## 3 Results
### Characteristics of the NAI Temporal and Spatial Distribution  
In this study, an RF model was developed to estimate the spatio-temporal distribution of NAIs in Zhejiang Province during 2020. The mean NAI concentration in Zhejiang Province in 2020 was 1550 ions cm\({}^{-3}\), which ranged from the lowest value of 296 ions cm\({}^{-3}\) to the highest of 9046 ions cm\({}^{-3}\). According to a previous study, NAI concentrations were classified into four grades based on their benefits to human beings (Table 2). Overall, few regions in Zhejiang Province were classified as grade IV (0.04%), and most were classified as grade II (54.65%). NAI concentrations in cities and towns were mostly classified as grade III (32.51%), which basically meets the needs of human health maintenance.  
\begin{table}
\begin{tabular}{c c c c} \hline
**NAI Concentrations** & **Air Freshness** & **Relationship with Human Body** & **Grades of NAI** \\ \hline \(\leq\)500 & Not fresh &
\begin{tabular}{c} Induces various diseases or \\ physical disorders \\ \end{tabular} & IV \\
500–1000 & Generally & Maintains health & III \\
1000–2000 & Fresh & Improves immunity & II \\ \(>\)2000 & Very fresh & Prevents or cres diseases & I \\ \hline \end{tabular}
\end{table}
Table 2: The relationship between NAI concentration level standards and human health.  
Figure 7: Path analysis diagram illustrating relationships among terrain (DEM, aspect, slope), environmental factors (T, CO, RH), plant photosynthesis (SIF), and NAIs and the total effects of the drivers on NAIs for all land types (**a1,a2**), forest land (**b1,b2**), urban and built-up lands (**c1,c2**), and cropland (**d1,d2**).  
The spatial-temporal distribution of NAI concentration in Zhejiang Province in 2020 was achieved (Figure 8). Spatially, the NAI concentration in Zhejiang Province decreased from southwest to northeast (Figure 8). Higher NAI concentrations were found in southwestern Zhejiang, where human activities are quite weak and vegetation is widely distributed. Low values of NAIs were mainly observed in northern and middle Zhejiang, where cities and arable land are dominant with strong human activity.  
Figure 8: Estimated results of NAI concentrations in Zhejiang Province from January to December 2020. Roman numerals refered to the NAI grades, I: NAI > 2000 ions cm\({}^{-3}\); II: 1000 < NAI \(\leq\) 2000 ions cm\({}^{-3}\); III: 500 < NAI \(\leq\)1000 ions cm\({}^{-3}\); IV: NAI \(\leq\) 500 ions cm\({}^{-3}\).  
The global Moran's I of NAIs showed a strong positive spatial correlation (0.566, \(p<0.005\)). The LISA aggregation map indicated that the H-H accumulation areas of NAI concentrations were mainly distributed in woodlands, especially in southwestern Zhejiang (Figure 9). The L-L accumulation areas of NAI concentrations were mainly distributed in the northeastern plains of Zhejiang, the Central Basin, and the southeastern coastal areas, which have experienced intensified human interference (Figure 9).  
Among these land types, NAIs in woodlands were much higher than in the other land types, indicating the critical role of vegetation in promoting NAI formation (Figure 10). The NAI concentrations of water bodies followed woodlands, by providing abundant water vapor for NAI formation. Comparably, arable land and urban and built-up land had the lowest NAI concentrations.