Oral Session 10 | Thursday, October 6, 14:20–14:40 | Abstract 571

Quantifying CDOM photobleaching and its effects on apparent optical properties in the global surface ocean

Photobleaching is an important degradation pathway of CDOM optical properties. As a result, it can play a major role in regulating UV-blue solar exposure and photochemical processes in the surface ocean. However, despite its importance, photobleaching rates in the global ocean remain poorly quantified due to: 1) the difficulty of determining photobleaching apparent quantum yields matrices (AQY-M) that adequately capture the dual spectral dependency of this process, 2) the poorly understood and constrained variability of the AQY-M in marine waters. We recently developed a new experimental method that facilitates the determination of photobleaching AQY-M of natural water samples. Here, we applied this method on samples collected in estuarine, coastal and open-ocean waters and we analyzed the variability in the measured AQY-M and its dependence on environmental conditions. The magnitude and spectral characteristics of the measured AQY-Ms showed a strong dependence on three quantities amenable to remote sensing: 1) CDOM degradation state as indicated by the spectral slope coefficient S 275-295, 2) water temperature, 3) the dose of quanta absorbed by CDOM. The measured AQY-Ms were then used to build a machine-learning model capable of reproducing >90% of the observed AQY-M variability from the three remotely sensed variables. Integration of this model in a spectrally resolved photochemical modeling framework facilitated the calculation of accurate photobleaching rates in the global surface ocean. The modeled rates were then used to quantify the daily impacts of photobleaching on the apparent optical properties of the upper ocean and on the relationship between CDOM and DOC.

*Xiaohui Zhu, Boston University, 0000-0002-9000-312X

Cédric Fichot, Boston University, 0000-0002-1099-5764

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