## 3 Results and Discussion
### Optimal Combination of Polarization Indicators  
Based on the above experimental results, it is clear that various properties of aerosol can affect the process of polarized light scattering, so we can classify and identify different types of aerosols by polarization characterization. The complex properties of aerosol samples and in-situ measurements in real environments make it difficult to obtain enough aerosol information only by a few specific polarization indicators, which implies the important potential of the fine recognition of aerosol types by the combination of multipolarization parameters. It should be noted that different polarization indices have different sensitivity and discrimination. In general, monodisperse particles show the smallest PFR values as shown in Figure 6b, but the polydispersity in aerosol scale and irregularity in morphology will cause a larger PFR. According to four groups of experimental results, it is easy for polarization characterization to distinguish the microphysical properties of aerosol samples. In our first group of experiments, the PSL particles with different sizes were clearly identified. Besides, polarization characterization also works well in distinguishing different aerosol compositions, especially for absorption characteristics, which can be supported by our second and third groups of experiments.  
Finally, we select four polarization parameters from the above experimental results and show the scatter data distribution of the 12 types of aerosol samples used in this paper. Five boxes are used to mark some regular phenomena of polarization indices between different samples. If we observe the green frame A in Figure 13a, we can find that three water-soluble salts are completely indistinguishable based on P-Pdop (60\({}^{\circ}\)). Actually, according to the above figures, it can be noticed that P-Pdop (60\({}^{\circ}\)) is easily influenced by many aerosol features, such as particle size, morphology, and optical absorption. Based on their distribution perpendicular to the horizontal axis (P-Pdop (60\({}^{\circ}\))), the key attribute difference among three water-soluble aerosol samples is the refractive index connected with different compositions. The H-Pdop (60\({}^{\circ}\)) of various aerosol samples marked by the yellow frame B and purple frame C also can be supported by the above experimental data. H-Pdop (60\({}^{\circ}\)) is a good indicator to distinguish aerosol composition but is insensitive to aerosol size and morphology; therefore, in group C, the polarization index distributions of four types of carbon samples with respective morphological characteristics are perpendicular to the vertical axis (H-Pdop (60\({}^{\circ}\))). The case of three PSL particles with different sizes in frame B can be explained in a similar way. Furthermore, based on the above experiments, R-Hdop (60\({}^{\circ}\)) is sensitive to aerosol absorption and insensitive to aerosol morphology, and R-Pdop (30\({}^{\circ}\)) is not easily affected by optical absorption. So, in blue frame D, the polarization data distribution of three aerosol samples with different morphologies is perpendicular to the horizontal axis (R-Hdop (60\({}^{\circ}\))). If combined with the regular pattern in Figure 13a, we can easily estimate whether morphology or size is the key difference of aerosol properties among various samples. For the red frame E in Figure 13b, the polarization data are distributed perpendicular to the vertical axis (R-Pdop (30\({}^{\circ}\))), corresponding to the inherent absorption attributes from strong to weak.  
In summary, Figure 13 shows the potential of multidimensional polarization indicators in aerosol classification and identification analysis. It is generally known that the properties of suspended aerosols can change dynamically with environmental factors. Figure 13 implies a feasible way to evaluate the changes of aerosol microphysical characteristics by the distribution of the polarization indices, and follow-up studies are in progress.