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

Abstract

Non-algal particles and chromophoric dissolved organic matter (CDOM) are two major classes of seawater constituents that contribute substantially to light absorption in the ocean within the ultraviolet (UV) and visible (VIS) spectral regions. The similarities in the spectral 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 absorption coefficient, adg(λ), in optical remote-sensing applications. Given the different biogeochemical and ecological roles of non-algal particles and CDOM in the ocean, it is important to determine and characterize the absorption coefficient of each of these constituents separately. We describe an ADG model that partitions adg(λ) into ad(λ) and ag(λ). This model improves upon a recently published model [Appl. Opt. 58, 3790 (2019)APOPAI0003-693510.1364/AO.58.003790] through implementation of a newly assembled dataset of hyperspectral measurements of ad(λ) and ag(λ) from diverse oceanic environments to create the spectral shape function libraries of these coefficients, a better characterization of variability in spectral shape of ad(λ) and ag(λ), and a spectral extension of model output to include 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, which determines solutions over the spectral range from 350 to 700 nm, and the ADG_VIS model, which determines solutions in the VIS but can also be coupled with an independent extrapolation model to extend output to the near-UV. This specific model variant is referred to as ADG_VIS-UVExt. Evaluation of the model with development and independent datasets demonstrates good performance of both ADG_UV-VIS and ADG_VIS-UVExt. Comparative analysis of model-derived and measured values of ad(λ) and ag(λ) indicates negligible or small median bias, generally within ±5% over the majority of the 350–700 nm spectral range but extending to or above 10% near the ends of the spectrum, and the median percent difference generally below 20% with a maximum reaching about 30%. The presented ADG models are suitable for implementation as a component of algorithms in support of satellite ocean color missions, especially the NASA PACE mission.