## 3 Results
### OC Characteristics of PM\({}_{2.5}\) in Orchard Areas
Additionally, in terms of secondary products, WSOC involves the conversion of gaseous compounds into particles through the oxidation of VOCs [39,45,50,51]. Therefore, in this study, VOCs were measured for 31 compounds in the study area during the study period (Table 3). Table 2 shows that the total of the average concentration of VOCs (\(\Sigma\)VOCs) in the study area was 10.2351 ppb. The average concentration of Toluene, m&p-Xylene, o-Xylene, ethylbenzene, benzene, and styrene presented 5.62, 1.11, 1.02, 0.68, 0.65 and 0.20 ppb, respectively. In addition, among the 31 compounds analyzed, BTEXS accounted for 90.73% of \(\Sigma\)VOCs, accounting for most of the VOCs in the study area. It is known that BTEX is mainly emitted through the combustion of fossil fuels, such as car exhausts, industrial activities, such as petrochemicals, and the combustion of heating fuel for houses [52, 53, 54]. However, the research area was an orchard area where apples are harvested, and there are almost no residential or industrial complexes nearby, as it is more than 300 m away from the main road, without much traffic. In addition, the study result of OC also showed that the main causes of WSOC, WIOC, WSOCbb, and WSOCnbb were not clearly confirmed. On the other hand, BTEXS, which showed a high proportion of VOCs observed in the study area, was consistent with the major VOCs components of pesticides, germicides, and herbicides analyzed in this study. Therefore, it is considered that the VOCs observed in the study area are derived from the agricultural chemicals used in the agricultural area. VOCs in the air are known to be important precursor gases of SOA [55]. According to previous studies, semi-volatile or non-volatile aerosols are originated by photochemical oxidation reaction (mainly OH radical reaction), and then react with nitrogen oxide in the presence of solar radiation to generate SOA [56,57]. Therefore, VOCs in the air can generate secondary PM\({}_{2.5}\) based on photochemical reactions. Therefore, in this study, to check the effect of VOCs on PM\({}_{2.5}\) generation in the study area, the data were categorized into rainy days, and sunny days without rain, during the study period. This was performed to confirm the relationship between the OC originating from the photochemical reaction, and the concentration of VOCs in the air. As a result, Table 2 shows that the OC in PM\({}_{2.5}\) was (4.66 and 2.01) \(\mathrm{\SIUnitSymbolMicro m}\)/\(\mathrm{m}^{3}\) on a sunny and rainy day, respectively, which showed decrease of about 56.89% in rainy days compared to sunny days. Table 3 confirms that \(\Sigma\)VOCs were (11.29 and 7.30) ppb on sunny and rainy days, respectively, which is a 35.35% reduction. Although there is a large difference between the reduction rate of VOCs and the reduction rate of OC, the correlation between the daily VOCs concentration and the daily OC concentration is 0.8 or higher, confirming that VOCs have a significant impact on the OC concentration. Judging from the reduced concentration of OC and VOCs on rainy days compared to sunny days with active photochemical reactions, it was found that OCs in the study area originated from photochemical reactions of VOCs. The production yield for VOCs was calculated with the assumption that WSOCnbb in the study area originates by secondary products by VOCs, and it was estimated by using the yield production of each component suggested in a previous study with the assumption that under normal atmospheric conditions, the measured BTEXS fully reacts [58; 59; 60; 61; 62; 63; 64]. As a result, particles that originated from BTEXS were found to be 2.35 and 4.01 \(\mathrm{\SIUnitSymbolMicro m^{3}}\) when the minimum and maximum coefficients of yield production were applied, respectively. When the number of particles calculated from the daily BTEXS was compared with WSOCnbb, the determination coefficient \(\mathrm{r^{2}}\) was 0.4125, which was relatively low.