Laboratory experiments for inter-comparison of three volume scattering meters to measure angular scattering properties of hydrosols

Abstract

Measurements of the volume scattering function (VSF) of hydrosols is of primary importance to investigate the interaction of light with hydrosols and to further interpret in situ and remote sensing data of ocean color. In this paper, a laboratory inter-comparison experiment of three recently developed VSF meters that are able to measure the scattered light for a wide range of scattering angle at 515 nm wavelength is performed using phytoplankton cultures and mineral-like hydrosols. A rigorous measurement protocol was employed to ensure good quality data. In particular, the protocol enabled removing the influence of bacteria on the hydrosols within the sample. The differences in the VSF measurements between the instruments vary from 10 to 25% depending on the composition of the hydrosols. The analysis of the angular features of the VSF revealed a sharp increase of the VSF beyond the scattering angle of 150° for some phytoplankton species. Such behavior is observed for two of the three VSF meters, thus suggesting that it is not due to instrumental artifacts but more likely to phytoplankton optical properties themselves. Moreover, comparisons with currently used theoretical phase functions show that the models are not able to reproduce satisfactorily the directional patterns in the backscattering region. This study suggests that a better modelling of the VSF shape of phytoplankton at high scattering angles is required to correctly represent the angular shape of the VSF in the backscattering hemisphere. Tabulated values of the measured phase functions are provided for scattering angles from 0.1 to 175°.

Obtaining Phytoplankton Diversity from Ocean Color: A Scientific Roadmap for Future Development

“…PT specific absorption properties are available but large spectral variability is related to algal culturing and variations in size, pigment composition and pigment packaging due to physiological responses of PT. In contrast, due to high measuring uncertainties spectral scattering properties (including back-scattering and volume scattering function) are still less known (Tan et al, 2015;Harmel et al, 2016). Thus, PG related specific IOPs are not adequately represented in RTMs.…”

Phytoplankton Group Identification Using Simulated and In situ Hyperspectral Remote Sensing Reflectance

“…One simplification regards the scattering properties of phytoplankton; some natural variability is included in the simulations, but due to the lack of reliable specific information, all phytoplankton groups are modeled with the same scattering assumptions. But due to different particle shapes and size distribution, it is evident that the spectral scattering properties vary (e.g., Morel, 1987;Evers-King et al, 2014;Harmel et al, 2016). Robertson Lain et al (2017 showed the potentially considerable influence of different phytoplankton phase functions on modeled remote sensing reflectance over the entire visible range.…”

Optical Backscattering Measured by Airborne Lidar and Underwater Glider

“…Some of these report χ(170 • ) > 1 [15,17], while others report χ(170 • ) < 1 [12,13,16]. More recent laboratory instruments are even lower than most previous measurements, with χ(175 • ) < 0.01 for several algal species [21]. There are also uncertainties in the wavelength dependence of this quantity [41][42][43].…”

“…Some studies reported lower values for χ(170 • ) [12,13,16], but most do not [13,[15][16][17]. The most recent laboratory measurements, made out to 175 • , are significantly lower, reporting χ(175 • ) < 0.01 for several phytoplankton species and χ(175 • ) = 0.2 for Arizona road dust [21]. In all cases, the variability is higher at larger scattering angles, which is partially a reflection of difficulties in scattering measurements near 180 • .…”

“…This can be improved to about 3% using measurements at two scattering angles [20] and to under 1% with a geometry that provides the integrated scattering with a diffuse cavity [19]. Both field [12][13][14][15]17] and laboratory [13,16,21] studies have shown that the variability in χ is minimum at a scattering angle near 120 • . Most studies report χ(120 • ) = 1.10-1.13 with a variability of a few % [12][13][14][15][16][17].…”