## 2 Methods
### Laboratory Experiments and Mie Scattering Calculations to Evaluate LISST-VSF
#### 2.1.3 Data Processing  
Processing of LISST-VSF data was done with a standard processing code provided by manufacturer (version of 2013) to determine \(\beta_{p}(\psi)\), \(DoLP_{p}(\psi)\), and \(c_{p}\), denoted hereafter as \(\beta_{p}^{LISST*}(\psi)\), \(DoLP_{p}^{LISST*}(\psi)\), and \(c_{p}^{LISST*}\) respectively (the asterisk indicates that the variable is derived from the standard processing code without additional corrections developed in this study). Some details specific to the processing and quality control of our experimental data are provided below. As a first step in data processing, the baseline values in raw counts were subtracted from each LISST-VSF measurement of raw counts acquired on samples of bead suspensions. To account for light attenuation along the path between the scattering volume and the detector, an attenuation correction factor was calculated using the average \(c_{p}^{LISST*}\) from the series of measurements and the length of the path for each scattering angle. Further, to account for the difference in sensitivity of the two Roving Eyeball PMT detectors, a factor \(\alpha\) is used to adjust the measured counts of one PMT detector relative to the other [67]. The value of \(\alpha\) = 0.9335 was determined by averaging all median values of \(\alpha\) derived from each series of measurements for each bead size, particle concentration, and PMT gain. The \(\alpha\) parameter was observed to be nearly constant over the period of experiments (\(\sim\)18 months, the coefficient of variation \(<\) 5%). For each series of measurements a specific scaling factor was determined to convert \(p_{11}(\psi)\) in PMT counts to \(\beta_{p}^{LISST*}(\psi)\) in absolute units [m\({}^{-1}\) sr\({}^{-1}\)] for scattering angles 14-155\({}^{\circ}\) measured by the Roving Eyeball sensor. First, for each measurement from a given series of measurements, a scaling factor was determined by matching the PMT counts measured with Roving Eyeball sensor between 15\({}^{\circ}\) and 16\({}^{\circ}\) with \(\beta_{p}^{LISST*}(\psi)\) in absolute units obtained from measurements with the last two ring detectors at 13.01\({}^{\circ}\) and 15.17\({}^{\circ}\). Then, using these determinations, the average scaling factor for a given series of measurements was calculated and used for further data processing. Note that this scaling was not needed for the determination of \(DoLP_{p}^{LISST*}(\psi)\) for the Roving Eyeball angular range, which is calculated from \(p_{11}(\psi)\) and \(p_{12}(\psi)\) determined in PMT counts following Equation (2). Quality control of data was performed by removing the first set of 20 measurements (the remaining four sets with 20 measurements each were retained) and the first 20 measurements from the set of 100 measurements. We observed that this was necessary to ensure reasonable stability in the measured scattering signal. The mean and standard deviation values for each angle based on all of the 160 remaining measurements in the series were determined, and the outlying single measurements within the series were identified and rejected from subsequent analysis. Typically, 120 to 130 measurements from a given series of 200 measurements passed the quality criteria. Example data of uncorrected \(\beta_{p}^{LISST*}(\psi)\) for 200 nm and 2000 nm bead suspensions are shown in Figure 1. The series of measurements that remained after quality control and the median values of \(\beta_{p}^{LISST*}(\psi)\) derived from the series of measurements are shown. We also note that the median values were very close to mean values for our data (\(<\)1% difference for most scattering angles). The results for 2000 nm beads show a distinct pattern with several scattering maxima and minima due to constructive and destructive interference of the scattered light from a nearly monodisperse population of beads that are large relative to the wavelength of light. The 200 nm beads are smaller than the wavelength of light leading to a more featureless shape of \(\beta_{p}^{LIST*}(\psi)\).