Linking UV optical properties and photochemical rates using remotely sensed ocean color
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Solar ultraviolet radiation (UVR) is a critical factor regulating photo processes in the ocean with the majority of UVR absorbed by chromophoric dissolved organic matter (CDOM). Accurate knowledge of UVR and CDOM distributions is desired to better model the UV–CDOM interactions that drive photochemistry. This dissertation contains four studies that develop optical techniques to model photochemistry and fluorescent components in the ocean. First, a composite set of remote sensing algorithms was developed to retrieve UV attenuation coefficients (Kd(UV)) from remote sensing reflectance (Rrs), allowing improved estimates of UVR penetration in a variety of water types (e.g. mean relative error of 13% for Kd(340)). Second, I used a suite of statistical approaches to develop a novel algorithm (SeaCDOM) for accurate, direct retrieval of fully resolved UV absorption spectra for CDOM, ag(275-450), from Rrs, obtaining a mean relative error of ~25%. It has the advantage of being free of the assumed CDOM exponential extrapolations from modeled visible wavelengths that have hampered previous work. This advance should provide new insight about the chemical composition, origins, transformation, and cycling pathways of CDOM in the surface ocean with a synoptic view. Third, I introduced a new approach for calculating photochemical rates by blending the composite (Kd(UV) and SeaCDOM algorithms to examine carbon monoxide photoproduction using a single, high resolution coastal satellite image. This novel product demonstrates the complex spatial variability of depth-specific and depth-integrated photoproduction on an estuarine scale. This new capability for independent retrievals of ag and Kd allows quantitative partitioning of UV photons between CDOM and other optical constituents, producing greatly improved estimates for photochemistry in complex waters. Finally, I explored the spatial (0 to ~5000m) distribution and dynamics of DOM throughout the Gulf of Alaska using absorbance and fluorescence measurements. Biogeochemical and physical processes driving DOM optical property distributions are explored using multi-linear relationships, revealing significant control by both in situ production and mixing of water masses. Strong relationships between dissolved organic carbon (DOC) and protein-like fluorescent components, suggests a new tool to trace bulk dissolved organic carbon in the deep ocean, especially within the bio-refractory pool.