A model for partitioning the nonphytoplankton absorption coefficient of seawater in the ultraviolet and visible spectral range into the contributions of non-algal particulate and dissolved organic matter

• Matthew Kehrli, Dariusz Stramski, Rick Reynolds, and Ishan Joshi
• received 01/25/2024; accepted 04/12/2024; posted 04/15/2024; Doc. ID 517706
• Abstract: Non-algal particles and chromophoric dissolved organic matter (CDOM) are twomajor classes of seawater constituents which contribute substantially to light absorption in theocean within the ultraviolet (UV) and visible (VIS) spectral region. The similarities in thespectral shape of these two constituent absorption coefficients, ad(λ) and ag(λ) respectively,have led to their common estimation as a single combined non-phytoplankton absorptioncoefficient, adg(λ), in optical remote sensing applications. Given the different biogeochemicaland ecological roles of non-algal particles and CDOM in the ocean, it is important to determineand characterize the absorption coefficient of each of these constituents separately. We describean ADG model that partitions adg(λ) into ad(λ) and ag(λ). This model improves upon a recentlypublished model [Appl. Opt. 58, 3790 (2019)] through implementation of a newly assembleddataset of hyperspectral measurements of ad(λ) and ag(λ) from diverse oceanic environments tocreate the spectral shape function libraries of these coefficients, a better characterization ofvariability in spectral shape of ad(λ) and ag(λ), and a spectral extension of model output toinclude the near-UV (350–400 nm) in addition to the VIS (400–700 nm) part of the spectrum.We developed and tested two variants of the ADG model; the ADG_UV-VIS model whichdetermines solutions over the spectral range from 350 to 700 nm, and the ADG_VIS modelwhich determines solutions in the VIS but can also be coupled with an independentextrapolation model to extend output to the near-UV. This specific model variant is referred toas ADG_VIS-UVExt. Evaluation of the model with development and independent datasetsdemonstrate good performance of both ADG_UV-VIS and ADG_VIS-UVExt. Comparativeanalysis of model-derived and measured values of ad(λ) and ag(λ) indicates negligible or smallmedian bias, generally within ±5% over the majority of the 350–700 nm spectral range butextending to or above 10% near the ends of the spectrum, and the median percent differencegenerally below 20% with a maximum reaching about 30%. The presented ADG models aresuitable for implementation as a component of algorithms in support of satellite ocean colormissions, especially the NASA PACE mission.