). The novel technique Frit inlet/frit outlet-flow field-flow fractionation (FIFO-FIFFF) with absorbance and fluorescence detectors was used to determine these properties. The SR receives organic material from the Everglades. By contrast, the CR arises from Lake Okeechobee in central Florida, receiving anthropogenic inputs, farming runoff, and natural organics. Both rivers discharge to the Gulf of Mexico. Fluorescence identified, for both rivers, two different MM distributions in low salinity water samples: the first was centered at approximately 1.7 kDa (CR) and approximately 2 kDa (SR); the second centered at approximately 13 kDa for both rivers, which disappeared gradually in the river plumes to below detection limit in coastal waters. Absorbance detected only one MM distribution centered at approximately 2 kDa (CR) and 2.2-2.4 kDa (SR). Fluorescence in general peaked at a lower MM than absorbance, suggesting a different size distribution for fluorophores vs chromophores. A photochemical study showed that, after sunlight, irradiated freshwater samples have similar characteristics to more marine waters, including a shift in MM distribution of chromophores. The differences observed between the rivers in the optical characteristics, MM distributions, and D values suggest that the CDOM sources, physical, and photochemical degradation processes are different for these two rivers.

Estimation of the inherent optical properties of natural waters from the irradiance attenuation coefficient and reflectance in the presence of Raman scattering

By means of radiative transfer simulations we developed a model for estimating the absorption a, the scattering b, and the backscattering b(b) coefficients in the upper ocean from irradiance reflectance just beneath the sea surface, R(0-), and the average attenuation coefficient for downwelling irradiance, <K(d)>1, between the surface and the first attenuation depth. The model accounts for Raman scattering by water, and it does not require any assumption about the spectral shapes of a, b, and b(b). The best estimations are obtained for a and b(b) in the blue and green spectral regions, where errors of a few percent to <10% are expected over a broad range of chlorophyll concentration in water. The model is useful for satellite ocean color applications because the model input, R(0-) and <K(d)>1, can be retrieved from remote sensing and the model output, a and b(b), is the major determinant of remote-sensing reflectance.

Remote sensing reflectance and its relationship to optical properties of natural waters

Abstract Monte Carlo simulations of photon propagation through natural water have been utilized to determine the sub-surface remote sensing reflectance, R RSW (the sub-surface value of the ratio of upwelling radiance from the nadir to the downwelling irradiance) as a function of water type (defined by the ratio of the backscattering coefficient to the absorption coefficient Bb/a), solar zenith angle, and incident radiation distribution (direct or diffuse). R RSW, as opposed to volume reflectance, R V (the sub-surface value of the ratio of upwelling to downwelling vector irradiance), is directly applicable to remotely sensed data collected over natural waters. It is shown that, for a nadir viewing direction, (a) R RSW is essentially independent of solar zenith angle and incident radiation distribution and (b) the dominant factor in determining R RSW is the optical nature of the water body itself (expressed as Bb/a). A relationship between the sub-surface remote sensing reflectance averaged over solar zenit...

A comparison of methods for the measurement of the absorption coefficient in natural waters

In the spring of 1992 an optical closure experiment was conducted at Lake Pend Oreille, Idaho. A primary objective of the experiment was to compare techniques for the measurement of the spectral absorption coefficient and other inherent optical properties of natural waters. Daily averages of absorption coefficients measured using six methods are compared at wavelengths of 456, 488, and 532 nm. Overall agreement was within 40% at 456 nm and improved with increasing wavelength to 25% at 532 nm. These absorption measurements were distributed over the final 9 days of the experiment, when bio‐optical conditions in Lake Pend Oreille (as indexed by the beam attenuation coefficient cp(660) and chlorophyll a fluorescence profiles) were representative of those observed throughout the experiment. However, profiles of stimulated chlorophyll a fluorescence and beam transmission showed that bio‐optical properties in the lake varied strongly on all time and space scales. Therefore environmental variability contributed significantly to deviations between daily mean absorption coefficients measured using the different techniques.

