Effect of optically active substances and atmospheric correction schemes on remote-sensing reflectance at a coastal site off Kochi

Freshwater lakes provide many important ecosystem functions and services to support biodiversity and human well-being. Proximal and remote sensing methods represent an efficient approach to derive water quality indicators such as optically active substances (OAS). Measurements of above-ground remote and in situ proximal sensors, however, are limited to observations of the uppermost water layer. We tested a hyperspectral imaging system, customized for underwater applications, with the aim to assess concentrations of chlorophyll a (CHLa) and colored dissolved organic matter (CDOM) in the water columns of four freshwater lakes with different trophic conditions in Central Germany. We established a measurement protocol that allowed consistent reflectance retrievals at multiple depths within the water column independent of ambient illumination conditions. Imaging information from the camera proved beneficial for an optimized extraction of spectral information since low signal areas in the sensor’s field of view, e.g., due to non-uniform illumination, and other interfering elements, could be removed from the measured reflectance signal for each layer. Predictive hyperspectral models, based on the 470 nm–850 nm reflectance signal, yielded estimates of both water quality parameters (R² = 0.94, RMSE = 8.9 µg L−1 for CHLa; R² = 0.75, RMSE = 0.22 m−1 for CDOM) that were more accurate than commonly applied waveband indices (R² = 0.83, RMSE = 13.2 µg L−1 for CHLa; R² = 0.66, RMSE = 0.25 m−1 for CDOM). Underwater hyperspectral imaging could thus facilitate future water monitoring efforts through the acquisition of consistent spectral reflectance measurements or derived water quality parameters along the water column, which has the potential to improve the link between above-surface proximal and remote sensing observations and in situ point-based water probe measurements for ground truthing or to resolve the vertical distribution of OAS.

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Impact of Monsoon-Transported Anthropogenic Aerosols and Sun-Glint on the Satellite-Derived Spectral Remote Sensing Reflectance in the Indian Ocean

Remote Sensing ◽

10.3390/rs13020184 ◽

2021 ◽

Vol 13 (2) ◽

pp. 184

Author(s):

Rongjie Liu ◽

Jie Zhang ◽

Tingwei Cui ◽

Haocheng Yu

Keyword(s):

Remote Sensing ◽

Indian Ocean ◽

Infrared Imaging ◽

Atmospheric Correction ◽

Spatiotemporal Pattern ◽

Anthropogenic Aerosols ◽

Remote Sensing Reflectance ◽

The Indian Ocean ◽

Moderate Resolution Imaging Spectroradiometer ◽

Terra Modis

Spectral remote sensing reflectance (Rrs(λ), sr−1) is one of the most important products of ocean color satellite missions, where accuracy is essential for retrieval of in-water, bio-optical, and biogeochemical properties. For the Indian Ocean (IO), where Rrs(λ) accuracy has not been well documented, the quality of Rrs(λ) products from Moderate Resolution Imaging Spectroradiometer onboard both Terra (MODIS-Terra) and Aqua (MODIS-Aqua), and Visible Infrared Imaging Radiometer Suite onboard the Suomi National Polar-Orbiting Partnership spacecraft (VIIRS-NPP), is evaluated and inter-compared based on a quality assurance (QA) system, which can objectively grade each individual Rrs(λ) spectrum, with 1 for a perfect spectrum and 0 for an unusable spectrum. Taking the whole year of 2016 as an example, spatiotemporal pattern of Rrs(λ) quality in the Indian Ocean is characterized for the first time, and the underlying factors are elucidated. Specifically, QA analysis of the monthly Rrs(λ) over the IO indicates good quality with the average scores of 0.93 ± 0.02, 0.92 ± 0.02 and 0.92 ± 0.02 for VIIRS-NPP, MODIS-Aqua, and MODIS-Terra, respectively. Low-quality (~0.7) data are mainly found in the Bengal Bay (BB) from January to March, which can be attributed to the imperfect atmospheric correction due to anthropogenic absorptive aerosols transported by the northeasterly winter monsoon. Moreover, low-quality (~0.74) data are also found in the clear oligotrophic gyre zone (OZ) of the south IO in the second half of the year, possibly due to residual sun-glint contributions. These findings highlight the effects of monsoon-transported anthropogenic aerosols, and imperfect sun-glint removal on the Rrs(λ) quality. Further studies are advocated to improve the sun-glint correction in the oligotrophic gyre zone and aerosol correction in the complex ocean–atmosphere environment.

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Plasmochemical Synthesis of Optically Active Substances

IEEE Transactions on Plasma Science ◽

10.1109/tps.2021.3064692 ◽

2021 ◽

Vol 49 (3) ◽

pp. 1050-1054

Author(s):

Valeriy Ya. Chernyak ◽

Vitalii V. Iukhymenko ◽

Kostia V. Iukhymenko ◽

Yevgen A. Oberemok ◽

Daniil D. Tretiakov ◽

...

Keyword(s):

Optically Active ◽

Active Substances ◽

Optically Active Substances

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XVIII.—The Physical Properties of Mixed Solutions of Independent Optically - Active Substances

Proceedings of the Royal Society of Edinburgh ◽

10.1017/s0370164600017375 ◽

1907 ◽

Vol 27 ◽

pp. 172-180

Author(s):

Clerk Ranken ◽

W. W. Taylor

Keyword(s):

Physical Properties ◽

Active Substance ◽

Optically Active ◽

Present Communication ◽

Points Of Interest ◽

Systematic Examination ◽

Mixed Solutions ◽

Two Systems ◽

Active Substances ◽

Optically Active Substances

The two systems containing one of a pair of optically-active stereoisomers and an independent optically-active substance present many points of interest and importance, but have not hitherto been investigated with any degree of completeness. What is known about them may fairly be said to consist of a series of isolated facts. The present communication also contributes a few more isolated observations, and is, in reality, a preliminary to a more systematic examination of the whole subject.

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A 40-year global dataset of visible channel remote sensing reflectances and coccolithophore bloom occurrence derived from the Advanced Very High Resolution Radiometer catalogue

10.5194/essd-2018-81 ◽

2018 ◽

Author(s):

Benjamin R. Loveday ◽

Timothy Smyth

Keyword(s):

Remote Sensing ◽

High Resolution ◽

Atmospheric Correction ◽

Global Ocean ◽

Climate Data ◽

Ancillary Data ◽

Remote Sensing Reflectance ◽

Data Files ◽

Climate Data Record ◽

Very High

Abstract. A consistently calibrated 40-year length dataset of visible channel remote sensing reflectance has been derived from the Advanced Very High Resolution Radiometer (AVHRR) sensor global time-series. The dataset uses as its source the Pathfinder Atmospheres – Extended (PATMOS-x) v5.3 Climate Data Record (CDR) for top-of-atmosphere (TOA) visible channel reflectances. This paper describes the theoretical basis for the atmospheric correction procedure and its subsequent implementation, including the necessary ancillary data files used and quality flags applied, in order to determine remote sensing reflectance. The resulting dataset is produced at daily, and archived at monthly, resolution, on a 0.1° × 0.1° grid at https://doi.pangaea.de/10.1594/PANGAEA.892175. The primary aim of deriving this dataset is to highlight regions of the global ocean affected by highly reflective blooms of the coccolithophorid Emiliania Huxleyi over the past 40 years.

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