## 2 Methods
### Laboratory Experiments and Mie Scattering Calculations to Evaluate LISST-VSF
#### 2.1.3 Data Processing
The variability between the individual measurements is largest at very small scattering angles, i.e., approximately \(<\)4\({}^{\circ}\), where the scattering signal for submicron particles is low relative to our baseline measurements. Apart from small scattering angles, the coefficient of variation (\(CV\)) for each scattering angle calculated from a series of measurements on 200 nm beads is generally very small, ranging from \(\sim\)3% to \(<\)1%, with the smallest values at angles greater than 64\({}^{\circ}\) where full laser power is used. The measurements of 2000 nm beads exhibit somewhat higher \(CV\), i.e., between about 3% and 6%. The higher values of \(CV\) are observed mostly near the angles where minima of \(\beta_{p}^{LIST*}(\psi)\) occur. The DAWN-EOS measurements for four bead sizes were also used to calculate \(\beta_{p}(\psi)\) and \(DoLP_{p}(\psi)\), denoted as \(\beta_{p}^{\rm DAWN}(\psi)\) and \(DoLP_{p}^{\rm DAWN}(\psi)\). First, for each time series of 1440 measurements with DAWN-EOS, the highest 2% of data was rejected, as these data are assumed to result from sample contamination with rare, larger particles. Each set of measurements then consists of 1411 measurements of both \(I_{s\parallel}(\psi)\) and \(I_{s\perp}(\psi)\) for a specific orientation of sample vial. These measurements were averaged to represent that orientation. Such results were then averaged for three vial orientations. This protocol was applied to both the sample and baseline measurements, with the exception that  
Figure 1: Measurements of the particulate volume scattering function, \(\beta_{p}(\psi)\), at light wavelength of 532 nm for 200 nm (**a**,**b**) and 2000 nm (**c**,**d**) diameter polystyrene beads suspended in water. The left panels depict the angular range of 1-50\({}^{\circ}\) with logarithmic scaling, and the right panels depict the range 50–160\({}^{\circ}\) with linear scaling. The expected reference value, \(\beta_{p}^{REF}(\psi)\), obtained from Mie scattering calculations is indicated as a dashed line. Quality-controlled but uncorrected measurements obtained with the LISST-VSF (gray lines, number of measurements \(N=128\)) and the median value (solid black line) are shown. baseline values were calculated by averaging the lowest 5% of data. The final \(I_{\mathrm{s}\parallel}(\psi)\) and \(I_{\mathrm{s}\perp}(\psi)\) for the beads were calculated by subtracting the average baseline from the average sample data. These particulate \(I_{\mathrm{s}\parallel}(\psi)\) and \(I_{\mathrm{s}\perp}(\psi)\) were then used to determine \(p_{11}(\psi)\) and \(p_{12}(\psi)\) according to Equations (3) and (4), from which \(\beta_{p}^{\mathrm{DAWN}}(\psi)\)[61] and \(DoLP_{p}^{\mathrm{DAWN}}(\psi)\) (Equation (2)) were determined. Note that two dilutions of the master suspension for 400 and 700 nm beads were measured with DAWN-EOS and the average of the two was used to represent these bead sizes. As a final step, the determined \(\beta_{p}^{\mathrm{DAWN}}(\psi)\) values were rescaled using relevant dilution factors to obtain final results representing the particle concentration in LISST-VSF samples and enable direct comparisons with LISST-VSF measurements. Note that such rescaling is not necessary for \(DoLP_{p}^{\mathrm{DAWN}}(\psi)\). With regard to processing of data acquired with a Lambda 18 spectrophotometer, the spectral data of measured optical density OD\((\lambda)\) (i.e., measurements made in the absorbance mode of the spectrophotometer) were converted (after subtraction of baseline measurement) into the particulate beam attenuation coefficient [m\({}^{-1}\)] using the relationship \(c_{p}(\lambda)=\mathrm{In}(10)\) OD\((\lambda)/0.01\), where \(\mathrm{In}\) is the natural logarithm and 0.01 is the path length in meters. The final particulate beam attenuation coefficient obtained from spectrophotometric measurements is denoted as \(c_{p}^{SPEC}\). The estimates of \(c_{p}^{LISST*}\) from LISST-VSF measurements were calculated with the standard manufacturer's processing code.