Evaluating The Size Distribution Of Submicron Particles In Seawater

Particles of sizes \u3c 1 µm or submicron particles in seawater can contribute up to 60% of optical backscattering that reflect sunlight measured by remote sensors. Better understanding of their optical effect in ocean color requires the knowledge of their particle size distributions (PSDs). However, data on PSDs of submicron particles is limited. We examined an instrument, ViewSizer 3000, for its capability of measuring PSDs of submicron particles. The working theory of the instrument is to track the Brownian motion of particles and relate the diffusivity of each particle to its size. A Monte Carlo simulation was conducted to prove that the instrument can be used to quantify PSDs of naturally occurring particles. Calibrations in the lab using NIST-traceable microbeads indicates two corrections are needed: (1) the sample volume varies with the size of particles and (2) particles of sizes \u3c250 nm cannot be estimated reliably due to their low signal-to-noise ratio. With the corrections, the instrument can resolve submicron particles of sizes \u3e250 nm with a mean absolute error of 3.9% in size and 38% in concentration. The ViewSizer 3000 was deployed in two NASA EXPORTS field campaigns in the North Pacific Ocean in August 2018 and the North Atlantic Ocean in May 2021. A total of 373 water samples were collected from 5 m to 500 m and measured in the ship’s lab for their PSDs. The PSDs of submicron particles at different depths and waters exhibit up to 3 orders of magnitude variations at each size bin, with the mean concentration in NA 30% greater than that in NP. The slopes of PSDs, representing the relative proportion of small versus large particles, have maximum values coincident with the depths of the maximum backscattering coefficients. Concurrent measurements of angular scattering indicate that submicron particles account for up to 60% scattering at angles \u3e 90 degrees, which directly determines the magnitude of remote-sensing reflectance and its bidirectional reflectance distribution function (BRDF). Simulation using HydroLight shows that the shape of angular scattering at angles \u3e 90 degrees can explain approximately 97% of BRDF of remotely sensed reflectance