## 3 Results and Discussion
### Analysis of Aquagram  
The analysis was based on raw spectra taken at a fixed time of 8 h. The spectra were preprocessed and underwent Savitzky-Golay second-order-derivative processing, and an aquagram of 12 selected wavelengths was calculated and plotted using the processed spectral data. The results are presented in Figure 7. It indicates an overall increase in spectral variation across 12 wavelengths, with some fluctuations observed at intermediate time (2.5-6 h). This is due to the presence of only a few highly reactive short-lived species during spectroscopic detection. Long-lived species such as H\({}_{2}\)O\({}_{2}\), NO\({}_{2}^{-}\), and NO\({}_{3}^{-}\) fluctuate in concentration within the water. In particular, H\({}_{2}\)O\({}_{2}\) reacts with NO\({}_{2}^{-}\) to form peroxynritte, which is unstable in the PAW of acidic pH and decomposes into the end product NO\({}_{3}^{-}\), and NO\({}_{2}^{-}\) also disproportionates under acidic conditions to form NO\({}_{3}^{-}\)[17; 18]. The concentrations of H\({}_{2}\)O\({}_{2}\) and NO\({}_{2}^{-}\) in the PAW are gradually decreasing, while the concentration of NO\({}_{3}^{-}\) constantly increases [16; 31]. There is an increasing/decreasing balance in the PAW and the general tendency increases with time when the equilibrium is broken.  
Studies have revealed that H\({}_{2}\)O\({}_{2}\), NO\({}_{2}^{-}\), and NO\({}_{3}^{-}\) have an impact on water molecule structure through mechanisms like solvation and hydrogen bonding [32; 33; 34; 35]. This can lead to hydrogen bonding between water molecules, between water and solute, and the stretching or bending vibration of the OH bonds of water [36]. Figure 7 presents an aquagram that demonstrates 12 wavelengths with a high level of regularity and repeatability in their  
Figure 7: Aquagram of NIR spectral data containing 12 water matrix coordinates.  
spectral variation. The 1360-1410 nm and 1800-1860 nm wavelengths are of special interest. The former corresponds to the range of weakly hydrogen-bonded water, free water, and solvent shell absorption of water [27], while the latter corresponds to the range for OH-bond stretching and bending vibration absorption of water [29]. Each wavelength corresponds to the absorption of a specific water molecular species: 1360 nm and 1366 nm--OH-(H\({}_{2}\)O)\({}_{1,2,4}\); 1378 nm--\(\upsilon_{1}+\upsilon_{3}\)'; 1385 nm and 1390 nm--2\(\upsilon_{1}\), OH-(H\({}_{2}\)O)\({}_{1,4}\) and O\({}_{2}\)-(H\({}_{2}\)O)\({}_{4}\); 1394 nm and 1398 nm--water trapped in the local field of ions; 1403 nm and 1407 nm--free water molecules (S\({}_{0}\)) [27]; 1833 nm, 1844 nm and 1852 nm--OH bond stretching and bending vibrations [29].  
To measure the ROS concentration in PAW samples treated for 15 min, we employed a reactive oxygen kit and conducted the assay both before and after 8 h. The final experimental results were expressed as the ratio of the fluorescence intensity of the sample to that of the blank control, as shown in Figure 8. It confirms that the ratio of fluorescence intensity decreases with the increase in time, implying a decrease in ROS concentration in the PAW over time.  
Aquaphotomics has some advancements to indirectly study the changes of other molecules in the water system by using water as the mirror. The concentration of active substances in PAW is a millimolar gradient [16], and it needs specific analytical tools and complicated procedures for active-substance determination. And, in this study, the small changes in the active substance in PAW can be measured through the significant changes in the aquagram. Therefore, the aquagram could be used as a tool to visualize the PAW in the future.