## 2 Experimental Setup and Methods
### Signal Processing and System Calibration  
In this paper, for the scattering signals of one individual detected particle, the detected partial Stokes vector at each scattering angle can be calculated as follows:  
\[S=\left[ \begin{array}{c}s_{0}\\ s_{1}\\ s_{2}\end{array} \right]=\left[ \begin{array}{c}I_{0}+I_{90}\\ I_{0}-I_{90}\\ I_{45}-I_{135}\end{array} \right] \tag{1}\]  
Figure 2: Schematic and experimental setup: **(a)** 3D view of the optical system; **(b)** schematic diagram of the experimental device. PSG: polarization state generator; C: cylindrical lens; I: intensity channel at 10\({}^{\circ}\); P1–P4: polarization signal receivers at 30\({}^{\circ}\), 60\({}^{\circ}\), 85\({}^{\circ}\), and 115\({}^{\circ}\) respectively; L: a spatial filtering lens module; PMT array: silicon photomultiplier tubes (SiPMT) array; PSA: polarization state analyzer. Each PSA is composed of four film polarizers (FP) fixed at the receiving end where the four FPs work as a four-quadrant polarization analyzing channel to separate the scattering light with different polarization states. The red arrows are the sample inlet and sheath inlet. where \(I_{0}\), \(I_{90}\), \(I_{45}\), and \(I_{135}\) are the intensity detected by four linear polarization analyzers at the four scattering angles. After normalization by the intensity components \(S_{0}\), the polarization components (\(S_{1}\), \(S_{2}\)) can be transferred as follows:  
\[Hdop=\frac{S_{1s}}{S_{0s}};Pdop=\frac{S_{2s}}{S_{0s}} \tag{2}\]  
Six polarization indices at each scattering angle can be obtained, including H-Hdop, H-Pdop, P-Hdop, P-Pdop, R-Hdop, and R-Pdop, where the capitals before \"-\" refer to the polarization state of the incident light, including horizontal (H), 45\({}^{\circ}\) (P) linear polarization and right (R) circular polarization, respectively. So for each sample, by the parallel measurements of three devices with a four-angle synchronous measuring system, we can get a dataset composed of 24-dimensional polarization index groups. Before detecting real aerosols, we calibrated our experimental apparatus with the Mie theory as follows. Firstly, we compared the forward scattering intensity of three kinds of monodisperse polystyrene microspheres (PSL) with different sizes with the theoretical calculations based on Mie theory. Secondly, using PSL with a diameter of 1 \(\upmu\)m as the standard particle sample, we further compared the experimental multi-angle scattering polarization parameters with the theoretical calculations. The above agreement between experiments and theoretical calculations can roughly confirm the validity of our polarization measurements. However, due to the oscillating scattering angle spectrum of a single particle size, it is difficult to achieve accurate calibrations of polarization signals at several angles. Therefore, we propose a calibration method for the multi-channel polarization signals of our measurement system based on a monodisperse polystyrene sample. By comparison with the theoretical curves shown as a dotted line, we can get a quantitative calibration accuracy of the measured Stokes vectors. As shown in Figure 3, the red markers and the pink markers are experimental results of monodisperse polystyrene particles before and after numerical calibration. The mean deviation of the absolute value of all Stokes parameters is less than 0.07, as shown in Figure 3a, which is determined by the only hardware modification of the optical system, and the final mean deviation of the absolute value of all Stokes parameters is less than 0.01, as shown in Figure 3b, which is the optimal result according to the subsequent numerical calibration. In addition, during every group of experiments of the same sample, we repeat the measurements several times to observe the stability of the original polarization analyzing signals and their distribution curves. To verify the performances of our instrument, we tested three kinds of polystyrene latex (PSL) spheres and recorded polarization scattering pulses.