## 3 Results and Discussion
### PM10-Bound PAH Profiles
While PM10 concentrations were not significantly different in the summer and rainy season, the mean concentrations of most PAHs were noticeably lower in the summer, except for IP at the urban background site and DBA at the roadside and industrial sites. Most pPAH concentrations in the winter and rainy season at the roadside and industrial sites were higher than summer concentrations. At the background site, some  
Figure 2: Annual mean concentrations of 15 PAHs found at the roadside, urban background and industrial sites in the BMA from May 2013 to May 2014.  
pPAHs including BaA, BaP and BP were found at higher concentrations in the rainy season than in the winter. BP, IP, BBF and BaP were the most abundant pPAHs at all sampling sites. The previous study found that IP and BP were the highest pPAHs in Bangkok ambient PM10 from November 1999 to November 2000 [30]. BaP concentrations were the fourth highest among 15 PAHs concentrations measured but appeared to be significantly lower than the top  
Figure 3: Seasonal mean concentrations of individual PAHs at the roadside site, industrial site and urban background site respectively. Weather conditions were hot and humid in the summer, frequent rain in the rainy season and dry and cool in the winter. three PAHs. In this study, BP, IP, BBF and BaP appeared in the same order; however, they were at significantly lower concentration ranges, despite similar annual average PM10 concentrations. The annual mean BaP concentrations in 2000 were found at 4 to 5 ng m\({}^{-3}\) at roadside sites. The results of roadside PAH concentrations pointed to a significant reduction of traffic-related pPAHs over the past decade. The time series variability of daily PM10 concentrations evaluated for BaP and carcinogenic PAHs (seven PAHs: BaA, CHR, Bbf, Bbf, Bbf, Bap, IP and DBA) showed no major insight and are provided to the reader within the Supplementary Information (Figure S2a-c). Though variable, there is a general increasing trend of PM10 concentrations that began during the dry season in November 2013 and reached the highest concentration in January 2014 at all study sites. Total PAH concentrations were highest in the rainy season at the roadside and urban background sites, and in the winter at the industrial site. The total PAH and PM10 concentrations exhibited similar variation patterns throughout the measurement, indicating consistent emission sources. In contrast, daily PM10 and PAH concentrations were inconsistent at the roadside and urban background sites, which implied that they originated from various emission sources. Seasonal mean BaP concentrations were compared at three sampling sites during summer (mid-February to mid-May), rainy season (mid-May to mid-October) and winter (mid-October to mid-February), as shown in Figure 4. The dominant wind direction and the relationship between PM10 and PAHs for each season can be found in the Supplementary Information (Figures S2-S4). The mean BaP concentrations at the roadside site were significantly higher in the winter (0.68 \(\pm\) 0.28 ng m\({}^{-3}\)) and rainy season (0.55 \(\pm\) 0.44 ng m\({}^{-3}\)) than other sites. Seasonal variations of BaP showed the highest mean concentration at the roadside site in winter at 0.68 \(\pm\) 0.28 ng m\({}^{-3}\) together with the highest mean PM10 concentration of 92.03 \(\pm\) 35.27 \(\mu\)g m\({}^{-3}\). Seasonal mean BaP concentrations were lowest in the summer and no significant difference was found among summer mean concentrations at all sites. Figure 4: Boxplots comparing seasonal mean BaP concentration (ng m\({}^{-3}\)) at three sampling sites (Rd: roadside; Ind: industrial; Bg: urban background) in the winter (W), summer (S) and rainy season (R) [providing median (horizontal line), 25 th to 75 th percentile (box), data range (vertical line), and outliers (* points)]. At the roadside site, the mean BaP concentration was significantly lower in the hot and humid conditions of summer (0.24 \(\pm\) 0.25 ng m\({}^{-3}\), \(p\) = 0.015) than in other seasons.