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The index of refraction (n) of particles is an important parameter in optical models that aims to extract particle size and carbon concentrations from light scattering measurements. An inadequate choice ofncan critically affect the characterization and interpretation of optically-derived parameters, including those from satellite-based models which provide the current view of how biogeochemical processes vary over the global ocean. Yet, little is known about hownvaries over time and space to inform such models. Particularly, in situ estimates ofnfor bulk water samples and at diel-resolving time scales are rare. Here, we demonstrate a method to estimatenusing simultaneously and independently collected particulate beam attenuation coefficients, particle size distribution data, and a Mie theory model. We apply this method to surface waters of the North Pacific Subtropical Gyre (NPSG) at hourly resolution. Clear diel cycles innwere observed, marked by minima around local sunrise and maxima around sunset, qualitatively consistent with several laboratory-based estimates ofnfor specific phytoplankton species. A sensitivity analysis showed that the daily oscillation innamplitude was somewhat insensitive to broad variations in method assumptions, ranging from 11.3 ± 4.3% to 16.9 ± 2.9%. Such estimates are crucial for improvement of algorithms that extract the particle size and production from bulk optical measurements, and could potentially help establish a link betweennvariations and changes in cellular composition of in situ particles.