## 3 Results and Discussion  
### Atmospheric Concentrations of PAHs and OH-PAHs  
The highest concentration of total PAHs was found in winter 2018 (daily average \(\pm\) standard deviation: 1.00 \(\pm\) 0.26 ng/m\({}^{3}\)), followed by spring 2017 (0.86 \(\pm\) 0.56 ng/m\({}^{3}\)), autumn 2017 (0.66 \(\pm\) 0.23 ng/m\({}^{3}\)), spring 2018 (0.55 \(\pm\) 0.30 ng/m\({}^{3}\)), and summer 2017 (0.30 \(\pm\) 0.09 ng/m\({}^{3}\)) (Figures 1 and S1) [2,34]. The total PAH concentrations in spring 2017 and winter 2018 were significantly higher than those in summer 2017 (\(p=0.02\) and \(<\)0.01, respectively). These results are consistent with previous reports of higher PAH concentrations with combustion-derived PAH emissions in the winter and lower concentrations in the summer [4,13,14]. The concentrations of atmospheric PAHs in this study were similar to or lower than those in previous atmospheric surveys in Japan [14,30,37], China [38,39], and other Asian countries [40]. The current results were considered to reflect the regulation of emissions in Japan and neighboring countries [4]. Since 1966, the Japanese government has gradually strengthened regulations regarding the emissions of pollutants from new vehicles [41]. Similarly, the Chinese government has adopted a series of policies aimed at reducing emissions [42]. Pyr was the dominant PAH (23.7 \(\pm\) 5.5%), followed by Flt (19.6 \(\pm\) 5.3%), BbF (12.6 \(\pm\) 1.9%), and BgPe (12.1 \(\pm\) 2.8%). This trend was almost consistent throughout the year. Low molecular weight PAHs (4 rings PAHs) had higher percentages than high molecular weight PAHs (5 and 6 rings PAHs), which was considered to be caused by emission source and differences in the vapor pressures among PAHs. Similarly, the total OH-PAH concentration was highest in winter 2018 (75.06 \(\pm\) 23.38 pg/m\({}^{3}\)), followed by autumn 2017 (57.36 \(\pm\) 33.84 pg/m\({}^{3}\)), spring 2017 (49.81 \(\pm\) 24.12 pg/m\({}^{3}\)), spring 2018 (36.62 \(\pm\) 18.33 pg/m\({}^{3}\)) and summer 2017 (17.08 \(\pm\) 4.83 pg/m\({}^{3}\)) (Figures 2 and S2). The total OH-PAH concentration in autumn 2017 was significantly higher than in summer 2017 (\(p\) = 0.02), and in winter 2018 was significantly higher than those in summer 2017 and spring 2018 (\(p\) < 0.01 and \(p\) = 0.03, respectively). Similar to previous studies [22; 43], the concentration of total OH-PAHs was approximately 6.9-24.8 times lower than that of the total PAHs (11.93-129.3 and 177.9-1757 pg/m\({}^{3}\), respectively). On average, 2 OH-Nap was the dominant PAH (20.1 \(\pm\) 11.7%), followed by 2/3 OH-Phe (16.5 \(\pm\) 3.0%), 1 OH-Flu (14.8 \(\pm\) 3.4%), and 2/3 OH-Chr (12.0 \(\pm\) 2.9%). Atmospheric OH-PAHs (OH-Pyr and OH-Flu) were detected at lower concentrations (1 OH-Pyr: 5.53 \(\pm\) 4.22 and 2 OH-Flu: 1.45 \(\pm\) 1.12 pg/m\({}^{3}\)) to those previously reported for Nagasaki, Japan, in 1997-1998 (1 OH-Pyr: 21.48 \(\pm\) 14.97 and 2 OH-Flu: 10.40 \(\pm\) 10.71 pg/m\({}^{3}\)) [21]. The dominant component was 2 OH-Nap in spring 2017/2018 (17.4%/22.8%) and summer 2017 (35.9%), but 2/3 OH-Phe in autumn 2017 (19.6%) and winter 2018 (15.9%). This was probably caused by differences in the emission source [22; 23] and atmospheric conditions [26; 27]. [PERSON] et al. [24] reported more than 40 times lower concentration of 2 OH-Nap compared to 2/3 OH-Phe and these were considered to be emitted from exhaust gas. Moreover, [PERSON] et al. [17] found the same levels of 2/3 OH-Phe as 2 OH-Nap from wood combustion. The current results may reflect these emission source differences. Compared to the results reported by [PERSON] et al. [24] for samples collected in Kanazawa in July 2013, the concentration of 2 OH-Nap detected in the present study (6.14 \(\pm\) 2.59 pg/m\({}^{3}\) in August) was much higher (20 \(\pm\) 16 pg/m\({}^{3}\) in July in previous study). Additionally, the proportion of 1 OH-Pyr in the present study was much lower than in the study by [PERSON] et al. (8.9% in the present study, and 48% calculated from reported values in the previous study).