## 3 Results and Discussion
Figure 3 shows the optical and structural changes in the PVA NF-based filter (electrospinning time: 300 s) during repeated chamber tests. The chamber test was repeated using an air purifier equipped with the PVA NF-based filter, and the changes in the reflectance spectrum and structural morphology of the filter surface were investigated. The inset in Figure 3a shows that the color of the PVA NF-based filter changed from white to dark brown as the PM capture time increased. As shown in Figure 3a, as the PM collection time increased, the reflectance of the PVA NF-based filter gradually decreased. In particular, the reflectance of the filter decreased significantly at wavelengths from 400 to 600 nm. This is because the PM captured by the PVA NFs selectively absorbs light at wavelengths from 400 to 600 nm [9,13]. The surface morphology of the PVA NF-based filter was investigated using FE-SEM to confirm the PM collection and mechanism. Figure 3b shows the PVA NF-based filter surface before capturing PM. The change in the filter surface morphology during continuous PM capture is shown in Figure 3c-e. Furthermore, the mechanism of the PM capture at different stages is described with the schematics shown in Figure 3f-i. At the initial PM capture stage, PM was captured by the PVA NFs and bound tightly to the NFs (see Figure 3c,g). As the duration of filter usage increased, more PM particles attached to the PVA NFs. The PM particles were able to move along the PVA NFs and aggregate to form larger particles. Additionally, the new incoming PM particles could directly attach to the PM that were already on the PVA NFs and then merge (see Figure 3d,h). As the PM capture continued, the surface of the PVA NF-based filter filled with large aggregated PM particles. The NF junctions accumulated more PM, which formed spherical particles of larger sizes (see Figure 3e,i) [20]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Mean Fiber** & **\multirow{2}{*}{**\(\mathbf{E}\) [\%]**} & \multirow{2}{*}{**\(\mathbf{\Delta P}\) [Pa]**} & \multirow{2}{*}{**\(\mathbf{Q_{\ell}}\) [Pa\({}^{-1}\)]**} \\  & **Diameter [nm]** & & & \\ \hline Nonwoven fabric substrate & & & & \\ (without PVA NF) & \(\sim\)20,000 & 42.10 & 63 & 0.008674 \\ \hline PVA NF-based filter & & & & \\ (Electrospinning time: 60 s) & & & & \\ \hline PVA NF-based filter & & & & \\ (Electrospinning time: 180 s) & & & & \\ \hline PVA NF-based filter & & & & \\ (Electrospinning time: 300 s) & & & & \\ \hline PVA NF-based filter & & & & \\ (Electrospinning time: 600 s) & & & & \\ \hline Cabin air filter & \(\sim\)22,000 & 40.27 & 49 & 0.010517 \\ \hline Dust mask (KF80) & \(\sim\)13,000 & 85.61 & 213 & 0.009102 \\ \hline HEPA filter & \(\sim\)16,000 & 94.55 & 189 & 0.015394 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measured average fiber diameter, filtration efficiency, pressure drop, and calculated quality factor of PVA NF-based filters and commercial air filters used in this study. Finally, as shown in Figure 4a, the filter was soaked in water to selectively discard the contaminated PVA NF layer. Here, the time required to completely remove the contaminated PVA NF layer from the nonwoven substrate was less than 5 s. The remaining nonwoven fabric was dried in an oven and reused as a substrate for the fabrication of a PVA NF-based filter. Figure 4b shows the surface FE-SEM images of the contaminated PVA NF-based filter observed during washing and recycling. In this process, there were no serious problems with the surface morphology and QF of the remade PVA NF-based filter. The result was equivalent to that of the chamber test with the first PVA NF-based filter (see Figure S5). Figure 3: Optical and structural changes in PVA NF-based filter (electrospinning time: 300 s) during repeated chamber tests.