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Particulate optical scattering coefficients along an Atlantic Meridional Transect
The particulate optical backscattering coefficient (bbâ‚š) is a
fundamental optical property that allows monitoring of marine suspended
particles both in situ and from space. Backscattering measurements in the
open ocean are still scarce, however, especially in oligotrophic regions.
Consequently, uncertainties remain in bbâ‚š parameterizations as well as
in satellite estimates of bbâ‚š. In an effort to reduce these uncertainties,
we present and analyze a dataset collected in surface waters during the
19th Atlantic Meridional Transect. Results show that the relationship
between particulate beam-attenuation coefficient (câ‚š) and chlorophyll-a
concentration was consistent with published bio-optical models. In contrast,
the particulate backscattering per unit of chlorophyll-a and per unit of
câ‚š were higher than in previous studies employing the same sampling
methodology. These anomalies could be due to a bias smaller than the
current uncertainties in bbâ‚š. If that was the case, then the AMT19 dataset
would confirm that bbâ‚š:câ‚š is remarkably constant over the surface open
ocean. A second-order decoupling between bbâ‚š and câ‚š was, however,
evident in the spectral slopes of these coefficients, as well as during diel
cycles. Overall, these results emphasize the current difficulties in obtaining
accurate bbâ‚š measurements in the oligotrophic ocean and suggest that, to
first order, bbâ‚š and câ‚š are coupled in the surface open ocean, but they are
also affected by other geographical and temporal variations
Determination of the absorption coefficient of chromophoric dissolved organic matter from underway spectrophotometry
This is the final version. Available on open access from Optical Society of America via the DOI in this recordMeasurements of the absorption coefficient of chromophoric dissolved organic matter (ay) are needed to validate existing ocean-color algorithms. In the surface open ocean, these measurements are challenging because of low ay values. Yet, existing global datasets demonstrate that ay could contribute between 30% to 50% of the total absorption budget in the 400-450 nm spectral range, thus making accurate measurement of ay essential to constrain these uncertainties. In this study, we present a simple way of determining ay using a commercially-available in-situ spectrophotometer operated in underway mode. The obtained ay values were validated using independent collocated measurements. The method is simple to implement, can provide measurements with very high spatio-temporal resolution, and has an accuracy of about 0.0004 m−1 and a precision of about 0.0025 m−1 when compared to independent data (at 440 nm). The only limitation for using this method at sea is that it relies on the availability of relatively large volumes of ultrapure water. Despite this limitation, the method can deliver the ay data needed for validating and assessing uncertainties in ocean-colour algorithms.European Space Agency (ESA)National Atmospheric and Space Administration (NASA