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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-259
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-259
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 19 Jul 2019

Submitted as: research article | 19 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

A Global End-Member Approach to Derive aCDOM(440) from Near-Surface Optical Measurements

Stanford B. Hooker1, Atsushi Matsuoka2, Raphael M. Kudela3, Youhei Yamashita4, Koji Suzuki4, and Henry F. Houskeeper3 Stanford B. Hooker et al.
  • 1NASA Goddard Space Flight Center, Ocean Ecology Laboratory/Code 616.2, Greenbelt, Maryland 20771
  • 2Université Laval, Takuvik Joint International Laboratory, Québec City, Canada G1V 0A6
  • 3University of California Santa Cruz, Ocean Sciences Department, Santa Cruz, California 95064
  • 4Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan 060-0810

Abstract. This study establishes an optical inversion scheme for deriving the absorption coefficient of colored (or chromophoric, depending on the literature) dissolved organic material (CDOM) at the 440 nm wavelength, which can be applied to global water masses with near-equal efficacy. The approach uses a ratio of diffuse attenuation coefficient spectral end members, i.e., a short and long wavelength pair. The global perspective is established by sampling "extremely" clear water plus a generalized extent in turbidity and optical properties that each span three decades of dynamic range. A unique data set was collected in oceanic, coastal, and inland waters (as shallow as 0.6 m) from the North Pacific Ocean, the Arctic Ocean, Hawaii, Japan, Puerto Rico, and the east and west coasts of the United States. The data were partitioned using subjective categorizations to define a validation quality subset of conservative water masses, i.e., the inflow and outflow of properties constrain the range in the gradient of a constituent, plus 15 subcategories of water masses that were not evolving conservatively. The dependence on subcategories was confirmed with an objective methodology based on cluster analysis techniques. The latter defined five distinct classes with validation quality data present in all classes, but which also decreased in percent composition as a function of increasing class number and optical complexity. Four different algorithms based on different validation quality end members were validated with accuracies of 1.–6.2 %, wherein pairs with the largest spectral span were most accurate. Although algorithm accuracy decreased with the inclusion of more subcategories containing non-conservative water masses, changes to the algorithm fit were small when a preponderance of subcategories were included. The high accuracy for all end-member algorithms was the result of data acquisition and data processing improvements, e.g., increased vertical sampling resolution to less than 1mm and a boundary constraint to mitigate wave focusing effects, respectively. An independent evaluation with a historical database confirmed the consistency of the algorithmic approach and its application to quality assurance, e.g., to flag data outside expected ranges, identify suspect spectra, and objectively determine the in-water extrapolation interval by converging agreement for all applicable end-member algorithms. The legacy data exhibit degraded performance (as 44 % uncertainty) due to a lack of high-quality near-surface observations, especially for clear waters wherein wave-focusing effects are problematic. The novel optical approach allows the in situ estimation of an in-water constituent in keeping with the accuracy obtained in the laboratory.

Stanford B. Hooker et al.
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Status: final response (author comments only)
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Stanford B. Hooker et al.
Stanford B. Hooker et al.
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Latest update: 20 Oct 2019
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Short summary
A Kd(lambda) and aCDOM(440) data set spanned oceanic, coastal, and inland waters. The algorithmic approach, based on Kd end-member pairs, can be used globally. End members with the largest spectral span had an accuracy of 1.2–2.4 % (RMSE). Validation was influenced by subjective "non-conservative" water masses. The influence of subcategories was confirmed with an objective cluster analysis.
A Kd(lambda) and aCDOM(440) data set spanned oceanic, coastal, and inland waters. The...
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