Abstract
Marine microscopic particles profoundly impact global biogeochemical cycles, but our understanding of their dynamics is hindered by lack of observations. To fill this gap, optical backscattering measured by satellite sensors and in-situ autonomous platforms can be exploited. Unfortunately, these observations remain critically limited by an incomplete mechanistic understanding of what particles generate the backscattering signal. To achieve this understanding, optical models are employed. The simplest of these models—the homogeneous sphere—severely underestimates the measured backscattering and the missing signal has been attributed to submicron particles. This issue is known as the missing backscattering enigma. Here we show that a slightly more complex optical model—the coated sphere—can predict the measured backscattering and suggests that most of the signal comes from particles >1 µm. These findings were confirmed by independent size-fractionation experiments. Our results demonstrate that the structural complexity of particles is critical to understand open-ocean backscattering and contribute to solving the enigma.
Highlights
Marine microscopic particles profoundly impact global biogeochemical cycles, but our understanding of their dynamics is hindered by lack of observations
Optical scattering is quantified by the volume scattering function (VSF) that measures the fraction of incident light deflected by particles in a given direction per unit distance[10]
Modelled bbp values could be forced to match observations by increasing n to 1.11 (Fig. 1d), but this increase in n enhanced the bias in cp predictions by about 10 times (Fig. 1c). These results were robust to variations in the refractive index within the particle size distributions (PSDs) (Supplementary Fig. 2), as well as to the presence of absorbing particles (Supplementary Fig. 3)
Summary
Marine microscopic particles profoundly impact global biogeochemical cycles, but our understanding of their dynamics is hindered by lack of observations. To fill this gap, optical backscattering measured by satellite sensors and in-situ autonomous platforms can be exploited. Optical backscattering measured by satellite sensors and in-situ autonomous platforms can be exploited These observations remain critically limited by an incomplete mechanistic understanding of what particles generate the backscattering signal. To achieve this understanding, optical models are employed. Difficulties in interpreting bbp arise due to an incomplete mechanistic understanding of which particles are detected by bbp
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