Toward closure of the inherent optical properties of natural waters

A fundamental relationship of inherent optical properties (IOP) is that the beam attenuation coefficient is the sum of the volume absorption and scattering coefficients (c = a + b). A relative calibration of a set of instruments can be provided using this IOP closure equation. Measurement of the true beam attenuation coefficient c is not practical as all attenuation instrumentation has some finite acceptance angle in which scattered light is collected. We provide a theoretical framework for measuring the attenuation and scattering coefficients in a consistent manner. Using this framework, we provide a practical version of the IOP closure equation. We apply the practical IOP closure equation to measurements made at Lake Pend Oreille, Idaho, in the spring of 1992. Results of this IOP closure indicate that the practical closure equation is a useful approach. Closure was achieved during some measurement sets but not at others. The intermittent lack of closure may be due to the method of determining the scattering coefficient from the general angle scattering meter or that the calibration of at least one of the instruments drifted during the time of the experiment.

Optical absorption and fluorescence properties of chromophoric dissolved organic matter in natural waters

Complete optical absorption and fluorescence spectra were collected for a diverse suite of 0.2‐µm‐filtered marine, riverine, and estuarine waters, as well as for colored dissolved organic matter (CDOM) isolated from several of these waters by solid‐phase C18 extraction. Absorption and fluorescence parameters for these samples arc reported. For surface waters, variations in the fluorescence quantum yields obtained with 355‐ and 337‐nm excitation fell within a narrow window (&lt;2.5‐fold variation about the mean values), demonstrating that fluorescence measurements can be used to determine absorption coefficients of CDOM in the ultraviolet region with reasonably good accuracy. Methods for predicting absorption coefficients and line shapes from the fluorescence data are introduced and tested. The absorption and fluorescence spectra of CDOM extracted from some seawaters differed significantly from those of the original waters, demonstrating that material isolated by hydrophobic adsorption is not necessarily representative of the suite of colored organic matter present in aquatic systems. These results clearly illustrate that great care must be taken when extracted material is used to infer the optical properties of natural waters.

Dependence of relationship between inherent and apparent optical properties of water on solar altitude

A computer simulation study, using the Monte Carlo calculation procedure, has been carried out to determine in what way the relationships between the apparent and the inherent optical properties of natural waters are affected by the angle of incidence of the photons on the surface. With the particular normalized volume scattering function used in these simulations, the vertical attenuation coefficient for downward irradiance at the midpoint of the euphotic zone, Kd(zm), can be expressed as a function of the absorption (a) and (b) coefficients, and the cosine of the incident photons just below the surface (µo), in accordance with urn:x-wiley:00243590:media:lno19842920350:lno19842920350-math-0001 Thus, as the direction of the incident light departs increasingly from the vertical, Kd(zm) increases but becomes progressively less responsive to increases in scattering at constant absorption. The irradiance reflectance just below the surface [R(0)] at all angles of incidence increases linearly with the ratio of the backscattering coefficient (bb) to the absorption coefficient, and at any value of bb/ a increases as the angle of the light departs further from the vertical, in approximate conformity with urn:x-wiley:00243590:media:lno19842920350:lno19842920350-math-0002

Terminology and units in optical oceanography

Abstract This document presents fundamental terms describing the transfer of radiative energy and relevant optical properties of natural waters. These are primarily based upon the terminology of the “Systeme International d'Unites (SI)”; and the “International Commission on Illumination (CIE).’’ Quantities and Symbols as proposed in the present terminology follow and extend the terminology that was recommended by the Committee on Radiant Energy in the Sea (of the International Association of Physical Oceanography, IAPO), as published by Jerlov (1968, Optical Oceanography Elsevier Oceanography Series, Vol. 5; 1976, Marine Optics Elsevier Oceanography Series, Vol. 14).

Some relationships between Secchi depth and inherent optical properties of natural waters

Relationships between the inherent and optical properties of the ocean (Gorden et al., 1975 and Preisendorfer, 1961) are combined with the Duntley-Preisendorfer equation to show the dependence of these properties on the depth at which a Secchi disk disappears from view. An expression relating the Secchi depth to the limiting contrast of the disk is derived in terms of the average beam attenuation coefficient, the average diffuse attenuation coefficient for downwelling irradiance, the albedo of the disk, and the reflectance functions at the Secchi depth and just below the surface. It is shown that combining Secchi depth observations with other optical properties yields significant information about the constituents of the medium.

A survey of experimental hydrologic optics

This paper describes current experimental methods and instrumentations used to measure the constructs of radiative transfer in the sea as specified by theory. Experimental data are presented to illustrate the salient properties of the underwater light fields and to show the application of radiative transfer theory to the determination of some of the optical properties of natural waters. Applications of the theory to practical problems in biological and physical oceanography are discussed briefly.