Measuring the size distributions of submicron particles in ocean by tracking the Brownian motion

The size distributions of particles (PSDs) in aquatic environment have been measured for decades, but the size ranges were mainly limited for particles greater than 1 µm while only a few studies reported the size distributions less than 0.2 µm. The direct measurements of submicron particles between 0.2 µm and 1 µm in ocean is lacking. We utilized a particle size analyzer called ViewSizer^TM to track the Brownian motion to estimate the size of the submicron particles. We calibrated the instrument using polystyrene beads of standard sizes from 100 nm to 900 nm, with a mean percentage error of ~5% for the peak sizes and < 55% for the concentrations. During the EXPORTS cruise surveying the area near the Station Papa (50°N, 145°W) in 2018, we measured the size distribution of submicron particles in water samples collected at various depths from surface to 3000 m. The concentrations of submicron particles vary from 3×10⁵ to 2×10⁷ /ml in the entire survey area. Detailed analyzing is still ongoing, but the preliminary results show that > 60% difference of the PSD values were found between the surface layer (< 15 m) and the deeper layer (> 500 m), and the Junge slopes of the submicron particles vary from 1.95 to 4.13 for all samples. This result is within the range of the Junge slopes (from approximate 2 to 6) previously reported with larger particles worldwide, while our direct measurements provide detailed features of the PSDs in the submicron sizes for further analyzing.