## 3 Results and Discussion  
### Back Trajectory Analysis  
Figures 2-5 show back trajectory analysis results for the forest site for each season during the study period. Figure 2 shows the frequency of back trajectories in winter. In all years, the back trajectories were drawn more frequently in the northwest direction of Nagasaki, indicating that advection from the northwest was predominant in winter. In other words, in winter, there were many cases in which air masses that passed through cold continental regions such as northeastern China and the Korean peninsula reached Nagasaki. Figure 3 shows the frequency of back trajectories in spring. Although advection from the northwest direction of Nagasaki was also common in spring, advection from directions other than the northwest, including the Pacific side, was increasing compared to winter. Figure 4 shows the frequency of back trajectories in summer. There was less advection from the continental direction in summer. Advection from the Pacific and other oceanic regions, where there was less land and less anthropogenic activity, was dominant. Figure 5 shows the frequency of back trajectories in autumn. As in spring, advection from the northwest of Nagasaki was common in autumn, while advection from directions other  
than the northwest was increasing compared to winter. These results were almost the same as those of the previous study [19]. Figure 3: Frequency of residence of back trajectories in a grid cell in spring calculated by HYSPLIT. Model of the National Oceanic and Atmospheric Administration, USA. The analysis period was set from 29 March to 3 May in each year. Figure 2: Frequency of residence of back trajectories in a grid cell in winter calculated by HYSPLIT. Model of the National Oceanic and Atmospheric Administration, USA. The analysis period was set from 30 December–3 February in each year. 12.2 ng/m\({}^{3}\) and 0.012 to 9.43 ng/m\({}^{3}\) in the suburban and forest sites, respectively. The mean value of the suburban site was 2.12 ng/m\({}^{3}\), which was 1.25 times greater than that of the forest site (1.69 ng/m\({}^{3}\)). In addition to wide-area pollution, including transboundary pollution, suburban site was likely affected by local automobile emissions and other factors. Conversely, there were a few cases in which PAH concentrations were higher at the forest site even though there were no nearby pollution sources. This is presumably because they do not capture exactly the same air mass due to the shift in wind direction and the slight deviation in the sampling period. As observed in previous studies, PAH concentrations were higher in winter and lower in summer [19; 22; 23]. The reasons for the high PAH concentration in winter at this study area according to the previous report are (1) an increase in the burning of coal and biomass for thermal heating, especially in the cold northern parts of China; (2) the prevailing winter wind from the northwest from the stable atmospheric pressure chart pattern; (3) a lower altitude for mixing of atmospheric zones in winter; (4) a decreased decomposition of chemical substances due to low temperatures and weak sunshine in winter; and (5) less breakthrough of PAHs with high vapor pressure from the particulate phase to the gas phase due to low temperatures [19]. The average values for each year are shown in Figure 7 to clarify the changes over time. There was an overall decreasing behavior, although \(\Sigma\)PAHs increased from 2017 to 2018 at both sites. During the entire period, \(\Sigma\)PAHs decreased by 60% in suburban site and by 57% in forest site. There are few reports on atmospheric PAH concentrations during the same period as this study. [PERSON] et al. reported a 40-50% decrease in atmospheric PAH concentrations in Wajima, Japan, during the COVID-19 outbreak period compared to the same period of the previous year [24]. Although the study period was different, the results of this study were similar to the report.