Analysis of oceanic suspended particulate matter in the western North Pacific using the complex amplitude sensor

Abstract

Oceanic suspended particulate matter (SPM) plays important roles in the coupling of climate and biogeochemical cycles via ocean-atmosphere interactions. However, methods for quantifying the properties of SPM in seawater have not yet been well established. Here we present the application of the recently developed complex amplitude sensor (CAS) for analyzing the complex forward-scattering amplitude of individual SPM (0.2-5.0 µm in diameter) obtained at depths of 0-100 m during a research cruise in the western North Pacific. The measured distribution of the complex amplitude indicated that the CAS-derived SPM data could be roughly classified into five major types. Comparison with reference sample's complex amplitude data and scanning electron microscopy analysis suggested that these types could be attributed mainly to diatom fragments, carbonaceous materials (likely organic matter), mineral dusts, iron oxides, or black carbon. Depth profiles revealed that relatively high concentrations of SPM, presumably dominated by diatom fragments and carbonaceous materials with peak diameters of 0.7-1.0 µm, were typically associated with elevated turbidities and chlorophyll a concentrations. Based on this case study, we discuss the practical advantages and limitations of using the CAS to measure size-resolved concentrations of SPM in seawater and to characterize its composition.

Fig. 1

Research area and track of cruise. The locations of collecting seawater samples are denoted as black points. This figure was generated by Matplotlib (version 3.8.0, Hunter, J. D., Matplotlib: a 2D graphics environment. Computing in Science and Engineering 9, 2007, https://ieeexplore.ieee.org/document/4160265) and Cartopy (version 0.21.1, MetOffice, UK, https://scitools.org.uk/cartopy/docs/latest/index.html).

Fig. 2

(a–d) Distribution of S data points combined for (a,b) all in situ samples and (c,d) all filter samples. Boundaries for classifying S data points are represented as dashed curves and lines. The boundary curves are theoretical S curves for sphere particles with complex refractive index of m. (e,f) Median of ReS values as functions of ImS values for reference samples. The black rectangle boxes at lower left in l-channel panels show the outer boundaries of s-channel panels.

Fig. 3

SEM–EDS analyses of representative particles found in CAS34 and CAS35 samples. (a) Representative SEM images of carbonaceous and Si-rich (mineral dust and diatom) particles (indicated by arrows). Red arrows indicate particles whose spectra are shown in Supplementary Fig. 4. (b) Comparison of size-resolved concentrations of the particles determined by CAS and SEM–EDS.

Fig. 4

Size-resolved number concentrations of Type 0–4 particles obtained by the in situ measurements shown by depth. Solid line and shaded area represent median and 25–75 percentile values, respectively.

Fig. 5

Vertical profiles of number and volume concentrations of SPM obtained by the in situ measurement, comparing with the vertical profiles of turbidity and Chl-a concentration.

Fig. 6

Vertical profiles of volume concentrations for Types 0 and 1 obtained by the in situ measurements, comparing with the vertical profiles of turbidity and Chl-a concentration. Types 2–4 are not shown due to their low concentrations.

Fig. 7

Temporal variations in the volume concentration of each Type of SPM at a depth of 0 m and 10 m. The date are displayed in JST.