## 3 Results and Discussion
### Air Analysis
#### 3.2.1 Case Selection Using Vanadium (V) Concentration  
Figure 4 shows the V concentrations in PM2.5 depending on the wind direction, and the proportion of V in PM2.5 to remove the dependence of PM2.5 concentration change. The mass concentration of PM2.5 does not significantly depend on the wind direction (Figure 2). However, the proportion of V in PM2.5 was higher than the mean when the wind direction ranged between 90\({}^{\circ}\) and 180\({}^{\circ}\). V is a naturally occurring transition element. Previous studies have shown that humans are exposed to V by breathing in the atmosphere, and that most of it is emitted together with Ni from the combustion of fossil fuels in diesel vehicles, ships, and industrial complexes [53; 54; 55]. Moreover, the locations of industrial complexes and monitoring stations are insufficient evidence to estimate the effects of PM2.5 and PM10 from the industrial complex. Thus, V, a rare metal component of PM2.5, was used as an airflow tracer in this study, according to the wind direction. The V concentrations changed depending on the wind direction. In particular, the concentration of V in the southeasterly wind direction, where the industrial complex is located (Figure 1), was significantly higher than that in other directions. Therefore, it can be inferred that the concentration of V is affected by the industrial complex. Many studies have used V in PM2.5 as a tracer of a combustion source [56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66;  
measurement limitation of PM2.5 components that occurs when PM2.5 concentration is low, and to remove the cases in which the concentration of V rises owing to high PM2.5 concentration. When SCs were excluded, the PM2.5 concentration decreased to 17.6 ug/m\({}^{3}\) (annual mean: 18.5 ug/m\({}^{3}\)), and the V concentration decreased to 3.95 ng/m\({}^{3}\) (annual mean: 7.39 ng/m\({}^{3}\)). These values are similar to the PM2.5 and V concentrations reported in Milan, Italy (18.7 \(\pm\) 6.5 ug/m\({}^{3}\) and 3 \(\pm\) 2 ng/m\({}^{3}\), respectively) [50]. Furthermore, these values are lower than those reported in Beijing, where the reported PM2.5 mean concentration was 55.4 ug/m\({}^{3}\) and the V concentration was 15.3 ng/m\({}^{3}\); however, the proportion of V in PM2.5 in these two studies was similar [61]. Following the SC selection, it was assumed that there would be changes in the correlations between heavy metals in PM2.5 due to the presence of a V smelting plant in Mipo National Industrial Complexes (C in Figure 1); thus, the correlations between heavy metals were analyzed to examine the representativeness of the effect of SC on the air quality of the industrial complexes. First, we examined the correlations between the proportions of V and Ni in PM2.5 between SCs and the cases excluding SCs (ALL-SC) (Figure 5). We found that in the ALL-SC, the correlation coefficient (r) of the ratio between Ni and V was 0.933, which is higher than the correlation (0.762-0.790) reported in a previous study [62]. When only SCs were examined, r was very high at 0.980. This correlation suggests that the heavy metals were emitted from the same source where the proportions of V and Ni remained constant based on previous studies, showing that the V/Ni ratio in PM2.5 varies by the fuel type and combustion facility [54; 63], as well as by city [51]. The correlations between different combinations of Zn, Fe, Mn, and Ti were higher in SCs than those in ALL, except the correlation between Ti and Fe (Figure 6). However, the concentrations distributed around the 11:1 line between Ti and Fe were eliminated in SCs. Moreover, both extreme observations of the correlation between Zn and Ti as well as Zn and Fe were eliminated in SCs. Figure 5: Comparison of vanadium (V) and nickel (Ni) concentrations in the cases excluding selected cases (ALL-SC) and those in the selection cases (SCs).