## 3 Results and Discussion
### Back Trajectory Analysis
Figure 6: Total PAH concentrations (\(\Sigma\)PAHs) in the suburban and forest sites of Nagasaki from 2017 to 2021. The gray areas indicate winter data. Figure 7: Behaviors in the annual average of total PAH concentrations in the suburban and forest sites of Nagasaki from 2017 to 2021. The error bars indicate the magnitude of the standard error. To understand this downward behavior, Figure 8 shows the results aggregated by season. The winter value changed more markedly than the other seasons, with a decrease of 72% in suburban site and 84% in forest site. Since the items (2) to (5) which are the reasons for the high PAH concentration in winter listed above would not have changed during this period, it is assumed that the results were due to an alteration in relation to item (1). Within China, measures are being taken to convert the domestic coal used for heating and cooking to natural gas, and renewable energy is being used extensively. In addition, according to [PERSON] et al., PAH emissions from 2016 to 2020 from domestic coal across China have been nearly halved [25]. Thus, considering also the results of the back trajectory analysis, one of the main reasons for the markedly downward behavior in winter values is thought to be the decrease in the amount of biomass and coal burned for heating and other operations in cold continental regions such as northern China. The \(\Sigma\)PAH values in the suburban and forest sites decreased by 37% and 49% in 2020, and by 53% and 73% in 2021, respectively, compared to 2019. One reason for this may be the frequent curtailment of industrial activities in China, such as the factory shutdowns that occurred for several months beginning in January 2020, as China's novel coronavirus outbreak led to lockdowns throughout the country, which suspended economic activities. In 2021, community-acquired infections still occurred frequently in China, and factory operations were suspended each time. As a result, the concentration of PAHs emitted in China decreased, which is presumed to have affected the overall decrease in the concentration of PAHs in Nagasaki in 2020 and 2021. In spring, PAHs decreased by 50% in suburban site and by 20% in forest site, but with a smaller range than that observed in winter. This may be due to more air mass advection in spring from directions other than northeastern China, and the reduced amount of coal and biomass burning for heating compared to that in winter. Similarly, the autumn PAH concentrations did not decrease significantly during this period. Figure 8: Behaviors in seasonal averages of total PAH concentrations in the suburban and forest sites of Nagasaki each year from 2017 to 2021: (**a**) winter; (**b**) spring; (**c**) summer; (**d**) autumn. In summer, the concentrations decreased by 60% in suburban site and by 62% in forest site. One of the main reasons for this may be the decrease in emissions from sources in the vicinity of Japan because of the predominance of air masses originating from the Pacific Ocean in summer. In particular, for 2020 and 2021, movement restrictions in Japan could cause a decrease in PAH emissions from automobiles, aircraft, ships, etc. However, because concentrations are much lower in summer than in other seasons, the effect of this decrease on the annual mean is small. Overall, the recent declining behavior in PAH concentrations in Nagasaki is considered to be mainly due to less PAHs originating from cold continental regions, such as northern China. The decline in total PAH emissions in China could be influenced by the replacement of residential coal with natural gas, widespread use of new energy sources such as solar and wind energy, and efforts to improve air quality in China including sophistication of exhaust gas treatment technology. In addition, restrictions on economic activities in response to COVID-19 may have impacted these values.