## 3 Results and Discussion
### Days of High Concentration and Transport Paths  
Human health is considered to be adversely affected when concentrations of PM and its components are high. In this section, we will examine the transport paths using back trajectory analysis when PM\({}_{2.5}\) and other chemical components are high. The method used to extract high concentration days is as follows. The average and standard deviation were calculated for PM\({}_{2.5}\) and chemical components from February to April 2020. The average and standard deviation of PM\({}_{2.5}\), nitrate, sulfate, total ions, total PAHs, total metals, OC, EC, and ozone are 12.4 \(\pm\) 5.50 ugm\({}^{-3}\), 1.20 \(\pm\) 1.13 ugm\({}^{-3}\), 2.62 \(\pm\) 1.73 ugm\({}^{-3}\), 5.60 \(\pm\) 3.71 ugm\({}^{-3}\), 0.75 \(\pm\) 0.56 ngm\({}^{-3}\), 1.35 \(\pm\) 1.02 ugm\({}^{-3}\), 2.65 \(\pm\) 1.34 ugm\({}^{-3}\), 0.24 \(\pm\) 0.25 ugm\({}^{-3}\), and 37.8 \(\pm\) 16.2 ppb, respectively. The high concentration days were defined as those greater than the average concentration plus one standard deviation. Then, the backward trajectory was calculated using HYSPLIT for each high concentration day for each component. The pathways along which air masses were transported were examined to investigate whether there was a relationship between high concentration and transport pathways. Table 1 shows the days of high concentration of each component for February, March, and April, respectively. Each number is the respective concentration. All the concentrations for each component were high in some days, but in other cases, the concentration was high for a single or few components. Typical examples of days with high concentrations for almost all components are 2-3 and 10-11 February. The maps of backward trajectory are shown in Figure 5a. The air masses reached Fukuoka from southern Korea, from the Shandong Peninsula through the northern East China Sea, or from northern China through the Yellow Sea. The backward trajectory suggests that transboundary air pollution from the Asian continent influences the Fukuoka air quality, which was seen before in Fukuoka [7; 8; 9]. Since Fukuoka is located in the western area of Japan, the seasonal monsoon prevails in the winter-spring season. The weather chart provided by the Japan meteorological Agency showed that a high-pressure system remained in the East China Sea area on 2-3 and 10-11 March, which is a typical atmospheric pressure pattern in winter season, and northerly wind prevails under this pattern. Then, the air pollutants emitted in the Asian continents are transported to Japan, especially in the west part of Japan [2; 3; 4; 5; 6; 7; 8; 9]. However, local influences are also likely to affect the Fukuoka air quality. This is probable because the population in Fukuoka is about 1.5 million, and there are seaports and airports near the city in addition to the large industrial and commercial area. Therefore, the local emissions from the city itself affects the air quality in Fukuoka. The ozone shows diurnal variation and the wind direction indicates a sea-land wind pattern on those days [9; 22; 23], suggesting that the local emissions also affect the local air quality. Figure 4: Correlation plots of various chemical components with respect to PM\({}_{2.5}\). (**a**) Sulfate, nitrate, and ammonium. (**b**) OC and EC. (**c**) Total trace metals and total PAHs. Typical examples of days when only sulfate was high were on 26 March and 17 April. The air mass moved northward from the south of the Kyushu area to reach Fukuoka, as shown in Figure 5b. This pattern probably affects the air quality in Fukuoka by increasing sulfate, which was generated by the oxidation of SO\({}_{2}\) from volcanoes such as Sakurajima and Suwanosejima. On 26 March, the back trajectory passed over both volcanoes. The weather chart shows that a high-pressure system remained in the south of Japan, which is a typical atmospheric pressure pattern for the southernly wind in the Kyushu area. A similar pattern was seen on 17 April.