## 3 Results  
The prepared solutions were filled into a 5 mL syringe with a feeding rate of 1.0 mL/h. The applied voltage on the syringe was 15 kV and the distance between syringe tip and the collector was 13 cm. The nanofiber membranes were then collected on a stainless-steel mesh (150 mesh), as shown in Figure 2. The time for electrospinning was 10 min for each kind of FME. The temperature was maintained at 25 \({}^{\circ}\)C and the relative humidity was controlled at 40-60%. Figure 3a-c show the scanning electron microscopy (SEM) and diameter statistics of nanofibers fabricated from pure shellac, pure PVP and P-Shelllac, respectively. The pure shellac nanofibers exhibited rough edges and a broad diameter distribution with the range of 400-1000 nm with a mean diameter of around 650 nm (Figure 3a), while PVP nanofibers had a smooth surface and a more uniform diameter in the range of 250-350 nm with a mean diameter of around 270 nm (Figure 3b). In comparison to pure shellac nanofibers, P-Shelllac nanofibers possessed a smoother surface and a more uniform diameter, mainly (about 73%) distributed in the range of 200-500 nm with a mean diameter of around 439 nm (Figure 3c). The standard deviations of the mean diameters at different positions in the FME were less than 10 nm for all the three FME. The P-Shelllac FME became translucent, while the shellac FME exhibited a white color (Figure S2). At the same time, the mechanical properties of P-Shelllac nanofibers were obviously improved, as indicated by the tensile stress-strain curves (Figure S3). When we drew lines on the surface of the nanofiber membranes with tweezers, there was a clean scratch on the P-Shelllac FME, but much shellac powder could be seen on the pure shellac FME (Figure S1). Figure 2: Schematic diagram for fabricating P-Shelllac air filter by electrospinning method. The results of the FTIR characterization of pure shelllac nanofibers, pure PVP nanofibers, and P-Shelllac nanofibers are shown in Figure 3d. Firstly, for pure shelllac nanofibers, the peaks at 3420 cm\({}^{-1}\), 1716 cm\({}^{-1}\), and 1255 cm\({}^{-1}\) were due to -OH, C=O and C-O stretching vibration, respectively. For pure PVP nanofibers, the characteristic peaks appearing at around 3449 cm\({}^{-1}\) were ascribed to the -OH stretching vibration [32], while stronger absorption peaks at 1655 cm\({}^{-1}\), 1420 cm\({}^{-1}\) were ascribed to C=O and C-N stretching vibration, respectively. Compared with pure shellac, the mechanical properties of P-Shellac nanofibers were greatly improved, owing to the hydrogen bond formed between PVP (-OH, C-N and C=O) and shellac (C=O, C-O, -OH), as shown in Figure S1. The addition of PVP in the P-shellac nanofibers should contribute to improved surface smoothness, diameter distribution, and mechanical properties. There is a clear effect of air filtration for P-Shellac FME, as shown in Figure 4. Two transparent bottles were put together mouth to mouth, with P-Shellac FME between the two bottles. PM fog was collected from burning incense, whose particle size, chemical composition including CO, NO\({}_{2}\), SO\({}_{2}\), and organic compounds attached to the PM were quite similar to those found on hazy days [33]. The top bottle was filled with PM fog, while there was only clean air in the bottom bottle. After 20 min, the bottom bottle with P-Shellac FME was still clear, and the PM fog remained only in the upper bottle (Figure 4a). By contrast, the bottom bottle without P-Shellac FME became foggy (Figure 4b). The translucent P-Shellac FME turned yellow after PM filtration (Figure 4c,d), indicating its good filtration efficiency. We found that P-Shellac FME has quite good hydrophilicity with a water contact angle of only 13 degrees (Figure S4), which is between PVP (10.7 degrees) and shellac (22 degrees). Hence, the wettability of nanofiber membrane is good and the air pollutants easily spread on the nanofibers, which could be one of the reasons for high filtration efficiency [34; 35].