A method for determining oceanic particle size distributions and particle composition using scanning electron microscopy coupled with energy dispersive spectroscopy

To understand the behavior of light scattered in seawater, it is necessary to know the size distribution of particles in seawater, as well as their composition (refractive index distribution) and complex shape. A method has been developed to determine marine PSDs and simultaneously characterize their chemical compositions by utilizing a scanning electron microscope (SEM) coupled with an energy dispersive spectrometer (EDS) and applying sophisticated image analysis techniques that minimized user bias including automatic image thresholding. The method was validated by verifying the PSD and chemical composition of Arizona test dust, which has a well-documented size distribution and chemical composition. PSDs of field samples collected from the coastal Long Island Sound and the remote South Pacific Ocean were also determined. Where applicable, PSDs agreed well overall with other PSD determining methods such as electroresistive counting and near-forward diffraction theory inversions. The method performed optimally when the particle mass on the filter was between 0.4mg and 1.0mg. With this in mind, measuring particle beam attenuation coefficient at 650nm (c(650)) can provide immediate feedback in the field to determine filter volumes for sample preparation.