## 3 Results
Figure 3: SEM images of nanofibers made from pure shellac (**a**), pure PVP (**b**), and P-Shellac (**c**). The insets in (**a–c**) are the diameter statistics of the nanofibers. (**d**) FTIR of three kinds of nanofibers. To exactly evaluate air filtration efficiency, we designed a setup especially for air filtration in our lab as shown in Figure 5a. Polluted air (mainly PM\({}_{2.5}\) and PM\({}_{10}\)) was produced by burning incense in a closed acrylic box. Suction pump and dust analyzer (PGM) was used to control the rate of air flow and measure the concentration of PM, respectively. Dust analyzer 1 (PGM-1) was connected to collection bottle 1 (CB1') with unfiltered air, while dust analyzer 2 (PGM-2) was connected to collection bottle 2 (CB2) filled with filtered air. Filtration efficiency was calculated according to the following equation:  
\[E(\%)=\frac{C_{\text{PGM-2}}}{C_{\text{PGM-1}}}\times 100 \tag{1}\]  
where \(C_{\text{PGM-2}}\) is the concentration of PM\({}_{2.5}\) and PM\({}_{10}\) measured by PGM-2, and \(C_{\text{PGM-1}}\) is the concentration of PM\({}_{2.5}\) and PM\({}_{10}\) measured by PGM-1. We first measured the concentration of PM in collection bottle 1 (CB1') after incense burning for 6 h. In addition, the average concentrations of PM\({}_{2.5}\) and PM\({}_{10}\) of PM fog were 2150 \(\text{\mu g}/\text{m}^{3}\) and 5600 \(\text{\mu g}/\text{m}^{3}\), respectively (Figure S5). We then measured the concentrations of PM\({}_{2.5}\) and PM\({}_{10}\) in collection bottle 2 after air filtration lasting 3 h. The filtration efficiency of commercial PP was also introduced for comparison. The filtration efficiencies of PM\({}_{2.5}\) for FMEs made from PVP, shellac, P-Shellac and PP were 73%, 93.1%, 95% and 55.2%, respectively. The filtration efficiencies of PM\({}_{10}\) for FMEs  
Figure 4: A simple filtration setup with (**a**) and without (**b**) P-Shellac FME between the top and bottom bottles. The top bottle was filled with PM fog. Optical images of P-Shellac FME before (**c**) and after (**d**) PM filtration. Figure 5: (**a**) Schematic of air filter setup and (**b**) pressure drop measurement. made from PVP, shellac, P-Shellac and PP were 84.5%, 96.3%, 98.1% and 72.7%, respectively. Among the four kinds of nanofiber membranes, P-Shellac FME gave the highest filtration efficiency for both PM\({}_{2.5}\) and PM\({}_{10}\), much higher than PP and PVP, and a little higher than shellac (Figure 6a, Table 1). The nanofiber structure of P-Shellac and pure shellac with varying diameters should be the main reason for the high filtration performance, in which the large-diameter nanofibers with large pores play a supporting role, and they will be further divided into smaller pores by small-diameter nanofibers. The BET analysis showed that P-Shellac FME have the largest surface area, at 67.9 m\({}^{2}\)/g, and the most mesoporous structure, much higher than the other two FMEs (Table S1 and Figure S6). The hierarchical pore structure thus could improve the filtration efficiency, delay PM aggregation, and finally result in a large pollutant carrying capacity. Besides filtration efficiency, air permeability is another important feature for air filters. Pressure drop measurement was then carried out as demonstrated in Figure 5b. At an air flow rate of 0.25 m/s, the pressure drops (\(\Delta P\)) of FMEs for PVP, shellac, P-Shellac and PP were measured to be 114 Pa, 133 Pa, 101 Pa and 76 Pa, respectively (Figure 6b), which exhibited only 0.11%, 0.13%, 0.1% and 0.07% pressure drops with reference to atmospheric pressure (101,325 Pa). It was found that all the three FMEs have comparable air permeability to commercial filter PP. The filtration performance of FME is evaluated by a quality factor (QF) [36], which is defined as:  
\[QF=\frac{-\ln(1-E)}{\Delta P} \tag{2}\]  
where \(E\) is the filtration efficiency of PM and \(\Delta P\) represents the pressure drop.