## 2 Methods
### Laboratory Experiments and Mie Scattering Calculations to Evaluate LISST-VSF
#### 2.1.2 Experimental Procedure  
Baseline measurements of 0.2 \(\upmu\)m filtered water were collected with all three instruments used in the experiments; LISST-VSF, DAWN-EOS, and Lambda 18 spectrophotometer. These baseline measurements were subtracted from subsequent measurements taken on particle suspensions to determine the optical properties associated with suspended particles only, i.e., the particulate volume scattering function, \(\beta_{p}(\psi)\), the particulate degree of linear polarization, \(DoLP_{p}(\psi)\), and the particulate beam attenuation coefficient, \(c_{p}\). Original manufacturer's stock samples of standard polystyrene beads (100, 200, 400, 500, 700, and 2000 nm in diameter) were used to generate master samples using 0.2 \(\upmu\)m filtered, deionized, and degassed water as a medium (with the exception of 2000 nm beads which used 0.2 \(\upmu\)m filtered seawater). In the process of preparation of master samples, the particle concentration was optimized to ensure that spectrophotometric measurements of beam attenuation coefficient can be performed either directly or with small dilution factor (\(\sim\)3) on these samples over 1-cm path length with sufficiently high signal but negligible multiple scattering effects. The \(c_{p}\) values for master samples ranged from about 18 m\({}^{-1}\) to 58 m\({}^{-1}\) (Table 1). The master sample was diluted for measurements with the LISST-VSF to avoid oversaturation of PMT detectors and multiple scattering over the longer path length (15 cm). For baseline measurements, the LISST-VSF sample chamber was filled with 1900 mL of 0.2 \(\upmu\)m filtered water. The final samples of particle suspensions were created by addition of 20 to 100 mL of master sample to the LISST-VSF chamber. For most beads examined in our experiments, more than one particle suspension differing in terms of particle concentration was measured with LISST-VSF (Table 1). The different particle concentrations were achieved by different dilution of master sample within LISST-VSF chamber. Owing to different dilution factors ranging from 20 to 96 (labeled as DF1, DF2, and DF3 in Table 1), the \(c_{p}\) values of LISST-VSF samples ranged from about 0.5 m\({}^{-1}\) to 1.8 m\({}^{-1}\). For a single bead size, concentration, and PMT gain, a series of LISST-VSF measurements was composed of 200 measurements taken in rapid succession (recall that a measurement refers to two scans, each with a different polarization of incident beam). This measurement series was divided into five sets of 20 measurements and one set of 100 measurements to enable manual gentle mixing of sample before each set of measurements. In addition, for the 2000 nm bead suspensions a magnetic stir bar which operated on low speed and changed direction of rotation every 30 s was used to prevent particle settling during the measurement. Several LISST-VSF baseline measurements of 0.2 \(\upmu\)m filtered water were collected for each experiment, i.e., for each examined bead size. However, for reasons of consistency and out of the desire to use an optimal baseline representative of the least contaminated 0.2 \(\upmu\)m filtered water, a single baseline was used for processing of all experimental data collected for various bead sizes and concentrations except for 2000 nm sized beads which used 0.2 \(\upmu\)m filtered seawater. This baseline was determined on the basis of finding a measurement which exhibited minimal scattering signal detected by Roving Eyeball and ring detectors and maximum directly transmitted light detected by the laser transmission sensor. We note, however, that for each PMT gain setting of the Roving Eyeball sensor a separate baseline was determined. Measurements using the DAWN-EOS instrument were collected for 100, 200, 400, and 700 nm beads. Dilution factors of master samples for DAWN-EOS measurements were between 300 and 3000, depending on bead size. The gain settings for each detector were adjusted to the highest setting that would avoid saturation of signal with incident perpendicular polarization of light.