Estimation of scattering error in spectrophotometric measurements of light absorption by aquatic particles from three-dimensional radiative transfer simulations

Abstract

We present an approach based on three-dimensional Monte Carlo radiative transfer simulations for estimating scattering error in measurements of light absorption by aquatic particles with a typical laboratory double-beam spectrophotometer. The scattering error is calculated by combining the weighting function describing the angular distribution of photon losses that are due to scattering on suspended particles with the volume scattering function of particles. We applied this method to absorption measurements made on marine phytoplankton, a diatom Thalassiosira pseudonana and a cyanobacterium Synechococcus. Assuming that the scattering phase function is described by the Henyey-Greenstein formula, we determined the backscatter probability of phytoplankton, which yields the best correction for scattering error at a light wavelength of 750 nm, where true absorption is null. The backscattering ratio estimated for both phytoplankton species is significantly higher than previously reported data based on Mie-scattering calculations for homogeneous spheres. Depending on the type of particles, the corrected absorption spectra obtained with our method may be similar or significantly different from spectra obtained with the null-point correction based on wavelength-independent scattering error.

Evaluation and Improvement of an Iterative Scattering Correction Scheme for in situ Absorption and Attenuation Measurements

“…The effect of imperfections in the structure of the glass cuvette walls of the ac-9 absorption tube was modeled by randomly imposing a percentage error loss term r w on photons striking the water-glass interface from both directions. (Stramski and Piskozub 2003). This was confirmed by a series of simulations with varying absorption, scattering, and scattering phase functions that were performed for a given ac-9 absorption configuration (Fig.…”

“…The HOBI Labs HS-6 and a-beta measure the spectral volume scattering function (VSF), b (c, l), at an effective scattering angle of c 5 1408 (Maffione and Dana 1997). The backscattering coefficient was determined from the measured b (1408, l) using the procedures described in detail by Stramski et al (2008). Theoretical estimates of pure seawater scattering, adjusted for temperature and salinity effects, were calculated from the relationships described in Buiteveld et al (1994) and Twardowski et al (2007).…”

“…whereb p (l, c) is the wavelength-dependent particulate scattering phase function and W a (c) is a wavelengthindependent weighting distribution function that describes the angular scattering collection efficiency of the absorption sensor (Stramski and Piskozub 2003). Piskozub et al (2004) have shown that the equivalent weighting function for attenuation sensors is reasonably approximated by assuming that all photons scattered between zero and the collection angle of the transmissometer (c c ) contribute equally to the attenuation scattering error and that photons scattered outside this range make zero contribution to the error.…”

“…Monte Carlo simulations were performed with code previously used to study ocean optics phenomena and instrumentation (Stramski and Piskozub 2003;Piskozub et al 2004;Flatau et al 1999;. The ac-9 absorption and attenuation sensors were simulated using accurate instrument dimensions provided by the manufacturer.…”

“…However, measuring the absorption of turbid media is considerably more complicated because of the effect of scattering on measured signals (Stramski and Piskozub 2003). Various approaches to resolve this issue have been attempted, with the reflecting-tube absorption meter [Western Environmental Technologies Laboratories (WET Labs) ac-9/ ac-spectra (ac-s)] becoming a widely adopted instrument within the optical oceanography community for measuring in situ absorption.…”