Significant contribution of submicron particles to the volume scattering functions in the open ocean

We measured the volume scattering functions of three particle size fractions (bulk, < 0.7 µm, and < 0.2 µm) in North Pacific Ocean during three field experiments (Line P 2017, Line P 2018, and EXPORTS 2018 cruises) using a recently commercialized LISST-VSF that measures angular scattering from 0.08° to 150°. Our results showed that (1): The contribution to the scattering by submicron particles, and particularly the particles in the dissolved domain (<0.2 μm), could be significant (up to 60% of bulk scattering). (2) The contribution to the bulk scattering by submicron particles generally increases with scattering angle, with the greatest contribution at angles around 110° - 120° then decreases towards larger angles. (3) Submicron particles constitute a more or less constant background backscattering independent of chlorophyll concentrations whereas backscattering by larger particles increases with chlorophyll concentration. (4) As a result, the fractional contribution to backscattering by submicron particles generally increases with decreasing trophic level, from approximately 10% in coastal water, to 20% in near surface open waters (depth < 100 m), and to 40% in deeper waters. These findings confirm that the variability of bulk particle scattering is mainly controlled by phytoplankton particles. However, in clear oceanic waters, the contribution by submicron particles is significant and cannot be ignored.