Laboratory measurements of light scattering from marine particles

Abstract

Oceans cover about 70% of the surface of the Earth and represent an important source of food. To obtain information about primary production, propagation of phytoplankton blooms, nutrient status, and temperature, the marine environment is continuously monitored both in situ and from satellites. In this context, in situ optical measurements are indispensable because they are fast and suited for automated routine operation. Among the most commonly measured parameters in situ are the absorption and attenuation coefficients, the diffuse attenuation coefficient for downwelling irradiance, and natural fluorescence. From satellite measurements, one can estimate the water-leaving radiance, and in the laboratory, the chlorophyll concentrations of water samples can be measured by fluorometric or spectrophotometric methods.The volume-scattering function (VSF) is an important optical property that is not routinely measured. Thus, the measurements of Petzold (1977), which date back to the 1970s, are still commonly used to represent the VSF for seawater. Among the laboratory measurements of the VSF relevant for our study, the data obtained by Privoznik et al. (1978) and Morel and Bricaud (1986) AbstractWe present an improved technique for measurements of the volume-scattering function (VSF) for marine particles, which include added spectral information, a novel optical design of the sample container, and two new ways of eliminating unwanted reflections. The novel optical design enables us to measure an angular range comparable to the largest angular range previously reported. Our improved setup eliminates the need for an empirically based data correction and reveals interesting characteristics of the VSF for different phytoplankton species. Measurements with our improved setup provide information that is important to understand and simulate radiative transfer in the ocean.

Backscattering by very small particles in coastal waters

“…(1) Stramski et al [2004] reviewed various seawater constituents for their roles in light backscattering in the ocean and suggested a ''reductionist'' approach to better understanding variability and biogeochemical sources of optical backscattering by studying separately optical properties of particles of various types. During the last decade, further progress has been made in quantifying the angular scattering by phytoplankton particles [Vaillancourt et al, 2004;Lotsberg et al, 2007;Svensen et al, 2007;Whitmire et al, 2010;Tan et al, 2013], particulate inorganic matter and organic matter [Zhang et al, 2014], and oceanic bubble populations [Czerski et al, 2011;Twardowski et al, 2012]. However, few studies have been conducted to assess the backscattering of very small particles (VSPs) of sizes <0.2 mm, also described as small colloidal particles [Wells and Goldberg, 1991].…”

A new approach to measure the volume scattering function

“…However, this setup requires additional delicate calibrations between sensors. Zhang et al, Lee and Lewis, and Lotsberg et al [12][13][14] presented a more advanced VSF meter with improved angular resolution. However, fundamental problems affecting VSF measurement, described below, remained.…”

“…As a consequence, instruments for VSF measurements became very complicated in their mechanical design. The measurements are typically quite timeconsuming, determining scattering step by step over the entire range of angles [14], and the spectral resolution is often limited to one or only several wavelengths [20]. The spectral distribution of the VSF for particulate suspended matter is still mostly unknown due to the lack of direct measurement.…”

Effects of an Arctic under‐ice bloom on solar radiant heating of the water column

“…This asymmetry factor was chosen as it gives a close approximation to Petzold's average-particle phase function [Mobley, 1994]. However, as algae cause a high degree of forward scattering [e.g., Lotsberg et al, 2007], 0.92 may not be appropriate after the bloom onset. Implications of higher asymmetry factors will be discussed further in section 3.2.…”