The open-ocean missing backscattering is in the structural complexity of particles

This is the final version. Available on open access from Nature Research via the DOI in this recordData availability This study uses data from the Atlantic Meridional Transect Consortium (NER/0/5/2001/00680), provided by the British Oceanographic Data Centre and supported by the Natural Environment Research Council. Particle size distribution data collected during the AMT26 cruise can be downloaded at https://doi.org/10.5285/79103bda-8913-39f3-e053-6c86abc0567a. All other data are freely available at https://www.bodc.ac.uk/projects/uk/amt/ and https://www.bodc.ac.uk/data/bodc_database/nodb/data_collection/207/, or from the authors upon request.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.European Union Horizon 2020Natural Environment Research Council (NERC)NAS