## 2 Materials and Methods
### PAH Analysis  
Methods of PAH analysis in this study were almost the same as in the previous study, although the analytes were somewhat different. In this case, 15 PAHs and one methylated substance were selected for quantification: acenaphthylene, phenanthrene, anthracene (Ant), 3-methylphenanthrene (Mph), fluoranthene (Flt), pyrene (Pyr), benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene,  
Figure 1: Location of the sampling sites. Suburban site: 32\({}^{\circ}\)47\({}^{\prime}\)07.9\({}^{\prime\prime}\) N, 129\({}^{\circ}\)51\({}^{\prime}\)52.6\({}^{\prime\prime}\) E; Forest site: 32\({}^{\circ}\)54\({}^{\prime}\)32.4\({}^{\prime\prime}\) N, 129\({}^{\circ}\)44\({}^{\prime}\)33.5\({}^{\prime\prime}\) E.  
dibenz[a,h]anthracene, indeno [1,2,3-cd]pyrene, benzo[ghi]perylene, and coronene (Cor). In this study, these were denoted as \"PAHs\". These standard reagents, apart from Ant, Mph, and Cor, were purchased from AccuStandard Inc. (New Haven, CT, USA). Ant was purchased from NACalai Tesque, Inc. (Kyoto, Japan), while Mph and Cor were purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan). Acenaphthene-d\({}_{10}\), phenanthrene-d\({}_{10}\), pelyrene-d\({}_{12}\), and pyrene-d\({}_{10}\) were used as surrogate compounds for precise measurements. The first three were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), and pyrene-d\({}_{10}\) was acquired from Kanto Chemical Co. Inc. (Tokyo, Japan). Silica gel C-200 was purchased from Wako Pure Chemical Industries, Ltd. for cleanup and the analytical-grade solvents were purchased from NACalai Tesque, Inc.  
The filters were stored in a desiccator for more than 2 days. The filter was cut into pieces, placed in a centrifuge tube, and 100 ng of each of the four surrogates was added. Further 30 mL of acetone was introduced, and ultrasonic extraction was performed for 10 min. After centrifugation, the extract was filtered through a cartridge with a pore size of 0.45 \(\upmu\)m. This procedure was repeated three times. The filtrate was concentrated to approximately 1 mL using nitrogen blow. Polar compounds in the solution, components that interfere with the analysis, were removed by adsorption on 15 mL of 5% H\({}_{2}\)O deactivated silica gel. The component containing PAHs was eluted with 50 mL of an acetone-dichloromethane mixture (3:2, \(v/v\)). This eluate was collected and concentrated for GC/MS analyses.  
The eluted samples were dehydrated and concentrated to 1 mL using nitrogen blow. The PAHs were determined using a 7890A gas chromatograph (Agilent Technologies, Santa Clara, CA, USA) equipped with a 7000A mass spectrometer (Agilent Technologies). An aliquot of 1 \(\upmu\)L was injected into an HP-1 MS capillary column with 1.4 mL/min helium gas. The injector temperature was maintained at 300 \({}^{\circ}\)C. The column temperature was programmed as follows: 60 \({}^{\circ}\)C for 1 min, heat to 300 \({}^{\circ}\)C at 10 \({}^{\circ}\)C/min, and hold for 5 min. The molecular weight of each PAH and deuterium-labeled PAH was used for quantification in selected ion monitoring (SIM) mode. The concentration of the target component was quantified by the internal standard method using the standard for each PAH and the surrogate described above.