## 3 Results
### Characteristics of Water-Soluble Ions in PM\({}_{2.5}\) at Orchard Areas  
The 43.20% of rePM\({}_{2.5}\) in the study area were identified as water-soluble ionic components, and the concentration for each compound was 0.73, 1.07, and 1.78 ug/m\({}^{3}\) for NH\({}_{4}\)\({}^{+}\), NO\({}_{3}\)\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\), respectively. The ratio of NO\({}_{\rm x}\) to SO\({}_{\rm x}\) was examined to estimate the emission source of ionic aerosols affecting PM\({}_{2.5}\) generation in the study area. In general, the higher the NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) mass ratio, the greater the effect of mobile pollutants. Previous studies reported the ratio of NO\({}_{\rm x}\) and SO\({}_{\rm x}\) originating during gasoline and diesel combustion to be 13:1 and 8:1, respectively [73]. Therefore, the mass ratio of NO\({}_{3}\)\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) can represent the contribution of fixed and mobile pollutants affecting the generation of PM\({}_{2.5}\). The NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) mass ratio measured in this study was 0.6, suggesting that the source of PM\({}_{2.5}\) and water-soluble ions in the study area had a greater effect from the fixed source, than the effect from the mobile source. This is consistent with the estimated result of the WSOC emission source described above, and it can be suggested that the combustion of fossil fuels by vehicles has relatively little effect on PM\({}_{2.5}\) in the study area. To confirm the formation process of PM\({}_{2.5}\) by water-soluble ions, the concentration of gaseous aerosols, the molar ratio of NH\({}_{4}\)\({}^{+}\) and SO\({}_{4}\)\({}^{2-}\), and the molar ratio of NO\({}_{3}\)\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) among the water-soluble ions of PM\({}_{2.5}\) SUM were measured. NH\({}_{3}\) in the atmosphere, which is a dominant factor in the SOA formation reaction, neutralizes sulfuric acid (H\({}_{2}\)SO\({}_{4}\)) with alkaline gas to generate ammonium sulfate ((NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\)) or ammonium bisulfite ((NH\({}_{4}\))HSO\({}_{4}\)), and then the remaining NH\({}_{3}\) in the air reacts with nitric acid (HNO\({}_{3}\)) to form ammonium nitrate (NH\({}_{4}\)NO\({}_{3}\)) [74, 75]. Ammonium sulfate, ammonium bisulfite, and ammonium nitrate originated are all SOA, and their production is dominated by NH\({}_{3}\) in the air. Based on this, the effect of NH\({}_{3}\) in the atmosphere can be confirmed by using the molar ratio of NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), and NH\({}_{4}\)\({}^{+}\) contained in PM\({}_{2.5}\). According to previous studies, if the molar ratio of NH\({}_{4}\)\({}^{+}\)/SO\({}_{4}\)\({}^{2-}\) in PM\({}_{2.5}\) is 1.5 or more, it can be defined as ammonia-rich, in which the NH\({}_{3}\) concentration in the air is sufficient for SOA formation; and when this molar ratio is 1.5 or less, it is reported that the contribution of SO\({}_{4}\)\({}^{2-}\) to the PM\({}_{2.5}\) SOA is high [76, 77]. That is, as the molar ratio of NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) increases, the contribution of NO\({}_{3}\)\({}^{-}\) to SOA increases. In this study, it was confirmed that the molar ratio of NH\({}_{4}\)\({}^{+}\)/SO\({}_{4}\)\({}^{2-}\) was 2.23, and the molar ratio of NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) was 0.95. Therefore, it was determined that the study area was an ammonia-rich state, where the concentration of NH\({}_{3}\) in the air was high enough to form SOA, and when NO\({}_{3}\)\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) were compared, it could be confirmed that SOA had a greater effect on SO\({}_{4}\)\({}^{2-}\) than on NO\({}_{3}\)\({}^{-}\). To examine the process of SOA formation in the study area in detail, the data were divided into sunny days and rainy days, to collect the measurement result. During the study period, the molar ratio of NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) was in the range (0.1-4.0), and the period when the molar ratio of NO\({}_{3}\)\({}^{-}\)/SO\({}_{4}\)\({}^{2-}\) was 1.5 or more was mostly sunny days.