Scott A. Freeman

Use of optical scattering to discriminate particle types in coastal waters

The particulate scattering characteristics of coastal waters were examined at nine locations around the United States, including near-shore sites in the Gulf of Mexico and the Atlantic and Pacific oceans. The scattering measurements were used in conjunction with inversion models to estimate particle size distributions and bulk refractive indices of the suspended particles. The relationships between various scattering properties and chlorophyll concentration were also investigated and compared with previous relationships described for case I waters. Although the general patterns of scattering and particle characteristics in coastal waters were fairly consistent, fine-scale variability within the water column was substantial. Combining optical measurements with inversion techniques provided a more informative view of the environment and a better understanding of the nature of particle populations in the coastal ocean.

Spectra of Particulate Backscattering in Natural Waters

Hyperspectral profiles of downwelling irradiance and upwelling radiance in natural waters (oligotrophic and mesotrophic) are combined with inverse radiative transfer to obtain high resolution spectra of the absorption coefficient (a) and the backscattering coefficient (b(b)) of the water and its constituents. The absorption coefficient at the mesotrophic station clearly shows spectral absorption features attributable to several phytoplankton pigments (Chlorophyll a, b, c, and Carotenoids). The backscattering shows only weak spectral features and can be well represented by a power-law variation with wavelength (lambda): b(b) approximately lambda(-n), where n is a constant between 0.4 and 1.0. However, the weak spectral features in b(b)b suggest that it is depressed in spectral regions of strong particle absorption. The applicability of the present inverse radiative transfer algorithm, which omits the influence of Raman scattering, is limited to lambda < 490 nm in oligotrophic waters and lambda < 575 nm in mesotrophic waters.

Determining Size Distributions and Composition of Particles Suspended in Water: A New SEM-EDS Protocol with Validation and Comparison to Other Methods

Knowledge of particle size distributions (PSDs)in seawater is importantfor understanding several facets of marine science, such as the behavior of light scattering in seawater, phytoplankton dynamics, and biogeochemical cycling. Here, a method has been developed to quantify the size distribution of particle suspensions and characterize their chemical composition utilizing a scanning electron microscope (SEM) coupled with an energy dispersive spectrometer (EDS) and applying image analysis techniques, including automatic thresholding. The method was validated by verifying the PSD and chemical composition of the Arizona Test Dust (ATD), which has a well-documented size distribution and chemical composition. Size distributions of ATD particles containing specific elements important in the marine environment, such as silicon, iron, calcium, aluminum,andpotassium, were quantified. PSDs determinedwith thetechnique infield samples from coastal Long Island Sound and the remote South Pacific were compared with other sizing methods, including electroresistivity and laser diffractometry. Most accurate results for PSD determinations occurred when the particle mass loading on the filter was between 0.04 and 0.1 mg cm 22 . With this in mind, immediate feedback in the field can be provided to prepare appropriate filtration sample volumes due to alinearrelationshipbetweenthebeamattenuationcoefficientat650 nm(c650)andthetotalsuspendedmatter (TSM).Overall,themethodpresentstwodefiningadvantagesin 1)minimizinguserbias,becausethemajority of the analysis is automated, and 2) providing an elemental distribution in the context of a particle size distribution.

Platform effects on optical variability and prediction of underwater visibility.

We present hydrographic and optical data collected concurrently from two different platforms, the R/P FLoating Instrument Platform and the R/V Kilo Moana, located about 2km apart in the Santa Barbara Channel in California. We show that optical variability between the two platforms was due primarily to platform effects, specifically the breakdown of stratification from mixing by the hull of R/P FLIP. Modeled underwater radiance distribution differed by as much as 50% between the two platforms during stratified conditions. We determine that the observed optical variability resulted in up to 57% differences in predicted horizontal visibility of a black target.

Multi-method approach to quantify uncertainties in the measurements of light absorption by particles.

Through technological and research advances, numerous methods and protocols have emerged to estimate spectral absorption of light by particles, ap, in an aquatic medium. However, the level of agreement among measurements remains elusive. We employed a multi-method approach to estimate the measurement precision of measuring optical density of particles on a filter pad using two common spectrophotometric methods, and the determination precision, or uncertainty, of the computational techniques for estimating ap for six ocean color wavelengths (412, 443, 490, 510, 555, 670 nm). The optical densities measured with the two methods exhibited a significant, positive correlation. Optical density measurement precision ranged from 0.061%-63% and exhibited a significant, positive correlation. Multi-method uncertainty ranged from 7.48%-119%. Values of ap at 555 nm and 670 nm exhibited the highest values of uncertainty. Poor performance of modeled ap compared to determined ap suggest uncertainties are propagated into bio-optical algorithms.

Diver visibility measured with a compact scattering-attenuation meter (SAM) compatible with AUVs and other small deployment platforms

An appropriate determination of water clarity is required by defense and security operations assessing subsurface threats compromising harbor and coastal security. For search and inspection operations involving divers, underwater imaging, and electro-optical identification (EOID) systems such as laser line-scanners, the key environmental parameter needed is the optical attenuation coefficient (directly related to diver visibility). To address this need, a scattering-attenuation meter (SAM) measuring attenuation and diver visibility was developed for integration on new compact surveying platforms such as ROVs and the REMUS and glider AUVs. The sensor is compact (18X8X6 cm3), low power, robust, and hydrodynamic with a flat sensing face. The SAM measures attenuation using a novel dual-scattering approach that solves the paradox of making high-resolution attenuation measurements over the long pathlengths required for natural waters with a compact sensor. Attenuation and visibility data is presented from San Diego harbor in coordination with video images of bottom topography collected with a REMUS vehicle, from around New York harbor with a SAM mounted in an autonomous Slocum glider, and from Narragansett Bay. Results show that 1) visibility and/or attenuation in harbor and coastal regions can change rapidly over small scales (meters), especially near the bottom, 2) turbid bottom nepheloid layers are common, 3) typical visibility and/or attenuation levels fall in a range where knowledge of visibility and/or attenuation can be essential in the decision making process for security operations, and 4) attenuation is a significantly more accurate proxy for diver visibility than backscattering.

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 (c650) can provide immediate feedback in the field to determine filter volumes for sample preparation.